Receptor and signal molecule as lock and key

Lock and key model of receptor and signal molecule

During a physiological response (whether it be sweating, increasing the heart rate, or thinking), our cells need to talk to each other, in order that they may act in a coordinated fashion.  Hormones and neurotransmitters are examples of signal molecules that our cells use to communicate with one another.  This communication is so important, that most of the known drugs (and poisons) are molecules that either enhance, or block, the action of signal molecules.

This is a really “key concept” in physiology, so the idea of a “lock and key” provides an especially fitting metaphor.  Just like a door key, the signal molecule has a structure that must fit precisely into the receptor (the “lock”) in order to activate it.  Some drugs, known as agonists, are designed to mimic (or even exceed) the effect of the signal molecule, by having a similar enough structure that they are able to bind and activate the receptor — essentially “picking the lock”.

Other drugs, known as antagonists, have a similar enough structure to bind to the receptor, but fail to activate it.  These drugs prevent the signal molecule from gaining access to the receptor, and thus prevent the normal response from occurring — essentially “jamming the lock”, which is what would happen if you were foolish enough to try picking it with a twig.

Goldilocks and the quest for homeostasis

Goldilocks and the quest for homeostasis

The fairy tale of Goldilocks and the Three Bears has a long history and has been interpreted in various ways.  It’s unlikely that it was intended as an object lesson in physiology, but the story provides a perfect metaphor for the basic response system that keeps us alive.

Like Goldilocks, the cells inside your body are fussy about many things, and require a relatively constant internal environment, a dynamic condition known as homeostasis.  If your internal temperature or body chemistry are not “just right”, the intricate chemical pathways that sustain us will begin to fail.

In order to achieve homeostasis, the body makes constant small corrections.  For example when you exercise, the metabolic heat produced increases your body temperature, causing your body temperature to exceed the optimum.  In response, you produce sweat to bring your temperature back down.  Conversely, if you find yourself underdressed in a blizzard, your body temperature will drop below normal, causing you to shiver and generate more heat.

This process of counteracting a deviation by bringing about an opposing change is called negative feedback.  It’s a vital process that’s commonly misunderstood by physiology students.  In our day-to-day lives, we have grown accustomed to seeking out “positive” feedback from friends, colleagues and customers.  But it’s the negative feedback that plays a vital and sustaining role, by correcting perturbations to our physiological state.  Without negative feedback, not only our body temperature, but pH, osmolarity, acid/base balance and many other internal properties would easily spin out of control.

So next time you receive negative feedback from someone, remember it’s a blessing in disguise!  By letting you know when things aren’t right, it can set you back on the right course.

Pectoralis major, the bodybuilder’s muscle

Pectoralis major, the bodybuilder's muscle

We all know the image of a well-muscled boxer or weightlifter doing the “self-clasping handshake” as a gesture of victory; and sometimes politicians have used this gesture to recognize a spirited response from the crowd.  But this position also presents an opportunity for isometric exercise, in which muscles contract but are prevented from shortening by an opposing force — in this case, the opposite arm.  So a bodybuilder might use the occasion of one triumph, as a chance to work out for the next competition.

The pectoralis major might be considered the “bodybuilder’s muscle” because it is the largest of the chest muscles, enhancing the male physique.  It performs several actions, by pulling (at what is known as the insertion point) on the front side of the humerus (arm bone).  The action shown here is horizontal adduction — bringing raised arms toward the midline.  An alternative example would be a musclebound villain strangling his puny foe.

But despite its caricature status, the pectoralis major is one of few muscles with a special ability:  It can undo its own action!  Since all muscles work by shortening (contracting), this is a rare feature indeed.  The key is that the muscle has two divisions (heads) — the origin (attachment point on the body) of one is above the insertion, at the clavicle (collarbone), the other is below the insertion, at the sternum and ribs.  Depending on which head is used, you can move the arm in opposite directions.

When the clavicular head contracts, the arm is brought forward and upward (flexion).  You can test this by facing a wall, with your arms at your sides.  Push against the wall with one arm, and use the other hand to feel the muscle contract, just below your collarbone.  The front wall of your armpit, formed by your entire pectoralis major, hardens as well.

