3-31. THE AUTONOMIC (VISCERAL EFFERENT) NERVOUS SYSTEM
The autonomic nervous system is composed of the sympathetic nervous system
(concerned primarily with emergency mechanisms) and the parasympathetic nervous
system (concerned with the conservation of resources). The autonomic system controls
visceral functions rather than somatic motor functions.
3-32. CENTRAL NERVOUS SYSTEM
a. General. The central nervous system presents two regions: gray matter and
white matter. The gray matter contains mainly nerve cells and nonmyelinated fibers and
the white matter, primarily myelinated nerve fibers. A concentrated mass of nerve cells
(gray matter) in any part of the central nervous system is called a nucleus. A similar
grouping of nerve cells in the peripheral nervous system (outside the brain and spinal
cord) is called a ganglion. A tract is a bundle of nerve fibers with a common origin and
destination. The nerve fibers are either ascending or descending, depending on
whether they carry impulses to or from the brain.
b. Embryonic Development. In very early embryonic development, the central
nervous system emerges as a continuous tube along the dorsal aspect of the embryo.
As growth progresses, a small cranial portion of this neural tube develops into the brain
and the remaining section becomes the spinal cord. In its early stages of development,
the cranial portion of the neural tube presents three-enlargements: the forebrain, the
midbrain, and the hindbrain. These three embryonic subdivisions give rise to the
various regions of the adult brain. As the brain develops, the original cavity of the
neural tube expands into four communicating chambers called ventricles that are
continuous with the central canal in the spinal cord.
c. Protective Coverings. The brain and spinal cord are well protected within
the skeleton, the brain in the cranium and the spinal cord in the vertebral canal. In
addition, three connective tissue membranes called meninges (the dura mater, the
arachnoidea, and the pia mater) enclose them.
(1) The dura mater, the outermost covering, is a tough fibrous membrane. It
adheres to the inner surface of the cranium, but in the spinal canal it is separated from
the bony housing by the small epidural space.
(2) The arachnoidea, the middle meningeal tissue, is a delicate serous
membrane that loosely envelops the brain and the spinal cord. The potential space
between the arachnoid and the dura mater contains cerebrospinal fluid.
(3) The pia mater, the innermost meningeal membrane, is a loosely
arranged network of vascular tissue containing a plexus of fine blood vessels that permit
the passage of blood to and from the central nervous system. Between the arachnoid
and the pia mater is the subarachnoid space, which contains cerebrospinal fluid. The
ventricles of the brain are lined by the pia mater.