The somatosensory system relays sensory information from the skin, mucous membranes, limbs, and joints. Somatosensation is more familiarly known as the sense of touch. A typical somatosensory pathway includes three types of long neurons: primary, secondary, and tertiary. Primary neurons have cell bodies located near the spinal cord in groups of neurons called dorsal root ganglia. The sensory neurons of ganglia innervate designated areas of skin called dermatomes.
In the skin, specialized structures called mechanoreceptors transduce mechanical pressure or distortion into neural signals. In hairless skin, most disturbances can be detected by one of four types of mechanoreceptors. Two of these, Merkel disks and Ruffini endings, are slow-adapting and continue to respond to stimuli that remain in prolonged contact with the skin. Merkel disks respond to light touch. Ruffini endings detect deeper static touch, skin stretch, joint deformation, and warmth.
The other two major cutaneous mechanoreceptors, Meissner corpuscles and Pacinian corpuscles, are rapidly-adapting. These mechanoreceptors detect dynamic stimuli, like those required to read Braille. Meissner corpuscles are responsive to delicate touch and pressure, as well as low-frequency vibrations. Pacinian corpuscles respond best to deep, repetitive pressure and high-frequency vibrations. Information detected by these mechanoreceptors is propagated towards the cell body in the dorsal root ganglion.
Primary neurons from the dorsal root ganglia extend axons into the spinal cord, continuing the propagation of somatosensory information from the body to the brain. The axons terminate in the medulla, where they synapse, or communicate, with secondary neurons. At this point, the signal has remained ipsilateral, on the same side of the body that initially detected the stimulus. Secondary neurons, though, have axons on the opposite side of the medulla and decussate (cross) the information. Thus, information detected on the left side of the body is initially processed in the right hemisphere of the brain. From the opposite side of the medulla, the axons from secondary neurons continue up to the thalamus, where they synapse with tertiary neurons. Tertiary neurons have axons that terminate in the somatosensory cortex.
Each part of the body, to some degree, is represented in this cortical area on a somatosensory map called a homunculus. Body areas with a higher density of mechanoreceptors, like the fingertips, have greater representations in the cortex than areas with a lower mechanoreceptor density, such as the palms and arms.
When a particular bodily region does not function as intended, the sensory cortex can undergo cortical reorganization. For instance, Braille readers have larger finger representations in the somatosensory cortex than individuals who cannot read Braille. In forearm amputees, some data suggests that the cortical region previously connected to the amputated arm can be remapped to the adjacent cortical region (in this case, the face). This can cause phantom limb experiences, in which an amputee feels stimulation from the missing arm when certain areas of the face are stimulated.
