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The somatosensory system

Last updated: October 18, 2021

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The somatosensory system is a complex of neural structures involved in the perception, transmission, and processing of external and internal stimuli. It is divided into the following categories: proprioception (sense of the body position and movement in space), exteroception (sensation of external stimuli), and interoception (sensation of internal stimuli). Proprioceptors are localized in the joints, tendons, and muscles, and proprioceptive stimuli are transmitted to the primary somatosensory cortex via the dorsal column pathways. Exteroception can be further subdivided according to the modality of the perceived stimulus into the senses of touch and vibration, which are transmitted via the dorsal column pathways, and nociception (pain sensation) and thermoception (temperature sensation), which are transmitted via the anterolateral system. Interoceptive stimuli are transmitted through the sensory nervous system towards the sensory cortex.

The somatosensory system is a complex system of neural structures that respond specifically to stimuli within or on the surface of the body. It serves three major functions:

  • Proprioception: a sense of body position and movement in space
  • Exteroception: a sense of external stimuli applied to the body
    • Tactile sense
    • Thermoception: the sense of temperature differences inferred from heat flux
    • Nociception: the sense of pain
  • Interoception: a sense of stimuli applied to or arising in the internal organs and structures of the body
Overview of sensory modalities
Modality Function Receptors Pathways

Proprioception

Conscious proprioception
  • Sense of body position and movement in space
  • Sense of strength
Unconscious proprioception
Exteroception Tactile sense
  • Pressure
  • Touch
  • Vibration
Thermoception
  • Heat
  • Cold
  • Anterolateral system
Nociception
Interoception
  • Sensation and integration of information regarding the internal state of the body

The sense of vibration is often first to be affected in disorders that involve the somatosensory system, e.g., polyneuropathies and multiple sclerosis.

Types of receptors

Mechanoreceptors

Overview of receptors involved in exteroception
Type of Mechanoreceptor Receptor location Receptor adaptation Fiber type Type of sensation
Merkel discs
  • Slowly adapting
  • and A-beta
  • Direct pressure
  • Texture
  • Proprioception
  • Static touch (e.g., edges, corners, shapes)
Ruffini corpuscles
  • Stretching of tissue
  • Pressure
  • Changes in joint angles
Meissner corpuscles
Pacinian corpuscles
  • Rapidly adapting
  • Pressure
  • Vibration
Hair plexus
Mechanical nociceptors
  • Variable
    • Slowly adapting
    • Intermediate adapting
    • Rapidly adapting

Mechanoreceptors beginning with “M” (Merkel discs and Meissner corpuscles) are located in the superficial layers of the skin, while those whose name contains “ini” (Ruffini corpuscles and Pacinian corpuscles) are located in the deep layers.

A Delta plane is fast, but a Cab is slow”: A-delta fibers have fast conduction velocity and C fibers have slow conduction velocity.

Proprioceptors

Overview of proprioceptors
Characteristics Muscle spindles Golgi tendon organs Joint receptors
Description
  • Spindle-shaped
  • Consist of intrafusal muscle fibers (a type of muscle fiber located within the muscle spindle that serves as a proprioceptor), nerve fibers, and a surrounding perineural capsule
  • Spindle-shaped
  • Consist of collagen strands, with nerve fibers and perineural capsule in between
Location
  • Pass at the junction between the muscle fibers and tendon
Activation
  • Stretching and contracting of muscle (muscle tension)
    • Ib afferents of Golgi tendon organs transmit information about muscle tension to the spinal cord
    • Stimulation of inhibitory interneurons → inhibition of the α-motor neurons through which they were activated → termination of contraction, partly to prevent over-stretching (Golgi tendon reflex)
      • In comparison to muscle spindles, Golgi tendon organs are only stimulated by more intense stretching.
      • Very intense stretching of muscles does not lead to myotactic reflex (stretch reflex) but rather to the termination of the reflex.
Axons

Types of nerve fibers [1]

Overview of nerve fiber types
Fiber type Information carried Myelinated Diameter Conduction velocity
A-alpha
  • Yes
  • 13–20 μm
  • 80–120 m/s
A-beta
  • Touch
  • 6–12 μm
  • 35–90 m/s
A-delta
  • Pain (mechanical and thermal)
  • Partially
  • 1–5 μm
  • 5–30 m/s
C-fibers
  • Pain (mechanical, thermal, and chemical)
  • No
  • 0.5–1.5 μm
  • 0.5–2 m/s
Overview of sensory pathways
Characteristics Dorsal column Spinothalamic tract Trigeminal lemniscus Ventral spinocerebellar tract Dorsal spinocerebellar tract
Area of sensation
  • Trunk and extremities
  • Face
  • Trunk and extremities
Sensory modalities
First-order neuron
Second-order neuron
Third-order neuron
  • N/A
Target structure
Additional features
  • Crosses the midline after the second-order neuron

Somatosensory cortex
Characteristics Primary somatosensory cortex Secondary somatosensory cortex
Functions
  • Differentiates intensity and type of stimulus
  • Determines stimulus localization
  • Recognition of stimuli
  • Interpretation of stimuli
Location
Structure
Afferent input
Efferent output

Damage of the secondary somatosensory cortex can lead to sensory agnosia, in which a sensory stimulus can be felt but not linked to existing sensory memories and, thus, recognized.

  1. Hall JE. Guyton and Hall Textbook of Medical Physiology. Elsevier ; 2016