Control of Ventilation

Now we're moving onto learning about respiration! Yay...? Also, this lecture has a hell of a lot of aims, so hold onto your hat!

Describe the central generation of respiratory rhythm, including the three common hypothesis.

As mentioned here, generation of respiratory rhythm originates from centres in the medulla of the brain. There are several different hypotheses about how these centres interact:

1. Off-switch model: In this model, inspiratory motor neurons stimulate inspiration. Inspiration then feeds back onto some integrating neurons, which then stimulate an "off-switch" to stop inspiration. Eventually, the lack of inspiration stops the "off-switch" neurons, causing the inspiratory neurons to start up again.
2. Oscillator model: In this model, both inspiratory and expiratory neurons are constitutively activated. Inspiratory neurons can activate interneurons that turn the expiratory neurons off, and vice versa. If the timing is right, they can inhibit each other at the right times, generating a rhythm.
3. Pacemaker kernel model: This model suggests that there are some cells that act as a pacemaker. Indeed, some cells in the pre-Bötzinger complex do show synchronised pacemaker spikes. Glutamate inhibitors can block this synchrony.

The off-switch model and oscillator models are also known as "distributed network models," as they require multiple different groups of cells to work together.

Define apnoea, hyperpnea, hypopnea, gasping, apneusis.

• Apnoea: Lack of breathing. May result from damage to the medulla.
• Hyperpnea: Increased breathing
• Hypopnea: Reduced breathing
• Hyperventilation: Increased breathing that goes beyond what your body actually needs. (Note that hyperpnea and hyperventilation both involve increased breathing, but hyperventilation is inappropriate to the situation, whereas hyperpnea is totally appropriate.)
• Apneusis: Prolonged inspirations with short expirations. May occur due to damage to the pons. (This is where the pneumotaxic centre is, as I'll explain in a bit.)
• Gasping: The opposite of apneusis: prolonged expirations with short inspirations.

Describe the brain regions involved in the respiratory rhythm.

See earlier post: Control of Ventilation

Explain the function of the pneumotaxic centre.

See earlier post: Control of Ventilation. Also, as mentioned earlier in this post, the pneumotaxic centre is located in the pons, so damage to the pons causes apneusis.

Identify simple respiratory patterns and how they arise

I'm not really sure what I'm meant to know for this. Apneusis maybe? But I just wrote about that...

Describe the location of the central and peripheral chemoreceptors.

The central chemoreceptors are located in the ventral medulla, whereas the peripheral chemoreceptors are located in the carotid body and aortic arch.

Describe the stimuli that these receptors respond to.

Central chemoreceptors respond to the pH of the cerebrospinal fluid, which actually allows them to respond to CO₂ in a very roundabout way. CO₂ can diffuse through the blood-brain barrier, where it can form H⁺ and HCO₃^- by reacting with water. An increase in H⁺ decreases the pH, which is detected by the central chemoreceptors. It's unclear exactly how the central chemoreceptors are activated. To add even more confusion, there are many cells in the brain that can detect pH: aside from the chemoreceptors in the ventral medulla, cells in the dorsal and ventral respiratory groups, pons and hypothalamus can all respond to pH. Le sigh.

Peripheral receptors can respond to CO₂, H⁺ and O₂, though to my understanding they respond mostly to O₂. Also, carotid bodies may be better at sensing H⁺ than aortic bodies.

Peripheral receptors can respond more rapidly than central chemoreceptors as they don't have to worry about gases diffusing through the blood-brain barrier and whatnot.

Recall the sensory input into control of ventilation, the effectors controlled and where control occurs.

Not sure what exactly I'm supposed to put here that I haven't put under another heading, so I'm just going to shove a link to my old post on ventilation control here and call it a day.

Explain how the peripheral chemoreceptors sense P_O2 and P_CO2

The cells of the peripheral chemoreceptors that detect O₂, CO₂ and pH are called glomus cells. Glomus cells are excitable and can release neurotransmitters, just like nerve cells.

CO₂ can diffuse into glomus cells and cause a change in pH, just like they do in the brain. The increased H⁺ ions can protonate and close calcium-activated potassium channels in the membrane of these cells. As potassium can no longer leave, the cell becomes more and more depolarised, eventually leading to action potentials.

O₂ is detected via a different mechanism. O₂ can be converted into carbon monoxide by an enzyme called haemoxygenase. Carbon monoxide is usually known as a poisonous gas, but it can also serve as a signalling molecule in the cells. In this case, carbon monoxide causes CO-gated potassium channels to open. This keeps the membrane potential low, inhibiting action potentials. If O₂ levels drop, CO levels also drop, allowing these channels to close. (This usually happens when P_O2 < 60mmHg.) As mentioned before, stopping potassium from leaving causes the cell to depolarise and action potentials to be produced.

Explain the response to changes in blood oxygen, carbon dioxide and pH.

As alluded to before, low levels of oxygen (<60mmHg) cause a large increase in ventilation. Aaaaand I don't really have much else to say here.

Most of the response to carbon dioxide (60-80%) occurs via central chemoreceptors, though peripheral receptors play a part too, as discussed above.

Another interesting phenomenon to take note of is that of "synergistic drives." Essentially, this means that when oxygen levels are low, your cells become more responsive to high CO₂, and vice versa. This is because the detection of oxygen and carbon dioxide occur through similar mechanisms: the opening or closing of potassium channels of glomus cells.

Smooth Muscle Mechanics and Adhesion

So far, I've talked a lot about things that can trigger contraction in smooth muscle, but I haven't spoken about how that contraction is actually caused. Well, now's the time!

Define dense bodies, dense bands and adherens.

Dense bodies are so-called because they look like dense, dark splotches under an electron microscope (I think). These are locations where actin filaments in the cell are cross linked with α-actinin.

Dense bands are very much like dense bodies, in that they also contain actin filaments cross-linked with α-actinin. Dense bands, however, also help form junctions between the cell and the extracellular matrix, or between the cell and other cells. These junctions are also known as adherens.

Explain the how smooth muscle cells attach to other cells and extra-cellular matrix.

As I just mentioned, junctions form at dense bands, which consist of actin filaments cross-linked with α-actinin. There are a few more proteins involved, however.

In a cell-matrix junction, actin and actinin associate with other proteins such as vinculin and talin. Talin is probably the more important one for our purposes, as it also binds to integrin. Integrin is a transmembrane protein that can connect to proteins in the extracellular matrix, such as fibronectin. Hence, integrin is sometimes known as the fibronectin receptor. In this way, actin filaments are anchored to the extracellular matrix.

In a cell-cell junction, actin and actinin still associate with vinculin, but they associate with catenin instead of talin. Catenin binds to cadherin, which, like integrin, is a transmembrane protein. Cadherin's binding site, however, is the cadherin on a neighbouring cell. Hence, cadherin serves as a "bridge" that links multiple cells together.

Recall and explain the crossbridge cycle and how it is regulated in smooth muscle.

As you may or may not know, contraction in cells relies on the movement of different filaments. Firstly, I'm going to take a step back and describe the structure of the filaments.

Thick filaments are polymers of myosin II, which has two heads. (There's also a myosin I, but that's not important for contraction. Myosin I has only one head, and it can bind and transport vesicles down actin.)

Thin filaments are polymers of actin and tropomyosin. Another fun fact to store for later (by later I mean probably a future lecture so you don't have to hold this in your head for now): monomers of actin are also known as G-actin, whereas polymers of actin are known as F-actin, or filamentous actin.

Now it's time to explain the crossbridge cycle!

Before binding can occur, the myosin head has to be in the "cocked" position. This occurs when ATPases break down ATP into ADP and Pi (phosphate). From this position, it can bind to actin, releasing a phosphate at the same time. The release of this phosphate causes a conformational change in the myosin from the cocked to the uncocked position, and it's this that causes the movement (also known as the "power stroke"). This movement triggers the release of ADP from the myosin head, which causes unbinding of myosin and actin. (Fun fact: in rigor mortis, the ADP is unable to unbind, causing stiffness.) Release of ADP also allows ATP to bind, which can then be broken down by ATPases, cocking the myosin head for the next cycle.

In smooth muscle, the rate-limiting step is the ATPase part. ATPases in smooth muscle require the regulatory light chain of myosin to be phosphorylated by myosin light chain kinase (MLCK), as mentioned here.

