Sunday, January 25, 2015

The rough guide to the stress response

[Note: This post is intended as reading material for my upcoming online course, "Canine Hormones: From molecules to behavior." This is an entirely online course offered through APDT, begins Februrary 11, and is worth 12 CEUs. I posted with more information. I encourage you to sign up!] 

The series of organs working together to form to stress response are called the hypothalamic-pituitary-adrenal (HPA) axis. This post is a reference to them. The major players are:

The hypothalamus

The hypothalamus
Licensed under CC BY-SA 2.1 jp
via Wikimedia Commons
What it is: part of the brain, an important link between the nervous system and the endocrine (hormonal) system

What it does in the HPA: in response to input from other parts of the brain, releases cortocotropin-releasing hormone (CRH) into blood vessels which take it directly to the pituitary and not into the rest of the body

The pituitary
Emplacement de l'Hypophyse
Patrick J. Lynch, medical illustrator
via Wikimedia

The pituitary

What it is: a little gland hanging off the bottom of the brain. Some people consider it part of the brain and some don't.

What it does in the HPA: in response to hormones coming through the blood directly from the hypothalamus (not going out through the rest of the body first), sends adrenocorticotropic hormone (ACTH) out to the rest of the body

The adrenals

What they are: small organs next to the kidneys responsible for sending all kinds of important hormones out into the body

What they do in the HPA: in response to ACTH in the bloodstream, release cortisol into the bloodstream so that it can alert different organs and tissues around the body to the need to respond to a stressor

The minor players

Those are the three organs which are part of the name of the stress response: the hypothalamic-pituitary-adrenal (HPA) axis. But to some extent, humans just chose those three as the central parts of the axis because we understood their functions first. Other organs are important in the functioning of the stress system too.

The hippocampus

What it is: a part of the brain associated with learning and memory

What it does in the HPA: assesses the amount of cortisol in the bloodstream and sends a negative feedback message to the hypothalamus to tell it to slow down the HPA axis (resulting, eventually, in the release of less cortisol from the adrenals). This is probably part of how socialization works: the hippocampus undergoes epigenetic changes early in life which make it more or less able to send the “slow down” message to the hypothalamus and put the brakes on the stress response.

The amygdala

What it is: a part of the brain associated with fear

What it does in the HPA: the amygdala is part of the system that sends that initial message of fear when an animal encounters something scary, triggering the initial HPA axis stress response.

The liver

What it is: an organ that makes a lot of useful substances used for various things in the body

What it does in the HPA: makes corticosteroid-binding globulin (CBG), the little protein that carries cortisol around in the blood stream. CBG does more than just ferry cortisol about; it actively spits it out in locations where it's needed, and when an animal has very low levels of CBG, the entire HPA axis becomes less reactive. Very young animals have low levels of CBG, which may contribute to their early lack of fear.

Thursday, January 22, 2015

The stress of life

[Note: This post is intended as reading material for my upcoming online course, "Canine Hormones: From molecules to behavior." This is an entirely online course offered through APDT, begins Februrary 11, and is worth 12 CEUs. I posted with more information. I encourage you to sign up!]

Stress isn't good or bad. Stress is life. Stress is some change in your environment that means your body has to work a little harder. Stress is a blast of cold, missing a meal, not getting enough sleep. But stress is also going for a run, seeing a loved one after a long absence, thinking through a hard problem and getting it right. All of these things might mean your body has to rev up: your heart might beat faster, you might spend less energy on digestion, your immune system might adapt to meet expected coming challenges. An extra challenge can be good or bad. If life were one long nap, it wouldn't be much of anything.

Your body uses the hormone often referred to as the stress hormone, cortisol, to manage its response to stress. Going to an agility trial today? Need a little more cortisol to deal with all the extra energy you're going to spend. Cortisol affects our bodies profoundly, regulating our immune systems, our metabolism, and our behavior. Without enough of it, we would die, and indeed there is a disease (called Addison's in humans and hypoadrenocorticism in dogs, but it is the same thing) which is simply a lack of sufficient cortisol. Without treatment, it is often fatal.

