General Organization and Functions of the Nervous System

GENERAL ORGANIZATION AND FUNCTIONS OF THE NERVOUS SYSTEM:-

                      The nervous system is divided into two parts:the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord.The PNS consists of all afferent (sensory) neurons, which carry nerve impulses into the CNS from sensory end organs in peripheral tissues,and all efferent (motor) neurons, which carry nerve impulses from the CNS to effector cells in peripheral tissues.The peripheral efferent system is further divided into the somatic nervous system and the autonomic nervous system. The effector cells innervated by the somatic nervous system are skeletal muscle cells.The autonomic nervous system innervates three types of effector cells:
(1) smooth muscle
(2) cardiac muscle
(3) exocrine glands.

While the somatic nervous system can function on a reflex basis, voluntary control of skeletal muscle is of primary importance. In contrast, in the autonomic nervous system voluntary control can be exerted, but reflex control is paramount.

Both somatic and autonomic effectors may be reflexly excited by nerve impulses arising from the same sensory end organs. For example, when the body is exposed to cold, heat loss is minimized by vasoconstriction of blood vessels in the skin and by the curling up of the body.At the same time,heat production is increased by an increase in skeletal muscle tone and shivering and by an increase in metabolism owing in part to secretion of epinephrine.

ANATOMIC DIFFERENCES BETWEEN THE SOMATIC AND AUTONOMIC NERVOUS SYSTEMS:-

                      Anatomical differences between the peripheral somatic and autonomic nervous systems have led to their classification as separate divisions of the nervous system. 

.   The axon of a somatic motor neuron leaves the CNS and travels without interruption to the innervated effector cell. In contrast, two neurons are required to connect the CNS and a visceral effector cell of the autonomic nervous system. The first neuron in this sequence is called the preganglionic neuron. 

   The second neuron, whose cell body is within the ganglion,travels to the visceral effector cell;it is called the postganglionic neuron.

AUTONOMIC NERVOUS SYSTEM :-

The preganglionic neurons of the sympathetic nervous system have their cell bodies in the thoracic and lumbar regions of the spinal cord,termed the thoracolumbar division. The preganglionic neurons of the parasympathetic division have their cell bodies in the brainstem and in the sacral region of the spinal cord, termed the craniosacral division.The cranial part of the parasympathetic nervous system innervates structures in the head, neck, thorax, and abdomen (e.g., the stomach, part of the intestines, and pancreas). 

Location of the Autonomic Ganglia:-

The sympathetic ganglia consist of two chains of 22 segmentally arranged ganglia lateral to the vertebral column. The preganglionic fibers leave the spinal cord in adjacent ventral roots and enter neighboring ganglia, where they make synaptic connections with postganglionic neurons.Some preganglionic fibers pass through the vertebral ganglia without making synaptic connections and travel by way of splanchnic nerves to paired prevertebral ganglia in front of the vertebral column, where they make synaptic connections with postganglionic neurons. In addition, some sympathetic preganglionic fibers pass through the splanchnic nerves into the adrenal glands and make synaptic connections on the chromaffin cells of the adrenal medulla.

         Because sympathetic ganglia lie close to the vertebral column, sympathetic preganglionic fibers are generally short. Postganglionic fibers are generally long, since they arise in vertebral ganglia and must travel to the innervated effector cells.