Peripheral nervous system
Also known as: pns
The peripheral nervous system is everything in the nervous system outside the brain and spinal cord — the cranial and spinal nerves, their branches, and the associated ganglia. It carries sensory information inward and motor commands outward.
The nervous system divides into the central nervous system (brain and spinal cord) and the peripheral nervous system (PNS), which comprises 12 pairs of cranial nerves, 31 pairs of spinal nerves, the plexuses and named nerves they form, and the ganglia housing peripheral cell bodies. Functionally the PNS is the wiring between the central nervous system and the rest of the body.
It splits along two axes. By direction, afferent (sensory) fibers carry information from skin, muscle, joints, and viscera toward the central nervous system, while efferent (motor) fibers carry commands outward. By control, the somatic division innervates skeletal muscle under voluntary control, and the autonomic division regulates smooth muscle, cardiac muscle, and glands involuntarily through its sympathetic and parasympathetic branches. Peripheral axons are myelinated by Schwann cells rather than the oligodendrocytes used centrally — a difference that matters because peripheral nerves can regenerate after injury while central axons largely cannot.
Clinically, peripheral lesions produce a recognizable pattern: flaccid weakness, muscle atrophy, fasciculations, decreased or absent reflexes, and sensory loss in a dermatomal or specific-nerve distribution. That lower motor neuron picture contrasts with the spasticity and hyperreflexia of a central lesion, and distinguishing the two is the first step in localizing any neurologic complaint. Common peripheral disorders include radiculopathy, carpal tunnel syndrome and other entrapment neuropathies, diabetic polyneuropathy, and Guillain-Barré syndrome.
The NPTE for both physical therapists and physical therapist assistants devotes neuromuscular content to peripheral nervous system anatomy, peripheral nerve conditions, and differential diagnosis, since therapists routinely screen for nerve root versus peripheral nerve involvement. USMLE Step 1 approaches the same material through central and peripheral nervous system pathology. Learn the upper versus lower motor neuron distinction first — it anchors nearly every question in this area.
Key takeaways
- The peripheral nervous system includes all nerves and ganglia outside the brain and spinal cord.
- It contains 12 pairs of cranial nerves and 31 pairs of spinal nerves, carrying both afferent and efferent fibers.
- It divides functionally into the somatic division controlling skeletal muscle and the autonomic division controlling involuntary functions.
- Peripheral axons are myelinated by Schwann cells and can regenerate, unlike most central nervous system axons.
- Peripheral lesions cause a lower motor neuron pattern — flaccid weakness, atrophy, and reduced reflexes — which the NPTE and USMLE Step 1 both test.