But we think of stress as a bad thing, and indeed when it goes on too long, it is. Our bodies have developed to expect brief, passing stressors. A predator's attack. A few days of icy weather. Then safety and warm sun. When stress goes on and on, our bodies try to adapt, but high cortisol levels over weeks or months have side effects. Our immune systems become suppressed -- look at any college campus during final exams and you'll see rampant sneezing and coughing as students' high cortisol levels leave them unable to fight off infections. Our metabolism changes — we store fat for famines that never come, and eventually succumb to diabetes. And our brains suffer: ongoing high levels of cortisol can actually cause certain parts of our brains (associated with learning and memory) to become smaller, and other parts (associated with fear) to become larger. Ongoing stress leads to depression.

The stress system is among the more complex of the various hormonal systems. It is called the hypothalamic-pituitary-adrenal (HPA) axis because of the three main organs which secrete its hormones: the hypothalamus (part of the brain), the pituitary (maybe part of the brain and maybe a little stalk hanging off of it, depending on who you ask), and the adrenals (tiny glands down by the kidneys). But there are all kinds of other parts to this system: other parts of the brain which feed in to it from the top level (the hippocampus, site of learning and memory and many other things; the amygdala, site of fear and many other things); corticosteroid binding globulin, the little carrier protein which carries cortisol around in the blood and is made by the liver. The reproductive hormones, estrogen and testosterone, also affect the HPA axis. So does serotonin, the chemical targetted by so many anti-depressants.

All of these different parts of the system work together to regulate the amount of cortisol in the bloodstream: up when there is stress (good or bad), down when there is not. Our bodies are a chemical soup of hormones and these hormones are both cause and effect: cortisol rises when we experience stress. But high levels of cortisol also seem to cause us to feel distress. The stress system is enormously complicated, as mysterious in some ways as the brain itself, and yet a huge part of what makes each of us who we are. Different cortisol profiles (usually high, usually low, very reactive, very unreactive) are associated with different personality types in animals: bold, shy, proactive, reactive. The human research has some more complex findings but the basic truth remains that our personalities are, in part, chemical. We are our hormones.

[The title of this post, The Stress of Life, is also the title of a book by Hans Selye, who first isolated cortisol.]

Saturday, January 17, 2015

Hormone regulation: it's all about control

[Note: This post is intended as reading material for my upcoming online course, "Canine Hormones: From molecules to behavior." This is an entirely online course offered through APDT, begins Februrary 11, and is worth 12 CEUs. I posted with more information. I encourage you to sign up!]

One of the wisest things my introductory biology teacher ever said to us was “The body always has one foot on the gas pedal and one foot on the brake.” I think of this statement a lot when I am frustrated by the great complexity of methods that the body uses to regulate its systems. Why not just an on/off button?

Because the body is both a control freak and highly democratic. The body wants to be able to manage all the hormonal systems at a very fine level. But it doesn’t tend to manage at a central level; instead, it manages by taking votes from all the different processes participating in a system. So instead of determining that the car needs to go a particular speed and pushing down the gas pedal until that speed is reached, the body lets individual processes all have a go at hitting the gas pedal and the brake, all at the same time. One process hits the gas a few times and ramps up the car’s speed, another hits the brake and slows it back down, a couple few more do the same thing, and eventually the car is going at the speed it needs to go. That’s what my teacher meant about the gas pedal and brake at the same time. Complicated, but it’s how the body keeps control of things.

"Drawing Hands," M.C. Escher. Source: Wikipedia

One arm of the stress system, known as the HPA axis, is a great example of this. I’ll go into the HPA axis in a lot more detail in later posts, but for now the important point is that there are several different hormones that get sent out in response to a stressful situation. One hormone triggers the release of another hormone which triggers the release of another hormone. It’s sort of like a game of dominoes, where you push the first domino and it causes the next ones to fall, in a self-propagating line.

"HPA Axis Diagram (Brian M Sweis 2012)" by BrianMSweis - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

The body sets things up this way because, unlike with a line of dominoes, each new step in the hormone cascade can boost or decrease the volume of the message. So the brain releases the first hormone, CRH, and it filters into another part of the brain, which says, “I see hormone #1! Time to release hormone #2!” This part of the brain can then decide to release a lot of hormone #2 or a little of hormone #2, making the message louder or softer. Hormone #2, ACTH, goes out to the adrenal glands, and they release hormone #3, CORT — again with a chance to boost (or muffle) the signal. By the end of the cascade, the message may have been amped way up (high levels of CORT in the blood), or it may have petered out (low levels of CORT in the blood), depending on whether the gas pedal got pushed more or the brake got pushed more.

This cascade system suggests that hormones farther down in the cascade are powerless over their own fates. But in fact they do get a chance to feed back into the system. Just like students who are asked at the end of a course for their feedback on their professor’s performance, hormones at the bottom of a cascade often provide information which is then used back at the top of a cascade.

