What part of the body does the peripheral nervous system refer to?

The Peripheral Nervous System: Structures Outside the Brain and Spinal Cord

The peripheral nervous system (PNS) refers to all nervous structures located outside the brain and spinal cord (central nervous system), including cranial nerves, spinal nerves, and their branches that extend throughout the body to the extremities. 1, 2

Anatomical Components of the PNS

The peripheral nervous system consists of:

• Cranial nerves: 12 paired nerves that emerge directly from the brain and brainstem, providing sensory and motor innervation primarily to the head and neck region 3

• Spinal nerves: Nerves that emerge from the spinal cord through the intervertebral foramina, forming major plexuses:

  • Brachial plexus: Formed from C5-T1 ventral rami (sometimes including C4 and/or T2), supplying the upper extremities 3
  • Lumbosacral plexus: Comprised of lumbar (L1-L4) and sacral (L4-S4) plexuses, supplying the lower extremities 3

• Peripheral nerves: The extensions and branches of spinal and cranial nerves that innervate muscles, skin, and organs throughout the body 4

Functional Components of the PNS

The PNS can be functionally divided into:

1. Sensory (afferent) division: Transmits sensory information from the periphery to the CNS

   • General sensory: Provides tactile, pain, temperature sensations 3
   • Special sensory: Includes smell, vision, taste, hearing, and balance 3
   • Visceral sensory: Supplies sensory input from internal organs 3

2. Motor (efferent) division: Carries signals from the CNS to effector organs

   • Somatic motor: Innervates skeletal muscles 3
   • Autonomic motor: Further divided into sympathetic and parasympathetic systems that innervate viscera, glands, and smooth muscle 3, 5

Clinical Significance

Understanding the PNS is crucial for diagnosing and managing various neurological conditions:

• Peripheral neuropathies: Can be classified as 1:

  • Mononeuropathy: Affecting a single nerve (usually from compression or trauma)
  • Multiple mononeuropathy: Affecting multiple individual nerves (often indicating systemic disease)
  • Polyneuropathy: Diffuse and symmetric involvement (commonly from metabolic or toxic causes)

• Plexopathies: Abnormal neurological symptoms localizing to a nerve plexus, manifesting as neuropathic pain, dysesthesia, weakness, and sensory loss in regions innervated by multiple nerves 3

• Cranial neuropathies: Dysfunction of cranial nerves that can result from various pathologies including trauma, neoplasm, inflammatory, infectious, or autoimmune etiologies 3

Diagnostic Approaches

When evaluating PNS disorders, diagnostic tools include:

• Electrodiagnostic studies: Essential for differentiating between patterns of nerve involvement 1

  • EMG (electromyography) for assessing muscle fiber action potentials
  • Nerve conduction studies for evaluating nerve function

• Imaging: MRI is the standard modality for imaging neural plexuses and peripheral nerves 3

  • MR neurography: High-resolution T2-weighted sequences of peripheral nerves
  • Should include orthogonal views with T1-weighted, T2-weighted, and fat-saturated sequences

Key Distinctions from the CNS

While the PNS and CNS share many similarities in neuronal structure and function, key differences include 5:

• Glial cell types: PNS contains Schwann cells rather than oligodendrocytes for myelination
• Regenerative capacity: PNS has greater potential for regeneration than CNS
• Absence of blood-brain barrier: PNS lacks the protective blood-brain barrier present in the CNS
• Different modes of synaptic connections

Understanding these distinctions is important for properly diagnosing and treating neurological conditions affecting different parts of the nervous system.