Endocrine System

Second post for PHYL2001! I'm going to finish off talking about the basics of communication and control around the body before I go on to talk about the respiratory system. (We actually did cardiovascular before respiratory, but I feel that I need a refresher already!)

Be aware of the different types of hormones found in the human body and how their release is controlled by the brain.
Discover the role of the hypothalamus and pituitary gland in the body’s hormonal response to a change in the internal or external environment to allow the body to cope with or adapt to the change.

The endocrine system, just like the nervous system, plays roles in sending messages around the body. However, instead of releasing neurotransmitters into synapses to be picked up by other neurons or by target organs, the endocrine system achieves its effects by releasing signalling molecules, called hormones, into the blood. ("Endocrine" generally means that a substance is being released into the blood, as opposed to "exocrine" which means that a substance is being secreted into a duct or to the outside of the body.)

There are three main types of hormones: peptides (i.e. chains of amino acids), steroids (derived from cholesterol) and tyrosine derivatives. They all work in pretty much the same way, however: by travelling around the blood until finding a receptor that they can fit into. They then bind and have their effect. The main difference would probably be that some bind to receptors on the cell surface, whereas the more lipophilic molecules that can cross the cell membrane may bind to receptors within the cell.

Release of many hormones is controlled by the brain, in particular the hypothalamus of the brain. The hypothalamus is pretty much below the anterior part of the thalamus ("hypo" does mean "below" after all), and below that is the pituitary gland, which is actually made up of two quite different parts. The posterior part, a.k.a. the neurohypophysis, is pretty much just an out-pouching of the brain. Neurons go directly between the hypothalamus and the neurohypophysis. The anterior pituitary, however, was originally derived from surface ectoderm (Rathke's pouch) and has no direct neural connections with the hypothalamus. Instead it shares a blood vessel connection, called the median eminence. Hormones from neurons in the hypothalamus can be released into the median eminence, allowing signals to travel to the anterior pituitary, where other hormones are made and released.

Chances are, you're probably going to hear a lot more about the anterior pituitary than the posterior pituitary in your studies. That's because the anterior pituitary releases far more hormones than the posterior pituitary. The posterior pituitary only releases oxytocin (which stimulates lactation, along with prolactin I think) and vasopressin/ADH (which increases retention of water and also helps regulate blood pressure). The anterior pituitary, on the other hand, releases FSH (follicle-stimulating hormone), LH (luteinising hormone), growth hormone, TSH (thyroid-stimulating hormone, prolactin and ACTH (adrenocorticotropic hormone).

One example of how hormones help the body respond to outside stimuli is the role of oxytocin in lactation. When the baby suckles on the mother's breasts, stretch receptors are activated. These send signals to the CNS, stimulating release of oxytocin, which leads to contraction of myoendothelial cells in the breast. This ejects milk into the hungry baby's mouth.

Gain an understanding of the role of feedback pathways in the control of hormonal signalling.

There have to be ways for the amount of hormone released to be controlled within limits, otherwise you'll end up with an overload of something (or not enough) and that isn't always good. For example, too much growth hormone can lead to gigantism; too little can lead to dwarfism.

The most common feedback pathway is negative feedback. In negative feedback, the end products (or sometimes intermediates) prevent more hormone from being released, as if to say "There's enough of us, you can stop making more clones now."

A good example of negative feedback inhibition occurs in the reproductive system (both male and female). GnRH (gonadotropin releasing hormone) from the hypothalamus stimulates the anterior pituitary to release FSH (follicle-stimulating hormone) and LH (luteinising hormone), which stimulate the developing follicles in the gonads. These follicles then produce inhibin as well as testosterone (in males) and oestrogen (in females). Inhibin prevents further production of FSH and LH in the anterior pituitary whereas testosterone and oestrogen provide negative feedback to both the hypothalamus and the anterior pituitary. This keeps hormones at an optimal level for follicle growth (in males, at least: things get slightly more complicated in females as we shall see).

Positive feedback is also a thing. Positive feedback is where production of products leads to more products being produced until something catastrophic happens. This sounds counterintuitive, but it actually works in certain situations. One of these situations is ovulation in females. As the follicle grows, it secretes more and more oestrogen. When the oestrogen levels rise past a certain threshold, they cause positive feedback to occur. More and more FSH and LH are released until eventually ovulation occurs. Ovulation is the "catastrophic event" that stops further positive feedback from occurring.

Gain a understanding of the role of the hormone cortisol, in daily life and adapting to stress.

Cortisol, like adrenaline and noradrenaline, is secreted from the adrenal glands. Unlike adrenaline and noradrenaline, however, it is secreted from the zona fasciculata (a part of the adrenal CORTEX) rather than from the adrenal medulla. Also like adrenaline and noradrenaline, it contributes to the "fight or flight" response, but its action takes a longer time. There are always basal levels of cortisol that fluctuate throughout the day in order to help us get out of bed etc. These levels tend to be highest in the morning when we need them to get out of bed and lower at night when it's time to sleep.

The release of cortisol requires several steps. Firstly, CRH (corticotropin releasing hormone) is released from the hypothalamus to the anterior pituitary. The anterior pituitary then releases ACTH (adrenocorticotropic hormone) which is then picked up by the adrenal glands, which then release cortisol, among other hormones such as aldosterone (but they're not important for now).

As I said, cortisol is important in the stress "fight or flight" response. It increases glucose and oxygen supply to the heart, brain and skeletal muscles. It increases glucose availability by limiting amino acid uptake everywhere except for the liver, and by breaking down muscle to release more amino acids to be sent to the liver. Aside from amino acids, it also breaks down lipids to release fatty acids. The liver can then produce glucose from amino acids in a process known as gluconeogenesis ("gluco"- sugar, "neo"- new, "genesis"- to make something). It also antagonises insulin in order to decrease peripheral glucose utilisation.

Aside from these effects, cortisol can also promote analgesia (i.e. stop you from feeling pain) and increase the vascular response to catecholamines (adrenaline and noradrenaline), thereby compounding the response.

But it's not all good, though. Cortisol does also suppress immune, reproductive and digestive functions. This is good for short-term stress as it allows energy to be diverted somewhere else. In the long-term, however, high levels of cortisol can adversely affect your health.

One example of a disease with problematic cortisol levels is Cushing's syndrome, in which too much cortisol is being secreted. Some of the symptoms of this disease include diabetes (due to antagonism of insulin), central obesity, thinning and bruising of skin and muscle wasting (due to muscle being broken down to release their amino acids).

So that's it from me on the basics of physiological control symptoms! Next up I'll be revising the reproductive system.