Professor Dave here, let’s check out the brain.
In this course, we will be talking quite extensively about the brain, so we have to know what’s
it made of and how it works.
We covered this in reasonable detail in the anatomy and physiology course while introducing
the central nervous system, which consists of the brain and spinal cord.
If this is your first time learning about the brain, I strongly advise clicking on the
card you see now to view this tutorial on the structure of the brain, and ideally the
two that follow, covering the rest of the nervous system, as much of that knowledge
will be assumed in this course.
If you’ve already viewed those, we can then recall that the brain consists of five divisions.
First, the myelencephalon, or medulla, and the metencephalon, consisting of the pons,
and the cerebellum.
These first two regions comprise the hindbrain.
Next, there is the mesencephalon, or midbrain.
Continuing, we have the diencephalon, comprised of the thalamus and hypothalamus, as well
as the telencephalon, which together comprise the forebrain.
The telencephalon is the largest part of the brain and the most familiar looking, with
its two cerebral hemispheres covered by the cerebral cortex.
This can be put into a crude evolutionary context with the three-scoop model of the
brain.
We see the oldest structures at the bottom, those being the brainstem and cerebellum that
are sometimes referred to as the lizard brain.
Stacked on top like another scoop of ice cream is the limbic system, or the so-called rat
brain.
And all the way on top is the cortex, or monkey brain, since this region evolved most recently
once primates came onto the scene.
We will discuss these divisions and the smaller subdivisions within as necessary throughout
the course.
Zooming in further, inside all of these regions we will find specialized cells called neurons,
which we also learned about in the anatomy and physiology course.
Basic knowledge regarding neuronal structure and the generation of the action potential
that is used to relay signals through the body will also be assumed from here on out,
so if you need a refresher on those topics, be sure to check out this tutorial on nervous
tissue before moving forward.
In addition to the neurons of various types, we can find neuroglia, also called glial cells,
or simply glia.
These are the other cells present in nervous tissue that are not neurons.
In the brain, the first of these are called oligodendrocytes, which form myelin sheaths
that wrap around one or more axons to protect them, and which speed up conduction along
the axon, just like Schwann cells do in the peripheral nervous system.
Next are astrocytes.
These are heavily branched cells that attach to neurons, supporting them, and also anchoring
them to nutrient supply lines.
They absorb and secrete various substances, thus allowing them to perform a variety of
functions.
After that we have microglial cells.
These have long processes that monitor the health of the neurons, and can perform phagocytosis
to get rid of dead ones.
And lastly, we have ependymal cells.
These are the cells, typically ciliated, that line the central cavities of the brain and
spinal cord, forming a barrier between the cerebrospinal fluid and the fluid surrounding
the cells of the central nervous system.
This cerebrospinal fluid cushions the brain and protects it from physical trauma.
Speaking of protecting the brain, we must also mention something called the blood-brain
barrier.
As we have learned in our study of the circulatory system, throughout the body, substances readily
diffuse through the walls of blood vessels so as to service tissues with nutrients and
collect waste.
Between hormones, ions, and many other things, lots of exchange is taking place.
In the brain, many such substances would interfere with the ability of neurons to fire properly,
so blood vessel walls within the brain are not as permeable.
Most large molecules, like proteins and many drugs, are not able to get through, while
molecules that are critical to the survival of neuronal cells, like oxygen and glucose,
can make it, as well as psychiatric drugs.
The endothelial cells in these blood vessel walls form tight junctions, which are what
restrict the permeability.
As we said, this blood brain barrier is critical in preventing certain substances from entering
brain tissue that would interfere with the firing of neurons, so let’s go ahead and
get a closer look at the mechanism of neural transmission next.