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.

You have six Achilles tendons

Cartoon representation of colloquium talk by Jason Franz at Integrative Physiology Department, CU Boulder, November 4, 2019

Your calf muscles are attached to your heel by a tendon — the Achilles tendon.  What you might not know (nor did I) is that each of your major calf muscles — the soleus, and the two heads (divisions) of the gastrocnemius — exerts force through its own subtendon within the Achilles tendon.  These three subtendons (six including both legs) can slide past each other, which allows each muscle to work independently.  That’s good news for walking performance, because each muscle is free to “do its own thing” without having to remain in “lock step” with the others.  Unfortunately, as we age, adhesions form among the subtendons, reducing their independence, and walking performance is reduced.

In his talk, Dr. Franz explained the problem and then introduced his laboratory’s current work on biofeedback techniques (using sensors on the calf muscles and, yes, a futuristic pair of glasses) which holds the promise of restoring some of that youthful gait performance, and thus, a longer period of independent living into advanced age.

The title of his talk was “Mechanics, Energetics, and Stability: Modifiable Factors to Preserve Independent Mobility in Old Age”.  Dr. Franz directs the Applied Biomechanics Laboratory at the Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University.  The talk was presented at the weekly Colloquium at the Integrative Physiology (IPHY) Department at CU Boulder.

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.

 

 

 

 

Hepatocyte, Jack of 500 trades

hepatocyte, jack of 500 trades

Our largest internal organ, the liver, is also one of the most versatile — it performs over 500 different functions.  Virtually all its functions are performed by hepatocytes (literally, “liver cells”).  Here, one of the liver’s 200 billion hepatocytes looms greatly enlarged, busily carrying out five of these vital functions — represented by familiar visual metaphors.

  • Conversion of protein (and other compounds) to glucose — a group of processes known as gluconeogenesis.  Here, a ham (high in protein content) is converted to some candies (mostly sugar).
  • Glucose storage and release — the conversion of glucose to glycogen (and  back again) — plays a major role in the regulation of blood sugar levels.  (Here, glycogen is represented as a slice of bread — not quite glycogen, but it’s made of starch, another long-chain carbohydrate.)
  • Secretion of bile, containing among its components bile salts, molecules that bind to fats on one side, and water on the other.  In doing so, they stabilize — in other words, emulsify — small drops of fat, making them more available for efficient enzymatic digestion.  The green dish detergent is an apt metaphor in two ways.  First, it works much the same way as bile salts, emulsifying the grease on your dishes so it can be washed away.  Second, bile is in fact green!  The color comes from bilirubin, another component of bile, which serves to excrete broken down red blood cells and has a strong color (which changes depending on the exact compound) owing to its iron content.
  • Secretion of blood proteins, such as albumins — represented here by egg whites (which do contain albumins as a major component).  Among other roles, blood proteins modify the osmotic balance of your blood, preventing it from losing too much fluid in your capillary beds.
  • Metabolism of drugs and poisons, typically converting them into a form that can be more easily excreted by the kidney into the urine.

Cool as a sea cucumber

Cool as a sea cucumber

Sea cucumbers made news not long ago for a new conservation effort in Sri Lanka. It seems they’ve been over-harvested in many tropical regions across the world.  This came as a surprise to me – I saw quite a few while snorkeling in Hawaii. It turns out some sea cucumbers are being depleted – it’s not so much that they taste good; they’re said to be so bland they’re often boiled in meat broth to add flavor.  But they  contain some unusual nutrients, and their resemblance to a certain human anatomical part has given them an unfounded reputation for increasing virility.

Let’s hope sustainable practices prevail, because there’s plenty to admire about living sea cucumbers. Belonging to the class Holothuroidea, they stand out among the other classes in the phylum Echinodermata. Starfish are a good example of a “typical” echinoderm – radially symmetrical, with five arms (pentameral symmetry).  Echinoderms are, in general, pretty slow-moving. Most travel on tiny feet – tube feet — which are operated hydraulically, using the water vascular system that is another unique feature of this phylum.

Unlike other echinoderms, sea cucumbers have adopted a worm-shaped (or really, sausage-shaped) body plan. But the five divisions of the body are still evident, separated by grooves, ridges, or rows of tube feet, running lengthwise along the body.  Their elongated, five-part shape can indeed bear a remarkable resemblance to  a cucumber, although the color varies widely.

Sea cucumbers bear five sticky tentacles, which they use to collect organic matter, either from the sand or drifting down through the water.  These they insert alternately into the mouth, to pull off food particles. Because the mouth is often occupied, the other end of the digestive tube has become their main respiratory organ.  The sea cucumber’s anus, opening and closing as it breathes  throughout the day, has proved too inviting for the pearlfish to resist.  This long, narrow fish, with a pointed tail, has evolved just the right body shape to easily retreat into the sea cucumber for safety.

