CorpusHuman Anatomy
NervousNerve group

Somatic nervous system

Also known as: voluntary nervous system, SNS (somatic)

The part of the PNS serving the body wall and limbs: somatic sensation from skin, muscles and joints, and voluntary control of skeletal muscle through single lower motor neurons.

Overview

Neurons from CNS to effector1 (lower motor neuron)
Transmitter / receptorACh / nicotinic (N_M)

Somatic afferents carry exteroceptive sensation (touch, pressure, vibration, pain, temperature) and proprioceptive information (muscle spindles, Golgi tendon organs, joint receptors) through dorsal roots and cranial sensory ganglia. Somatic efferents are the axons of lower motor neurons (α and γ) in the ventral horn and the cranial motor nuclei. Each runs without interruption from the CNS to the muscle, releases acetylcholine onto nicotinic (N_M) receptors at the neuromuscular junction, and is always excitatory.

This contrasts with the autonomic efferent pathway, which uses a two-neuron chain with a peripheral ganglion, can excite or inhibit, and targets smooth muscle, cardiac muscle and glands. Somatic control can be conscious and voluntary, but much of it is reflex (stretch, withdrawal) or automatic (posture, breathing, gait).

Parts & subdivisions

  1. Somatic afferents
    Cell bodies in dorsal root and cranial sensory ganglia; myelinated Aα/Aβ (proprioception, touch) and Aδ/C (pain, temperature) fibres.
  2. Somatic efferents
    α motor neurons to extrafusal fibres (force) and γ motor neurons to intrafusal spindle fibres (sensitivity). One motor neuron and the fibres it supplies form a motor unit.
  3. Somatic vs autonomic efferents
    Somatic: 1 neuron, ACh → nicotinic N_M, always excitatory, skeletal muscle, heavily myelinated (fast). Autonomic: 2 neurons (preganglionic thinly myelinated B fibre, postganglionic unmyelinated C fibre), ACh or noradrenaline, excitatory or inhibitory, smooth and cardiac muscle and glands.

Function

  • Voluntary and reflex contraction of skeletal muscle
  • Conscious perception of touch, pain, temperature and limb position
  • Provides the sensory and motor limbs of somatic reflexes
Physiology

Force is graded by recruiting motor units in order of size (Henneman size principle: small, fatigue-resistant units first) and by increasing their firing rate (rate coding). The γ-motor system keeps spindles taut during contraction (α–γ co-activation).

Clinical significance

Neuromuscular blocking drugs

Drugs acting at the somatic N_M receptor paralyse skeletal muscle during anaesthesia while largely sparing the autonomic ganglionic (N_N) receptor:
• non-depolarising competitive antagonists (rocuronium, vecuronium) are reversed by neostigmine or sugammadex;
• the depolarising agent suxamethonium causes fasciculations and then a phase I block, and can cause hyperkalaemia and malignant hyperthermia.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.