236. CEREBROSPINAL FLUID
a. Formation and Reabsorption. Cerebrospinal fluid (CSF) is produced
predominantly by small masses of blood vessels known as choroid plexuses. Choroid
plexuses are found in chambers of the brain (first, second, third, and fourth ventricles).
The composition of the choroid secretion is similar to interstitial fluid with the exception
that the sodium concentration is 7 percent greater, potassium 40 percent less, and
glucose usually 30 percent less. Cerebrospinal fluid fills the four ventricles of the brain
and the subarachnoid space and surrounds the spinal cord. Within the subarachnoid
space are arachnoid villi, which absorb cerebrospinal fluid and return lost proteins and
electrolytes to cerebral veins. Approximately 750 mL of CSF are formed per day while
only about 135 mL are needed to fill the subarachnoid space.
b. Functions. Cerebrospinal fluid functions primarily to protect the brain by
cushioning it against external blows to the skull. Secondly, the cerebrospinal fluid
system is similar to the lymphatic system in that it returns lost proteins and nutrients to
the bloodstream.
c. Pathology. Occasionally, inflammation of the membranes (meningitis)
surrounding the brain and spinal cord occurs due to a number of types of
microorganisms. Spinal taps are therefore performed by physicians, and CSF is
analyzed for bacterial or viral infections. Another type of malfunction associated with
the CSF system is the disease hydrocephalus. Generally, one of the ducts between the
ventricles in the brain becomes blocked. The CSF accumulates within the brain causing
chronic edema. In young children, bones of the skull separate and the head become
greatly enlarged.
237. PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system, although it communicates with the brain and
spinal cord, is distributed outside of, and around (peripheral to) the central nervous
system, hence its name. The peripheral nervous system includes the 12 pairs of cranial
nerves and the 31 pairs of spinal nerves, stemming from the brain and spinal cord
respectively. The nerve fibers carry both voluntary and involuntary impulses. (The
latter belong to the autonomic system and will be discussed separately.) Not every
nerve necessarily has both types of fibers. The cranial nerves are mostly voluntary,
except for those few involuntary fibers going to the ciliary eye muscles, the salivary
glands, the heart, the smooth muscles of the lung, and to the gastrointestinal tract. The
spinal nerves have voluntary fibers going to all skeletal muscles of the trunk and
extremities, and involuntary fibers going to the smooth muscles and to the glands of the
gastrointestinal tract, the genitourinary system, and the cardiovascular system. The
voluntary fibers carry impulses that are stimulatory, and cause a contraction of a
muscle.
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