But perhaps the strangest thing about the sea cucumber is its ability to expel many of its internal organs when harassed. The benefits are debated, but it likely serves as a distraction in many cases – similar to  a lizard’s tail tip that breaks off when attacked.  Afterward, the sea cucumber slowly regenerates its discarded organs.  There’s plenty of diversity in when, why, and from which end, evisceration occurs.  Sad to say, none of this deters the sea cucumber fisherman.  But we can all do our part to prevent these vulnerable creatures from being driven to extinction — I’ll have my cucumbers from the garden, please.

Smooth muscle, ace of tubes

Smooth muscle, ace of tubes

What’s your favorite muscle tissue?  Odds are you’ll say skeletal muscle, the type used in all voluntary movements.  Or, you might be partial to cardiac muscle, the main tissue component of your heart.  But there’s much to appreciate in the third muscle tissue, smooth muscle.  It’s a major component of your tubular organs – those of the digestive, urinary, reproductive, and respiratory systems, as well as your blood vessels – and for good reason.

The name “smooth muscle” refers to the lack of striations – the stripes visible on skeletal and cardiac muscle cells.  Those stripes reflect a highly regular, organized arrangement of protein filaments that give great strength and efficiency to striated muscle tissues.  But it comes at a cost – if you overstretch a skeletal or cardiac muscle cell, it becomes completely unable to contract.  That’s because muscle contraction depends on the sliding of myosin and actin filaments past one another.  Without any overlap to start with, the myosin molecules have nothing to grab onto.

Smooth muscle gets around this problem with a loose, net-like arrangement of myosin and actin.  When the cell is stretched, this network starts to straighten out, which means each group of myosin and actin suffers little tension.  The result is that much more overlap is maintained and these cells remain functional.

Why is this so important for a tubular organ?  Many of your tubes undergo stretching – think of your stomach after a big meal — which in turn, stretches the muscle cells.  But many other tubular organs undergo fluctuations in diameter, and smooth muscle allows them to contract under a wide variety of conditions.

Smooth muscle is also the only type of muscle cell that can divide after birth – a crucial feature in repairing a damaged wall after the passage of a chicken bone or a kidney stone.  Also, blood vessels can grow and change shape in response to changing demands – made possible by the production of new smooth muscle tissue.

Let’s give smooth muscle a little respect.  It may lack obvious “sex appeal” at first.  But considering smooth muscle makes up a big part of your reproductive organs, maybe it’s the “sexiest” muscle tissue of all!

Dense connective tissue — diverse hardware for the body machine

Dense connective tissues

If bones are the “steel frame” of the body and cartilage forms the “rubber shock absorbers” between your bones, what name do we give for all the nuts, bolts, stitches, pulleys, cords, housings, laces, bindings, springs and bungees that connect our parts together?

These tough, durable attachment structures are provided by the dense connective tissues of the body.  Like connective tissues in general, these tissues have few living cells (here, fibroblasts, shown traveling among the fibers).  But in contrast to other connective tissues, the word “dense” here refers to an especially high density of collagen fibers.  Collagen fibers provide a strong “steel cable” that is difficult to tear apart, and thus is used to provide tension resistance in body tissues.  Collagen is such an important structural component that it makes up 25% of body protein – your most abundant protein of all.

There are three types of dense connective tissue:

  • In dense irregular connective tissue, the collagen fibers lie in all different directions. This type is useful in tissues that are subject to unpredictable forces.  The deep part of your skin (dermis) is a good example – there’s no telling what part of your face your grandma is going to pinch, and in what direction!  This type also forms a fibrous capsule around joints and various organs.
  • In dense regular connective tissue, the collagen fibers are all lined up together, providing tremendous strength against tension, but only in one direction. It’s the tissue used in our tendons – cord-like structures that attach a muscle to a bone and allow the muscle to pull on the bone.  It’s also used by ligaments – cord-like structures that attach one bone to another, and prevent the two bones from being ripped apart from each other.
  • In elastic connective tissue, we also find a high density of collagen, but with an important difference. Large numbers of elastic fibers dominate the behavior of the tissue.  The result is like an elastic band — you can stretch it, but when you let go, it recoils right back to its original shape.  It’s an important component of arteries, allowing them to stretch when the blood pressure varies.  It also allows your lungs to exhale without using any energy, saving energy in ventilation.  There are also elastic ligaments in your neck that give your head a little bounce when you start to fall asleep in class — perhaps saving you from injury, while providing the rest of the class with an entertaining demonstration!