Peripheral Nervous System

The nervous system is often described in two broad parts: the central nervous system, made up of the brain and spinal cord, and the peripheral nervous system. The PNS extends from the central system into the head, neck, trunk, arms, and legs.

It acts as a two-way communication route. Sensory information travels towards the brain and spinal cord, while motor instructions travel outwards to muscles, organs, and glands.

Somatic nervous system

The somatic nervous system carries sensory information from the skin, muscles, joints, and special senses. It also sends commands to skeletal muscles, supporting movements that can usually be controlled consciously.

Anatomical organisation

Two groups of nerves are especially important:

• Cranial nerves arise from the brain or brainstem and serve structures in the head and neck, as well as some internal organs.
• Spinal nerves arise from the spinal cord and distribute sensory and motor signals throughout the trunk and limbs.

There are 12 pairs of cranial nerves. The 31 pairs of spinal nerves are grouped into cervical, thoracic, lumbar, sacral, and coccygeal regions. Networks called plexuses allow selected spinal nerves to combine and redistribute their branches.

Examples include the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus.

Autonomic nervous system

The autonomic nervous system regulates many functions that usually happen without deliberate attention. It helps coordinate the heart, blood vessels, airways, digestive tract, bladder, and glands.

Two complementary divisions

• The sympathetic nervous system helps the body respond to challenge or increased demand.
• The parasympathetic nervous system supports quieter functions such as digestion and recovery.

Autonomic pathways commonly use a two-neuron chain. A preganglionic neuron communicates with a ganglion, where it connects with a postganglionic neuron that continues to the target tissue.

How signals work

Specialised receptors detect changes such as touch, temperature, pressure, stretch, or tissue irritation. Sensory neurons carry these signals towards the spinal cord or brainstem, where further pathways help the brain interpret them.

For movement, the brain and spinal cord send motor instructions through peripheral nerves to skeletal muscle. Some responses, such as a reflex, can be organised rapidly through circuits involving the spinal cord before the brain has fully processed the event.

Autonomic signals adjust body activity continuously. Heart rate, digestion, pupil size, sweating, and blood-vessel tone can all be influenced by this branch of the peripheral nervous system.

When peripheral nerves are affected

A nerve can be affected by physical injury, pressure, inflammation, or disease. The effects depend on which nerve is involved and whether sensory fibres, motor fibres, autonomic fibres, or a combination are affected.

Possible effects include altered sensation, weakness, changes in reflexes, or difficulty controlling a particular body function. Symptoms can have many causes, so persistent or concerning changes should be discussed with a qualified healthcare professional.

Summary

The peripheral nervous system consists of nerves and related structures outside the brain and spinal cord. It links the central nervous system with the body.

• The somatic division carries sensory information and supports skeletal muscle movement.
• The autonomic division helps regulate involuntary activities in organs and tissues.
• Cranial nerves, spinal nerves, ganglia, and plexuses form important parts of the peripheral network.
• Injury or disease can affect sensation, movement, reflexes, or automatic body functions.