Isometric muscle contraction – achieving a perfect stalemate

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Isometric muscle contraction: each myosin pulls on actin but also slips a little bit, so there is no overall progress and the muscle does not change length

Next time you hold some interesting creature in front of you, against the force of gravity, take a moment to consider what you have done. This type of muscle contraction is called isometric because the muscle length stays the same (“iso” = “same”). To prevent itself from either shortening or lengthening, the muscle must produce an internal force (tension) that perfectly balances the gravitational force. It does this by activating just the right number of muscle cells to balance the load. Within each active cell, millions of proteins are engaged like soldiers in a desperate tug-of-war against gravity. Recruiting the right number of “regiments” (muscle cells) ensures that the army neither wins, nor loses.

On a microscopic level, your muscles look like a “honeycomb” of flattened hexagons. A stack of of these hexagons is called a sarcomere. The outer frame of each hexagon is made of actin, and surrounds an inner core made of myosin. During muscle contraction, it is the myosin that performs the “tug-of-war”, pulling on the actin. If it is able to bring the two sides of each hexagon together, this will shorten the sarcomeres, and thereby shorten the entire muscle. Just like a real tug-of-war, each myosin goes through a cycle of reaching out, gripping the actin, pulling it, and then releasing again to grip at a different point.

In an isometric contraction the tension is equal to the load, so the tug-of-war reaches an impasse and the sarcomeres are frozen in position. Each myosin molecule is able to pull its patch of actin a little bit, but its grip may slip due to the external force, and also, when it releases its grip to pull at another patch, the external force tends to pull the actin back the other way. It’s analogous to someone who is trying to climb a rope but keeps slipping back to where they started. Our myosin foot-soldier finds itself in a stalemate – but does not realize that it is all part of a carefully orchestrated plan. Your brain commands many isometric contractions throughout the day, for example in the muscles that maintain posture.

And of course there are other ways to use your muscles. When you first picked up that creature, your muscle was able to “win” against gravity because your brain recruited enough cells for the task, with a large enough army of myosin molecules to make progress on their rope climb. This is called a concentric muscle contraction because the muscle’s ends move “toward the center” and the muscle shortens. Conversely, placing that creature gently back on the ground means allowing your muscles to lengthen in a controlled manner (eccentric contraction = “away from the center”). In this case the muscle “loses” to gravity – the myosin molecules slide right down to the bottom of their rope, while maintaining some grip to slow their descent.

Receptor and signal molecule as lock and key

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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

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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.

Delayed dusk, Seattle schools and the Qom

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Two studies of delayed dusk and its consequences

Sleep seems to be more and more in the news, the less and less we get!  The adverse effects of electric lights are by now well-known, but most studies are performed on volunteers in strictly controlled conditions.  Depicted here are two extraordinary studies that took place in the “natural environment” of human societies, taking advantage of changing conditions over space or time.

The first study compared subpopulations of the indigenous Qom people of far-northern Argentina.  Electricity has many benefits for developing societies, but sleep apparently is not one of them!  The second study was an exceptional example of science impacting policy-makers, and yielding data afterward to confirm the benefits.  The 2018 publication of the “Sleepmore in Seattle” study led to follow-up articles in the New York Times, Washington Post, Seattle Times and NPR.

These studies were carried out by Dr. Horacio de la Iglesia at the University of Washington, and presented in his talk “Sleep after Natural and Electric Dusk” in our weekly Colloquium series at the Integrative Physiology (IPHY) Department at CU Boulder.

Pectoralis major, the bodybuilder’s muscle

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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!

The kangaroo rat’s adaptable hopping mechanism

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The kangaroo rat's adaptable hopping mechanism

This cartoon summarizes the versatile ways that kangaroo rats deploy their hopping machinery in the desert.  It is based on a presentation by Dr. Craig McGowan of his research.  In studies of animal movement, biomechanicians often describe the muscles and connective structures using a machine metaphor, which helps to identify key adaptations in how animals get around.  Faced with a diverse terrain and hungry rattlesnakes, kangaroo rats use their gastrocnemius or “gastroc” muscle (homologous to our own “calf muscle”) in at least three different ways.  These discoveries may also inform future developments in the design of more versatile lower limb prostheses for humans.

Dr. McGowan directs the Comparative Neuromuscular Biomechanics Lab at the University of Idaho.  His presentation “Built to Hop: Meeting the Mechanical Demands of Locomotion in the Desert”, was a part of the weekly Colloquium at the Integrative Physiology (IPHY) Department at CU Boulder.

Serratus anterior, the boxer’s muscle

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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

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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

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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

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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.