In the case of the stress system, the brain sends out CRH, in other words, the message “release more stress hormones,” in response to a stressful event. This message goes through several levels at which it could be amplified or muffled, so the brain needs a way to know whether to keep pushing the gas pedal by sending the message “more stress hormones! That’s nowhere near enough for this situation!” or decide that the message has been sent sufficiently and hit the brake. The brain does this by monitoring the levels in the blood of CORT, the final hormone in the cascade. If the brain sees more of this bottom-level hormone, it releases less of the top regulatory hormone, which allows the system to settle down. If it sees less, it releases more of the top regulatory hormone, keeping the system ramped up. This is called a negative feedback loop: higher levels in later parts of the system result in lower levels in the earlier parts of the system. What goes up must come down — systems that have negative feedback loops are characterized by an initial increase in response to a stimulus (an increase in stress hormone levels in response to a stressful event) followed by gradual reduction (the levels of stress hormones gradually decrease as time passes after the stressful event).


A few systems have positive feedback loops, in which increased levels of hormones lower in the cascade lead to increased, not decreased, levels of hormones higher in the cascade. The only hormonal example of a positive feedback loop that I know of (though I imagine there are more) occurs during birth, when hormones which lead to uterine contractions cause more of the same hormones to be released. This particular positive feedback loop is halted by the completion of the birth process.

Cattle? Hormones? Both have positive feedback loops.
Source: Wikipedia

The consequence of having such a complex regulation process for managing hormone levels is that there are a lot of places for things to go wrong. For veterinarians trying to diagnose endocrine diseases (diseases relating to hormone imbalances), this means a lot of tests often have to be run to find the root of a particular hormone problem. From my perspective as someone who’s trying to figure out how the stress system behaves differently in fearful dogs, this means there is rarely or never a clear answer to questions I want to ask. CORT levels might be higher in one dog compared to another, less fearful dog. But why — because of what point in the cascade? Or because of the process that triggers the cascade in the first place? Or because of some side process influencing the cascade (the passenger who reaches over and grabs the hand brake while you’re driving)?

It is useful to try understand hormonal processes, but we have to remember that we’re understanding them only at a very high, abstracted level. Inside a real dog’s body, there are a zillion little processes interacting in ways we can’t ever see or completely understand. Meaningful understanding of these processes isn’t impossible, but complete understanding might be!

Monday, January 12, 2015

How I got my dog back

I was in Boston, seeing some old friends. My dogs were two hours away in Connecticut, with the friend I was staying with. My shy dog Jenny had been doing so well, really relaxing around my friend. So I wasn't prepared when my friend called me. “How far away are you? You need to come back right now. Jenny ran away.”

My husband and I were in the car in a few minutes, heading back to Connecticut. It wasn’t going to be a good scene when we got there. Jenny had slipped out of the door and bolted, and had disappeared. I suspected she wouldn’t come to anyone but me, and worried that she would be so scared that she wouldn’t come to me either. When we finally arrived, I walked around and called for her for an hour, but she didn’t come. She was gone.

Jenny was gone for four days before we got her back. This is what we did:

  • Called professionals. We went to Missing Pet Partnership and looked at their directory to find a local non-profit that specializes in finding lost pets. We called SMART because they were located nearby, and they were incredibly helpful. They came out with their tracker dog and got us a lot of information about where Jenny had been, which helped us figure out where to put our posters. They gave excellent advice and provided emotional support when I felt that I was starting to crumble.
  • Postered. SMART emphasized that posters should be visible from cars, should be big (bigger than 8.5 x 11!), and brightly colored. They provide detailed instructions.
  • Used Facebook. The local Animal Control Officer posted a photo of Jenny on Facebook, and we let the neighborhood group for the place where we were staying know so that they could share it.
  • Stayed in close touch with local Animal Control Officers. I was very lucky to be staying with a friend who is the medical director at a local shelter, so she had relationships with them already.
The community response was amazing. On the morning of Jenny’s fourth day lost, we got a phone call in response to the Facebook post. Jenny was in the caller’s yard. My husband and I left in the middle of breakfast and drove over. (We would realize around dinnertime that we had never gotten around to brushing our teeth that day.) We saw her — for the first time in four days! She didn’t come when I called her, and we lost her again. But then the phone calls started coming one after the other, some in response to Facebook, some in response to our posters. We marked her progress on a map as we followed the calls; she was travelling fast. We finally caught up with her just before dark, thanks to the Animal Control Officer who saw her but did not chase her, and stayed to point out to us where she had gone.

