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.
