Chapter 3: Anatomy of the nervous system
General layout of the nervous system
The vertebrate nervous system if composed of two divisions: the central nervous system and the
peripheral nervous system.
The central nervous system (CNS): is the division of the nervous system that is located within the skull
The Peripheral nervous system (PNS): is the vision that is located outside the spine and skull
The CNS is comprised of two divisions: the brain and the spinal cord. The brain is the part of the CNS
that is located in the skull
The PNS is also composed of two divisions: the somatic nervous system and the autonomic nervous
system. The Somatic nervous system (SNS) is the part of the PNS that interacts with the external
environment. It is composed of afferent nerves that carry sensory signals from the skin, skeletal
muscles, joints, eyes, ears, and so on to the nervous system and efferent nerves that carry motor signals
from the central nervous system to the skeletal muscles. The autonomic nervous system (ANS) is the
part of the peripheral nervous system that regulates the body's internal environment. It is composed of
afferent nerves that carry sensory signals form internal organs to the CNS and efferent nerves that
carries motor signals form the CNS to internal organs. Hint- going toward CNS starts with an a and going
away from something begins with an e.
The ANS as two kinds of efferent nerves: sympathetic nerves and parasympathetic nerves. The
sympathetic nerves are those autonomic motor nerves that project from the CNS in the lumbar and
thoracic regions of the spinal cord. The parasympathetic nerves are those autonomic motor nerves that
project from the brain and sacral region of the spinal cord. All sympathetic and parasympathetic
neurons project from the CNS and go only part of the way to the target organs before they synapse on
other neurons that carry the signals the rest of the way. However, the sympathetic and parasympathetic
systems differ in that the sympathetic neurons that project from the CNS synapse on second stage
neurons at a substantial distance from their target organs, whereas parasympathetic neurons that
project from the CNS synapse near their target organs on very short second stage neurons
The conventional view of the respective functions of these two systems stresses three important
1. that sympathetic nerves stimulate, organize, and mobilize energy resources in threatening situations,
whereas parasympathetic nerves act on conserve energy
2. that each autonomic target organ receives opposing sympathetic and parasympathetic input and its
activity is thus controlled by relative levels of sympathetic and parasympathetic activity
3. that sympathetic changes are indicative of psychological arousal, whereas parasympathetic changes
are indicative of psychological relaxation.
Most of the nerves of the peripheral nervous system project from the spinal cord, but there are 12 pairs
of exceptions: the 12 pairs of cranial nerves, which project from the brain. They are numbered in
sequence from front to back. The cranial nerves include purely sensory nerves such as the olfactory
nerves (1) and the optic nerves (11), but most contain both sensory and motor fibbers. The longest
cranial nerves are the vagus nerves (X), which contain motor and sensory fibers traveling to and from
-the brain and spinal cord are the most protected organs in the body. They are encased in bone and
covered in three protective membranes, the three meninges. The outer meninx is a tough membrane called the Dura mater. Immediately inside the Dura mater is the fine arachnoid membrane. Beneath the
arachnoid membrane is called the subarachnoid space, which contains many large blood vessels and
cerebrospinal fluid: then comes the innermost meninx, the delicate pia matter which adheres to the
surface of the CNS. Also protecting the CS is the cerebrospinal fluid (CSF), which fills the subarachnoid
space, the central canal of the spinal cord, and the cerebral ventricles of the brain. The central canal is a
small central channel that runs the length of the spinal cord; the cerebral ventricles are the four large
internal chambers of the brain, the two lateral ventricles, the third ventricle and the fourth ventricle.
The CSF supports and cushions the brain; the CSF is continuously produced by the choroid plexuses-
networks of capillaries that protrude into the ventricles from the pia matter.
