The Central Nervous System
This page outlines the basic physiology of the central nervous system,
including the brain and spinal cord.
Separate pages describe the nervous system in
sensation, control of
skeletal muscle and control of internal
The central nervous system CNS is responsible for integrating sensory
information and responding accordingly. It consists of two main
The spinal cord serves as a conduit for signals between the brain
and the rest of the body. It also controls simple musculoskeletal
reflexes without input from the brain.
The brain is responsible for integrating most sensory
information and coordinating body function, both consciously and
unconsciously. Complex functions such as thinking and feeling as
well as regulation of homeostasis are attributable to different
parts of the brain.
The brain and spinal cord share some key anatomic features:
Living nervous tissue has the consistency of jelly and requires
special protection from physical damage. The entire CNS is encased in bone.
is within the cranium, while the spinal cord runs within a
canal through the vertebrae.
Within its bony case, the entire CNS is bathed in a cerebrospinal
fluid (CSF), a colorless fluid produced by special structures in the brain.
CSF provides a special chemical environment for nervous tissue, as well
as an additional buffer against physical damage.
The special chemical environment of nervous tissue is maintained by the
relatively impermeable membranes of capillaries in the CNS. This feature
is known as the blood-brain barrier.
There are two general types of tissue in the CNS:
Gray matter consists of nerve cell bodies, dendrites, and axons.
Neurons in gray matter organize either in layers, as in the cerebral cortex,
or as clusters called nuclei.
White matter consists mostly of axons, causing it to look white
due to the myelin sheathing of the axons.
In the early embryo, the CNS forms as a relatively uniform tube. The
major regions of the brain develop as enlargements at the head end of
The medulla oblongata appears as a swelling at the upper end of
the spinal cord. Besides being a conduit for fibers running between the
spinal cord and higher regions of the brain, it contains control centers
for involuntary functions such as blood pressure, breathing, swallowing
Just above the medulla are the pons and cerebellum. The
pons relays information between higher regions of the brain and the
cerebellum, which processes sensory information and helps coordinate movement.
The next segment, the midbrain, is primarily responsible for eye
Above the midbrain lies the diencephalon, which is composed of
two major parts:
The thalamus processes and integrates all sensory information
going to the higher regions of the brain.
The hypothalamus is critical for homeostasis, the maintenance of
the body's internal environment. It influences nervous control of
all internal organs and also serves as the master regulator of endocrine
function by its control over the pituitary gland.
The highest region of the brain is the cerebrum, which includes both
the cerebral cortex that is visible on the outside of the brain as well
as other internal structures. The cerebrum is responsible for conscious
sensation and voluntary movement, as well as advanced functions such
as thinking, learning and emotion.