PNS

The peripheral nervous system (PNS) comprises all neural elements outside the brain and spinal cord, including cranial and spinal nerves, peripheral ganglia, and the neuromuscular junctions that link nerves to muscles and glands. It connects the central nervous system to the body's limbs and organs and participates in reflexes and autonomic regulation.

The PNS is organized into the somatic nervous system, which conveys sensory information from the body to the CNS and conducts voluntary motor commands to skeletal muscles, and the autonomic nervous system, which regulates involuntary functions. The autonomic system includes sympathetic and parasympathetic divisions; the enteric nervous system governs the gut and is often described as part of the autonomic system.

Nerves are bundles of axons that travel outside the CNS and are myelinated by Schwann cells. Dorsal root and autonomic ganglia contain the cell bodies of sensory and autonomic neurons, respectively. Cranial and spinal nerves form regional plexuses such as the cervical, brachial, lumbar, and sacral plexuses.

Functions include transmitting sensory signals to the CNS, carrying motor commands to muscles, and mediating reflexes. Autonomic nerves regulate heart rate, digestion, respiration, and glandular activity. Peripheral nerve injuries can cause sensory loss, weakness, neuropathic pain, and autonomic dysfunction.

Developmentally, peripheral nerves arise from neural crest cells; Schwann cells myelinate PNS axons. Compared with the CNS, the PNS has greater regenerative capacity after injury, though recovery is variable. Common clinical issues include peripheral neuropathies, radiculopathies, carpal tunnel syndrome, and Guillain-Barré syndrome.