Peripheral nervous system
The peripheral nervous system includes 12 cranial nerves 31 pairs of spinal nerves. It can be sub-divided into the somatic and autonomic systems. It is a way of communication from the central nervous system to the rest of the body by nerve impulses that regulate the functions of the human body. The twelve cranial nerves are:
- Olfactory nerve for smell
- Optic nerve for vision
- Oculomotor for looking around
- Trochlear for moving eye
- Trigeminal for feeling touch on face
- Abducens to move eye muscles
- Facial to smile, wink and help us taste
- Glossopharyngeal for swallowing and gagging
- Vestibulocochlear to help with balance, equilibrium and hearing
- Spinal accessory for shrugging shoulders.
- Vagus for swallowing, talking and parasympathetic actions of digestion .
- Hypoglossal for tongue more divided into different regions as muscles.
The 10 out of the 12 cranial nerves originate from thae brainstem and mainly control the functions of the anatomic structures of the head with some exceptions. CN X receives visceral sensory information from the thorax and abdomen and the CN IX is responsible for innervatin sternocleidomastoid and trapezius muscles, neither of which is exclusively in the head.
Cerebral (Cranial nerves)
The nerves that arise from or join the brain are called cerebral, or cranial, nerves. Man has twelve pairs of cranial nerves.* They are numbered I to XII in Roman numerals. They leave cranium through apertures, called foramina, in its wall, and mainly innervate the parts of the head.
Name, origin, distribution, nature and functions of various cranial nerves are described in table.
The spinal nerves arise from the spinal cord.
Number. There are 31 pairs of spinal nerves in man. These are classified into five groups : cervical, 8 pairs; thoracic, 12 pairs; lumbar, 5 pairs; sacral, 5 pairs ; and coccygeal, 1 pair.
Nature. All spinal nerves are mixed, having sensory and motor fibres in approximately equal numbers. They are composed primarily of medul- lated nerve fibres.
Connection with Spinal Cord . Each spinal nerve is joined to the spinal cord by two roots: poterior (dorsal) root and anterior (ventral) root. The sensory fibres of a spinal nerve enter the spinal cord as the posterior root, whereas the motor fibres of a spinal nerve emerge from the spinal cord as the anterior root. The nerve cell bodies of the sensory neurons are located outside the spinal cord in the swellings of the posterior roots, the posterior root ganglia. The nerve cell bodies of the motor neurons are located in the anterior columns of the grey matter of the spinal cord. The posterior and anterior roots of a spinal nerve join in the vertebral canal of the vertebral column.
Exit. The spinal nerves come out of the neural canal through paired apertures, the intervertebral foramina, present laterally between the successive vertebrae. The first cervical nerve, unlike others, arises from the medulla oblongata. It passes out between the cranium and the first cervical vertebra called atlas . Each of the other cervical nerves emerges through an intervertebral foramen above the vertebra of the corresponding number. The eighth cervical nerve comes out through intervertebral foramen below the seventh cervical vertebra. All other spinal nerves leave through the intervertebral foramina below the corresponding vertebrae. Since the spinal cord is shorter than the vertebral column, the roots of the lumber, sacral and coccygeal spinal nerves descend for varying distances to reach their corresponding foramina . These roots along with the filum terminale form a bundle , called cauda equina , due to its resemblance to the horse’s tail.
Autonomic nervous system
The autonomic nervous system deals with the visceral organs, like the heart, stomach, gland and the intestine. It regulates systems that are unconsciously carried out to keep our body alive and well, such as breathing, digestion (peristalsis) and regulation of the heartbeat. The autonomic system consists of the sympathetic and the parasympathetic divisions. Both divisions work without conscious effort and they have similar nerve pathways, but the sympathetic and parasympathetic systems generally have opposite effects on target tissues (they are antagonistic). By controlling the relative input from each division, the autonomic system regulates many aspects of homeostasis. One of the main parasympathetic autonomic system is cranial nerve X, the nerves for the vagus nerve.
Sympathetic nervous system
The sympathetic nervous system activates what is often termed the fight or flight response, as it is most active under sudden stressful circumstances (such as being attacked). This response is also known as sympathetico-adrenal response of the body, as the pre-ganglionic sympathetic fibres that end in the adrenal medulla (but also all other sympathetic fibres) secrete acetylcholine, which activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it. Therefore, this response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.
Parasympathetic nervous system
The parasympathetic nervous system is one of three divisions of the autonomic nervous system. Sometimes called the rest and digest system, the parasympathetic system conserves energy as it slows the heart rate, increase intestinal and gland activity and relaxes sphincter muscles in the gastrointestinal tract.
Parasympathetic nervous system is composed of
i. Preganglionic parasympathetic fibres These are long sizesd axons of neurons present in the midbrain, brainstem and lateral funiculus of sacral part of spinal cord. These come out either through some cranial nerves like oculomotor (III), facial (VI), glossopharyngeal (IX) and vagus (X) or (II), (III) and (IV) sacral spinal nerves, so these form cranio-sacral outflow. These form synapses with neurons of the parasympathetic ganglia.
(ii) Postganglionic parasympathetic fibres These are small sized axons of neurons of para sympathetic ganglia and supply smooth muscles and glands of visceral organs. These form a network in the wall of the visceral organs, called aurebach plexus. These have limited effect.
Postganglionic parasympathetic fibres release acetylcholine at their nerve endings, so these are called cholinergic nerve fibres. It involves conservation of energy and brings about relaxation, comfort, pleasure, etc., at the time of rest.
(iii) Parasympathetic ganglia These are present either near or inside the visceral organs. Each is a group of neurons. These are isolated ganglia and are not interlinked to form a chain.