There are twelve pairs of cranial nerves in man. They leave the cranium through foramina, and mainly innervate the head region. Their origin, supply and nature are given.
In man there are 31 pairs of spinal nerves (all are mixed i.e. sensory + motor), which are formed by the union of dorsal and ventral roots shortly after they leave the spinal cord. They are classified into five groups.
(i) 8 pairs of cervical nerves
(ii) 12 pairs of thoracic nerves
(iii) 5 pairs of lumbar nerves
(iv) 5 pairs of sacral nerves and
(v) One pair of coccygeal.
They leave the vertebral column through intervertebral foramina, and all of them are mixed nerves containing both sensory and motor neurons.
Shortly after the spinal nerves leave the vertebrae, they divide into the branches known as rami (sing, ramus). These are
– Innervates the deep back muscles and skin
– Innervates the superficial back muscles and all of the structure of the extremities and lateral and ventral trunk
– Re-enters the spinal canal through the inter vertebral foramen to supply nerves to vertebrae, vertebral ligaments, blood vessels and the meninges
– Components of autonomic nervous system.
2. Autonomic (Or Visceral) Nervous System:
Autonomic nervous system automatically regulates the activities of involuntary smooth muscle, cardiac muscles and glands without consulting the will.
This system is entirely motor having efferent fibres. The system consists of preganglionic fibres from CNS, ganglia and post ganglionic fibres.
It is also called visceral nervous system because the autonomic nervous system innervates the viscera. It consists of two antagonistic components: sympathetic and parasympathetic.
It is represented by a chain of 21 sympathetic ganglia on either side of spinal cord. It receives preganglionic sympathetic fibres from the spinal cord which make their exit along with thoracic and lumbar nerves and constitute thoraco-lumbar outflow.
These fibres synapse with the neurons present in sympathetic ganglia. From the sympathetic ganglia arise post ganglionic fibres which terminate on the viscera.
Some preganglionic fibres pass through the sympathetic chain without synapsing and then join to form splanchnic nerves eminating in some collateral ganglia which include a coeliac ganglion, an anterior mesenteric ganglion and a posterior mesenteric ganglion.
The post ganglionic fibres arising from collateral ganglia supply digestive system and urinogenital system.
It consists of preganglionic parasympathetic fibres, parasympathetic ganglia and postganglionic parasympathetic fibres.
Preganglionic parasympathetic fibres make their exit along with the 3rd, 7th, 9th and 10th cranial nerves, and 2nd, 3rd, 4th sacral nerves.
They together form cranio-sacral outflow. The parasympathetic ganglia do not form any chains and instead lie on or near the viscera. The post ganglionic parasym-pathetic fibres arise from these ganglia and supply the viscera.