Explain relationship between length and tension in smooth muscle.

The optimum length for contraction occurs when myosin and actin overlap each other, allowing the maximum number of cross-bridges between the two to occur. When the muscle is made longer, less overlap occurs and less force is produced. When the muscle is made shorter, force can still be generated, but if you make the muscle too short, dense bodies might get in the way of contraction.

It's also important to note that, in smooth muscle, all of the actin/myosin combinations within the muscle are in different stages of contraction (i.e. it's not coordinated as nicely as skeletal muscle).

Define pre-load and after-load, isometric and isotonic contractions.

Pre-load and after-load have pretty much the same definitions in smooth muscle as they did in cardiac muscle. Pre-loads are loads that stretch the muscle to its starting point, and after-loads are loads that the muscle contracts against.

Isometric contractions are contractions in which the length stays the same. Just like in cardiac muscle, this is due to after-loads being too great. Muscles undergoing isometric contraction generate force without shortening.

Isotonic contractions are contractions in which the force stays the same. These may occur when there is little to no afterload, allowing the muscle to shorten without generating force.

Why does this happen? Well, force is generated when actin and myosin are bound. Muscle can't shorten when actin and myosin are bound- you need the full crossbridge cycle to occur multiple times for muscle shortening to occur. So, in short: when actin and myosin are attached, force is generated; when actin and myosin are moving, movement is generated.

Explain the relationship between force and velocity for smooth muscle in terms of crossbridge function

This is pretty much related to the point that I just made. If crossbridges are all attached and are therefore not moving, maximal force is generated, but velocity is essentially 0. As crossbridges begin to move, force decreases but velocity increases.

Overview of Metabolism

This lecture was a nice little refresher on concepts from 1st and 2nd year. Enjoy :)

Explain the processes of cellular energy generation - the three phases of metabolism and the cell compartments involved.

The phases mentioned in the lecture are glycolysis (in the cytosol), the citric acid cycle (in the mitochondria) and oxidative phosphorylation (in the inner membrane of the mitochondria). I've blogged about all of these before, so here are some posts that explain each of these processes:

• Glycolysis and the citric acid cycle- Cellular Respiration and Protein Synthesis
• Oxidative phosphorylation- Cell Biology- Introduction and Energy Production

Explain the relationship between anabolic and catabolic reactions and the energy requirements of the cell.

See earlier post: Cellular Respiration and Protein Synthesis

Explain redox potentials.

See earlier posts: Basics of Redox and Redox again- Electrolytic cells and some other stuff

Understand the role of activated carrier molecules.

The carrier molecules covered in this lecture were NADH, NADPH, FADH₂ and acetyl CoA. I've covered the first three in an earlier post: Overview of How the Cell Works. Acetyl CoA (acetyl coenzyme A) has a high-energy bond which facilitates the transfer of two carbon atoms to another molecule. This is important in processes such as fat anabolism, as mentioned here.

Exercise Metabolism

Now I've finally gotten through that post with all the graphs, I can move on with my life! I'm going to write a bit about biochemistry, because surprisingly enough, that's been the easiest subject to write about so far.

This is the last lecture about nutrition, and it probably has more new content in it than all of the other nutrition lectures combined (not like that's an amazing achievement). Enjoy :)

Be familiar with the different fiber types and their metabolic characteristics

There are two main types of muscle fibres: type I and type II. Type II can be further subdivided into type IIa and IIb. Most muscles have a mixture of different types of fibres- it's the proportions that matter.

Type I fibres have a low power output, but they are very fatigue resistant. Muscles that have a high proportion of these fibres include postural muscles and other muscles that work pretty much all the time without you even noticing. They use mostly oxidative metabolism due to their abundant mitochondria, which contributes to their fatigue resistance.

Type IIa fibres have a higher power output. They are not as fatigue resistant as type I fibres, but they still do reasonably well. They generally use a mixture of oxidative and glycolytic metabolism.

Type IIb fibres also have a high power output. They are readily fatigued as they mostly use glycolysis to generate energy. I've mentioned here that glycolysis on its own can result in lactic acid production, resulting in pain and fatigue.

Be able to explain the different types of fuels which can be used during exercise.
Know the advantages and limitations of the different types of metabolic pathways.

As you should know by now, ATP is basically the energy currency of the cell. The main fuels used to produce ATP are glycogen, fatty acids and phosphocreatine (PCr). The first two are probably already pretty familiar to you, so let's talk about phosphocreatine first.

Phosphocreatine (PCr) can be broken down rapidly into creatine and ATP during bursts of heavy activity. It can produce ATP pretty quickly, which is nice, but unfortunately we only have small reserves of PCr to draw on. Fortunately, however, it is fairly easy to restore phosphocreatine levels once you're resting: creatine and ATP can combine back together to form PCr, ADP and H⁺.

Now let's go back into the familiar stuff!

Glycogen stores can be broken down to form glucose and, as mentioned here, glucose can be broken down further via glycolysis. If limited oxygen reserves are available, the process pretty much stops once the glucose becomes pyruvate, at which stage it gets converted into lactic acid. This still produces some energy, and it produces it fairly quickly (in seconds), but this pathway can cause fatigue. If oxygen is available, pyruvate can enter the citric acid cycle to produce more ATP. This process doesn't cause fatigue, but is slower and has a limited capacity to supply ATP (possibly due to limitations on oxygen supply or mitochondria ability).

Fatty acids can also become oxidised to release ATP. Some of the pathways involved, like the citric acid cycle, are similar to those involved in the breakdown of glucose. The limitations are also similar: the process is slow, especially because releasing fatty acids and delivering them to the muscle takes time. It might take around 30min for fatty acid oxidation to become optimal. Also, we only have a limited ability to oxidise fatty acids, so more carbohydrates get used as exercise intensity increases.

Know which metabolic pathways (eg aerobic and anaerobic) are used during different types of exercise
Be able to explain which metabolic pathways are used as exercise intensity increases

All of the pathways pretty much get started as soon as the muscles move, but obviously the faster pathways (the anaerobic ones) will kick in sooner. Hence, short, intense exercise (~5sec) will use mainly PCr at first, which gradually gets replaced by mostly glycolysis at around 30sec (i.e. just the glycolysis to lactate part, not the citric acid cycle part) to eventually mostly oxidative pathways at around 2-3min.

Be able to explain VO₂ max.

This wasn't actually in the lecture outline, but it looks important, so I'm going to include it. VO₂ max is a measure of the maximum capacity to transport and utilise oxygen, as well as an indirect measure of aerobic ATP consumption. It can be measured by exercise tests. Well-trained subjects will usually have a higher VO₂ max as they have a higher density of mitochondria, more enzymes for the citric acid cycle, a higher maximum cardiac output and more muscle and cardiac capillaries.

Be able to explain how the different types of exercise affect protein synthesis in muscles.

The main types of exercise are aerobic ("cardio") and resistance (weight-lifting etc.). Aerobic exercise increases the synthesis of mitochondrial proteins and of type I and IIa fibres. Resistance exercise increases muscle mass and type IIb fibres.

Venous Pressure and Vascular Function Curves

This lecture was more complicated than it looked at first, which isn't all too helpful given that I can feel a headache coming on :P Oh well. This will be our last lecture on cardiovascular physiology for this unit for a little while.

Explain how central venous pressure determines cardiac pre-load and output.

As you should hopefully know by now, blood flows from a place of higher pressure to a place of lower pressure until the pressures are equal. Hence, blood will flow from the veins into the ventricles of the heart during diastole until the pressures are equal. The central venous pressure thus determines the amount of filling, which determines cardiac pre-load. As covered here and here, increased end-diastolic volume/increased pre-load increases stroke volume and, by extension, cardiac output.

Understand how central venous pressure is regulated through the vascular function curve, including the actions of blood volume, vasotone and veno-tone.

Blood volume alters venous pressure pretty much the same way it alters arterial pressure: increasing blood volume increases pressure and decreasing blood volume decreases pressure. So far, so good.

The effects of vasotone (arterial contraction) on central venous pressure are less intuitive. If vasotone increases, less blood can get through to the venous side. (Remember, flow is inversely proportional to resistance, and resistance increases as you constrict the blood vessels). Hence, increased vasotone will actually decrease venous pressure.

