Professor Dave here, let’s look at some brains.
We just learned about nervous tissue, and the structure of a neuron, as well as the
divisions of the nervous system.
The two main divisions are the central nervous system and the peripheral nervous system,
so let’s go over the first of these in more detail now.
As we said, the central nervous system consists of the brain and spinal cord.
The human brain is the single most complex object in the known universe.
With a dizzying number of neuronal connections, and the mechanism by which it produces consciousness
not yet well-understood, we will have to be satisfied with a mere introduction to this organ.
Rest assured, the brain and cognition will be discussed in far greater detail in the
upcoming biopsychology course, but for now, we will just cover the basics.
The best way to approach learning the structure of the brain is to examine early brain development.
Within an embryo, the brain and spinal cord begin as a single neural tube.
The anterior or rostral end begins to expand and constrictions soon demarcate the three
primary brain vesicles.
These are the prosencephalon, or forebrain, the mesencephalon, or midbrain, and rhombencephalon,
or hindbrain.
The posterior or caudal end of the neural tube will eventually become the spinal cord,
which we will discuss later.
From the primary brain vesicles, the secondary brain vesicles eventually develop.
The forebrain becomes the telencephalon, or endbrain, and the diencephalon, or interbrain.
The midbrain stays as it is, and the hindbrain becomes the metencephalon, or afterbrain,
and myelencephalon, or spinal brain.
These will then all develop further to become the regions of the adult brain.
The telencephalon sprouts two lateral regions called cerebral hemispheres, which together
form the cerebrum, and the midbrain and hindbrain segments collectively become the brain stem.
Confined to the volume of the skull, the fast-growing brain produces many folds, a process called
gyrification, in order to best occupy the available space.
This eventually results in the representation of the brain we are all familiar with, which
we typically divide into four main regions.
Those are the cerebral hemispheres, diencephalon, brain stem, and cerebellum.
There are also hollow cavities called ventricles, which are filled with cerebrospinal fluid
and lined with glial cells called ependymal cells.
Many of these have cilia to help circulate the fluid.
The other types of neuroglia we will find in the central nervous system include astrocytes,
with lots of branches to perform a variety of regulatory functions, microglial cells,
which monitor neuron health, and oligodendrocytes, which form myelin sheaths.
Getting back to brain structure, the majority of the mass of the brain sits in the cerebral
hemispheres.
The spongy appearance is produced by ridges called gyri that are separated by grooves
called sulci, while deeper grooves are called fissures, like the longitudinal fissure that
separates the hemispheres.
Each hemisphere is divided into five lobes, those being frontal, parietal, temporal, occipital,
and insula, the first four of which are named after the cranial bones that are adjacent
to them.
We can also describe each hemisphere as exhibiting three regions.
There is a cerebral cortex made of gray matter, consisting mainly of nerve cell bodies and
nonmyelinated fibers, an internal region of white matter, which is a dense collection
of myelinated fibers, and basal nuclei, or regions of gray matter within the white matter.
As it’s the most complex, let’s focus on the cerebral cortex.
This is the most recently evolved section of the animal brain, and as such it is where
the conscious mind is found.
It is made of gray matter comprised of six layers of interneurons, as well as glia and
blood vessels, and there are specific regions in the cortex called domains, which are responsible
for particular motor and sensory functions.
In other words, certain parts of the cortex are in charge of certain aspects of bodily
function.
We call these motor areas, sensory areas, and association areas.
The highest mental functions, however, like memory and language, are spread around much
of the cortex, and overlap numerous domains.
In addition, each hemisphere is responsible for the sensory and motor functions of the
opposite side of the body, so the left side of the brain controls the right side of the
body, and vice versa.
There are other aspects of the brain that are lateralized, meaning focused more on one
side of the cortex than the other, although that whole “left-brain/right-brain” personality
type is a complete myth.
Going back to the domains we mentioned, let’s discuss the motor areas first.
First is the primary cortex.
This region controls voluntary motion, and each part of the body is relegated to a particular
part of the primary cortex.
The most delicate voluntary motion occurs in the face, tongue, and hands, so a disproportionate
amount of this cortex is devoted to those areas.
The motor homunculus is an image that depicts the human body with all of its body parts
of a size that is proportional to the quantity of neurons that control them, which is why
some features seem dramatically oversized.
Then there is the premotor cortex.
This helps plan movements, and sequences them into complex tasks, like playing a musical
instrument.
Next is Broca’s area.
This controls muscles involved in speech production, among other things.
And then there is the frontal eye field, which controls voluntary eye movement.
Moving on, let’s list the sensory areas.
The primary somatosensory cortex receives information from receptors in the skin and
other areas.
This information goes to the somatosensory association cortex where it is integrated
to produce a rational understanding of an object that is being perceived.
The primary visual cortex and visual association area receive and integrate visual information,
the primary auditory cortex and auditory association area do the same for auditory information,
the olfactory cortex processes odors, the gustatory cortex perceives taste, the visceral
sensory area produces conscious perception of visceral sensations in the stomach and
other organs, while the vestibular cortex allows for our perception of balance or equilibrium.
There are also multimodal association areas that send and receive information to and from
multiple areas.
These are the anterior and posterior association areas, and the limbic association area.
Moving on from the cerebral hemispheres, we get to the diencephalon, which sits at the
very center of the brain.
This consists of the thalamus, hypothalamus, and epithalamus.
The thalamus receives and directs all of the information headed to the cerebral cortex.
This means it is intimately involved with learning and memory, among other things.
The hypothalamus sits immediately below the thalamus.
This controls the autonomic nervous system, regulates body temperature, hunger and thirst,
sleep cycles, physical response to emotions, and the endocrine system.
It also houses the pituitary gland.
Lastly the epithalamus houses the pineal gland, and helps regulate sleep.
After the diencephalon we get to the brain stem.
This consists of the midbrain, pons, and medulla oblongata, the last of which blends into the
spinal cord.
Finally, we get to the cerebellum.
This region, which consists of two hemispheres, regulates muscle contraction to generate smooth,
coordinated motion.
In addition, we should be aware of the structures that protect the brain.
Meninges are connective tissue membranes that sit between the brain and the skull.
On top is dura mater, consisting of a periosteal layer and a meningeal layer.
Immediately below is arachnoid mater, filled with blood vessels.
And lastly is pia mater, made of more delicate connective tissue.
That wraps up a basic survey of the brain.
We will go into more detail at another time, for now let’s finish off the central nervous
system with a quick look at the spinal cord.
We learned about the vertebral column when we looked at the skeletal system, and right
in the middle of the column sits the spinal cord, spanning from the base of the skull
to just past the ribs.
Other than the vertebral column, the spinal cord is protected by cerebrospinal fluid and
the same meninges that we saw for the brain.
Thirty one pairs of spinal nerves attach to the cord, and we can get a better look at
the cord by examining cross sections.
The gray matter towards the center takes on a butterfly shape, made of multipolar neurons.
From these dorsal horns and ventral horns, neurons connect with skeletal muscles and
other structures around the body, and these stem from four zones.
Somatic sensory, visceral sensory, visceral motor, and somatic motor.
Surrounding the gray matter is white matter, made of nerve fibers that allow for communication
between the cord and the brain.
These can be ascending, descending, or transverse, depending on their direction of travel.
These participate in an incredible number of pathways that we will investigate in more
detail later.
For now, let’s continue through a survey of the branches of the nervous system.