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Cutaneous Mechanoreceptors lation. Using this experiment, the threshold for the receptor may be found as the lowest stimulus intensity, i.e. degree of skin indentation, that evokes an action potential from one cycle of a sinusoidal stimulus.

Cutaneous receptors can be studied in humans using the technique of micro-neurography, in which a fine metal electrode is inserted percutaneously (i.e. through the skin) into a nerve, e.g. the radial nerve in the wrist, and recordings made from axons. The axons are stimulated electrically or a natural stimulus applied within the neuron's receptive field.

The modality of touch is mediated by skin mechanoreceptors. These may be slowly or rapidly adapting to a constantly applied stimulus. Rapidly adapting mechanoreceptors may respond at the onset and possibly at the offset of the stimulus, while slowly adapting mechano-receptors continue to respond throughout the duration of the stimulus.

Skin may be glabrous, i.e. smooth and hairless, or hairy, and this determines the distribution of mechanoreceptors. Each hair is innervated by a hair follicle receptor. In glabrous skin, the two main receptors are the rapidly adapting Meissner's receptor, and the slowly adapting Merkel receptor, also referred to as a 'cell' or 'disk'. Merkel's cells also occur in hairy skin. Both receptors have a relatively small receptive field of 2-4 ^m. Therefore these receptors have a fine power of resolution, especially at the fingertips, where innervation is dense. The receptive field may be defined as the area of skin within which the adequate stimulus for a sensory receptor excites the sensory neuron. There are also free nerve endings that act as mechanoreceptors.

Below the skin, in the subcutaneous tissues, lie two other mechanoreceptors, namely the rapidly adapting pacinian corpuscle, and the slowly adapting Ruffini's corpuscle. These two receptors have relatively large receptive fields. In contrast to the Meissner's corpuscle and Merkel cell, spatial resolution is relatively coarse.

The spatial and temporal resolving power of the mechanoreceptors can be measured using a sinusoidal oscillating pattern of skin indentation. It has been found that pacinian corpuscles, which occur in the deeper subcutaneous layer, respond to higher frequency stimuli, and may be more sensitive to applied stimuli, while the more superficial Meissner's corpuscles respond to lower frequency stimu-

Greenstein, Color Atlas of Neuroscience © 2000 Thieme

Merkel's celL

bare nerve_^

ending hair, receptor unmyelinated, nerve blood supply, to hair follicle hairy skin.

glabrous skin /

hairy skin.

glabrous skin /

blood supply, to hair follicle

Cutaneous Mechanoreceptors Glabrous

subpapillary plexus paciniai corpuscle

Meissner's corpuscles in glabrous skin Merkel's cells in glabrous and hairy skin free nerve endings in both skin types cutaneous receptors paciniai corpuscle

-papillary ridge

-epidermis dermis Meissners corpuscle myelinated subpapillary plexus

Meissner's corpuscles in glabrous skin Merkel's cells in glabrous and hairy skin free nerve endings in both skin types

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