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!

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.

The versatile pharynx

Pharynx depicted with legs, eyes, and nose.

Here’s a body part you’ve probably never seen in isolation — and almost certainly not with legs, and a face of its own!  But these embellishments serve to emphasize that the pharynx is a middle-man, straddling two different organ systems and negotiating the competing demands of each.

Ideally, air travels in through the nasal cavity, all three parts of the pharynx (nasopharynx, oropharynx, laryngopharynx), and then the larynx, on its way to the lungs.  While eating, breathing is paused while food travels through the oral cavity (mouth), the lower parts of the pharynx (oropharynx, laryngopharynx), and the esophagus on its way to the stomach.  But as you can see, these two pathways overlap in the pharynx.  As a result, the pharynx, along with associated structures, plays a vital role in regulating the passage of air and food.

There are four ways that food and air movement can differ from the ideal situation depicted above, and these “mistakes” vary all the way from innocuous (and perhaps entertaining) to life-threatening.

First, food could move through the upper pharynx (nasopharynx).  Sometimes food is expelled this way if you sneeze or laugh while food is in your mouth.  But most of the time, the soft palate, a muscular structure, closes against the back of the pharynx to prevent this.  If you want to “get in touch” with your soft palate, try snorting – you just closed your soft palate and then opened it while inhaling.  A word to the wise: Restrict your snorting to air and mucus!  But unfortunately, if you can think of it, somebody will try it — the FDA had to issue a warning not to snort chocolate powder.

Second, air can move between the middle pharynx (oropharynx) and the mouth.  Wait, you may say, isn’t this exactly how we breathe?  If that’s your impression, then you probably have a congested nasal cavity, and it’s true we can breathe through the mouth, although it’s less than ideal.  The reason is that the nasal cavity has complex folds that help to trap debris and pathogens and do an optimal job of “conditioning” the air before it reaches the lungs.  So if, as I often do, you find yourself walking behind a group of sneezing and coughing students, it may help to close your mouth and breathe through your nose.

Like the rest of your respiratory and digestive tracts, the pharynx is a moist habitat suitable for invading microbes, and being near the entrance to both tracts, it is a common place for infections to begin.  What’s known as a “sore throat” is generally a viral infection of the pharynx.  That’s likely why we have several immune structures — the tonsils — in the walls of the pharynx.

Third, air can move between the lower pharynx (laryngopharynx) and the esophagus – the food tube leading to the stomach.  According to healthline, in most of us about two quarts of air move into the stomach this way, each day, from the small amounts of air trapped in our food or drink.  And of course, when air leaves the body through this route it is called burping — technically known as eructation.

The fourth, and by far the most serious problem, is food traveling from the laryngopharynx into the larynx.  The larynx, or voicebox, sits in front of the esophagus and is the gateway for the lower respiratory tract.  “Inhaling” your food is dangerous — in the US, over 5,000 people die from choking each year. Our main protection from choking is the epiglottis — a laryngeal cartilage between the oropharynx and nasopharynx, which folds down to close off the entrance to the larynx, every time you swallow.

Your pharynx handles a lot of traffic through the body, and deserves some respect – definitely nothing to snort at!


How a long bone grows longer

How a long bone gets longer

The process by which your bones lengthen (during childhood and adolescence) is known as longitudinal bone growth.  It’s more complicated than you might think, so I’ve simplified it here.  There’s a strip of cartilage (the epiphyseal plate, or growth plate) embedded in each end of the bone, and that’s where all the lengthening growth occurs.

Within that plate, a remarkable process occurs.  First, as you might expect, the cartilage cells grow and divide, and this is what actually lengthens the bone.  But next, the cartilage cells essentially commit mass suicide, by creating a chemical “prison” that blocks the entry of nutrients and oxygen.  As the tissue dies it leaves a space behind, where bone cells can come in and add hard bone matrix.  Ultimately, this means the bone lengthens, and the whole “construction team” of cells follows the growth region as it extends further out.  It’s analogous to a road construction crew, where you have workers engaged in all steps of construction, but positioned at different points along the road.

But instead of painting a “center line,” the final step for our bone construction team is to lengthen the internal space of the bone.  This is known as the medullary cavity, and it’s there to make the bone lighter, and to house other important tissues (like bone marrow).  A group of “demolition” cells called osteoclasts lives near the end of the medullary cavity and continually eats away at the bone matrix.  This lengthens the medullary cavity, keeping its size in correct proportion, as the bone grows longer.

Why start with cartilage at all?  Unlike bone, cartilage is mostly water so it’s an easy tissue to rapidly assemble during early development.  Later in development, most of this cartilage is replaced by bone, but an important remnant of that early cartilage is the growth plate discussed here, which persists until adult height is reached.

