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Encyclopedia > Merkel nerve ending

Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that provide touch information to the brain. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal. This is sometimes referred to as a Merkel cell-neurite complex, or a Merkel disk receptor. A single afferent nerve fibre branches to innervate up to 90 such endings. They are classified as slowly adapting type I mechanoreceptors. A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. ... Diagram of the layers of human skin In zootomy and dermatology, skin is an organ of the integumentary system composed of a layer of tissues that protect underlying muscles and organs. ... The mucous membranes (or mucosa) are linings of ectodermic origin, covered in epithelium, that line various body cavities and internal organs. ... Subgroups †Conodonta Hyperoartia   Petromyzontidae (lampreys) †Pteraspidomorphi †Thelodonti †Anaspida †Cephalaspidomorphi   †Galeaspida   †Pituriaspida   †Osteostraci Gnathostomata (jawed vertebrates)   †Placodermi   Chondrichthyes (cartilaginous fish)   †Acanthodii   Osteichthyes (bony fish)     Actinopterygii (ray-finned fish)     Sarcopterygii (lobe-finned fish)       Actinistia (coelacanths)       Dipnoi (lungfish)       Tetrapoda (four-limbed vertebrates)         Amphibia (amphibians)         Amniota (amniotic embryo)           Sauropsida (reptiles)             Aves (birds)           Synapsida (mammal... This article needs to be cleaned up to conform to a higher standard of quality. ... Comparative brain sizes In animals, the brain, or encephalon (Greek for in the head), acts as the control center of the central nervous system. ... Merkel cells are large oval cells found in the skin of vertebrates. ... In the nervous system, afferent neurons, otherwise known as sensory or receptor neurons carry nerve impulses from receptors or sense organs towards the central nervous system. ...

Contents


Location

In mammals, Merkel nerve endings have a wide distribution. Merkel nerve endings are found in the basal layer of glabrous and hairy skin, in hair follicles, and in oral and anal mucosa. 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. (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 dermis. Wherever they are found, the epithelium is arranged to optimize the transfer of pressure to the ending. Orders Subclass Multituberculata (extinct) Plagiaulacida Cimolodonta Subclass Palaeoryctoides (extinct) Subclass Triconodonta (extinct) Subclass Eutheria (includes extinct ancestors)/Placentalia (excludes extinct ancestors) Afrosoricida Artiodactyla Carnivora Cetacea Chiroptera Cimolesta (extinct) Creodonta (extinct) Condylarthra (extinct) Dermoptera Desmostylia (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Litopterna (extinct) Macroscelidea Mesonychia (extinct) Notoungulata (extinct) Perissodactyla Pholidota Plesiadapiformes... The basal lamina (often erroneously called basement membrane) is a layer on which epithelium sits. ... A hair follicle is part of the skin that grows hair by packing old cells together. ... Merkel cells are large oval cells found in the skin of vertebrates. ... Meissners corpuscles (discovered by the anatomist Georg Meissner (1829-1903) are a type of mechanoreceptor and more specifically, a tacticle corpuscle (corpusculum tactus). ... A Pacinian corpuscle is a structure that functions as a mechanoreceptor. ... The subcutis is the layer of tissue directly underlying the cutis. ... Orders Many - see section below. ... The dermis is the layer of skin beneath the epidermis, that consists of connective tissue and cushions the body from stress and strain. ... In zootomy, epithelium is a tissue composed of a layer of cells. ...


Functions

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. They are the most sensitive of the four main types of mechanoreceptors to vibrations at low frequencies, around 5 to 15 Hz. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Biological tissue is a group of cells that perform a similar function. ... See Oscillator (disambiguation) for particular types of oscillation and oscillators. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...


Because of their sustained response to pressure, Merkel nerve endings are classified as slowly adapting. This is in contrast to (rapidly adapting receptors which respond only to the onset and offset of mechanical deflection, and to higher frequency vibrations.


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). Firing during the static phase can continue for more than 30 minutes. 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.


They fire fastest when small points indent the skin and fire at a low rate on slow curves or flat surfaces. Convexities reduce their rate of firing further still (Kandel et al., 2000).


Merkel nerve endings are extremely sensitive to tissue displacement, and may respond to displacements of less than 1 μm. Type I afferent fibres have smaller receptive fields than type II fibres. 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. for reading Braille). Receptive fields are areas of the retina, producing a change in the firing of cells in the visual system. ... PREMIER - first The braille system, named after Louis Braille, is a method that is widely used by blind people to read and write. ...


Receptive fields

A mechanoreceptor's receptive field is the area within which a stimulus can excite the cell. 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. If the two points touched span more than a single receptive field then both will be felt. 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. 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. Receptive fields are areas of the retina, producing a change in the firing of cells in the visual system. ...


References

  • Iggo, A. and Muir, A. R. (1969) "The structure and function of a slowly adapting touch corpuscle in hairy skin". Journal of Physiology (London) 200:763-796.
  • Kandel E.R., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science, 4th ed., pp.433. McGraw-Hill, New York.

 
 

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