Peripheral nervous system

The peripheral nervous system is one of the two major anatomical divisions of the nervous system in bilaterian animals, the other being the central nervous system. It comprises nerves and ganglia located outside the brain and spinal cord, forming an extensive network that links the central nervous system with organs, limbs, and tissues. Its overarching function is to relay sensory information to the central nervous system and convey motor commands back to the body. Owing to the absence of protection from the skull, vertebral column, and the blood–brain barrier, the peripheral nervous system is comparatively more vulnerable to mechanical injury and toxic exposure.

Structural Organisation

The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system, each containing sensory and motor components. The somatic portion is largely associated with voluntary movement and conscious perception, while the autonomic portion regulates involuntary functions of organs, smooth muscle, and glands.
Cranial nerves form an essential part of the somatic nervous system, though two exceptions—the olfactory nerve and the optic nerve, along with the retina—are considered extensions of the central nervous system due to their developmental origins. The remainder of the cranial nerve ganglia, as with all ganglia, belong to the peripheral nervous system.
The spinal nerves constitute the major peripheral pathways for the body outside the head and neck. Humans have 31 pairs of spinal nerves categorised into cervical, thoracic, lumbar, sacral, and coccygeal groups. These nerves emerge through intervertebral foramina and follow naming conventions based on their corresponding vertebrae. The arrangement of spinal nerve roots changes between the cervical and thoracic regions, producing the unique designation of spinal nerve C8 between vertebrae C7 and T1. Below the level of the second lumbar vertebra, spinal nerve roots travel within the dural sac, forming the cauda equina.

The Somatic Nervous System

The somatic system includes both motor and sensory pathways responsible for voluntary control and conscious perception. Sensory nerves carry information such as touch, temperature, and proprioception to the spinal cord and brain through somatosensory pathways. Motor nerves transmit signals to skeletal muscles, enabling movement.
In the head and neck, cranial nerves provide both motor and somatosensory innervation. Ten of the twelve cranial nerves originate in the brainstem. The vagus nerve is distinctive in that it supplies sensory and parasympathetic innervation to thoracic and abdominal organs, while the accessory nerve controls the sternocleidomastoid and trapezius muscles, which extend beyond the head region.
Spinal nerves delivering somatosensory signals often form plexuses—complex webs of interconnected roots that reorganise to create specific peripheral nerves.

• Cervical spinal nerves (C1–C4) form nerves supplying the neck and the back of the head. Spinal nerve C1, the suboccipital nerve, provides motor innervation to suboccipital muscles. C2 and C3 contribute to cutaneous nerves such as the greater and lesser occipital nerves, while the greater and lesser auricular nerves arise to supply the skin around the ears. The phrenic nerve, originating from C3–C5, is crucial for respiration as it innervates the diaphragm; injury above C3 results in loss of spontaneous breathing.
• Brachial plexus (C5–T1) forms an organised network innervating the upper limb and upper back. Despite its complex appearance, the brachial plexus shows consistent patterns with minimal anatomical variation.
• Lumbosacral plexus (L1–Co1) provides motor and sensory supply to the lower limbs, pelvis, and perineum. It is formed by anterior divisions of lumbar, sacral, and coccygeal nerves, with frequent contributions from the twelfth thoracic nerve.

The Autonomic Nervous System

The autonomic nervous system controls involuntary physiological processes, regulating smooth and cardiac muscle, as well as glands. It functions continuously but shifts between sympathetic and parasympathetic dominance depending on situational demands. Autonomic pathways consist of ganglionic neurons linking the central nervous system to visceral organs.
Autonomic activity influences a wide range of processes, including pupil diameter, heart rate, glandular secretion, and gastrointestinal motility. Neurotransmitter release varies depending on whether the sympathetic or parasympathetic division is engaged.

Sympathetic Nervous System

The sympathetic division orchestrates the fight-or-flight response during physical threat or stress. Activation triggers the release of norepinephrine and epinephrine, elevating heart rate and increasing blood flow to skeletal muscles while reducing non-essential activities such as digestion. Because sympathetic pathways are targeted, the body can selectively prepare specific tissues for immediate action.

Parasympathetic Nervous System

The parasympathetic division supports rest-and-digest functions. It uses acetylcholine as its primary neurotransmitter and promotes activities such as digestion, salivation, and metabolic storage, while slowing heart rate. Certain functions such as urination and defecation, though influenced by parasympathetic control, retain elements of voluntary regulation.

Enteric Nervous System

The enteric nervous system is a specialised division situated along the gastrointestinal tract. It can operate independently of the sympathetic and parasympathetic divisions, though it remains capable of receiving input from them. It regulates motility, secretion, and local reflexes within the digestive system, earning the informal description of a “second brain”.

Disorders of the Peripheral Nervous System

Diseases affecting the peripheral nervous system vary widely in cause and presentation. A mononeuropathy refers to damage confined to a single nerve, often resulting from trauma or compression. Compression neuropathies occur when nerves are mechanically constricted within restricted anatomical spaces, as in carpal tunnel syndrome or tarsal tunnel syndrome, where characteristic symptoms include numbness and pain in specific dermatomal distributions.
Peripheral neuropathy involves dysfunction of multiple nerves due to metabolic, toxic, infectious, autoimmune, or inflammatory mechanisms. Diabetes, alcohol misuse, heavy metal exposure, amyloidosis, and sarcoidosis are common contributors. A typical feature is sensory impairment in a glove-and-stocking pattern, beginning in distal extremities and progressing proximally. The autonomic nervous system may also be affected, producing symptoms of autonomic neuropathy, including cardiovascular, gastrointestinal, and genitourinary disturbances.