The blind spot in the human eye

The blind spot in the human eye

Normally, the blind spot goes unnoticed, for two reasons.  First, each eye fills in the gap in what the other eye is seeing.  So, if a mosquito is hidden in the blind spot of your right eye, you can still detect it with your left.  Each eye plays “backup” to the other.

So, does that mean that if you close one eye, you’ll see a blank “hole” in your vision, everywhere you look? While there really is a “hole” in your perception, it’s not that easy to detect. The reason is that the brain’s visual processing center fills in the “hole” by extrapolating from the surrounding area. So, if you look at a blue sky with one eye, the blind spot “sees” blue as well.

The way to “see” the blind spot is to both close one eye, and prevent your brain from extrapolating the surroundings. The way to do that is using an object that contrasts sharply with its surroundings but is small enough to “hide” inside the blind spot. In the example above, a round black spot is used.

The blind spot is one of my favorite topics for a classroom demonstration.  Using a document camera, I project an “X” on the left side of the screen, and a “dot” a short distance to the right.  I tell the entire class to close their left eye, focus their right eye on an “X” projected on the screen, and to raise their hand if the dot disappears.  I then slide the dot slowly to the right (on the document camera, this is easy to do if the “X” and the “dot” are on separate pieces of paper).  At first, only students in the front rows raise their hand, but as the visual angle between the shapes increases, hands go down in the front and start to go up further and further back in the room.

There’s a vas deferens between your epididymis and your ejaculatory duct

vas deferens mnemonic with Prof. Sperm at the chalkboard

You’ve almost made it through a semester of anatomy.  Like a sperm cell, you sometimes feel like just another face in the crowd, but you know there’s light at the end of the tunnel!

But first, you need to make sense of that tangle of tubes known as the male reproductive system.

The epididymis – which rhymes suggestively with “did he miss” – is the site of final sperm maturation.  Twenty feet of tiny tubing is coiled so tightly it fits in a structure less than two inches long, that hangs like a comma over the testis.  The epididymis is also where the sperm bide their time until ejaculation occurs.

But most of the semen is not sperm, and is instead produced by glands, found further down the assembly line.  The seminal vesicle, in particular, plays a seminal role, producing 60% of the semen, a concoction of nutrients, signals and other molecules.

On Professor Sperm’s chalkboard diagram, the ejaculatory duct represents the confluence where the wriggling sperm are doused with this sticky cocktail.  Other glands down the pipeline (prostate and bulbourethral glands) complete the mix, as it travels through the urethra on its way to the exit.

If all those tubes are starting to sound alike, take heart!  Between the epididymis and the ejaculatory duct, there’s truly a vast difference, and also, a vas deferens.  This longest of the male tubes (18 inches or so) is called into play only at the time of ejaculation, transporting the sperm all the way up the scrotum, along the inguinal canal and finally hurtling them into the ejaculatory duct.

In place of “vas deferens”, many books now use the term “ductus deferens”.  To that, I say, “same difference!”

Gray and white matter – the cities and highways of the brain

gray and white matter is city and highway

Your brain has around a hundred billion signaling cells called neurons, forming an astronomical number of connections (synapses).  The “wiring” is incredibly complex — an electrician’s nightmare!  Hundreds of functional brain regions and circuits have been identified, but to a large extent the brain is still a “black box”.

One thing that’s plain, though, as you look at a slice of brain, is that it’s divided into two general types of tissue.  Gray matter is where information is processed.  Here you’ll find dendrites, the “input cables” that a neuron uses to take in information.  These are all attached to the cell body, which therefore, in effect represents an “integration zone” for all the signals coming in.   If the neuron decides it has something to say about the matter, it fires off a signal through the axon, the single “output cable” of the cell.

You’ll also find white matter, distinguished by the presence of myelin, a wrapping provided by other cells (here, oligodendrocytes, seen hanging under the “freeway”).   Myelin is a fatty material so the white color is not surprising — think of the pale crust of grease on a refrigerated pot of chicken soup.

Unlike the water that fills and surrounds each cell (remember: keep that hair dryer out of the bathtub!), fat is a poor conductor of electricity, so myelin serves as an insulating covering for axons.  It prevents an axon from being “short-circuited” when two axons lie side by side, and it also greatly speeds up the signal transmission along an axon.  So, a group of myelinated axons (which can be up to several feet long!) is a good way to carry a large body of information from one place to another, quickly and uncorrupted.