The effects of venotone (venous contraction) are a bit easier to grasp. Increased venous constriction will increase the pressure, and vice versa.

The relationship between cardiac output and central venous pressure can be plotted on the vascular function curve:

This graph can be a bit tricky to interpret. For starters, you actually have to read it the "wrong way around": you have to find a venous pressure from a given cardiac output, not the other way around. It's done this way so that both the vascular function curve and the cardiac function curve can be plotted on the same axes, as you shall soon see.

Another thing you'll notice about this graph is that the graph levels out below a certain venous pressure. This is because ventricular filling depends on a pressure gradient between the veins and the ventricles, and if venous pressure drops to the same pressure or lower than ventricular pressure, then the ventricles cannot fill any more and cardiac output cannot be increased. One more thing to point out is the x-intercept at the graph: this point represents the venous pressure if there was no cardiac output (i.e. if the heart had stopped and the blood pressure was allowed to simply equilibrate throughout the entire system of blood vessels). This pressure is also known as the mean circulatory (MC) pressure, and is usually around 7mmHg.

When blood volume is increased, such as during a blood transfusion, the entire graph shifts up and right. A greater cardiac output can be produced, and the MC pressure is higher. (Remember, if you have more "stuff" in what is essentially the same amount of space, you're going to have a higher pressure.) The reverse is true for when blood volume is decreased.

Now, blood vessels are not just passive: they can constrict and relax. How does that affect the vascular function curve?

Venoconstriction has a similar effect to increasing the blood volume: it raises the curve up and right. It also changes the slope of the curve slightly (the above picture is possibly exaggerated), leading to a higher possible venous pressure at zero cardiac output. This makes sense: pressure is inversely proportional to volume, so if you constrict the veins, thereby reducing the space inside, you'll increase the pressure. Vasoconstriction, on the other hand, does not change venous pressure when cardiac output is zero. (I'm not sure why- I'm thinking it's because arteries are smaller than veins and thus vasoconstriction has less of an overall effect on volume than venoconstriction? That's something I'll have to check.) Maximum cardiac output is greatly reduced- as mentioned here, MAP = CO*TPR. Rearranging this equation gives CO = MAP/TPR, so it follows that an increased peripheral resistance, as occurs during vasoconstriction, will decrease cardiac output.

Understand how cardiac output is set by the interaction between the vascular and cardiac function curves.

Some of the stuff in this post might seem a bit unintuitive, especially given the relationship between diastolic volume (and venous pressure) and cardiac output. In previous posts, I've said that stroke volume, and therefore cardiac output, is dependent on ventricular filling, which in turn is dependent on venous pressure. However, venous pressure is also dependent on cardiac output! Increasing cardiac output increases the amount of blood taken out of the veins and put into the arteries, and thus decreases venous pressure. So how can we put all of this together into a coherent picture?

The answer is simply to use two graphs: the vascular function curve, which I mentioned above, as well as the cardiac function curve (the one I showed you when talking about the Frank-Starling Law). These two graphs can be plotted on the same axes. The place at which they overlap gives the cardiac output and central venous pressure at steady state.

When there is a deviation from steady state, the cardiac output and venous pressure will gradually adjust until steady state is reached. As an example, let's say that we have some venoconstriction, raising the venous pressure:

Venoconstriction raises the venous pressure to point A. From the cardiac function curve, that venous pressure will cause the cardiac output to increase to point B. However, from the vascular function curve, that cardiac output should cause the venous pressure to decrease to C'. Because of the conflicting results from these curves, the venous pressure decreases a bit, causing cardiac output to decrease, and then venous pressure decreases a bit more, and so on until steady state is reached again.

Any stimulus that causes one or both of these curves to change can change the steady state. For example, as mentioned here, increased inotropy will raise the cardiac function curve:

As can be seen in the graph above, changing the cardiac function curve will also change the intercept between the vascular and cardiac function curves.

Of course, there are many different stimuli that can affect the curves, but I feel like I've already drawn more than my fair share of crappy Paint diagrams for this post. If you want to see how a particular stimulus will affect steady state, why not draw a diagram for yourself and find out?

Pharmacological Targets for Controlling Hypertension

We finally get the lecture on the stuff we were quizzed on last week! I love it when that happens... not...

Ca²⁺-channel blocking drugs

I've explained quite a bit about the role of calcium in the cardiovascular system (see here for cardiac muscle and here, here and here for smooth muscle). In a couple of those posts, I also mentioned that there are drugs available that block this system. Here's a brief list:

• Cardioselective (i.e. selective on the heart): Verapamil
• Vascular selective (i.e. selective on the blood vessels): Dihydropyridines such as nifedipine, felodipine and amlodipine (all end with -dipine)
• Non-selective (i.e. act on heart and blood vessels): Diltiazem

RAAS blockers

An explanation on the RAAS system is given here.

• ACE (angiotensin-converting enzyme) inhibitors: Ramipril, perindopril, many other drugs ending in -pril
• AT₁ blockers/ARBs (angiotensin-receptor blockers): Candesartan, irbesartan, many other drugs ending in -sartan

α₁ and β₁ adrenoceptor blockers

Activation of β₁ receptors, present on the heart, increase heart rate and contractility in order to increase cardiac output and therefore blood pressure . Hence, drugs that inhibit these receptors decrease heart rate and contractility. Drugs that inhibit these receptors include metoprolol, atenolol and many other drugs ending in -olol.

Activation of α₁ receptors, present in many blood vessels, cause vasoconstriction. This increases total peripheral resistance and, by extension, blood pressure. Prazosin, which inhibits these receptors, decreases peripheral resistance and blood pressure.

Diuretics

Diuretics work by reducing your blood volume, generally by making you pee more. A certain class of diuretics, known as thiazides, work by blocking reabsorption of Na⁺ in the distal tubule. If this makes no sense to you, I've written quite a bit on reabsorption in the kidneys here and here.

Choice of Drugs

Given all of this, which drugs do we use? Here's the approach that seems to give the most benefit for the least risk:

• Start off with a thiazide, ACE inhibitor, ARB OR Ca²⁺ channel blocker
• If control is still inadequate, add a second or third drug
• If control is still inadequate, try an α- or β-blocker

Calcium Sensitisation

Recall and integrate the roles of Ca²⁺, calmodulin, MLCK and myosin RLC in activation of smooth muscle.

As mentioned here, Ca²⁺-calmodulin complexes can activate myosin light chain kinase (MLCK). MLCK can then phosphorylate serine 19 on the myosin regulatory light chain (RLC). This then allows it to interact with actin, which is important for contraction.

Another important molecule to take note of is MLCP, or myosin light chain phosphatase. This reverses the action of MLCK. Relative amounts of MLCP and MLCK are important in determining whether the cell is contracting or relaxing. I'll get to MLCP in a bit.

Explain the evidence for a role of Ca²⁺ and calmodulin in contraction.

We know that there is a role of Ca²⁺ as skinned fibres (i.e. muscle fibres with all of their lipid content removed- including membranes) only display contraction when calcium levels are high enough. We also know that there is a role of calmodulin in contraction because increased amounts of calmodulin, at the same concentration of Ca²⁺, can cause greater amounts of contraction.

Define calcium sensitisation and give evidence for its existence.

Calcium sensitisation refers to the generation of relatively large amounts of force from relatively small amounts of calcium. Some of the evidence for this comes from experiments done with calcium-binding dyes. Depolarisation-induced contractions show high levels of intracellular calcium that last for a while, whereas agonist-induced contractions show high levels of intracellular calcium that quickly drop off without having any deleterious effect on the contraction. Other studies have shown that agonist-induced contractions can display the same amount of force or greater than a depolarisation-induced contraction at the same concentration of calcium. These results, taken together, suggest a sensitisation effect: the cell becomes more "sensitised" to calcium so that less is required to have the same effect.

Explain how calcium sensitisation is produced through the rho-ROK and PKC pathways.

Rho is a G-protein, which refers to the fact that it binds GTP (it is NOT related to the trimeric G-proteins that I've covered when talking about G-protein coupled receptors). Rho activates Rho Kinase (ROK), which phosphorylates and inhibits MLCP. Hence, Rho makes it more likely that MLCK will overpower MLCP, maintaining contraction even at lower amounts of Ca²⁺.

