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!

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!

Who supports your support system? An unsung hero — connective tissue

Connective tissue, the inner part of the wall that conveys the body's infrastructure

If you’ve been reading this blog, by now you’ll know that the oxygen and nutrients your cells depend upon first enter the cell’s neighborhood by means of your crowded capillaries.  Molecules then diffuse outward across the interstitial fluid to reach most cells in your body.  The cardiovascular system is the most obvious “support system” in the body because it circulates the blood that keeps your cells alive.  It also works with the kidneys to remove wastes.  I’d also add the lymphatic system (see lymph nodes) which drains excess interstitial fluid and filter pathogens from it.  And, I’d include the nervous system which carries a crucial pipeline of information that many cells depend on to do their job.

But who supports the “support system”?  Do your blood vessels, lymph vessels and nerves simply meander through the body, or is there a structure – a scaffolding — that holds them in place?  Yes, there is – our unsung hero, the connective tissues of the body.

In a typical organ like your stomach, lungs, or skin, you’ll have one or more layers of epithelium at the surface (shown in yellow) to interact with your food, air, or the external environment.  Always behind that epithelium you’ll have connective tissue (shown in brown) which carries the blood vessels, lymph vessels, and nerves that serve that region of the body.  Even a muscle layer (for example in the stomach), as seen at far right, actually contains, between the muscle cells, a great deal of intervening connective tissue, which is where the vessels and nerves travel.  (This is the endomysium, the “first class seat” that supplies all the muscle cell’s needs.)

As you go about your business indoors, at work or home, think of the wall surfaces as an epithelium.  They provide electrical outlets, faucets, heating vents and other necessities for life.  But all of these things come to you through an infrastructure of electrical wires, water and sewage pipes, ventilation ducts, and internet cables that travel, unseen, through the walls.  Inside your body, that role of the “internal wall space”, which supports and conceals the “support system” of the body, is served by connective tissue.

Lymph nodes: Police stations of the body

Lymph node as a police stationA lymph node is a bit like a jungle gym for our microscopic police force.  A filter or dragnet, evolved to catch unsavory characters like bacteria and viruses — drifters who are up to no good.  Our hundreds of lymph nodes function like district police headquarters, situated around the body with concentrations in the armpits, neck and other areas.

Tissue fluids throughout the body make their way into the lymphatic vessels, carrying any infectious microbes along.  Lymph nodes like the one shown above are stationed along those vessels, and here is where the “crooks” meet a concentration of leukocytes, or white “blood” cells.  (I add quotes around “blood” because they spend most of their time patrolling through other body tissues.  It’s a bit like calling someone a “streetwalker” just because you caught them crossing the street now and then.)

The “jungle gym” in a lymph node is built of reticular connective tissue, specially evolved to provide “networking” opportunities (“reticular” means “net-like”).  This facilitates interaction among the cops while also giving them a good chance of nabbing a microbe.  Some leukocytes may respond immediately (cop reaching for gun), and will later serve as an antigen-presenting cell to communicate their findings to the others (the “wanted” slide shown at top).  After the initial encounter, certain leukocytes with a receptor for the specific microbe (“I was born for this assignment, boss!”) divide into many identical clones that are specifically on the lookout for said crook.  The production of clones is what produces the swollen lymph nodes during an illness.  Memory cells (the cop with the desk job) remain on high alert (perhaps with the help of coffee and a donut) even after the crooks have been executed, in case they show up again, when the body will be prepared for a more rapid response (adaptive immunity).  Some memory cells can live for decades – very old “cops” indeed — and they never forget.  Leukocytes communicate with a variety of signals (cops joking around at right), some of which encourage them to “bulk up” for the encounter, like the cop doing pull-ups.

We run into the larger lymph nodes (around 1-2 cm) with every dissection in the cadaver lab.  They’re nondescript, and firm, but with a dense, spongy texture that, with a touch of imagination, is suggestive of their “cop station” function.

Interstitial fluid: Our internal life aquatic

I would love to shrink down to microscopic size and greet the cells in my own body face-to-face, like something out of the movie Fantastic Voyage.  Of course, I won’t “hold my breath” waiting for such an opportunity to come along.  But even if there were some way to do this, I would quickly drown without SCUBA equipment!  This underscores the fact that our internal environment is an aquatic one.  Our living cells depend on the special properties of water, both to exchange gases, nutrients and wastes, and to facilitate the chemical reactions necessary for life.  Evolution has not yet found a way to allow life to thrive without internal water (a few creatures are capable of surviving complete desiccation, but they do so in suspended animation).

Indeed, a cell out of water is like a fish out of water.  This means that the dry skin surface of the human body is all dead tissue.  It also means that all our internal tissues are moist.  Even bone tissue, despite being largely composed of calcium phosphate, is permeated with water and houses living bone cells that live comfortably within tiny water-filled “lagoons” known as lacunae.

Some tissues contain more water than others.  Blood is the tissue with the greatest water content, but all other tissues have, between their cells, a fluid known as tissue fluid, or interstitial fluid.  The tissue depicted above might represent areolar connective tissue, a ubiquitous tissue found in many parts of the body, which contains a large amount of interstitial fluid. Because it’s found adjacent to all blood vessels, it plays an important “middleman” role allowing molecules to make their way between the blood and nearby tissues.  The two typical cells shown here would be fibroblasts, which produce the protein fibers giving structure to the tissue (collagen fibers are shown in gray, and reticular fibers in magenta); these in turn provide attachment points for the cells, as they go about living their aquatic lives. Unfortunately, the interstitial fluid is also a benign environment for bacteria (shown in pink) — but that’s a story for another day.

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.

The hierarchy of organ, tissue, cell in the human body

As another new semester begins, what better place to begin than the hierarchy of structure in the human body — organ systems, organs, tissues and cells. I hope this doesn’t sound old-fashioned, but it’s a strict hierarchy!

It sounds easy enough, but it’s easy to lose your bearings when you hear statements like “bone tissue has a lot of blood vessels”.  Sure, that’s true — and that’s why bone heals faster than, say, cartilage.  But this does not mean that blood vessels are a part of the bone tissue.  Blood vessels, in fact, are made of tissues like muscle, connective tissue, epithelium. So, if a blood vessel were a part of the bone tissue, then you’d have one tissue that’s part of another tissue — bad news for our hierarchy!