Understanding the Peripheral Nervous System

The peripheral nervous system includes the cranial nerves, spinal nerves and autonomic nervous system.

Cranial nerves
The 12 pairs of cranial nerves are the primary motor and sensory pathways between the brain and the head and neck. All cranial nerves except the olfactory and optic nerves exit from the midbrain, pons or medulla oblongata of the brain stem.

Spinal nerves
There are 31 pairs of spinal nerves, each named for the vertebra immediately below its exit point from the spinal cord.

The nerve of nerves
Each spinal nerve consists of afferent (sensory) and efferent (motor) neurones, which carry messages to and from specific body regions, called dermatomes.
Autonomic nervous system

The large autonomic nervous system supplies nerves to all internal organs. These visceral efferent nerves carry messages to the viscera from the brain stem and neuroendocrine system.

Two sympathetic systems
The autonomic nervous system includes two major parts:

• the sympathetic nervous system
• the parasympathetic nervous system.

Balancing act
When one part of the autonomic nervous system stimulates smooth muscles to contract or a gland to secrete, the other part of the system inhibits that action. Through such dual innervation, the sympathetic and parasympathetic systems counterbalance each other’s activities to keep body systems running smoothly.

Sympathetic nervous system
Sympathetic nerves, called preganglionic neurons , exit the spinal cord between the first thoracic and second lumbar vertebrae and enter relay stations (ganglia) near the cord. These ganglia form the links of a chain that sends impulses to postganglionic neurons, which reach the organs and glands.

Enormous responses
The postganglionic neurons of the sympathetic nervous system produce widespread, generalised responses sometimes called the 'fight or flight' response, including:

• vasoconstriction
• elevated blood pressure
• enhanced blood flow to skeletal muscles
• increased heart rate and contractility
• increased respiratory rate
• smooth-muscle relaxation of the bronchioles, GI tract and urinary tract
• sphincter contraction
• pupillary dilation and ciliary muscle relaxation
• increased sweat gland secretion
• reduced pancreatic secretion.

Parasympathetic nervous system
Fibres of the parasympathetic nervous system leave the CNS by way of the cranial nerves from the midbrain and medulla and the spinal nerves between the second and fourth sacral vertebrae.

After leaving the CNS, the preganglionic fibre of each parasympathetic nerve travels to a ganglion near a specific organ or gland. The postganglionic fibre of the nerve enters that organ or gland.

Subtle responses
The postganglionic fibres of the parasympathetic nervous system produce responses involving one specific organ or gland, and may be deemed as resting or usual responses, such as:

• reductions in heart rate, contractility and conduction velocity
• bronchial smooth-muscle constriction
• increased GI tract tone and peristalsis, with sphincter relaxation
• increased bladder tone and urinary system sphincter relaxation
• vasodilatation of external genitalia, causing erection
• pupil constriction
• increased pancreatic, salivary and lacrimal secretions.

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