Now try raising one arm up high and in front of you, as if eagerly answering a question in class.  Face the wall again, and push with your raised arm.  Now, you are attempting to extend the arm, bringing it back down to your side.  Using your other hand again, feel that the lower division (sternal head) is contracting, as is the front of the armpit again.  While the two heads can work together in some actions (“two heads are better than one”), they are antagonists in performing flexion vs. extension.

Repeat these exercises a few thousand times, and maybe you too can look like a cartoon!

Serratus anterior, the boxer’s muscle

Serratus anterior, the boxer's muscle

Boxers use a lot of muscles, but the quintissential boxing move is a simple forward punch.  Contraction of the triceps brachii straightens (extends) the elbow.  At the same, contraction of the pectoralis major and anterior deltoid muscles bring the arm forward at the shoulder joint, which is considered flexion.

But only one muscle can enjoy the “title” of The Boxer’s Muscle — serratus anterior.  Depicted here, the serratus anterior runs between the ribs and the shoulder blade (scapula).  When it contracts, it pulls the scapula forward (protraction), in the direction of the opponent’s jaw.  It also rotates the scapula upward.  In the illustration, the bottom of the scapula moves left, and the top moves to the right.  This raises the shoulder joint and orients it toward the opponent’s face.  This combination of movements provides a firm base for a forward punch, hence the epithet “boxer’s muscle”.

The name serratus comes from the “saw-like” appearance of the muscle (as in the word serrated), resulting from its several attachments along the ribs.


Levator scapulae, the “I don’t know” muscle

Levator scapulae, the "I don't know" muscle

What happens if you just can’t remember?  Not to worry — there’s a muscle for that!  The levator scapulae does as its name implies — it elevates your scapula, or shoulder blade.  So, next time you’re asked for a muscle action you don’t know, just shrug it off!  Do this a few times and not only will you benefit from the workout, but (if you’ve been paying attention) you’ll never forget the levator scapulae.

But this muscle in fact accommodates a broad range of confidence levels.  If the scapula is held in place by other muscles, then contracting both of your levator scapulae muscles will extend the neck, pulling your head back.  Let your head drop forward again, and repeat a few times — you have just nodded your head in the affirmative!

And finally if you might know, but you’re really not sure?  Then you’ll want to contract just one of your levator scapulae (while fixing the scapula in place) — this will tilt your head to the side, in an expression of quizzical puzzlement.

Biceps brachii, the sommelier’s muscle

biceps brachii, the sommelier's muscle

Probably you’ve heard of the “biceps”, but you might not have thought of it as the “sommelier’s muscle”!  And yet, the action of opening a wine bottle sums up the two major actions of this muscle.

But first of all, be warned that you have a biceps muscle in your thigh as well — so to be clear, the biceps in your arm is called biceps brachii (“two-headed muscle of the arm”).

The biceps brachii attaches to your forearm on the anterior side, and thus flexes the elbow — pulling the forearm toward the shoulder, and thus folding your upper limb in two.  But it also supinates the forearm — this is a rotational action that twists the forearm (and the hand with it) from a “palm backward” (or downward) position to a “palm forward” (or upward) position.

Supination has many uses, such as turning your cupped hands upward to “drink soup“, begging for mercy as you “supplicate”, or perhaps even expressing a certain attitude with “…’sup bro!” — and these can be helpful mnemonics for remembering this action.

The reason for this lesser-known action of the biceps brachii is that the muscle attaches to the inner surface of the radius (of the two long bones in your forearm, this is the one that sits on the lateral or thumb side).  As the muscle contracts, that surface is pulled toward the shoulder, rotating the radius laterally, which carries the hand with it.

When opening a wine bottle, supination is used to twist the corkscrew clockwise, inserting it into the cork.  This is followed by flexion at the elbow, as you pull the cork out of the bottle.  Be mindful, though, that this only works with your right hand!  Supination with your left hand achieves the opposite, which is helpful at the end — twisting counterclockwise, to get that corkscrew out of the cork.

Latissimus dorsi, the swimmer’s muscle

Latissimus dorsi, the swimmer's muscle

The latissimus dorsi, or “lat” for short, is often referred to as the “swimmer’s muscle”.  It’s the prime mover of arm extension — meaning it does most of the work when you bring your arm back from a forward position.  Such a movement is especially useful in swimming, because by pushing back against the water, it propels the body forward.  To see a well-developed latissimus dorsi, just visit your local swimming pool and look for someone who just swam some “laps” with their “lats”.  You can also use this muscle for pull-ups, or striking a blow with a hammer, but I’d prefer to let mine carry me across a coral reef.

