Merkel nerve endings (also Merkel's disks, or Merkel tactile endings) are

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into action potential, electrical signals tha ...

s situated in the basal

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

as well as around the apical ends or some

hair follicle The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction betwee ...

s. They are

slowly adapting A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into electrical signals that, in animals, ar ...

. They have small receptive fields measuring some millimeters in diameter. Most are associated with fast-conducting large myelinated axons. A single afferent nerve fibre branches to

innervate A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called ...

up to 90 such endings. Merkel nerve endings respond to light touch. They respond to sustained pressure, and are sensitive to edges of objects. Their exact functions remain controversial. The Merkel nerve endings consist of a nerve ending associated with a flattened epithelial cell (

Merkel cell Merkel cells, also known as Merkel–Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of th ...

); both the nerve ending and Merkel cell are independently mechanosensitive. The Merkel cell expresses the

PIEZO2 Piezo-type mechanosensitive ion channel component 2 is a protein that in humans is encoded by the ''PIEZO2'' gene. It has a homotrimeric structure, with three blades curving into a nano-dome, with a diameter of 28 nanometers. Function Piezos ...

mechanosensitive ion channels; mechanical activation of the channel causes depolarisation of the Merkel cell and consequent release of

serotonin Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...

into a synapse with the associated nerve ending, to also depolarise the later. The nerve ending, meanwhile, expresses an unknown mechanosensitive channel.

Location

Merkel nerve endings have a widely distributed in

glabrous Glabrousness () is the technical term for a lack of hair, down, setae, trichomes, or other such covering. A glabrous surface may be a natural characteristic of all or part of a plant or animal, or be due to loss because of a physical condition, ...

and hairy skin, in

s, and in oral and anal mucosa. Meckel nerve endings are most numerous beneath the ridges of the fingertips which make up fingerprints, and less so in the palms and forearm. They can be found at a depth of 900 μm in human fingertips. In hairy skin, Merkel nerve endings are clustered into specialized epithelial structures called "touch domes" or "hair disks". Merkel receptors are also located in the

mammary glands A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, human ...

. Wherever they are found, the

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

is arranged to optimize the transfer of pressure to the ending.

Functions

Merkel nerve endings provide information on pressure, position, and deep static touch features such as shapes and edges. They are

tactile sensor A tactile sensor is a device that measures information arising from physical interaction with its environment. Tactile sensors are generally modeled after the biological sense of cutaneous receptor, cutaneous touch which is capable of detect ...

s in the business of

mechanotransduction In cellular biology, mechanotransduction ('' mechano'' + '' transduction'') is any of various mechanisms by which cells convert mechanical stimulus into electrochemical activity. This form of sensory transduction is responsible for a number o ...

. They encode surface features of touched objects into perception, but also have to do with

proprioception Proprioception ( ) is the sense of self-movement, force, and body position. Proprioception is mediated by proprioceptors, a type of sensory receptor, located within muscles, tendons, and joints. Most animals possess multiple subtypes of propri ...

Electrophysiology

The Merkel cell's somewhat rigid structure, and the fact that they are not encapsulated, causes them respond to a sustained adequate mechanical stimulus with sustained "firing" of

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s (or spikes): they are

(contrast with the rapidly adapting Pacinian and Meissner's corpuscles). Merkel nerve endings exhibit an initial dynamic vigorous response to sustained mechanical stimulus, with a subsequent the static plateau phase of continuous lower-rate firing that may persist 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.

Sensitivity and receptive fields

Merkel nerve endings are the most sensitive of the four main types of mechanoreceptors to

vibration Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...

s at low frequencies, around 5 to 15 Hz. Merkel nerve endings are extremely sensitive to tissue displacement, and may respond to displacements of less than 1 μm. A mechanoreceptor's

receptive field The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms. Complexity of the receptive field ranges from the unidimensional chemical structure of od ...

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. 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 Braille ( , ) is a Tactile alphabet, tactile writing system used by blindness, blind or visually impaired people. It can be read either on embossed paper or by using refreshable braille displays that connect to computers and smartphone device ...

). Merkel's discs have small receptive fields which allow for them to detect fine spatial separation. They also have two point discrimination.

Eponym

Merkel's discs are named after German anatomist Friedrich Merkel (1845–1919), who was 30 years old when he described them.Merkel FS. (1875). Tastzellen und Tastkörperchen bei den Hausthieren und beim Menschen. Archiv für mikroskopische Anatomie, 11: 636-652.

Location

Functions

Electrophysiology

Sensitivity and receptive fields

Eponym

Diseases

References

External links