Merkel nerve ending
Encyclopedia
Merkel nerve endings are mechanoreceptor
s found in the skin
and mucosa of vertebrate
s that provide touch information to the brain
. The information they provide are those regarding pressure and texture. 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.
of glabrous and hairy skin, in hair follicle
s, and in oral and anal mucosa. In humans, Merkel cell
s (along with Meissner's corpuscle
s) occur in the superficial skin layers, and are found clustered beneath the ridges of the fingertips that make up fingerprints. In hairy skin, Merkel nerve endings are clustered into specialized epithelial structures called "touch domes" or "hair disks". (Some other types of mechanoreceptors, such as Pacinian corpuscle
s and Ruffini ending
s, are found primarily in subcutaneous tissue
.) Merkel receptors are also located in the mammary glands. Wherever they are found, the epithelium
is arranged to optimize the transfer of pressure to the ending.
s or spikes) to mechanical deflection of the tissue. They are the most sensitive of the four main types of mechanoreceptors to vibration
s at low frequencies, around 5 to 15 Hz.
Because of their sustained response to pressure, Merkel nerve endings are classified as slowly adapting. This is in contrast to Pacinian and Meissner's corpuscles, 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 of 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.
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 field
s 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
).
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.
Merkel Discs have small receptive fields which allow for them to detect fine spatial separation. They also have two point discrimination.
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...
s found in the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
and mucosa of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s that provide touch information to the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. The information they provide are those regarding pressure and texture. Each ending consists of a Merkel cell
Merkel cell
Merkel cells or Merkel-Ranvier cells are oval receptor cells found in the skin of vertebrates that have synaptic contacts with somatosensory afferents. They are associated with the sense of light touch discrimination of shapes and textures. They can turn malignant and form the skin tumor known as...
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
Afferent nerve
In the nervous system, afferent neurons , carry nerve impulses from receptors or sense organs towards the central nervous system. This term can also be used to describe relative connections between structures. Afferent neurons communicate with specialized interneurons...
fibre branches to innervate up to 90 such endings.
They are classified as slowly adapting type I mechanoreceptors.
Location
In mammals, Merkel nerve endings have a wide distribution.Merkel nerve endings are found in the basal layerBasal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
of glabrous and hairy skin, in hair follicle
Hair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s, and in oral and anal mucosa. In humans, Merkel cell
Merkel cell
Merkel cells or Merkel-Ranvier cells are oval receptor cells found in the skin of vertebrates that have synaptic contacts with somatosensory afferents. They are associated with the sense of light touch discrimination of shapes and textures. They can turn malignant and form the skin tumor known as...
s (along with Meissner's corpuscle
Meissner's corpuscle
Meissner's corpuscles are a type of mechanoreceptor. They are a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...
s) occur in the superficial skin layers, and are found clustered beneath the ridges of the fingertips that make up fingerprints. In hairy skin, Merkel nerve endings are clustered into specialized epithelial structures called "touch domes" or "hair disks". (Some other types of mechanoreceptors, such as Pacinian corpuscle
Pacinian corpuscle
Lamellar corpuscles or Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to vibration and pressure. Vibrational role may be used to detect surface, e.g., rough vs...
s and Ruffini ending
Ruffini ending
The Bulbous corpuscle or Ruffini ending or Ruffini corpuscle is a class of slowly adapting mechanoreceptor thought to exist only in the glabrous dermis and subcutaneous tissue of humans...
s, are found primarily in subcutaneous tissue
Subcutaneous tissue
The hypodermis, also called the hypoderm, subcutaneous tissue, or superficial fascia is the lowermost layer of the integumentary system in vertebrates. Types of cells that are found in the hypodermis are fibroblasts, adipose cells, and macrophages...
.) Merkel receptors are also located in the mammary glands. Wherever they are found, the epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
is arranged to optimize the transfer of pressure to the ending.
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 potentialAction potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s or spikes) to mechanical deflection of the tissue. They are the most sensitive of the four main types of mechanoreceptors to vibration
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
s at low frequencies, around 5 to 15 Hz.
Because of their sustained response to pressure, Merkel nerve endings are classified as slowly adapting. This is in contrast to Pacinian and Meissner's corpuscles, 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 of 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.
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 field
Receptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
s than type II fibres. Several studies indicate that type I fibres mediate high resolution tactile discrimination
Tactile discrimination
Tactile discrimination is the ability to differentiate information received through the sense of touch. This is often tested during neurological examination and represents a higher level of neurological function involving the cerebral cortex...
, and are responsible for the ability of our finger tips to feel fine detailed surface patterns (e.g. for reading Braille
Braille
The Braille system is a method that is widely used by blind people to read and write, and was the first digital form of writing.Braille was devised in 1825 by Louis Braille, a blind Frenchman. Each Braille character, or cell, is made up of six dot positions, arranged in a rectangle containing two...
).
Receptive fields
A mechanoreceptor's receptive fieldReceptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
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.
Merkel Discs have small receptive fields which allow for them to detect fine spatial separation. They also have two point discrimination.