Peripheral Nervous System (PNS)

Cranial Nerves

• 12 pairs of cranial nerves arise from the brain

• Most cranial nerves are mixed nerves; they contain fibres that carry impulses into the brain as well as fibres that carry impulses away from the brain

• Fibres that carry impulses into the CNS are called sensory fibres; those that carry impulses away from the CNS are motor fibres.

Spinal nerves

• 31 pairs of spinal nerves arise from the spinal cord

• They are all mixed nerves containing both sensory and motor fibres

• Each nerve is joined to the spinal cord by 2 roots

The Ventral root contains the axons of motor neurons that have their cell bodies in the grey matter of the spinal cord

And the dorsal root contains the axons of sensory neurons which have their cell bodies in a small swelling in the dorsal root known as the dorsal root ganglion.

Afferent division

The afferent division carrying information into the CNS. It is composed of:

1. Somatic sensory - neurons from skin and muscle

2. Visceral sensory - neurons from internal organs

Efferent division

The efferent division is carrying information away from the CNS. It is composed of

• the somatic division - carrying messages to the skeletal muscles

• Autonomic division - carrying messages to heart and involuntary muscles and glands.

  • This division further divides into the sympathetic and parasympathetic divisions.

Sympathetic division

• Prepares the body for fight or flight

• Produces noradrenaline

Parasympathetic division

• maintains the body during quiet times

• Acetylcholine