Anatomy and Physiology of the Nervous systemNerve cells (neurons) are unique among body cells in that, instead of being compact, theyconsist of a cell nucleus and extensions: one axon and several dendrites. he dendrites transmits impulses to the cell nucleus! the axon transmits impulses a"ay from the cell nucleus to body organs. hese cells vary in si#e ranging from a fe" inches to several feet long, reaching from distant body sites such as feet, through the spinal cord, and to the brain. Although their great length is vital to motor and sensory function, it also ma$es nerve cells more susceptible than other body cells to in%ury.he nervous system continues to mature through the first &' years of life. (t actually consists of t"o separate systems: the central nervous system and the peripheral nervous system. he PN) consists of the cranial nerves, the spinal nerves, and the somatic and visceral divisions. he visceral division includes the autonomic system.he *N) consists of the brain, the spinal cord and the surrounding membranes or meninges that protect the delicate tissues from normal trauma. hese tissues are also protected by the s$ull, the vertebral column, and the cerebrospinal fluid (*)+), the fluid in the subarachnoid space "hich serves as a cushion.he brain is covered by three membranes: the dura matter ( a fibrous, connective tissue structures containing many blood vessels), the arachnoid membrane (a delcicate serous membrane), and the pia matter ( a vascular membrane).+our fluid,filled cavities, or ventricles, lie "ithin the brain. *)+ forms in the t"o lateral ventricles in the choroid plexus of the pia matter and flo"s through the foramens of -onro into the third ventricle, then through a narro" canal (the aqueduct of )lyvius) to the fourth ventricle. (t leaves the fourth ventricles by the foramen of magendie and the t"o foramens of .ush$a and flo"s into the cisterna magna, the fluid circulates to the subarachnoid space of the spinal cord. he fluid is then absorbed by the arachnoid membrane. he time span for replacement is approximately / hours.he *)+ is basically a colorless, al$aline fluid "ith a specific gravity of approximately &.001 to &.002, containing traces of protein, glucose, lymphocytes and body salts. he fluid circulates do"n"ard to the level of the second sacral vertebra ()'). (n infants, the spinal cord ends at the third lumbar vertebra (.3)! in adolescents and adults, at .& or .'. his configuration leaves a space near the cord base containing *)+ that can be tapped safely (lumbar puncture) "ithout fear of causing spinal cord damage.