The blood-brain barrier- barrier so that many toxic substances cannot get into the brain. This barrier is
a consequence of the special structure of cerebral blood vessels
Cells of the nervous system:
-most of the cells of the nervous system are of two fundamentally different types: neurons and glial
cell membrane- the semipermeable membrane that encloses the neuron
dendrites- the short processes emanating from the cell body, which receive most of the synaptic
contacts from other neurons
axon hillock- the cone shaped region at the junction between the axon and the cell body
axon- the long, narrow process that project from the cell body
cell body- the metabolic center of the neuron; also called soma
myelin- the fatty insulation around many axons
nodes of Ranvier- the gaps between sections of myelin
buttons- the button like endings of axon branches, which release chemicals into synapse
synapses- the gaps between adjacent neurons which chemical signals are transmitted
cytoplasm- the clear internal fluid of the cell
ribosomes- internal cellular structures on which proteins are synthesized, they are located on the
Golgi complex- a connected system of membranes that packages molecules in vesicles.
neurotransmitters- molecules that are released from active neurons and influence the activity of other
microtubules- tubules responsible for the rapid transport of material throughout neurons
The neuron cell membrane is composed of a lipid bilayer or two layers of fat molecules. Embedded in
the lipid bilayer are numerous protein molecules that are the bassi of many of the cell membrane's
functional properties. Some membrane proteins are channel proteins, through which certain molecules
can pass; others are signal proteins which transfer a signal to the inside of the neuron when particular
molecules bind to them on the outside of the membrane.
A neuron with more than two processes extending form its cell body is classified as a multipolar neuron;
most neurons are multipolar. A neuron with one process extending from its cell body is classified as a
unipolar neuron, and a neuron with two processes extending from it cell body is a bipolar neuron.
neurons with shorter axon or no axon at all are called interneurons; their function is to integrate the
neural activity within a single brain structure, not to conduct signals from one structure to another.
There are two kinds of gross neural structures in the nervous system: those composed primarily of cell
bodies and those composed primarily of axons. In the central nervous system, clusters of cell bodies are called nuclei; in the peripheral nervous system, they are called ganglia. In the central nervous system,
bundles of axons are called tracts; in the peripheral nervous system they are called nerves. Glial cells are
found throughout the nervous system.
oligodendrocytes- glial cells with extensions that wrap around the axons of some neurons of the CNS
microglia- make up third class of glial cells. They are smaller than other glia, they respond to injury or
disease by multiplying, engulfing cellular debris, and triggering inflammatory responses
astrocytes- constitute a fourth class of glial cells, they are the largest, the extensions of some astrocytes
cover the outer surfaces of blood vessels that course through the brain, and they also make contact with
neuron cell bodies. They allow the passage of some chemicals into CNS neurons and in blocking other
Neuroanatomical techniques and directions
The Golgi stain was discovered by Camillo Golgi in the 1870s. His discovery made is possible to see
individual neurons for the first time, although on in silhouette.
The Nissl stain- the Golgi stain provided no indication of the number of neurons in an area of the nature
of their inner structure, the Nissl stain overcame these shortcoming and was developed by Franz Nissl.
Electron microscopy- a neuroanatomical technique that provides information about the details of
neuronal structure is electron microscopy.
Neuroanatomical tracing techniques- consists of two types: anterograde (forward) tracing methods and
retrograde (backward) tracing methods. Anterograde tracing methods are used when an investigator
wants to trace the paths of axons protecting away from cell bodies located in a particular area. The
investigator injects the brain with several chemicals which are taken up by cell bodies and then
transported forward along their axons to their terminal buttons. After a few days the brain is removed
and sliced. Retrograde tracing methods with in reverse, used when an investigator wants to trace the
paths of axons projecting into a particular area.
The vertebrate nervous system has three axes: anterior-posterior, dorsal-ventral, and medial-lateral.
Anterior means towards the nose end and posterior means toward the tail end. These same directions
are sometimes referred to as rostral and caudal. Dorsal means toward the surface of the back or the top
of the head, and ventral means toward the surface of the chest or the bottom of the head. Medial
means toward the midline of the body and lateral mea