Table of Contents
Mechanoreceptors – Definition
Mechanism of Sensation by Mechanoreceptors
Mechanoreceptors – Definition
Mechanoreceptors, also called mechanoceptors, are specialised neurons that respond to mechanical pressure and distortion. They are innervated with sensory neurons that convert mechanical pressure into electrical impulses that are transmitted to the central nervous system.
They are known to detect various stimuli such as touch, vibration, pressure and sound from both internal and external environments. Mechanoreceptors are located in the deep as well as superficial layers of the bone and the skin.
Types of Mechanoreceptors
Cutaneous Mechanoreceptors
The cutaneous mechanoreceptors are encapsulated neurons that transmit electrical impulses to the central nervous system in response to pressure, touch, vibration and cutaneous stimulation. The four types are:
Meissner’s Corpuscles
Meissner’s corpuscles, also known as tactile corpuscles, are located just beneath the epidermis in palms, fingers and soles. It was discovered by Georg Meisnner and Rudolf Wagner. They are encapsulated myelinated nerve endings that are embedded with Schwann cells. These nerve endings mostly respond to pressure on the skin. The encapsulation is made up of flattened cells arranged in horizontal lamellae that are surrounded by a connective tissue capsule.
The centre of the capsulated structure has a number of afferent nerve fibres that can generate rapid action potentials. They are the most abundant types of mechanoreceptors, accounting for about 40% of the total sensory neurons in a human hand. They are known to transduce signals at low vibration at the frequency of 30-50 Hz, such as when an object is moved across our skin.
Pacinian Corpuscles
Pacinian corpuscles, Vater-Pacini corpuscles or lamellar corpuscles are large nerve endings that are found in both glabrous (non-hairy) and hirsute (hairy) skins, joints and periosteum of bones, and mostly respond to vibrations on the skin. They are sensitive to sudden disturbances and transduce signals at the frequency of 250-320 Hz.
Structurally, the Pacinian corpuscle looks like an onion capsule in which the inner membrane lamella is separated from the outer membrane by a fluid-filled space. A few afferent axons are found in the centre of the capsule. They are encapsulated by a layer of connective tissue, inside which 20-60 constructive lamellae of fibroblast and fibrous connective tissue are found. The fluid is gelatinous in nature of which 92% is water.
They adapt quicker than the Meissner’s corpuscle and have a lower response threshold. They make up about 10-15% of the total cutaneous receptors present in the hand. These corpuscles can respond to vibrations even at a distance of few centimetres
Merkel’s Disks
Merkel’s discs are slow adapting mechanoreceptors that are found in basal layers of hairy and glabrous skin, hair follicles and anal and oral mucosa. They are large myelinated endings that help in identifying mechanical pressure, deep static touch features, position and shapes and edges. The Merkel cells store serotonin in the basal epidermis of the skin which they release in response to pressure.
Merkel discs along with Meissner’s corpuscles are clustered in the fingertips and are less scattered in the palms and forearms. They make up about 25% of the total mechanoreceptors in hand. They are densely found in the lips, fingertips and external genitalia. They can also be seen in mammary glands. Whatever their location, they function to pass the pressure felt to the nerve endings.
Ruffini’s Corpuscles
Ruffini’s corpuscles, Bulbous corpuscles or Ruffini’s endings are another kind of slow-adapting mechanoreceptors that are found deep in the skin, as well as ligaments and tendons. It gets its name after Angelo Ruffini who discovered it. They are spindle-shaped with enlarged dendritic endings and elongated capsules. They account for about 20% of total mechanoreceptors in the human hand and do not elicit any particular tactile sensations.
Other Mechanoreceptors
- Ligamentous mechanoreceptors are myelinated neurons found embedded in the ligaments. They function to transfer sensory information about joint movements to the central nervous system.
- Hair cells are sensory receptors that are important for both spatial and temporal balance of a person as well as the auditory system.
- Baroreceptors are another kind of mechanoreceptors that respond to stretching in blood vessels and send signals to CNS for the proper maintenance of blood pressure.
- Juxtacapillary (J) receptors are located in the alveolar walls of the lung and respond to conditions such as pulmonary embolism, pulmonary edema and pneumonia.
Mechanism of Sensation by Mechanoreceptors
Mechanoreceptors transduce electrical signals using ion channels. The mechanism by which mechanical stimulus is converted into electrical signals is known as mechanotransduction. This sensory transduction is elicited in response to touch, pain, hearing and balance. When one of these sensations is caused, the mechanically gated ion channels in the sensory cells are opened. This allows movement of Na+ ions into the cell, creating a depolarising effect and creating a receptor potential which is then transduced through the network of neurons. Many models of transduction of the electrical signals have been put forward, but it is beyond the scope of your syllabus.
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