A second possible method of sensitisation occurs through the PKC pathway. As I've mentioned before, phosphatidylinositol breaks up into IP₃ and diacylglycerol (DAG). I've already spoken about the effects of IP₃ in smooth muscle here, so let's talk about DAG. DAG can activate protein kinase C (PKC), which also phosphorylates and inhibits MLCP. PKC can also phosphorylate another protein called CPI-17, which also inhibits MLCP. CPI-17's method of inactivating MLCP is slightly different: it does its job by binding to MLCP's active site (rather than phosphorylating it).

Recall and explain the functions of rho, ROK, MLCP, DAG, PKC and CPI-17 in smooth muscle contraction

This is basically just a summary of the points I've touched on in this post:

• Rho- G-protein that activates ROK.
• ROK (Rho Kinase)- phosphorylates and inhibits MLCP.
• MLCP (Myosin Light Chain Phosphatase)- dephosphorylates myosin, stopping contraction.
• DAG (diacylglycerol)- activates protein kinase C.
• PKC (protein kinase C)- phosphorylates and inhibits MLCP. Also phosphorylates and activates CPI-17.
• CPI-17- inhibits MLCP by binding to its active site.

Hypertension Treatment- Success or Failure?

Another blog post for PHAR3303! This lecture was actually scheduled for this upcoming Tuesday, but since our lecturer can't make it, last year's lecture has been uploaded online. I've already watched it because I clearly had nothing better to do yesterday :P

I'm really not sure what to write about for this post- there's no lecture outline, and most of this lecture consists of random statistics and stuff. I'm assuming that we don't need to memorise the statistics, and that they're just there for context, so that doesn't leave me with much else to write about. Oh well.

History

Quite a few prominent figures over the past century, such as Stalin and President Roosevelt, suffered strokes and/or other complications related to hypertension. In the 30s to 40s, however, pretty much all we had to treat hypertension were salt free diets, sedatives and even removal of the adrenal glands. It wasn't until the 50s that hypertension research really took off, with some early drugs such as phenoxylbenzamine (an adrenergic blocker with serious side effects).

As mentioned here, the Framingham Study helped us to find out a lot about hypertension and cardiovascular disease. Another important initiative, called the National Health Blood Pressure Educational Program (NHBPEP) started in 1983 to help monitor the effects of drugs and lifestyle changes on hypertension. The NHBPEP also created some guidelines. Despite all of this, we still haven't seen that much improvement in the past 10 years or so. Hypertension, and its associated complications, are still very problematic today.

ASCOT

For some reason, half of this lecture was taken up with talking about the Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT), so I guess I'll talk about it for a little bit. This was a large study that enrolled 19 257 patients with hypertension. One group was taking atenolol (a β-blocker) and bendroflumethiazide (a diuretic), whereas the other group took amlodipine (a Ca²⁺-channel blocker) and perindopril (an ACE inhibitor). While both groups were relatively similar in terms of blood pressure control, the amlodipine/perindopril group had fewer CVD-related deaths, fewer strokes and improved outcomes on several other measures.

Moving forward

As mentioned, hypertension and its complications are still incredibly problematic. Even though malignant hypertension (see here) is rare, it'd be nice if we could do better.

It's been a while since the last "groundbreaking" class of antihypertensives (the ACE inhibitors). We haven't gotten anything better since then. There has, however, been a new kind of treatment suggested, called renal denervation (i.e. getting rid of some of the nerves between the kidney and brain as signalling between the two is partially responsible for influencing blood pressure). Evidence on this treatment, however, is still a bit mixed. Lifestyle changes are also important: smoking, less alcohol, weight loss, exercise and relaxation can all help reduce blood pressure.

And... that's it, I guess?

Cardiac Loads

According to the unit timetable, this is our second last lecture on the heart for a little while. How time flies...

Understand and explain changes in mean and pulse pressure.

Mean arterial pressure (MAP) and pulse pressure (PP) are measured by baroreceptors. (Note: baroreceptors do not measure systolic or diastolic pressure- only MAP and PP.) MAP and PP can be controlled by changes in the autonomic system, or directly via changes in the cardiovascular system such as end-diastolic volume (see here).

Explain and analyse changes in mean and pulse pressure in terms of changes in underlying CVS function.

The main determinants of mean arterial pressure are cardiac output and peripheral resistance. I've explained why this is so here. Main thing to note is that MAP = CO * TPR.

The main determinants of pulse pressure are stroke volume and arterial compliance. An increased stroke volume exerts more pressure on the arterial walls, causing an increased systolic pressure and thus an increased pulse pressure. The inverse is true for decreased stroke volume. As for arterial compliance, arteries that are not compliant (i.e. they are stiff) will require more pressure for the blood to pass through, thus also causing an increased pulse pressure. (At least, that's how I think this works. I'm not sure how accurate these explanations are.)

Predict changes in stroke volume and cardiac output from the pre-load, after-load and contractility of the ventricles.

I'm going to start off by defining pre-load and after-load in the context of muscles. Pre-load is the load that stretches the muscle. If you increase the pre-load, you increase the resting muscle length. After-load, on the other hand, is the load that a muscle contracts against. Increasing the after-load also increases the velocity and amount of muscle shortening.

Now that we've got that down, I'm going to introduce you to a length-tension curve:

Don't worry, I'll link the two together in a minute. Just let me explain this curve first.

For the diastole curve, you kind of need to read it the wrong way around, i.e. figure out what muscle fibre length (x-axis) you would get for a particular tension (y-axis). This is in contrast to the systole curve, in which you read it the right way around: you find out what maximum tension (y-axis) you could get from a particular muscle fibre length (x-axis).

Things get a bit more interesting when you consider the preload and afterload on the heart. Here's the same graph above but with a few more lines and symbols thrown in for funsies:

In the above curve, let's say we have some given pre-load, represented by the green circle. The muscle can begin to generate tension at this point, but while the tension is still less than the afterload, the muscle does not actually change in length. Hence, this period is known as isometric (same length) contraction. Isometric contraction continues until the tension generated exceeds the afterload, and then the muscle fibres begin to contract.

There's an issue, however. Since maximum tension is dependent on length, contraction of the muscle fibres also decreases the amount of tension that can be generated. When the maximum amount of tension falls below the afterload, the muscle stops contracting and isometric relaxation begins. Hence, the muscle has contracted from the point at which the pink and green lines intersect to the point at which the blue and green lines intersect. (Apologies for those who are colourblind.)

So what's this got to do with stroke volume and cardiac output? Well, the more a muscle can contract, the bigger the stroke volume. As stroke volume is also related to cardiac output, a muscle that can contract more can also generate a larger cardiac output.

How do preload and afterload affect contractility? Let's consider changing each of these factors in turn.

First, we'll start with preload:

Remember, the amount of contraction depends on the distance between the intersection of pink and green lines and the intersection between blue and green lines. In the picture above, you can see that this distance is shorter for line A than it is for line B. Hence, decreasing preload also decreases contraction, and vice versa. This goes in line with what I said back in second year: increasing end-diastolic volume also increases stroke volume.

Now let's look at the effect of changing the afterload:

As you can see from the above diagram, afterload A has a shorter distance (and smaller amount of contraction) than afterload B. Hence, increasing afterload decreases contraction, and vice versa.

Interpret cardiac pressure volume curves

A pressure volume curve is, simply put, a curve showing the relationship between volume and pressure throughout the cardiac cycle. Here is a crudely drawn example of a pressure volume curve:

At point A, the mitral valve opens, allowing the ventricle to fill with blood. Hence, the volume increases, but the pressure is still low. This continues until you get to point B, when the mitral valve closes. This final volume also serves as the pre-load for the heart. Isovolumetric contraction then occurs (see here), increasing the pressure without any change in volume. This happens until the force generated by the heart is able to overcome the afterload, allowing blood to leave via the now open aortic valve (point C). At the end of systole (point D), isovolumetric relaxation then occurs: the pressure is reduced while the volume stays the same. This brings us back to point A, and the cycle starts over again.

Describe cardiac work and predict changes in work using pressure volume curves

Okay, I've searched up definitions of "work" and hopefully I've understood them well enough to explain them here. Work is whenever a force moves something, and whenever work is done, energy is transferred from one place to another. Work is usually calculated by multiplying the force and distance, but when the force varies, you need to integrate the force with respect to distance.

