Peripheral nervous system
Also known as: PNS
All nervous tissue outside the brain and spinal cord: 12 pairs of cranial nerves, 31 pairs of spinal nerves, their ganglia, plexuses and sensory endings.
Overview
The PNS links the CNS to the rest of the body. Its afferent (sensory) division carries somatic sensation (skin, muscles, joints), visceral sensation (organs) and the special senses towards the CNS. Its efferent (motor) division has two parts: the somatic nervous system to skeletal muscle and the autonomic nervous system to smooth muscle, cardiac muscle and glands. A ganglion is a collection of neuronal cell bodies outside the CNS. Sensory ganglia (dorsal root ganglia and cranial nerve ganglia) contain pseudounipolar neurons and no synapses. Autonomic ganglia contain multipolar postganglionic neurons on which preganglionic fibres synapse.
A peripheral nerve is a bundle of axons bound by three connective-tissue sheaths:
• the endoneurium surrounds each axon with its Schwann cell;
• the perineurium wraps each fascicle and forms the blood–nerve barrier with tight-junction perineurial cells;
• the epineurium encloses the whole nerve, its fascicles and its blood vessels (vasa nervorum).
Most nerves are mixed, carrying sensory, motor and autonomic fibres. PNS myelin is made by Schwann cells, and ganglion neurons are wrapped by satellite cells.
Peripheral axons can regenerate. After injury the distal segment undergoes Wallerian degeneration, and Schwann cells form guiding bands of Büngner along which the axon regrows at ≈1–3 mm/day. Recovery therefore depends on the distance to the target and on whether the endoneurial tubes are intact.
Parts & subdivisions
- 12 pairs, I–XII, attached to the forebrain (I, II) and brainstem (III–XII).
- 31 pairs, each formed by a dorsal (sensory) and a ventral (motor) root; ventral rami form the plexuses.
- Somatic afferents and lower motor neurons to skeletal muscle.
- Sympathetic, parasympathetic and enteric parts supplying viscera.
- Sensory (dorsal root, trigeminal, geniculate, inferior vagal…) and autonomic (sympathetic chain, prevertebral, parasympathetic cranial and terminal).
- Nerve sheathsEndoneurium → perineurium (blood–nerve barrier) → epineurium.
Anatomy
- Histology
Nerve fascicles of myelinated and unmyelinated axons with Schwann cells, separated by endoneurial collagen; perineurium of flattened epithelioid cells; epineurium of dense irregular connective tissue with fat and vessels.
- Development
Sensory and autonomic ganglion neurons, Schwann cells and satellite cells derive from the neural crest (and cranial placodes for some cranial sensory ganglia). Motor axons grow out from basal-plate neurons of the neural tube.
Function
- Carries sensory information from receptors to the CNS
- Carries motor commands to skeletal muscle (somatic) and to smooth muscle, cardiac muscle and glands (autonomic)
- Provides reflex arcs with their afferent and efferent limbs
- Supports local regeneration of injured axons
Clinical significance
Patterns of peripheral neuropathy
The distribution of the deficit localises the problem:
• Mononeuropathy: a single nerve, e.g. carpal tunnel syndrome or common fibular palsy.
• Mononeuritis multiplex: several separate named nerves, typical of vasculitis or diabetes.
• Length-dependent polyneuropathy: symmetrical "glove-and-stocking" sensory loss with absent ankle jerks. Diabetes is the commonest cause; alcohol, B12 deficiency and drugs are others.
• Radiculopathy: dermatomal pain with myotomal weakness from a root lesion.
Nerve conduction studies separate demyelinating disease (slowed conduction) from axonal disease (reduced amplitude).
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.