Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Cutaneous, Deep, and Visceral Sensation
Other Sensations

Itch and Tickle

Relatively weak stimulation, especially when caused by something moving across the Skin, produces the sensations of itching and tickling. Itchy spots can be identified through detailed skin mapping; much like pain spots, they correspond to areas with a high concentration of unmyelinated free nerve endings. Itching is alleviated by scratching, which activates large, high-conduction afferents that gate impulse transmission in the dorsal columns, much like afferent stimulation inhibits pain (see above). Single-fiber recording experiments have shown that both itching and burning pain persist as long as impulses are conducted exclusively by C fibers, and both cease entirely upon sectioning of the spinothalamic tracts. Naturally, the anatomical distribution of pain and itch differs: itching can occur only in the skin, eyes, and certain mucous membranes, but not in deep Tissues or viscera. Furthermore, low-frequency stimulation of pain fibers elicits pain rather than itch; high-frequency stimulation of itchy skin spots can only increase the intensity of the itch without causing pain. These observations confirm that the C-fiber systems mediating itch and pain are distinct. Interestingly, tickling is perceived as pleasant, itching as annoying, and pain as unpleasant.

Itching can be triggered not only by repeated, localized mechanical irritation of the skin but also by a variety of chemical agents. The sensation of itch can be induced by the application of physiological concentrations of Bile salts to the skin. Histamine causes intense itching, and its release in the skin occurs following injury. However, in the vast majority of cases, histamine is not the underlying cause of the itch; intradermal administration of histamine in doses too low to elicit itching still produces erythema and edema, whereas intense itching frequently occurs in the absence of any visible skin changes. Kinins are potent Inducers of severe itching.

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Fig. 7-9. Schematic representation of various surgical interventions for pain relief: 1 - nerve section; 2 - sympathectomy (for visceral pain); 3 - myelotomy to section spinothalamic fibers in the anterior white commissure; 4 - posterior rhizotomy; 5 - anterolateral cordotomy; 6 - tractotomy within the Medulla Oblongata; 7 - tractotomy within the Midbrain; 8 - thalamotomy; 9 - cingulotomy; 10 - prefrontal lobotomy.

"Synthetic" Sensations

Cutaneous sensations with dedicated individual receptors include Touch, warmth, cold, pain, and possibly itch. The combination of these sensations, Characteristic Features of the stimulus, and, in some instances, cortical components are synthesized into the sensations of vibration, two-point discrimination, and stereognosis.

Vibratory Sensation

When a vibrating tuning fork is applied to the skin, a sensation of tremulousness or buzzing is perceived. This sensation is most pronounced over bony prominences, although it can also be felt at other sites. Such stimuli are detected by touch receptors, notably Pacinian corpuscles, though the temporal factor also plays a crucial role. Rhythmic pressure stimulation is perceived as vibration. The corresponding impulses ascend via the dorsal columns. Degeneration of these Spinal Cord tracts is observed in poorly controlled Diabetes Mellitus, pernicious anemia, certain vitamin deficiencies, and sporadically in other pathological conditions. An elevated vibratory perception threshold is one of the earliest clinical signs of such degeneration. Vibratory sense and Proprioception are closely related modalities; hence, impairment of one is typically accompanied by Impairment of the other.

Two-Point Discrimination

The minimum distance between two tactile stimuli at which they are perceived as separate points is referred to as the two-point threshold. This threshold depends on both peripheral touch receptors and the cortical mechanisms involved in identifying one versus two stimuli. Its value varies across different Regions of the body, being lowest in areas with the highest density of touch receptors. For example, on the back, the distance required for two points to be perceived independently must be at least 65 mm, whereas on the fingertips, it does not exceed 3 mm. On the palms, the two-point threshold roughly corresponds to the receptive field of an individual sensory unit. The peripheral neural mechanisms underlying two-point discrimination are not yet fully understood, particularly given the extensive overlap and interdigitating nature of sensory units.

Stereognosis

The ability to identify objects by touch alone, without visual inspection, is known as stereognosis. Under normal conditions, an individual can easily distinguish everyday objects such as keys or coins of different denominations. This capacity clearly relies on intact touch and pressure sensibility and is impaired by lesions of the dorsal columns. The cortical component is also essential: astereognosis is an early sign of Cerebral Cortex pathology and sometimes occurs in the absence of any deficits in touch and pressure sensation when the lesion involves the parietal lobe posterior to the postcentral gyrus.