Muscles

Following on from bones and joints, I'm going to talk about muscles, since they also play a vital role in allowing us to move around!

There are three main types of muscles, but I'm only really going to talk about one in this post. The type of muscle that I'm going to talk about is called skeletal muscles- the muscles that allow movement at the joints. Aside from these, there are also smooth/involuntary muscles, which are the muscles in the internal organs and cardiac muscle, which is the heart muscle.

All kinds of muscles, however, do have a few things in common. Firstly, they can only contract on their own. Contraction allows skeletal muscles to move bones, cardiac muscles to pump the heart, and smooth muscles to carry out functions specific to the organ in question. Although muscles cannot stretch on their own, they can be stretched. This ability to be stretched is extensibility. Elasticity, on the other hand, is the ability of muscles to return to their original length after stretching. Contractability, extensibility and elasticity are all vital for allowing muscles to create movement.

Now let's look at the stuff that's specific to skeletal muscles!

Skeletal muscles, as the name implies, are attached to the bones of the skeleton via tendons, which is a fibrous, inelastic connective tissue. Muscles are positioned in order to bridge the joints- the two ends of each muscle will generally be attached to different bones. The end attached to the stationary bone is called the origin, while the end attached to the moving bone is called the insertion. (The bit in the middle is called the belly.) Contraction of a muscle will therefore pull the two bones closer together. Since muscles can't stretch themselves out, moving the bones apart again generally requires a muscle on the other side of the limb to contract. Hence, muscles are generally grouped in pairs known as antagonists.

During movement, the muscle that causes the desired action is called the agonist or prime mover. The other muscle of the pair, on the other hand, is known as the antagonist (yup, there appears to be two definitions of that word here). There are sometimes other muscles involved called synergists or fixators, which help to steady joints, prevent unwanted movement in other areas, and allow the agonist to function more effectively.

Another way of looking at the movement of the bones and muscles is by looking at how the bones act as levers. A lever is a structure that moves around a fixed point called a fulcrum when force is applied. In the human body, the bones are the levers, the joints are the fulcrums, and the muscles provide the force.

Now for some more technical stuff...

The Structure of Skeletal Muscles

Skeletal muscles are made up of bundles of muscle fibres. Muscle fibres are surrounded by a sheath of connective tissue, which allows adjacent bundles to slide over each other during contraction. The sheaths of each bundle join each other, tapering towards the end of the muscle to form tendons. The amount of connective tissue increases with age, and is thought to contribute to the decrease in muscular strength that comes as we get older.

The muscle fibres themselves are elongated cylindrical muscle cells with many nuclei. Each cell is surrounded by a thin, transparent plasma membrane called the sarcolemma, which contains a kind of cell fluid called sarcoplasm (this is basically the cytoplasm of muscle cells- more on cytoplasm and other parts of the cell in a future post). The cells are 10-100 micrometres in diameter, and their length varies from a few millimetres to a few centimetres. The sarcoplasm contains hundreds to several thousands of thread-like myofibrils.

Myofibrils can be divided into units called sarcomeres, which contain many smaller myofilaments. Myofilaments come in two varieties and are the units actually responsible for the muscles being able to contract. Thick myofilaments are mainly comprised of a protein called myosin, while thin myofilaments are mainly comprised of a protein called actin. According to the sliding filament model (remember, a model is just a simplified representation of a scientific concept), these myofilaments can slide past each other to shorten the sarcomeres and allow the muscles to contract. They are then pulled past each other when the muscle relaxes.

Not all muscle fibres have to contract and relax at the same time: in fact, at any given time, some fibres will be relaxed while others will be contracted. Muscle tone is the maintenance of partial contraction of muscles. This is not due to the same fibres remaining contracted all the time, but rather by the muscle fibres "taking turns" at contracting so that contraction can be maintained for a long period of time.

Muscles of the Upper Limbs

In my post about the skeleton, I mentioned briefly that the pectoral girdle (shoulder bones), while allowing a wide range of movement, do not provide very strong support. To make up for this, the upper limb bones have a lot of muscular attachments to the axial skeleton (i.e. the ribs and spine). Here's a quick overview of only some of the muscles in our upper limbs:

• Trapezius- attaches scapula to axial skeleton. Allows us to move our shoulders around in various ways (e.g. shrugging).
• There are nine muscles that cross the shoulder joint to attach to the humerus. Seven of these are from the scapula. The shallow joint is held in place by ligaments, while the many muscles allow a wide range of movements.
• Biceps and triceps- you probably already know what these do. The biceps allows us to bend our arms at the elbow; the triceps allows us to straighten our arms out afterwards.
• The brachialis, a muscle that lies beneath the biceps, allows us to flex our forearms.
• Our forearms have many muscles in a number of layers: those on the lower layers allow us to move our fingers, while those on the upper layers move our wrists and palms. In order to reach the fingers and palms, the tendons of the muscles extend over the wrist.