In fact, you can use integration to figure out how much work is done by the heart. You can integrate the pressure (which is just force divided by area) between diastolic and systolic pressures with respect to volume:

This integral is also equal to the area within the pressure volume curve.

Because integration is hard when you don't have a nice, neat mathematical equation to integrate, you can simplify this a bit further. Instead of integrating the pressure, you can take the difference between systolic and diastolic pressure and multiply this by the overall change in volume (i.e. the stroke volume). If you want the work done on a minute to minute basis, you can multiply the blood flow by the arterial venous pressure difference.

Hopefully that made sense to you! (That barely made sense to me...)

Excitation Contraction Coupling

Second post on smooth muscle! In this post, you'll probably start to appreciate how weird smooth muscles are. (See here for my first post on smooth muscle.)

Describe how agonist binding to P_2X receptors or other receptors (e.g. M-cholinergic and α-adrenergic) produces contraction through electromechanical coupling using ROC and VOC.

Okay, I guess I should start by explaining some of the terminology. Electromechanical coupling is, in a nutshell, coupling of something electric (in this case the membrane potential) with something mechanical (contraction). ROCs are receptor-operated channels (which is basically a fancy name for a ligand-gated ion channel) and VOCs are voltage-operated channels (i.e. voltage-gated channels).

For some reason this lecture contains an example of how this works in skeletal muscle, which isn't really that relevant now, but will probably become relevant later when we move onto learning about skeletal muscle. In skeletal muscle, ACh stimulates N-cholinergic (i.e. nicotinic) receptors, which are non-selective ROCs. They allow both Na⁺ and K⁺ to pass through, causing depolarisation. Depolarisation then opens Na⁺ VOCs.

A similar principle occurs in smooth muscle, but with different receptors. In smooth muscle, ATP activates P_2X receptors, which are non-selective ROCs that allow the passage of Na⁺, Ca²⁺ and K⁺. Opening of these channels causes depolarisation, which opens L-type Ca²⁺ channels (which are VOCs).

P_2X receptors are possibly the only "true" ROCs in smooth muscle- there are other ROCs, but they seem to require intermediate steps. One example of this is the ACh ROC. ACh does not actually bind to the channel. It's not actually known how ACh activates its ROC, but it's been suggested that it might be through the use of G-proteins (see my post on G-protein coupled receptors here).

Explain the evidence for and differences between electromechanical and pharmacomechanical coupling including the use of extracellular versus intracellular calcium.

Electromechanical coupling seems to imply that a change in voltage, and something to sense the change in voltage (like VOCs), are required in order for contraction to occur. That doesn't happen in all muscles, however, and we know that because of experiments involving nifedipine. Nifedipine blocks L-type Ca²⁺ channels, so if only electromechanical coupling was involved, we'd expect that nifedipine would stop contraction, right?

Wrong.

Nifedipine does have an effect, but only a relatively modest effect. That means that something else must be coming into play here, and that something is pharmacomechanical coupling. Pharmacomechanical coupling occurs via G-protein coupled receptors rather than through ion channels.

Another thing that can be taken away from these experiments is that extracellular Ca²⁺ is not the be-all and end-all. If it was, then blocking Ca²⁺ channels would block contraction. Hence, intracellular Ca²⁺ stores are also important, and I'll get to them in a bit.

Explain how agonist including ACh and NA produce contraction using pharmacomechanical coupling, listing all enzymes, receptors, channels and second messages.

ACh and NA (noradrenaline) can bind to several different receptors. ACh can bind to muscarinic receptors, imaginatively named M₁, M₂, M₃, M₄ and M₅. NA can bind to α-adrenergic and β-adrenergic receptors.

Just to make things a bit confusing, these receptors have different effects. Odd-numbered muscarinic receptors and α-adrenergic receptors can activate a G_q protein, producing inositol triphosphate (IP₃). This causes contraction. Even-numbered muscarinic receptors can activate a G_i protein, inhibiting cAMP (which I assume results in the prevention of relaxation). β-adrenergic receptors activate G_s, activating cAMP and causing relaxation. (For more details on G-protein coupled receptors, please see here.)

When acetylcholine binds to an odd-numbered muscarinic receptor, or noradrenaline binds to an α-adrenergic receptor, G_q activates phospholipase C. Phospholipase C cleaves phosphatidylinositol into inositol triphosphate (IP₃) and diacylglycerol (DAG). For now, we only need to worry about IP₃ as it can bind to IP₃-receptors in the sarcoplasmic reticulum (SR) membrane (remember, the sarcoplasmic reticulum is basically just the endoplasmic reticulum of the muscle cell). These receptors are also Ca²⁺ ion channels, so when they are activated, they release Ca²⁺ stored in the SR. Note that none of this requires a change in membrane potential.

Describe the mechanisms for calcium entry into cells, release from the SR, clearance from cytoplasm, entry into SR and storage in smooth muscle.

As I just mentioned, odd-numbered muscarinic receptors and α-adrenergic receptors can cause release of Ca²⁺ from the SR via IP₃-activated channels. Some smooth muscle SRs also have ryanodine receptors (RyR), which, just like in the heart muscle, produce calcium-induced calcium release. Also, fun fact: RyR receptors can also be activated by caffeine. Ca²⁺ is later pumped back into the SR via the SERCA pump, again like in the heart muscle.

Smooth muscle, as I've alluded to, can also take up extracellular calcium via VOCs and ROCs. Calcium can also be pumped back into the extracellular fluid via Plasma Membrane Calcium ATPase (PMCA) pumps. There are also Na⁺/Ca²⁺ exchangers that can let calcium out in exchange for sodium.

There is also a mechanism for letting in more calcium when intracellular stores are low. STIM is a long transmembrane protein that runs from the inside of the SR, through the SR membrane and almost all the way to the plasma membrane of the cell. It is a Ca²⁺ binding protein which is bound and inactive whenever Ca²⁺ stores are adequate. When Ca²⁺ stores are depleted, it activates SOCCs (store-operated calcium channels) in the plasma membrane that allow entry of more Ca²⁺. SOCCs are conveniently located close to SERCAs so that the calcium can get its way into the SR quickly.

Explain how NA and NO produce relaxation including all enzymes, receptors, channels and second messages.

NA and NO (nitric oxide) have slightly different pathways. Noradrenaline activates adenylate cyclase via a G_s protein. Adenylate cyclase converts ATP to cAMP (cyclic AMP), which activates protein kinase A (PKA). NO, on the other hand, diffuses through the cell membrane and activates guanylate cyclase, which converts GTP to cGMP. cGMP activates protein kinase G (PKG).

PKA and PKG can phosphorylate and open K⁺ channels in smooth muscle, causing hyperpolarisation. They can also increase Ca²⁺ uptake into the SR and reduce Ca²⁺ release from the SR, though the mechanisms here are unknown. (Please note: all of this is completely different to the actions of PKA in cardiac muscle!) PKG can also activate PMCA, which, as I said earlier, pumps Ca²⁺ out of the cell. Since Ca²⁺ causes contraction, getting rid of it causes relaxation.

Weight Loss and Exercise

Okay, I'm starting to lose my ability to take this unit seriously due to the simplicity of the content, but here goes anyway.

Dieting

1. Be able to explain the difference between fasting and starvation
2. Be able to explain the key changes in metabolic pathways during fasting and starvation

Fasting is basically what happens when you stop eating for a short time. When you are fasting, fatty acids from adipose tissue are exported to the plasma and glycogen is broken down to maintain blood glucose.

When glycogen stores are depleted (takes ~2 days), you enter a starvation state. During this time, your body basically converts everything else into energy. Amino acids are broken down: carbon skeletons of glucogenic amino acids are converted to glucose, whereas the amino groups are converted to urea. Fatty acids are oxidised to ketone bodies, which, if allowed to accumulate, can cause ketosis. Ketosis decreases appetite and causes nausea, and ketones can be smelled on the breath. (See here for more details on ketoacidosis.)

