INTRODUCTION

The nervous system controls all the major functions of the body. It is divided into central and
peripheral nervous systems. The peripheral nervous system includes the somatic and
autonomic nervous systems which control voluntary and involuntary functions respectively.
The ANS controls the vegetative functions of the body. These include functions like circulation,
respiration, digestion and the maintenance of body temperature.
The ANS is subdivided into two major sub-divisions; this classification is based on both
anatomic and physiologic grounds; the two subdivisions are sympathetic (thoracolumbar) and
parasympathetic (craniosacral). Autonomic nerves are actually composed of two neuron
systems, termed preganglionic and postganglionic, based on anatomical location relative to the
ganglia. A preganglionic neuron has its cell body in the spinal cord or brain.
The sympathetic nervous system arises from the thoracic and lumbar areas of the spinal cord
and the preganglionic fibers for the parasympathetic nervous system arise from the cranial and
sacral nerves. The postganglionic neurons send their axons directly to the effector organs
(peripheral involuntary visceral organs). Autonomic innervation, irrespective of whether it
belongs to the parasympathetic or the sympathetic nervous system, consists of a myelinated
preganglionic fiber which forms a synapse with the cell body of a non-myelinated second neuron
termed post-ganglionic fiber. The synapse is defined as a structure formed by the close
apposition of a neuron either with another neuron or with effector cells.