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2Somatosensory System

Peripheral Components of the

Somatosensory System and

Peripheral Regulatory Circuits . . . 18

Central Components of the

Somatosensory System . . . . . . . . 39

Central Processing of Somatosensory Information . . . . . . . . . . . 50

Somatosensory Deficits due

to Lesions at Specific Sites along

the Somatosensory Pathways . . . 51

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218

2Somatosensory System

After a preliminary chapter on the structural elements of the nervous system, the discussion of its major functional components and mechanisms now begins with the perceptual processes mediated by receptor organs: as depicted earlier in Figure 1.1, these organs are the site of origin of information flow in the nervous system, in accordance with the basic organizing principle, perception processing response. Somatosensory impulses from the periphery are conducted along an afferent nerve fiber to its neuronal cell body, which lies in a dorsal root ganglion (spinal ganglion). The impulses are then conducted onward into the central nervous system, without any intervening synapses, along the central process (axon) of the same neuron. This axon makes synaptic contact with a second neuron in the spinal cord or brainstem, whose axon, in turn, proceeds further centrally, and crosses the midline to the opposite side at some level along its path. The third neuron lies in the thalamus, the so-called “gateway to consciousness”; it projects to various cortical areas, most importantly the primary somatosensory cortex, which is located in the postcentral gyrus of the parietal lobe.

Peripheral Components of the Somatosensory

System and Peripheral Regulatory Circuits

Receptor Organs

Receptors are specialized sensory organs that register physical and chemical changes in the external and internal environment of the organism and convert (transduce) them into the electrical impulses that are processed by the nervous system. They are found at the peripheral end of afferent nerve fibers. Some receptors inform the body about changes in the nearby external environment (exteroceptors) or in the distant external environment (teleceptors, such as the eye and ear). Proprioceptors, such as the labyrinth of the inner ear, convey information about the position and movement of the head in space, tension in muscles and tendons, the position of the joints, the force needed to carry out a particular movement, and so on. Finally, processes within the body are reported on by enteroceptors, also called visceroceptors (including osmoceptors, chemoceptors, and baroceptors, among others). Each type of receptor responds to a stimulus of the appropriate, specific kind, provided that the intensity of the stimulus is above threshold.

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Peripheral Components of the Somatosensory System and Peripheral Regulatory Circuits · 19	2

Sensory receptor organs are abundantly present in the skin but are also found in deeper regions of the body and in the viscera.

Receptors in the Skin

Most receptors in the skin are exteroceptors. These are divided into two classes: (1) free nerve endings and (2) encapsulated end organs.

The encapsulated, differentiated end organs are probably mainly responsible for the mediation of epicritic sensory modalities such as fine touch, discrimination, vibration, pressure, and so forth, while the free nerve endings mediate protopathic modalities such as pain and temperature. The evidence for this functional distinction is incomplete, however (see below).

Various receptor organs of the skin and its appendages are depicted in Figure 2.1, including mechanoreceptors (for touch and pressure), thermoreceptors (for warm and cold), and nociceptors (for pain). These receptors are located mainly in the zone between the epidermis and the connective tissue. The skin can thus be regarded as a sensory organ that covers the entire body.

Special receptor organs. The peritrichial nerve endings around the hair follicles are found in all areas of hair-bearing skin and are activated by the movement of hairs. In contrast, the tactile corpuscles of Meissner are found only on glabrous skin, particularly on the palms and soles but also on the lips, the tip of the tongue, and the genitals, and respond best to touch and light pressure. The laminated Vater-Pacini corpuscles (pacinian corpuscles) are found in deeper layers of the skin, especially in the area between the cutis and the subcutis, and mediate pressure sensations. The end bulbs of Krause were once thought to be cold receptors, while the corpuscles of Ruffini were thought to be warm receptors, but there is some doubt about this at present. Free nerve endings have been found to be able to transmit information about warmth and cold as well as about position. In the cornea, for example, only free nerve endings are present to transmit information about all of these sensory modalities. Aside from the receptor types specifically mentioned here, there are also many others in the skin and elsewhere whose function mostly remains unclear.

Free nerve endings (Fig. 2.1) are found in the clefts between epidermal cells, and sometimes also on more specialized cells of neural origin, such as the tactile disks of Merkel. Free nerve endings are present, however, not just in the skin but in practically all organs of the body, from which they convey nociceptive and thermal information relating to cellular injury. Merkel’s disks are mainly located in the pads of the fingers and respond to touch and light pressure.

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220 · 2 Somatosensory System

Fig. 2.1

Fig. 2.2

Fig. 2.1 Somatosensory receptors in the skin. a Free nerve ending (pain, temperature). b Tactile disk of Merkel. c Peritrichial nerve endings around a hair follicle (touch). d Tactile corpuscle of Meissner. e Vater−Pacini corpuscle (pressure, vibration). f End bulb of Krause (cold?). g Ruffini corpuscle (warmth?).

Fig. 2.2 Receptors in muscle, tendons, and fascia. a Annulospiral ending of a muscle spindle (stretch). b Golgi tendon organ (tension). c Golgi−Mazzoni corpuscle (pressure).

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