3. Be able to discuss the different types of diets.

This lecture covered four types of diets:

1. Low carb, low fat, high protein (e.g. Total Wellbeing Diet): Basically what it says on the box. Weight loss is initially rapid, and will continue so long as calories in < calories out (the most important part of any weight loss diet, as I'll reiterate soon). The high protein may help with satiety (feeling of fullness), which is probably why these diets are so popular. After all, if you feel full more quickly, you'll probably eat less.
2. Low carb, high fat (e.g. Atkins Diet): The key principle behind this is the development of ketones. I'm not sure why this is desirable, given what I just said about ketosis, and sure enough some of the common consequences of this diet include dehydration, weakness, headaches and dizziness. Furthermore, adherents to this diet may have inadequate amounts of fruit and vegetables.
3. Fad diets: These come in all shapes and forms, but most are pretty extreme. One example of a more extreme diet that comes to mind is the 30 Bananas a Day diet. Fad diets are generally not backed up by scientific evidence. They might help lose weight, however, if calories in < calories out.
4. Portion control: This is literally just calories in < calories out without getting too caught up in the details.

4. Understand the purpose of dieting in the context of fuel mobilisation and losing weight

Dieting, as I hope you've figured out by now, is simply trying to make sure you consume fewer calories than you expend. I've also touched on that here.

5. In the context of losing weight, understand the role of macronutrient proportion and macronutrient intake.

In the context of losing weight, macronutrient proportion isn't that important. As long as total calories in < total calories out, you will lose weight. Of course, adequate intake of macronutrients are important for other reasons (protein for building muscle, fats for absorbing fat-soluble vitamins, etc.), just not from a weight loss perspective. Hence, if you want to lose weight by going on a diet, anything that you will stick with is fine, as long as calories in < calories out.

Physical Activity

1. Be able to discuss the benefits of physical activity

Physical activity has plenty of benefits, including reducing "all-cause mortality," which I'm assuming is a misnomer, or else exercising would prevent us from dying in car crashes and so forth. (Don't get me wrong- it would be great if exercise could do that, but I'm not going to hold my breath.) It also reduces the rate of high blood pressure, some cancers, type 2 diabetes and so forth. Aside from preventing you from dying, it also improves your bone health, cognitive function, muscular fitness and so on.

2. Be aware of the guidelines for physical activity for Australians

It's recommended that you get at least 150min/wk of moderate physical activity. If you are exercising more vigorously, 75min might do the trick.

3. Be able to describe aerobic activity and its components

Aerobic activity is what is sometimes more colloquially known as "cardio": running, cycling, dancing, swimming, etc. It can be studied in terms of different components, such as intensity, frequency and duration.

4. Be able to describe muscle-strengthening activity and its components

Muscle-strengthening activity involves resistance training, lifting weights and so on. The components of muscle-strengthening activity are intensity, frequency and repetitions.

Cardiac Inotropy

Inotropy! Aside from being a nice word that I'm not 100% sure how to pronounce, inotropy is also the term used to refer to the amount of contraction for the same amount of load.

Summarise the excitation contraction coupling of cardiac cells

When an action potential comes along, it stimulates the opening of L-type Ca²⁺ channels. These let Ca²⁺ enter the cell. Ca²⁺ activates ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR), resulting in release of Ca²⁺ from the SR. (This phenomenon is known as "calcium-induced calcium release.") When Ca²⁺ is in the cytosol, it can bind to troponin. The troponin-Ca²⁺ complex can move tropomyosin, which normally blocks myosin binding sites on actin, allowing myosin to bind to actin and contraction to take place.

Eventually, Ca²⁺ is pumped back into the SR by SERCA, which is short for "Sarcoplasmic Endoplasmic Reticulum Calcium ATPase." Some calcium is also removed from the cell altogether via a Na⁺/Ca²⁺ antiport (a.k.a. NCX) on the cell membrane.

Describe the intracellular signalling pathway through which β-adrenergic receptors produce inotropy

β-adrenergic receptors are G-protein coupled receptors with G_s subunits. These activate adenylate cyclase, which produces cyclic AMP (cAMP) from ATP. cAMP goes on to activate protein kinase A, which indirectly results in the phosphorylation of L-type Ca²⁺ channels, increasing their opening probability, and SERCA, increasing the pump rate. This results in increased inotropy due to the increased Ca²⁺ influx, but also shortens the duration of contraction: increasing the activity of SERCA increases the rate at which Ca²⁺ gets "mopped up" by the sarcoplasmic reticulum. Both of these things combined also increases the amount of Ca²⁺ that gets stored by the sarcoplasmic reticulum so that even more can be released during the next contraction.

Fun fact- cAMP is usually degraded to AMP by phosphodiesterase. Phosphodiesterase can be inhibited by caffeine, leaving more cAMP to bounce around and produce inotropy.

Another fun fact- muscarinic receptors do the opposite. They are also G-protein coupled, but they have G_i subunits, which deactivate adenylate cyclase.

Explain how Digitalis and inhibition of Na⁺/K⁺ATPase produces inotropy

Digitalis, derived from the foxglove plant, is a drug that inhibits the Na⁺/K⁺ ATPase. This decreases the concentration gradient for Na⁺. The Na⁺/Ca²⁺ antiport (NCX), which brings in three Na⁺ ions for every Ca²⁺ ion removed from the cell, is very sensitive to changes in the sodium gradient. Hence, the effects of digitalis on the Na⁺/K⁺ ATPase also affects the Na⁺/Ca²⁺ antiport, resulting in less removal of Ca²⁺ from the cell. SERCA can also load more Ca²⁺ into the sarcoplasmic reticulum for later use. As increased Ca²⁺ results in increased inotropy, digitalis produces inotropy.

Pathological and Clinical Consequences of Hypertension

Yet another PHAR3303 lecture that covers a lot of content from previous posts! Makes things easy for me :)

Aetiology of Hypertension

See earlier post: Vascular Disorders

Hypertension and the Brain

See earlier post: Acute Neurological Conditions

Hypertension and the Heart

Hypertension can cause hypertrophy of the left ventricle (i.e. the left ventricle gets bigger and stronger because it has to do more work). Unfortunately, "bigger and stronger" doesn't always mean "better"- the thicker heart muscle can compress the arteries surrounding the heart, plus bigger muscles need a bigger blood supply, so the ventricle won't get the amount of blood that it needs. Also, the thickening of the ventricle walls means that there is less room for the ventricle to hold blood (which leads to the diastolic dysfunction briefly mentioned here). Over time, the left ventricle may weaken and thin out, leading to a condition called dilated cardiomyopathy. This can then cause heart failure, low blood pressure and death.

Hypertension and the Eye

I don't think I've covered this earlier (at least not in detail), but this lecture doesn't go into depth either. Basically all it says is that it's easy to see the blood vessels in the eye with an ophthalmoscope, and hypertension can make them look weird. The most important treatment method is to lower the blood pressure, but laser eye treatment can also help.

Hypertension and the Kidney

Kidneys have lots of little blood vessels that can be readily damaged by hypertension. This can result in renal failure, which has been described in detail (probably more detail than you need for this unit) here. Just like with the aforementioned eye problems, kidney problems related to hypertension can be treated by using drugs for high blood pressure, like ACE inhibitors and ARBs (angiotensin receptor blockers). One interesting thing, however, is that not all ethnicities seem to respond to these drugs in the same way (apparently they seem to be less effective for African-Americans).

Control of Blood Pressure

This lecture covered a lot of stuff from PHYL2001, and not a lot of new stuff, which is sad, because I'm pretty sure that the new stuff to be covered in this lecturer's next lecture would actually help us in the lab quiz this week. Oh well.

Also, this is the guy who doesn't provide a nice little lecture summary, so I'll have to figure out how to subdivide this myself. Damn, I hate making decisions.

Vascular Structure and Function

See previous post: Tubes

Control of Blood Pressure

Autonomic Nervous System

See previous post: Autonomic Nervous System

RAAS System

See previous post: Renal Physiology: Reabsorption part 2

Um wow, that's it I guess???

Physiology of Smooth Muscle

Now we're onto the second topic for PHYL3001! (I already feel like I've forgotten everything about electrophysiology...)

Identify the characteristics that distinguish smooth muscle from cardiac and skeletal.

