The somatosensory system

The body senses provide signals to the brain about the human body itself, including direct contact with the skin, the body's configuration and movement in the world, and the ambient temperature. Within this category, the vestibular system (Section 8.2) handles balance, and the somatosensory system handles touch, proprioception, and kinesthesis. Consider the human body and all of its movable parts, such as the legs, arms, fingers, tongue, mouth, and lips. Proprioception corresponds to the awareness of the pose of each part relative to others, whereas kinesthesis is the counterpart for the movement itself. In other words, kinesthesis provides information on velocities, accelerations, and forces.

Figure 13.1: Six major kinds of receptors in human skin. (Figure by Pearson Education.)

The somatosensory system has at least nine major kinds of receptors, six of which are devoted to touch, and the remaining three are devoted to proprioception and kinesthesis. Figure 13.1 depicts the six main touch receptors, which are embedded in the skin (dermis). Their names, structures, and functions are:

The first four of these receptors appear in skin all over the body. Meissner's corpuscles are only in parts where there are no hair follicles (glabrous skin), and the hair follicle receptors obviously appear only where there is hair. In some critical places, such as eyelids, lips, and tongue, thermoreceptors called the end-bulbs of Krause also appear in the skin. Yet another class is nocireceptors, which appear in joint tissues and cause a pain sensation from overstretching, injury, or inflammation.

Touch has both spatial and temporal resolutions. The spatial resolution or acuity corresponds to the density, or receptors per square area, which varies over the body. The density is high at the fingertips, and very low on the back. This has implications on touch perception, which will be covered shortly. The temporal resolution is not the same as for hearing, which extends up to 20,000 Hz; the Pacinian corpuscles allow vibrations up to a few hundred Hertz to be distinguished from a static pressure.

Regarding proprioception (and kinesthesis), there are three kinds of receptors:

Through these receptors, the body is aware of the relative positions, orientations, and velocities of its various moving parts.

The neural pathways for the somatosensory system work in a way that is similar to the visual pathways of Section 5.2. The signals are routed through the thalamus, with relevant information eventually arriving at the primary somatosensory cortex in the brain, where the higher-level processing occurs. Long before the thalamus, some of the signals are also routed through the spinal cord to motor neurons that control muscles. This enables rapid motor response, for the purpose of withdrawing from painful stimuli quickly, and for the knee-jerk reflex. Inside of the primary somatosensory cortex, neurons fire in a spatial arrangement that corresponds to their location on the body (topographic mapping). Some neurons also have receptive fields that correspond to local patches on the skin, much in the same way as receptive fields works for vision (recall Figure 5.8 from Section 5.2). Once again, lateral inhibition and spatial opponency exist and form detectors that allow people to estimate sharp pressure features along the surface of the skin.

Steven M LaValle 2020-01-06