What we have, then, is a sprawling “metropolitan area” consisting of “commercial city centers” joined together by high-speed, multi-lane “highways”.  Gray matter tends to have highly branched neurons with many connections, allowing intricate exchange of information.  The flow of “traffic” is complex, and stops are frequent.  White matter is all about rapid, long-distance signaling from one “city center” to another, very much like an expressway.  These axons are insulated against nervous input, but at intervals some will branch off to make a connection.

Proceeding with this fantasy, we see an exit approaching for “Las Vagus” – that’s a reference to the vagus nerve, a major nerve that extends from the brain to many of your internal organs.  You may also have noticed that on white matter, the ride is a bumpy one – That’s because there are gaps in the myelin (nodes of Ranvier).  Paradoxically, though, in real life these gaps actually speed up the signal!

Capillaries do all their work in bed – they’re not lazy, but they’re leaky!

Capillaries leaking in bed

Our tiny capillaries are the most important blood vessels in the body, responsible for supplying virtually all the oxygen and nutrients that our organs depend on.  So how do they behave, under the immense burden of this great responsibility?  They never even climb out of bed!

A capillary bed is the name for a group of capillaries that all receive blood from the same source.  The source vessel is called a metarteriole, and from there, a branching network of capillaries originates.  The large number of branches provides a huge surface area across which diffusion can rapidly deliver the materials demanded by the surrounding tissues.

But there’s something else going on in most capillary beds – they’re leaky.  But don’t worry, it’s perfectly normal!  There are holes in the capillary walls, allowing the fluid component of blood (plasma) to leak out, and this is how the body’s interstitial fluid is produced.  The bulk flow of fluid across the capillary wall, back and forth between the blood and surrounding tissues, permits more rapid exchange than diffusion alone can provide.

Capillary leakiness varies greatly from one organ to the next, depending on the need for exchange.  The liver is the “water purification plant” of the body, and requires tremendous amounts of exchange to do its job – it therefore has among the leakiest capillaries.  At the other extreme, the brain is like a “cleanroom” where contaminants are excluded to prevent “misfires” of the delicate neural machinery – so its capillaries are almost watertight (the blood-brain barrier).

Given that most capillaries are leaky, what happens to all that water?  Does the bedroom flood completely, until a plumber is called to the scene?  Well, not quite.  The answer to this problem is the lymphatic system – it’s the storm drain of the body, which collects all the excess interstitial fluid of the body, and carries it right back to the blood…to be leaked out, all over again, by our hard-working capillaries.

Don’t have a heart attack!

heart with obstructed coronary artery

This week’s image may seem melodramatic, but I’m sure if you were a heart undergoing a heart attack, you’d feel something like this.

Heart attacks teach us that the heart is like any other organ – it consists of cells that require oxygen and nutrients to survive.  The other thing we learn is that, despite the large amount of blood in the heart, almost none of its contents are directly available to the heart’s cells.  That’s because diffusion is only effective across short distances, so the thickness of the heart’s walls prevents all but the inner surface cells (endothelium) from acquiring their vital resources in this way.

The solution is for the heart to have its own network of blood vessels, the coronary arteries, penetrating deep into the heart walls to supply its own tissues with blood.  The suffering heart depicted here is actually anatomically correct (minus the face and heads), allowing us to see all the major blood vessels that deliver blood to and from the body.

Oxygenated blood, from the lungs, reaches the heart through the four pulmonary veins, entering horizontally from the sides.  It enters the left side of the heart, which ejects this oxygen-rich blood (shown in red) into the aorta, the largest artery in the body.  The aorta emerges diagonally from the left side and arches over the top of the heart, branching many times to provide oxygen to the entire body.

From the aorta, the right coronary artery curves around the heart’s right side (on the left side of the page), and the left coronary artery curves toward the heart’s left side, each providing many branches of their own.  If one of these becomes blocked by a blood clot or fatty deposits in the artery walls, then a heart attack results – meaning the heart tissue dies due to the obstructed blood supply, and the resulting lack of oxygen to that region.  This is different from cardiac arrest, in which the heart stops beating – a problem with the electrical conducting system of the heart.  (However, the two problems can be related, if a heart attack damages part of the conducting system.)

Let’s look at the rest of the blood flow, in a healthy heart.  From a coronary artery, tiny branches (capillaries) supply oxygen to the heart’s tissues by diffusion, and drain into cardiac veins (not shown) which drain into the right side of the heart.  This is also where oxygen-depleted blood enters from the rest of the body, through the superior and inferior vena cava (vertical vessels on the left side of the page).  From there, it is pumped through the pulmonary trunk (large vessel crossing in front of the aorta) back to the lungs.