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:

• Free nerve endings: These are neurons with no specialized structure. They have axons that extend up into the outer skin (epidermis), with the primary function of sensing temperature extremes (hot and cold), and pain from tissue damage. These neurons are special (called pseudounipolar) in that axons perform the role of both dendrites and axons in a typical neural cell.
• Ruffini's endings or corpuscles: These are embedded deeply in the skin and signal the amount of stretching that is occurring at any moment. They have a sluggish temporal response.
• Pacinian corpuscles: These are small bodies filled with fluid and respond to pressure. Their response is fast, allowing them to sense vibrations (pressure variations) of up to 250 to 350 Hz.
• Merkel's disks: These structures appear just below the epidermis and respond to static pressure (little or no variation over time), with a slow temporal response.
• Meissner's corpuscles: These are also just below the epidermis, and respond to lighter touch. Their response is faster than Merkel's discs and Ruffini's corpuscles, allowing vibrations up to 30 to 50 Hz to be sensed; this is not as high as is possible as the Pacinian corpuscles.
• Hair follicle receptors: These correspond to nerve endings that wrap closely around the hair root; they contribute to light touch sensation, and also pain if the hair is removed.

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:

• Muscle spindles: As the name suggests, these are embedded inside of each muscle so that changes in their length can be reported to the central nervous system (which includes the brain).
• Golgi tendon organs: These are embedded in tendons, which are each a tough band of fibrous tissue that usually connects a muscle to bone. The organs report changes in muscle tension.
• Joint receptors: These lie at the joints between bones and help coordinate muscle movement while also providing information to the central nervous system regarding relative bone positions.

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.