In perhaps the most amazing part of the entire story, Jenny, the dog who is so afraid of men, came when my husband called, walking right up to him and licking his hand. I was a block away at the time.

The next day, as we were walking around taking down posters, someone opened her window and called down to me from a second story room: “I’m so glad you got your dog back! I saw it on Facebook.” The Facebook post about our reunion got over 250 likes. Jenny was a local celebrity.

The most important advice we got from SMART was to use all the options you have to communicate to the community that your dog is missing: social media, traditional posters, calling ACOs, walking the street and talking to people. Get the word out any way you can! That is how we got her back: from the incredibly supportive community that looked for her, found her, and told us where she was.

I am deeply, deeply grateful to
  • The West Hartford, Hartford, and Simsbury Animal Control departments. Many of them worked during their time off to help us find Jenny. They went above and beyond.
  • The West Hartford and Hartford communities who were so supportive and friendly. We got zero crank calls, and we got Jenny back because these people really cared and really wanted to help.
  • My dear friend Katy and her boyfriend Zach, who tirelessly helped us poster and field calls.
  • SMART, who provided excellent advice and support; it was a privilege to watch their tracker dog at work.
  • Jenny herself, for not losing her cool and coming when we called!
A week after we got her back, I still feel amazed every time I look at her. Kiss your dog for me, and check to make sure that they have their collar and identification tag on them. You never know when something will happen.

About twenty seconds after Jenny and I were reunited.

Saturday, January 10, 2015

It takes a village to raise a ... hormone

[Note: this post is part of the course materials for my upcoming online class, Hormones: from molecules to behavior, with APDT.] 

Hormones don’t spring into existence from nowhere and they don’t do their jobs in a vacuum. They work in concert with many, many other molecules in the incredibly complicated canine (and human) body. This is a world we never see and rarely think about: a world of microscopic molecules interacting in chaotic but ultimately purposeful fashion, somehow operating in concert to influence nearly all of your dog's physiologic processes, like digestion and immune system, and, of course, behavior.

What is a protein hormone and what is a steroid hormone?

There are two main categories of hormones, proteins and steroids, and the mechanisms supporting each are a bit different.

First, some terminology. “Protein” means something different in biology than in nutrition; the University of Utah has a great explanation about what a protein is in the biology sense. (I assume you all know what it is in the nutrition sense — the thing that you cook or feed to your dogs.) When I use the word “protein” here, I’ll always mean the biological sense, never the nutrition sense.

Protein hormones are made directly from DNA instructions, just as all proteins are. They may be modified after they are first made — one protein, pro-opiomelanocortin (POMC), is actually split into three separate hormones after it’s constructed! (Hence the long name, which contains bits of the names of all three of the smaller protein hormones that are spliced out of it.) I don’t know of any other protein hormones with such a complicated origin story. Other protein hormones, such as luteinizing hormone (LH), are constructed by sticking two separate small proteins together to make one bigger one. In general, however, thinking of a protein hormone as being coded for by a single gene is a safe shortcut, even if not always completely true.

Steroid hormones, on the other hand, are made by modifying molecules of cholesterol. Dogs and humans both synthesize cholesterol from materials we get in our diets, and use it for a whole mess of different purposes. Too much if it might be bad for your heart, but if you didn't have enough of it, you would die. If you could see it, it would look something like this:

Molecule of cholesterol
Wikipedia: Cholesterol

A protein acting as a little machine takes a molecule of cholesterol and turns it into something slightly different. Then a different protein takes that product and modifies it a little more. Many of these proteins working in a chain (the synthesis pathway) will end up producing a steroid hormone. For example, here’s testosterone — can you see the similarities and differences in structure to cholesterol?

Molecule of testosterone
Wikipedia: Testosterone

There are a bunch of different steroid hormones, each with its own pathway of different proteins which construct it. Some steroids share some of the proteins in their pathways with other steroids, which leads to some steroids being related to others; for example, estrogen and testosterone are similar in structure and share parts of their synthesis pathways. Yes, testosterone is a steroid hormone, and sometimes abused by human athletes, leading to the shorthand of “steroids” to refer to a particular subset of steroid hormones, even though there are many others.