See previous post: Tissues
tl;dr: Smooth muscle is smooth. OMG

Oh wait, there is something else that I need to tell you. The following is apparently a tl;dr version of PHYL2002, which I didn't do. Contractions in all kinds of muscles ultimately result from Ca²⁺ binding to something, but that something is different in smooth muscle compared to the other two types of muscles. In cardiac and skeletal muscle, Ca²⁺ binds to troponin on the actin filament, whereas in smooth muscle, Ca²⁺ binds to calmodulin. This results in a Ca²⁺-calmodulin complex, which can activate myosin light chain kinase (MLCK). MLCK can phosphorylate the light chains in smooth muscle, activating ATPases and causing contraction of smooth muscle.

Define single and multi unit smooth muscle, tonic and phasic contraction for smooth muscle.

Tonic and phasic contractions

Tonic contractions are not rhythmic, whereas phasic contractions are. w00t w00t.

Single-unit smooth muscle

Single-unit smooth muscle all contracts as a single unit due to the presence of gap junctions between the cells (sorta like heart muscle). Some, such as the bladder and small blood vessels, can undergo tonic contractions, whereas others, such as the gut and uterus, can undergo phasic contractions. They often show something called "slow waves" or "basal electrical rhythm," which I will talk about in a bit.

Multi-unit smooth muscle

Multi-unit smooth muscle has discrete motor units that have to be activated separately, like skeletal muscle. These muscles only show tonic contraction. Examples of multi-unit smooth muscle include those in the eye, large blood vessels and airways.

List the receptors activated by the neurotransmitters ACh, NA/Adr, NO and ATP.

• ACh- Muscarinic cholinergic receptors. Causes contraction.
• NA/Adr- Can bind to α-adrenergic receptors to cause contraction, or β-adrenergic receptors to cause relaxation.
• NO- Can bind to guanylyl cyclase to cause relaxation.
• ATP- Can bind to P_2X receptors to cause contraction.

Explain the spontaneous contractions of gut, including the role of gap junctions, ICC, slow waves and action potentials.

As mentioned above, single-unit smooth muscle has gap junctions, making it easier for an action potential in one cell to propagate through to others. I also mentioned the basal electrical activity of this type of smooth muscle. But what sets all this off in the first place?

To my understanding, this is caused by ICCs, or Interstitial Cells of Cajal. They essentially act as the paceemaker cells of the gut. ICCs are located in the mucosa and submucosal plexuses, and are connected to smooth muscle cells via gap junctions. It is these cells that cause the "slow waves" of spontaneous, transient inward currents that are largely carried by Ca²⁺.

Slow waves do not cause contractions on their own. They can, however, cause action potentials if they depolarise the cell past the threshold potential. If the threshold is exceeded for long enough, many action potentials will result, which is good, because in smooth muscle many action potentials are required in order to cause a contraction. Neurotransmitters such as ACh can increase the strength of contraction by depolarising the cell to a larger extent, resulting in the cell spending more time above threshold and thus generating more action potentials. (Adrenaline does the opposite.)

Describe the channels and ions involved in smooth muscle action potentials and the actions of nifedipine/verapamil.

At rest, K_ir (inward rectifying) channels are open as they aren't being blocked by Mg²⁺ (see this post for more information). This allows potassium to move in the direction of its concentration gradient, which happens to be out of the cell.

When the cell depolarises, K_ir channels close. As mentioned in this post, these channels close, rather than open, when the cell becomes depolarised. At the same time, Ca²⁺ channels open, increasing Ca²⁺ inside the cell. As mentioned earlier, this allows Ca²⁺ to bind to calmodulin, activating MLCK and thus causing contraction.

Repolarisation occurs when K_v channels open. (You may also see them denoted as K_dr- delayed rectifier- channels.) This allows K⁺ to rush out and the cell to return to its resting potential.

Explain how some smooth muscle do not show action potentials including the relevant ion channels and ion movements.

Yeah, that's right, not all smooth muscles have action potentials. Airway smooth muscle, vascular muscle and others do not show action potentials, even when electrically stimulated. So I guess we'll all just have to throw out our ideas of "action potential = contraction." Damn.

A lack of action potentials is characteristic of many tonic smooth muscles. They can show graded depolarisations, but not full-on action potentials. This is because their K_v channels open really early- in fact, they open pretty much as soon as the cell depolarises. (There's a shit-ton of different K_v channels, so of course some of them have to be different >_>.) Additionally, these cells have K_Ca channels, or calcium-activated K⁺ channels, which I mentioned here but didn't discuss in any detail whatsoever. As their name suggests, they are K⁺ channels that open in response to an increase in Ca²⁺. Therefore, when depolarisation causes Ca²⁺ to come in, these K_Ca channels also open, increasing efflux of K⁺. All of this prevents the membrane potential from "spiking."

Cardiac Arrythmia

Structure and function of ion channels

See earlier post: Ion Channel Structure and Function

What is cardiac arrythmia?

See earlier post: Dysrhythmias and Congenital Heart Defects

Cardiac Long QT syndrome

Cardiac Long QT syndrome is, simply put, a prolongation of the QT interval. (If you don't know what a QT interval is, see here.) This can be inherited (autosomal dominant inheritance) or acquired, usually from various drugs.

Detecting abnormal electrical conduction- the ECG

See previous post: The Heartbeat

Also I'm not really sure where to put this, but one disorder that was covered in this lecture is Torsade de Pointes. It's basically a really erratic ECG that appears to be "turning around" on itself every so often. Because I'm terrible at describing this, here's the picture copy-pasted from the lecture slides to give you an idea of what I mean:

Torsade de Pointes is responsible for lovely stuff like ventricular fibrillation and sudden death.

Measurement of ion conductance- the patch-clamp technique

See previous post: Single Cell Electrophysiology Techniques

Current research investigating the effects of hypoxia on ion channels

For those of you who saw the lecture, don't panic- we don't need to know any of this in depth! This was mainly to show us the wider context of how different techniques and so on are used. As for those who didn't attend the lecture, I'll spare you the details, aside from that our lecturer's lab found that hypoxia seems to increase the sensitivity of the L-type Ca²⁺ channel to β-adrenergic receptor stimulation (mainly because that's the one sentence that I actually understood of this part). Oh and also myocardial ischaemia/hypoxia increases circulating and local catecholamines, increasing the risk of arrythmia and sudden death. What a cheerful note to finish this post on!

Cardiac Ion Channels and the Heart Beat

Tying in with my PHYL3001 post about ion channels, now you're going to get a bit more context and see how these ion channels operate in the heart!

Normal electrical conduction

See previous post: The Heartbeat.

Morphological/functional classification of cells

There are three main types of cells that you should know about. I'm going to use this as an excuse to make another table, because tables are great (despite Blogger's tendency to want to put around 20 lines of space before tables :P).

Name	Shape	Diameter (μm)	No. of myofibrils	Location	Other
Pacemaker cells	Round or oval	3-9	Reduced no.	SA node and AV node
Conducting cells	Cylindrical	50	Reduced no.	Bundle of His, bundle branches, Purkinje fibres	Many intercellular connections
Contractile cells	Cylindrical	10-15	Abundant	Atria and ventricles	Many intercellular connections, extensive T-tubule system

Characteristics of fast and slow action potentials

There are two kinds of action potentials in the heart: a fast and a slow action potential. The slow action potential is responsible for the pacemaker activity of the pacemaker cells, whereas the fast action potential is responsible for the actual contraction that takes place in the atria and ventricles.

Ionic composition of the fast action potential

Behold, a crudely drawn graph of the fast action potential!

Let's break this down into the different phases (labelled with the different numbers)!

Phase 0: Upstroke and Overshoot

This phase is due to a sharp increase in Na⁺ conductance. A large influx of Na⁺ increases the membrane potential from around -80 to around +20. At the end of this phase, Na⁺ conductance decreases rapidly as the channels close.

Phase 1: Initial Repolarisation

This phase is due to the activation and inactivation of a transient outward K⁺ channel (I *think* this is due to the transient outward rectifiers mentioned in my previous post, though unlike what I've said in my previous post, this is happening at a positive, not at a negative, membrane potential). This transient outward current is also known as I_to. As positive charges are let out of the cell, the membrane potential drops slightly, but this is short-lived due to the rapid inactivation of the channels.

Phase 2: Plateau Phase

This phase is due to L-type (long-acting type) Ca²⁺ channels. These open slowly and close slowly, allowing Ca²⁺ to enter the cell briefly and maintain the positive membrane potential.

