Chapter 3:Anatomy of the Nervous System
3.1: General Layout of the Nervous System
Divisions of the Nervous System
The vertebrate nervous system is composed of two divisions:
o Central Nervous System: Division of the nervous system that is located within the skull and
spine. Composed of two parts: the brain (protected by the skull) and the spinal cord (protected
by the spine)
o Peripheral Nervous System: Division of the nervous system that is located outside of the skull
and spine. Composed of two parts: the somatic nervous system and the autonomic nervous
Somatic Nervous System: the portion of the peripheral nervous system which interacts
with the environment.
Afferent Nerves: Nerves within the somatic nervous sysem which carry sensory signals
to the central nervous system.
Efferent Nerves: Nerves within the somatic nervous system which carry motor signals
from the central nervous system to the skeletal muscles.
Autonomic Nervous System: Portion of the peripheral nervous system which regulates the
body's internal environment.
Afferent Nerves: Nerves within the autonomic nervous system which carry sensory
information from the body's internal organs to the central nervous system.
Efferent Nerves: Nerves within the autonomic nervous system which carry motor
signals from the central nervous system to the internal organs.
o Sympathetic Nerves: Class of efferent nerve within the autonomic nervous system
which project from the central nervous system in the lumbar (back) and thoracic
(chest) regions of the spinal cord.
Sympathetic neurons synapse onto secondary neurons at a significant distance
from the target organ
o Parasympathetic Nerves: Class of efferent nerve within the autonomic nervous
system which project from the brain and sacral (lower back) regions of the spinal
o All sympathetic and parasympathetic nerves are two stage neural pathways.
The sympathetic and parasympathetic nerves project from the central nervous
system, but do not reach the internal organs of interest.
Parasympathetic nerves synapse with secondary nerves near to the target organ.
o Sympathetic nerves stimulate, organize and mobilize energy resources in threatening
situations. Parasympathetic nerves do the opposite, conserving and calming energy
o Each autonomic target receives opposite input from sympathetic and
parasympathetic systems. The result is controlled by the relative input of both
o Sympathetic changes are associated with arousal and parasympathetic changes are
associated with relaxation.
o These are general rules, however each as special cases and exceptions.
All neurons within the parasympathetic system except 12 cranial nerves extend from the
spinal cord. The cranial nerves are numbered from 1 to 12 from front to back.
Some cranial nerves are prely sensory such as the olfactory nerve and the optic nerve, but
most contain both motor and sensory neurons.
Longest cranial nerves are the vagus fibres which contain sensory and motor nerves for the
The autonomic motor fibres of the cranial nerves are parasympathetic.
Meninges, Ventricles, and Cerebrospinal Fluid
The brain and spinal cord are the most well protected areas of the body
Both are encased in bone and 3 protective membranes
Meninges: Group of three protective membranes which surround the brain and spinal cord.
o Dura Mater: Tough outer mininx protecting the brain and spinal cord. Means 'Tough Mother
o Arachnoid Membrane: Membrane located between the dura mater and the subarachnoid
space. Name means spiderweb-like membrane, and is named so because of its appearance like a
o Subarachnoid Space: Space directly below the arachnoid membrane and above the pia matter.
Contains many large blood vessels
Carries spinal fluid
o Pia Mater: Soft, delicate innermost meninx protecting the brain and spinal cord.Attaches
directly to the brain and spinal cord. Means 'Pious Mother'.
Cerebrospinal Fluid: Fluid which fills the subarachnoid space, the central canal of the spinal cord
and the cerebral ventricles of the brain.
Central Canal: Small channel which runs the length of the spinal cord. Contains cerebrospinal
Cerebral Ventricles: Four large internal chambers within the brain.
o Lateral Ventricles: Largest ventricles of the brain. Located symetrically on both sides within
the the cerebrum.
o Third Ventricle: Ventricle located in the center of the brain.
o Fourth Ventricle: Small lower ventricle located near the top of the cerebellum.
The subarachnoid space, central canal and cerebral ventricles are interconnected by a series of
openings and thus form a single resevoire.
Cerebrospinal fluid supports the brain.
Patients who have small amounts of cerebrospinal fluid removed often experience raging headaches
and sharp pain every time they turn their head.
Choroid Plexuses: Networks of capillaries that protrude into the ventricles from the pia mater.
Responsible for the production of cerebrospinal fluid.
Excess spinal fluid is drained into blood-filled spaces, or dural sinuses, which run through the dura
mater and drain into the large jugular veins of the neck.
The blood-brain barrier impedes many toxic substances from reaching the central nervous system
and disrupting its balance.
Cells that compose the walls of central nervous system blood vessels are tightly packed together
and do not allow very many molecules to pass through them. In contrast, the cells that make up the
walls of most other blood vessels are loosely packed together and allow many different molecules
to pass through. The degree to which therapeutic and recreational drugs are able to have an effect is often due to its
ability to cross the bloodbrain barrier.
Large molecules that are critical to the brain's survival are actively transported through the barrier
Certain areas of the bloodbrain barrier allow certain large molecules to pass through unimpeded.
3.2: Cells of the Nervous System
Most cells in the nervous system are of one of two types, neurons and glial cells
Anatomy of Neurons
Neuron: cells in the body specialized in reception, conduction, and transmission of electro-
o Neurons come in a wide variety of shapes and sizes.
ExternalAnatomy of Neurons
Cell Membrane: Semi-permeable membrane that encloses the neuron
Dendrites: The short processes emanating from the cell body, which receive most of the synaptic
connections from other neurons
Axon: Long, narrow process that projects from the cell body.
Axon Hillock: Cone-shaped region at the junction between the axon and the cell body.
Cell Body: The metabolic centre of the neuron; also called the soma.
Myelin: Fatty insulation around many axons.
Nodes of Ranvier: Gaps between sections of myelin.
Synapses: The gaps between adjacent neurons across which chemical signals ar