The variety of proteins used in constructing steroid hormones is quite dizzying; asking students to memorize these pathways is a favorite hazing ritual for biology and physiology teachers. Here is a typical steroid synthesis pathway diagram that veterinary students are expected to absorb and understand:

Häggström M, Richfield D (2014). "Diagram of the pathways of human steroidogenesis". Wikiversity Journal of Medicine 1 (1). DOI:10.15347/wjm/2014.005. ISSN 20018762.

Proteins, steroids, and genetics

Since proteins are made directly from DNA instructions, they may end up differing between individuals. All it takes is a small mutation to make a change in the DNA, and the protein which is generated is different. Genetic researchers have gotten quite good at identifying these small differences in DNA and the related proteins. However, we’re still quite bad at saying whether these small protein differences result in any differences in behavior. Many differences at the protein level don't make any difference in how the protein does its job — they’re the equivalent of a small typo that’s easy to recover from. Other differences completely change the ability of the protein to function. Just by looking at DNA, we can’t tell which is which. So genetics researchers are currently engaged in finding differences in protein hormones between different individuals and trying to figure out if the differences actually affect behavior.

Steroids, on the other hand, aren’t made directly from DNA instructions — they’re made by protein machines which are themselves made from DNA instructions. This means that steroid hormones are identical between individuals. Testosterone looks exactly the same in all of us, no individual differences. However, there can be differences in the protein machinery that makes testosterone, leading to people and dogs who make more or less of it, or who make it in different situations, or... So in the case of steroid hormones, genetics researchers are looking at the synthesis pathways for these hormones and trying to find interesting differences in the pathway proteins, rather than in the hormone proteins themselves, which relate to behavioral differences.

Water soluble or fat soluble?

Protein hormones are water-soluble, meaning they dissolve well in water (or, more to the point, blood); steroid hormones are fat-soluble, because they are basically cholesterol with some modification. Imagine dropping a mass of fat into a bowl of water: it wouldn't dissolve well at all, but would stay in goopy hunks. In the same way, steroid hormones, which are basically tiny molecules of cholesterol, don't dissolve well in blood.

Protein hormones, then, can hop in the bloodstream and easily be carried wherever they need to go. Steroid hormones have it a little harder. They can be free in the bloodstream, but often they rely on protein carrier molecules to shepherd them around. These molecules are designed just to carry one particular brand of steroid hormone.

The water versus lipid-soluble issue comes into play again when a hormone arrives at its destination cell. Animal cells are basically bags of water (the interior of the cell, called the cytosol) surrounded by a double layer of fats (the cell membrane). Now suddenly the fat-soluble steroid hormones are at the advantage: they can diffuse right on through the fatty cell membrane and into the interior of the cell. The water-soluble protein hormones, on the hand, are completely stalled by the cell membrane. They require help to get through it. Sometimes the cell’s receptors are on its outside so the protein hormones don't have to get inside. Sometimes there are transporter proteins which surround them and carry them through the cell membrane to the inside of the cell.

The macroscopic view

The details of how hormones inter-operate with the other parts of the body at a microscopic level aren't all that important. What is important is the complexity of this microscopic world that they operate in. Hormones aren’t just substances that exist for us to measure in the blood. They are very active little molecules which have their own life histories, from creation to modification to use in a cell. All the other substances that they interact with affect them. It's easy to think that the story of how testosterone or cortisol affect behavior is just about testosterone or cortisol — but it’s not. It's also about all the substances that they interact with. Understanding how biological processes are translated into behavioral differences is rarely straightforward. The next time you read a story about how a hormone affects behavior, remember that the hormone itself is only a small part of a symphony of molecules, all working together to make your dog function the way he does.

My dogs, Jack and Jenny, to remind us of the macro level that this is all about.

Thursday, January 8, 2015

Hormones: the public announcement system of the brain

[Note: this post is part of the course materials for my upcoming online class, Hormones: from molecules to behavior, with APDT.]

There your dog is, minding his own business, when suddenly another dog attacks him. His brain now has a bunch of physical changes to coordinate very quickly to deal with the emergency: routing energy away from the non-essentials (digestion, reproduction, certain parts of the immune system) and to what his body needs right now (muscle speed and strength). How to get out this message of impending danger to all the body's organs quickly and efficiently?