Phase 3: Final Repolarisation

This phase is due to delayed rectifier K⁺ channels, which, as I mentioned in my last post, are delayed in opening and allow K⁺ to exit (thus lowering the membrane potential).

Phase 4: Resting Membrane Potential

Finally, we have the action of inward rectifier K⁺ channels, which, as also mentioned in my last post, prevent excessive loss of K⁺. This current is also known as I_KI.

Ionic composition of the slow action potential

I'm going to go through this like I did for the fast action potential: first a graph, and then an explanation of each phase. This will be quicker, however, as the slow AP doesn't have phase 1 or 2.

Phase 0: Upstroke

Unlike in fast APs, Na⁺ is not involved in the slow AP. Instead, this phase is due to slow opening of L-type Ca²⁺ channels.

Phase 3: Repolarisation

Ca²⁺ channels close during this phase. Additionally, the outward potassium current increases during this time.

Phase 4: Max Diastolic Potential

During this phase, the cell has a lower conductance to K⁺ and a higher conductance to Na⁺. This results in a reduction in potassium current and/or a steady inward current of Na⁺. This inward current is also called I_h for some reason. Later on, Ca²⁺ conductance increases. This current is also known as I_Ca(T).

Refractory Periods

Just like other action potentials, the fast and slow action potentials of the heart also have refractory periods. There are two main types of refractory period: the early refractory period (ERP) and relative refractory period (RRP). During the early refractory period, between phase 0 and 3, another stimulus will not be able to set off another action potential. If, however, another stimulus comes along during the RRP, which immediately follows the ERP, early firing of the action potential may result.

Extrinsic influences on fast and slow action potentials

The autonomic system is one of the main extrinsic influences on action potentials. Sympathetic activity increases automaticity in pacemaker cells and contractility in contractile cells, whereas parasympathetic control decreases the rate of firing and conduction velocity. The sympathetic and parasympathetic systems also antagonise each other.

Whew! I find the fast and slow APs a little bit confusing, so hopefully I haven't confused you too much!

Ion Channel Structure and Function

Last post on electrophysiology!

Describe the structure and function of voltage-gated ion channels

Voltage-gated ion channels, as the name suggests, are ion channels that open in response to a specific voltage. Voltage-gated channels are made up of several subunits- usually four α₁-subunits that make up the pore, as well as a few other auxiliary subunits. In K⁺ channels, the auxiliary subunits consist of four cytoplasmic β-subunits. In Na⁺ channels, these are two transmembrane β-subunits. In Ca²⁺ channels, there is an extracellular α₂-subunit, a cytoplasmic β-subunit, and transmembrane γ and δ subunits. These auxiliary subunits may help modulate the gating activity of the channel, but we're still not 100% sure.

Since the α₁-subunits are probably the most important, we're going to focus on them. α₁-subunits have six transmembrane domains, imaginatively named S1 through to S6. S4 also serves as a "voltage sensing domain." Between S5 and S6 lies a P (pore) domain, which contains the selectivity filter (i.e. the thing that lets only the ion you want pass) as well as binding sites for other molecules.

Now let's have a look at how these channels work! As I just mentioned, the S4 region senses changes in voltage. That is because S4 domains are made up of largely positive residues, such as arginine, which are usually balanced out by negative charges on a neighbouring helix. When the cell is depolarised (more positive), however, the positive charge inside the cell repels the helix, pushing it around and up (like a screw). This movement also moves the S6 helices, thus opening the channel.

As the ability to sense voltage is clearly pretty important to a voltage-gated channel, it makes sense that this domain has been well conserved throughout evolution. Rats, fruit flies and electric eels share a lot of the same amino acid residues in their S4 regions.

The other key part of the ion channel is the pore region- i.e. the S5-P-S6 portion. In fact, Streptomyces bacteria only have this portion- they don't have the voltage sensing region. The P region, as mentioned above, contains a selectivity filter. This filter is located near the top of the pore and is quite short and narrow, minimising the distance that the ion in question requires in order to interact with the channel. It is lined with residues that will attract the ion in question (e.g. negative charges to attract positive ions), and the spacing is such that it is only energetically favourable for the right size of ion. Just below the selectivity filter is a water-filled cavity and some charged helix dipoles (negatively charged in potassium channels). As ions of the same charge pass through the channel, the repulsion between ions helps propel them through the channel more rapidly.

So far, what I've said applies mainly to K⁺ channels. Other ion channels are similar, but there are some differences. For example, in Na⁺ and Ca²⁺ channels, the four α₁-subunits are actually joined end-to-end to make a giant α subunit with four domains. Furthermore, each of these domains is slightly different- i.e. these are heterotetramers, not monotetramers like K⁺ channels. As I mentioned in a previous post, Na⁺ channels have inactivation gates, which are located between the third and fourth domains.

Understand the similarities and differences between different classes of ion channels

There are several different classes of ion channels. We're going to focus mainly on K⁺ channels for now.

Delayed outward rectifiers

Delayed outward rectifiers, as their name suggests, are delayed in opening and cause potassium to move outwards. The outward current rises steeply at positive voltages (i.e. as the voltage goes up, flow of positive ions out of the cell also goes up). This is probably where the "rectifier" part comes from.

Transient outward rectifiers

Transient currents, also known as A-type currents, are activated and inactivated over a relatively short time period. They tend to be activated when the membrane potential is very negative, such as during hyperpolarisation.

Ca²⁺-activated K⁺ currents

There doesn't seem to be anything in the lecture about these, so... moving on, I guess?

Inward rectifiers

Inward rectifiers, just like the potassium channels of Streptomyces, only has the S5-P-S6 part. As their name suggests, they control current going into, but not out of the cell. They do this with the help of Mg²⁺. When the inside of the cell is positive, Mg²⁺ is pushed towards the edge of the cell, where it blocks the pore of the ion channel. This prevents intracellular K⁺ from leaving, but allows it to enter from the outside. This is important for preventing excessive loss of K⁺ during repeated and/or lengthy action potentials.

(Note: When the inside of the cell is not positive, Mg²⁺ isn't blocking the channel and thus K⁺ will simply travel in the direction of the concentration gradient during this time. This will become more important when we start talking about smooth muscle.)

Become familiar with the impact of genetic mutations on ion channel function and how this impact membrane potentials and cell function

Ion channels, as hopefully you've realised over the past couple of weeks, are pretty important. Hence, genetic mutations can cause a range of problems, from pain disorders to long QT syndrome (which I'll talk about in one of my posts for PHYL3002). Here are some examples of ion channel problems:

Lambert-Eaton Syndrome

Lambert-Eaton Syndrome is an autoimmune disorder in which antibodies are produced against the S5-S6 region of voltage-gated Ca²⁺ channels. This decreases Ca²⁺ influx, which in turn decreases the amount of ACh released. As ACh is important for muscle contraction, this causes muscle weakness. It mainly affects proximal limb muscles, which can make it difficult to climb stairs, but can also affect respiratory muscles. Lambert-Eaton Syndrome can be treated in three main ways: by decreasing the breakdown of ACh (by using drugs such as pyridostigmine), increasing calcium influx (via 3,4-diaminopyridine) or by using immunosuppressants.

Myotonia

Myotonia is slow or relaxed relaxation after contraction. This manifests as difficulty in releasing grip on tools and so on. Myotonia is sometimes aggravated by cold and vigorous exercise.

There are several different causes of myotonia. Myotonia congenita results from mutations in the CLCN1 gene, causing reduced conductance of chloride. As chloride can't move around and balance out the positive charges, this causes repetitive firing of action potentials. Potassium-aggravated myotonia, or PAM, results from slow inactivation of certain sodium channels, also causing a chain of action potentials after the stimulation stops.

Hyperkalemic Periodic Paralysis (HyperPP)

HyperPP results from a large, persistent Na⁺ current. This causes the cell to lose excitability, resuting in paralysis.

Describe the effect of disruption to ion concentration gradients on membrane potential and cell function

When extracellular calcium is high, sodium channels become more likely to open at higher voltages than usual. Essentially, this means that if you want sodium channels to open, you'll need to depolarise the cell more than usual. This also means that a muscle, for instance, will become less excitable and weaker.

On the other hand, when extracellular calcium is low, muscles can become hyper-excitable and twitch spontaneously.