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.