A mechanoreceptor is a sensory receptor which detects the mechanical stimulus of stretch and distortion. It is also known as the tactile receptor. In human beings, four types of mechanoreceptors are present in the hairless skin: Pacinian corpuscles, Meissner’s corpuscles, Merkel’s cells and Ruffini endings. These types are discussed in detail below.
The afferent neurons carry the signals from the sensory receptors to the dorsal column nuclei, from where the second order neurons carry the message forward to the thalamus. The second order neurons synapse with the third order neurons in the ventrobasal complex. These third order neurons deliver the signals to the somatosensory cortex. The somatosensory cortex sends appropriate signals via the motor neurons.
It has also been suggested that the afferents from the Pacinian corpuscle, Meissner’s corpuscle and the Ruffini ending are directly linked to the muscle activation.
There are receptors present in the hair follicle which detect a change in the position of the hair. Mechanoreceptors are also present in the hair of cochlea which transmits vibration signals to the brain.
Baroreceptors are a type of mechanoreceptors present in the large blood vessels. They detect a rise in the blood pressure and send the signal to the vasomotor center in the brain.
Pacinian corpuscles, also known as lamellar corpuscles, are type II fibers which detect rapid vibrations of 200-300 Hz. They are around 1mm in length with an oval-cylindrical shape. There is an outer capsule present comprising of fibroblasts and fibrous connective tissue which is mainly type II and type IV collagen network.
They are rapidly adapting receptors and have a large receptive field. Any disruption in their lamellae causes opening of the sodium channels creating an action potential. The magnitude of the disruption decides the response. For a greater pressure change, a larger impulse is produced.
However, as mentioned earlier, they adapt very rapidly and the generation of impulses ceases after some time. Pacinian corpuscles are present in the skin and the fascia. The Pancreas also has these corpuscles to detect the vibration changes.
Meissner’s corpuscles, or tactile corpuscles, are type II fibers and respond to light touch. They have the lowest threshold and are sensitive to vibrations of 10-50 Hz. The length of the corpuscle is twice its diameter. It is an encapsulated, unmyelinated nerve ending, where the cells are arranged in the form of horizontal lamellae, surrounded by a capsule of connective tissue.
Any physical deformation of the corpuscle generates and action potential; however, they are also very rapidly adapting nerve endings. These nerve endings are concentrated in the thick hairless skin, such as finger pads and lips. Their number declines with an increase in age.
Merkel’s cells consist of type II fibers and detect sustained pressure and deep static touch. They are slowly adapting, unencapsulated and myelinated nerve endings.
Merkel’s disc or Merkel’s neurite complex refers to a group of Merkel’s cells linked to a single afferent nerve fiber. They have a small receptive field and therefore have the ability of two-point discrimination. They are found in the basal epidermis of the glabrous, as well as hairy skin and are usually lost in case of skin burn.
Ruffini endings or bulbous corpuscles are type II fibers which detect pressure tension deep in the skin and fascia. They are slow adapting, enlarged dendritic endings with an elongated capsule. They are responsible for the detection of angle change of the joints up to 3 degrees. They also detect slippage of an object or sustained holding. They are most abundantly found around the fingernails.
Encapsulated Nerve Ending
Encapsulated nerve endings are specialized structures consisting of non-neural components which enhance their physiological properties. The non-neural component is a cluster of connective tissue surrounding the axon.
Examples of the encapsulated nerve endings are the Meissner’s corpuscle, the Pacinian corpuscle, the Ruffini nerve endings and the neuromuscular junctions.
Unencapsulated Nerve Ending
Unencapsulated nerve endings consist of a terminal neuron which is not surrounded by any other tissue. Free nerve endings and the Merkel’s disc are examples of unencapsulated nerve ending.
Free Nerve Ending (FNE)
A free nerve ending is an unspecialized, unencapsulated nerve ending which usually carries the signal from the body’s periphery to the brain. They are frequently found in the skin where they penetrate the dermis and end in stratum granulosum.
They do not have any complex structure and resemble the branches or roots of a tree. Free nerve endings mostly detect pain. In addition, they may also recognize temperature, mechanical stimulus or danger (nociception). They express polymodality. The fibers are mostly type III or type IV.