Somatosensation

During somatosensation, the sense of touch, sensory neurons within the skin first detect mechanical disturbance via mechanoreceptors.

If the stimuli are light vibrations, Meissner's corpuscles respond. With an added constant presence, Merkel discs react, since they adapt slowly. Ruffini endings, also slow to adapt, respond to deeper, sustained stimuli. If this becomes repetitive, it is detected by Pacinian corpuscles.

Following detection, sensations are transduced into electric signals, and travel towards the spinal cord, specifically the dorsal root ganglion, where processes from the same area of the body, called a dermatome, make up a single dorsal root. Continuing on to the brain stem, the medulla in particular, the sensory information decussates, meaning it crosses over to the other side before being sent to the thalamus. From here, the message is finally sent to the somatosensory cortex.

This cortical region contains a map of the body called a homunculus. Because of decussation, the left side of the body is represented in the right cortex and vice versa.

Fingers occupy a large cortical area because they have a high density of mechanoreceptors with small receptive fields, which gives them better stimulus discrimination. In contrast, the back has fewer receptors and larger receptive fields, and thus a smaller cortical representation.