![]() ![]() Principles of Neural Science, 4th ed., pp.433. ^ Kandel E.R., Schwartz, J.H., Jessell, T.M.For this reason, Merkel nerve endings and Meissner's corpuscles are most densely clustered in the highly sensitive finger tips, and less so in the palms. The size of mechanoreceptors' receptive fields in a given area determines the degree to which detailed stimuli can be resolved: the smaller and more densely clustered the receptive fields, the higher the resolution. If the two points touched span more than a single receptive field then both will be felt. If the skin is touched in two separate points within a single receptive field, the person will be unable to feel the two separate points. for reading Braille).Ī mechanoreceptor's receptive field is the area within which a stimulus can excite the cell. Several studies indicate that type I fibres mediate high resolution tactile discrimination, and are responsible for the ability of our finger tips to feel fine detailed surface patterns (e.g. Type I afferent fibres have smaller receptive fields than type II fibres. Merkel nerve endings are extremely sensitive to tissue displacement, and may respond to displacements of less than 1 μm. Convexities reduce their rate of firing further still. They fire fastest when small points indent the skin and fire at a low rate on slow curves or flat surfaces. The inter-spike intervals during sustained firing are irregular, in contrast to the highly regular pattern of inter-spike intervals obtained from slowly adapting type II mechanoreceptors. Firing during the static phase can continue for more than 30 minutes. In mammals, electrical recordings from single afferent nerve fibres have shown that the responses Merkel nerve endings are characterized by a vigorous response to the onset of a mechanical ramp stimulus (dynamic), and then continued firing during the plateau phase (static). This is in contrast to Pacinian corpuscles (rapidly adapting receptors which respond only to the onset and offset of mechanical deflection, and to higher frequency vibrations. They are the most sensitive of the four main types of mechanoreceptors to vibrations at low frequencies, around 5 to 15 Hz.īecause of their sustained response to pressure, Merkel nerve endings are classified as slowly adapting. Their somewhat rigid structure, and the fact that they are not encapsulated, causes them to have a sustained response (in the form of action potentials or spikes) to mechanical deflection of the tissue. Wherever they are found, the epithelium is arranged to optimize the transfer of pressure to the ending. (Some other types of mechanoreceptors, such as Pacinian corpuscles and Ruffini endings, are found primarily in subcutaneous tissue.) In birds, Merkel receptors are located in the mammary glands. In humans, Merkel cells (along with Meissner's corpuscles) occur in the superficial skin layers, and are found clustered beneath the ridges of the fingertips that make up fingerprints. Merkel nerve endings are found in the basal layer of glabrous and hairy skin, in hair follicles, and in oral and anal mucosa. In mammals, Merkel nerve endings have a wide distribution. ![]()
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