Human societies sometimes solve this kind of problem with public announcement systems: a public broadcast in an airport to announce a flight change, a Facebook post to announce a change in relationship status, posters around a neighborhood about a lost dog. It’s not the most elegant of solutions. Having a direct connection to everyone you wanted to pass the message to would be better — the passengers on the flight with the changed departure time, your Facebook friends whom you think are particularly cute, the people who actually saw your lost dog running by. But in the absence of being able to call all these people directly, we just get the message out there and hope that the people who didn’t need it won’t be too annoyed, and the ones who do need it will respond appropriately.

Hormones are a similar low-elegance, high-efficiency answer to the same problem. In the case of your dog who is being attacked by another dog, his brain orders the release of stress hormones. Hormones are tiny molecules which can be carried by the bloodstream throughout the body. The message to each of the organs which see the hormones floating by is the same — get ready to deal with danger — but the response of each organ is different. In the case of stress hormones, the body alters its metabolism to prepare to use a lot of energy in fighting or running, the digestion slows, and the immune system is suppressed, among many other consequences.

Note that the brain is quite capable of sending direct messages without using hormones. The peripheral nervous system is a whole complicated network of nerves connecting the brain to individual parts of the body, so if a message needs to be sent directly and privately, it can be. But just as sending a hundred individual hand-addressed wedding invitations is annoying and time-consuming, sometimes the public broadcast approach is just easier.

I’ll go into more detail in future posts about how the brain causes hormones to be released, because it’s different for different systems. In short, the brain sends an executive order, and a hormone is released into the bloodstream. Just like pouring gallons of dye into a river on St. Patrick’s Day, there’s plenty to go around, even miles downstream, or, in this case, all the way to the most distant parts of the body.

The hormone message is carried through the bloodstream throughout the body, washing up against cells from every organ. This is where the story moves from macroscopic — things large enough to see with human eyes, like blood moving through vessels — to microscopic: molecules of hormone and individual cells. You’d be able to see a dog’s cell through a microscope, but not a molecule of hormone, so we’re entirely in the realm of our imagination here.

There is all kinds of stuff in a dog's (or a human's) blood. Not just molecules of cortisol. Molecules of other hormones — I’m personally particularly interested in the ones that are associated with behavior, like cortisol and oxytocin and vasopressin, but there are hormones that have nothing to do with behavior: insulin and glucagon for managing metabolism, lutenizing hormone (LH) and follicle-stimulating hormone (FSH) for reproduction, insulin-like growth factor for growth, and so on. And there are other things besides hormones — cells for the immune system (white blood cells), cells for carrying oxygen (red blood cells), antibodies, little proteins called cytokines released by individual cells to communicate with each other about the stuff cells care about (inflammation, infection, and so on), things that cells eat like sugar and free fatty acids — the list goes on and on. The bloodstream is a superhighway packed with traffic.

Cells need to be able to process all this stuff appropriately. In the case of hormones, they do it with receptors. A receptor is like a keyhole, tuned to exactly one key; the key is called a “ligand.” In the case of a receptor for cortisol (for which the technical name is glucocorticoid receptor), the ligand or key is cortisol. There is a very short list of molecules which will fit into this receptor like keys into a lock, and cortisol is on that list. Insulin might bump up against it as it is going about its way doing insulin things, but it won’t bind with the receptor: it won’t fit into the keyhole.

When cortisol bumps up against this receptor, on the other hand, it does bind. In technical terminology, it is the ligand for the receptor. The receptor is like a little robot which, when it gets its ligand of cortisol, suddenly activates, as if the key in the lock enables its switch to be flipped to “on.” It turns on and goes and does its preprogrammed job, interacting with many other little robots which are receiving other signals.

Somehow, in the chaos of a zillion little machines each doing a very simple job, some very complex biological processes get performed inside cells. It’s this complex interaction that explains why different kinds cells react differently to the same hormonal message. Remember that the muscles need to do one thing when they see cortisol and the intestines need to do something different: this is because their receptors are working in very different cell types, with different environments, and different cells may have more or fewer receptors. Of course there are some cells which don't much care about this particular message, and these cells don't have receptors for cortisol, allowing them to ignore it.

I’ll talk more about what glucocorticoid receptors do in a later post, but it's not all that important to understand this kind of minutia. The story at a high level is that the brain identifies a situation that requires action; it orders the release of a hormone to send a public broadcast message to the rest of the body; cells in the body which have specific receptors for this hormone get the message, and their receptors activate in response to binding to molecules of the hormone and perform the necessary work to respond to the message. The system is simple, but somehow allows for the bewildering complexity of the animal body.