Muscles of the Lower Limbs

While the muscles of the upper limbs are more about extending the range of movement, the muscles of the lower limbs are more about stability and strength. Although the pelvic girdle doesn't need any muscular attachments to the axial skeleton, strength is still required in order to help those leg muscles constantly resist the pull of gravity. Here's an overview of the muscles in the lower limbs:

• Three large gluteal muscles extend from the pelvis to the femur of each leg, each of which has a "neck" that bends inwards so as to provide optimal leverage for the muscles that pull on it. The gluteal muscles serve to extend and rotate the thighs. The largest gluteal muscle is called the gluteus maximus.
• There are two main muscle groups of the thigh: the hamstrings, which bend the knee and extend the thigh backwards, and the quadriceps, which can straighten the lower leg. All quadriceps muscles have a common tendon which crosses the knee joint to join with the tibia.
• The thigh also contains adductors, which move the thigh towards the centre line of the body (as mentioned in my post on joints, "adduction" is movement towards the centre line of the body). It's antagonistic (i.e. produces the opposite effect) with two of the gluteal muscles (which, surprise surprise, move the thigh away from the centre line of the body).
• The calf muscle is made up of two muscles, the more prominent being the gastrocnemius (which I have absolutely no idea how to pronounce without sounding stupid). It looks kinda like two muscles that merge into one, which then becomes the calcanean tendon, which you might know by its more common name- the Achilles tendon.
• The soleus is another muscle which is closely associated with the gastrocnemius, as the tendon of the soleus joins that of the gastrocnemius.
• The anterior tibialis is the front of the lower leg. It allows us to point our toes upwards (I think), taking weight onto the heels.
• The arch of the foot is made up of tendons from calf muscles. One of these muscles is called the posterior tibialis.
• There are many short muscles in the foot which support the arches, make up the fleshy part of the sole, and contribute to the suppleness and flexibility of the foot.

The next part of my book has a whole load of propaganda interesting facts on why we should exercise and stuff. I'm just going to cover the terms related to muscles:

• Muscular strength- the force that a muscle group can exert against a resistance
• Muscular endurance- the ability of a muscle to contract repeatedly or sustain a contraction for an extended period of time
• Flexibility- range of motion about a joint
• Atrophy- the decrease in size of a muscle (normally happens in muscles that aren't used or are only used for very weak contractions)

And now for the old obligatory "what can go wrong" part!

• Paralysis- occurs when the spinal cord is damaged. Any limbs below the break become paralysed (lose sensation and voluntary muscular movement). Paraplegia is paralysis of both of the lower limbs while quadriplegia is paralysis of all four limbs.
• Strain- normally occurs when a muscle or tendon is overstretched. Symptoms include a sudden pain and loss of power in the limb.
• Spasms- short, sudden, involuntary contractions
• Cramps- sustained involuntary contractions that lack even partial relaxation
• Convulsions- violent, involuntary contractions. May be caused by muscle fibres receiving impulses from nerves, which in turn might have been stimulated by fever, poisons and so on.
• Fibrillation- uncoordinated contractions of individual muscle fibres. This prevents the muscle from contracting smoothly
• Tics- involuntary, spasmodic twitching of muscles
• Muscular dystrophy- inherited diseases in which individual muscle cells degenerate. Leads to a progressive reduction in the size of the skeletal muscle, an increase in connective tissue and possibly the replacement of muscle fibres with fatty tissue. There are two forms: the Duchenne form and the fascioscapulohumeral form.

My Human Bio posts have been rather bland and picture-less as of late... but don't worry, this post isn't going to be one of them, as you shall soon see!

Back in Skills Week in Year 10, we had the option to go to a university and participate in one of three projects under the guidance of a researcher. The project my group did was looking at the effectiveness of different dietary interventions for Duchenne muscular dystrophy. What follows are some pictures that my group (or half of my group, to be precise, since we split into halves and took turns crunching numbers and taking pictures) took of muscle cells from both healthy mice and mice affected by Duchenne muscular dystrophy.

This first picture is titled "pikachu bulbasaur( absolutley [sic] nothing is in this image dont bother opening it)." (Yup, we were really mature back in Year 10.) It's a picture of normal muscle tissue. Those purplish dots are the nuclei of cells IIRC. (I think the tissues were stained with something beforehand to highlight the nuclei, but I can't remember.)

This second picture is also of healthy muscle tissue. It's titled "H***** [name blanked out to protect her privacy, not that I think you guys are going to stalk her or anything] is awesome (According to her...[Connecting Tissue in Normal Muscle])" so I'm assuming that that diagonal strip thing is some connective tissue. Or maybe it's not, and the other white bits are connective tissue (to be honest that makes more sense to me at the moment now that I know that bundles of muscle fibres are surrounded by connective tissue, but I could be wrong).

This next picture, titled "k***** is awesome(MDX fat and necrosis )" shows, as the title implies, fat and necrosis (cell death) in the tissue of an mdx mouse (i.e. a mouse affected by muscular dystrophy). The white bubble things are fat, while the clusters of purple nuclei indicate necrosis. (Presumably the breakdown of cells means that the nuclei originally inside them just end up floating around with nowhere to go.)

This picture is titled "awesome threesome (black stuff and fat and necrosis). The black stuff isn't important, by the way- we think it's just something that fell onto the slide.

Yet more necrosis from a picture titled "g******* is awesome (MDX NECROSIS)"

This last picture is titled "L**** is awesome no im not ( mdx purple splotches." This picture also displays necrosis- I'm wondering whether or not those big purple splotches indicate cells that are in the process of dying.

And that's it from me! Good night everyone!