The many faces of areolar connective tissue

The many faces of areolar connective tissue

Areolar connective tissue, like connective tissues in general, holds us together.  Like all connective tissues, it contains a lot of nonliving material — the extracellular matrix.  In this case the matrix is loose and unspecialized, with a large amount of interstitial fluid, making it an ideal “filler” between many structures in the body.  In particular, it is found on the back side of almost every epithelium in the body, including the lining of blood vessels.  As a result, every molecule that crosses between the blood and surrounding tissues, has to diffuse across areolar connective tissue — the “middleman” of exchange.

Epithelia line not just the blood vessels, but every other surface and cavity of the body.  This means they can function not only as an exchange surface, but also a barrier to microorganisms.  Here again, areolar connective tissue plays a vital role — as the “second line of defense”, harboring immune cells that attack any invaders that breach our defenses.

I’ve already paid homage to some of our other connective tissues.  The dense connective tissues are distinguished by large amounts of collagen, making them strong, though flexible.  Bone tissue contains a rigid mineral component making it an ideal structural support.  Areolar connective tissue, by comparison, is weak and shapeless.  But this unpretentious mass of matrix and cells is arguably even more important for our survival.

Large intestine — gotta stay hydrated

large intestine gotta stay hydrated

In a cast of organs that so far includes the pancreas, heart, pharynx, and assorted smaller beings, this week we’ll look at the large intestine.

First,  let’s set the record straight:  The only thing large about this intestine is its width — it needs a spacious interior, to accommodate the slowly desiccating remains of your meal.  By far the longest part of your intestines is the small intestine — the site where the great preponderance of digestion and nutrient absorption occur in the body.

What’s passed on to the large intestine is a soggy slurry of undigestible food bits — especially the undigestible long-chain carbohydrates known as “fiber”.  What remains of value in this mix is water and electrolytes (such as sodium and potassium), which the large intestine absorbs.

Don’t be misled, though — the large intestine is vital for your survival.  All the organs of the digestive system from the mouth to the small intestine secrete large amounts of fluid.  These secretions add digestive enzymes and other additives to process the food.  Enough fluid is released in this way, that you’d quickly dehydrate, without the large intestine’s help.  Indeed, the inability to absorb fluids in the large intestine, resulting in watery feces, or diarrhea, is a deadly condition that kills millions every year.

Incidentally, the mouth of our thirsty friend is accurately placed — it represents the opening where the small intestine attaches, and thus, releases its slushy contents into the large intestine.  At that junction, an ileocecal valve prevents backflow of feces into the small intestine.  Other parts are (more or less) anatomically correct as well — from the portly cecum (shown as the “body” of our absorptive acquaintance), and a tail-like vermiform appendix, through the ascending colon, transverse colon, descending colon, sigmoid colon (the S-shaped “zigzag” near the end), rectum, and anal canal.  The end!

The cerebellum — an athlete and a scholar

Cerebellum, athlete and scolar

The cerebellum, once thought to be simply a motor coordination center, is now understood to participate in both cognitive and emotional processing.  Somewhat resembling the cerebrum (with lobes and a highly folded cortex), but far smaller, it was given the name cerebellum meaning “little brain”.  After early studies showed its obvious role in motor coordination, the cerebellum was type-cast as a dedicated motor processor.

Even on a purely anatomical level, the cerebellum is an amazing structure.  While making up only 11% of the brain’s mass, it contains about half of all neurons in the brain.  It achieves this phenomenal density with vast numbers of tiny neurons called granule cells.  Indeed, their small size and density has slowed progress by making it difficult to record the activity of individual cells.  On the tissue level, the cerebellum has an impressively regular organization that’s suggestive of a printed circuit board.

So perhaps it’s no surprise that new research implicates the cerebellum as a “calculator”, not just for motor coordination, but in other roles.  A study last year (summarized here) showed greater involvement between the cerebellum and cognitive centers, lending credence to the notion that it plays  a general role in “quality control”, not just in movement but in thinking.  And a paper earlier this year (summarized here) showed powerful control by the cerebellum over an emotional reward center in the brain, thus controlling behavior.  Other studies have suggested roles for the cerebellum in autism and schizophrenia.  With this recent “sprint” in research, the cerebellum has begun to earn new respect.