The somatosensory system

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

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

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

Types of nerve fibers ^[1]

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.