The Peripheral Nervous System (PNS)

Composed of all other interconnecting nerves of the nervous system that are located outside of the brain and spinal cord, the peripheral nervous system (PNS) functions to take messages from receptors and to effectors.

It is primarily composed of:

• nerve fibres that carry information to and from the CNS

• ganglia - groups of nerve cell bodies that lie outside of the CNS

Cranial Nerves

12 pairs of nerves arise in the brain, mostly being mixed nerves, but some are motor or sensory nerves.

Spinal Nerves

31 pairs of nerves arise in the spinal cord, all being mixed and joined to the spinal cord via two roots. The ventral root contains motor nerves whereas the dorsal root contains sensory nerves.

• axons in the ventral root have their cell body in the grey matter of the spinal cord

• axons in the dorsal root have their cell body in the dorsal root ganglion

Afferent Division of the PNS

Nerve fibres that carry impulses into the CNS from receptors via sensory neurons.

Neurons in the afferent division are divided into two parts:

• somatic sensory neurons bring impulses from skin and muscles

• visceral sensory neurons bring impulses from internal organs

Efferent Division of the PNS

Composed of nerve fibres that carry impulses away from the CNS via motor neurons.

This is divided into two divisions:

• the somatic nervous system takes impulses to skeletal muscle - is voluntarily controlled.

• the autonomic nervous system takes impulses to cardiac and involuntary muscle and glands - is involuntarily controlled. It is further divided into two parts:

• the sympathetic nervous system brings about the “fight-or-flight” response when activated, leading to dilated pupils, an increased heart rate, and dilation of the bronchioles.

• the parasympathetic nervous system produces the “rest-and-digest” response when activated, leading to constricted pupils, increased salivary secretion and a reduced heart rate.