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.

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.

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!

How muscles work

muscle contraction

Here’s how we move, using the elbow as an example.  A muscle is attached to the bones on either side of your elbow joint.  Inside the muscle, proteins called myosin (red), which are arranged in tiny rows called thick filaments, have little arms that reach out and grab onto proteins called actin (blue), which are arranged in tiny rows called thin filaments.  Alternately grabbing, pulling, and releasing, the myosin, like a tug-of-war team, brings the actin on one side closer to the actin on the other side.  The shortening of the muscle, which results from this sliding filament mechanism, is called muscle contraction.  Because the muscle is attached to each bone by a tendon, the bones are pulled together and the elbow bends.

This arrangement of proteins, like a stack of flattened hexagons, is called a sarcomere.  It’s the “functional unit” of muscle contraction, meaning that in theory, if a muscle had just one sarcomere like in the cartoon, it would still work.

That’s the simple version.

The molecules, of course, are shown greatly enlarged.  To maximize efficiency, the muscle has an intricate structure of repeating units that will make your head spin.  Sarcomeres are attached end-to-end (about 10,000 per inch) to form contractile rods called myofibrils, and myofibrils are stacked side-by-side to fill each muscle cell, which is also known as a muscle fiber.  A unique feature of skeletal muscle tissue (the type that can be voluntarily controlled, as in the example) is that the individual cells are extremely long – almost as long as the muscle itself!  So a muscle in the arm has “only” around 250,000 muscle fibers – far fewer than the billions of cells one normally finds in an organ.  The extreme length of our skeletal muscle cells probably makes them quicker and more efficient.

Now let’s consider the entire muscle again.  Its whole purpose is to move a bone through space.  To do this, it has to have a stable attachment at one end, called the origin.  When the muscle contracts, the other attachment, known as the insertion, is moved closer to the origin, and this is what bends your elbow.  Within the muscle, all of the sarcomeres are shortening at once, but the ones closest to the origin are hardly moving at all.  Meanwhile the sarcomeres near the insertion are moving rapidly, pulled by the cumulative efforts of all the myosin molecules further up the myofibril, allowing the muscle to win the “tug of war” against gravity.

Which tissue do we need the most?

Which tissue do we need the most?

The entire body is composed of only four basic tissue types.  Muscle tissue, of course, allows you to move around.  But it’s also what moves your internal organs – the beating of the heart, the rumbling of the stomach.  Even your blood vessels have muscle tissue, which controls the distribution of blood in the body.  It’s hard to argue we could live without muscle.

But most of our muscles wouldn’t be much good without nervous tissue, which responds to stimuli and coordinates the activity of your organs.  It’s true that many of the slower, internal processes do not depend on nervous input – they may instead involve hormones, for example.  But what good is a body without a brain to give it meaning?

Epithelium, though, really is essential at the most basic level.  This is the tissue that lines all your external surfaces and your internal spaces.  Every substance that enters the body (food, water, oxygen) must cross an epithelium to do so.  These tissues are therefore the “gatekeepers” to the body, in charge of exchange with the environment – although, under the command of nervous tissue.

So the tissues must work together, and there’s no better example of this than the fourth basic type, connective tissue.  As the name suggests, it is the “putty” that holds the body together, filling in all the spaces between epithelium, muscle and nervous tissue.  But it also provides pathways for the movement of materials within the body.  This is the most diverse tissue type, including blood, the essential medium of transport, but many other types such as bone and cartilage.  The key feature of connective tissue is the presence of a large amount of nonliving “stuff” in between the living cells – the extracellular matrix.  Water is often abundant here, and this interstitial fluid forms another major transport medium for substances to move among all the tissues.

So of course, it’s hard to say any one basic tissue is more important than another.  I don’t know about you, but this “exchange” about the “connections” has been a “moving” subject for me that touches a “nerve”.  Pass the box of tissues!

Endomysium: A first-class seat for your muscle cells

Endomysium is like a first-class airplane seat for your muscle cells

Strapped into an economy seat as we fly across the country for the holidays, it’s hard not to appreciate life’s basic necessities — a cup of soda, a bag of pretzels, the relief of seeing “vacant” on the lavatory door. It’s also a good time to remember that the real protagonists in this story are your trillions of cells, each of whom has the same basic needs you have. Each muscle cell, for example, needs an oxygen supply, nutrients, a way to eliminate wastes, a command system telling it whether to contract (or just relax), and a physical attachment allowing it to work with the rest of the muscle. All of these things are provided by a thin sheath of connective tissue called the endomysium which surrounds each muscle cell. The endomysium, in effect, acts as a scaffolding to support the infrastructure of blood vessels and nerve cells that allow the muscle cell to function. What kind of airplane seat does a muscle cell occupy? Considering that the bloodstream keeps the cell supplied with a constant stream of goodies, I imagine it’s got to be a first-class seat.