Peripheral mechanosensory receptors
Peripheral mechanoreceptors are small structures found in the skin and other organs that detect displacement, velocity, and acceleration of the skin and internal organs. They provide information about the location, duration, and intensity of a stimulus. The brain perceives these stimuli as pressure, stretching, or vibrations. Mechanoreceptors are vital for a variety of purposes such as social interactions, manual dexterity (grip control and tool use), and locomotion (playing sports).
Functionally, mechanoreceptors can be thought of as dendrites for sensory neurons. Distorting the structure of the mechanoreceptor creates an action potential in the sensory neuron which will terminate in the dorsal horn of the spinal cord. From there, impulses are relayed to the thalamus through either the dorsal column pathway (or posterior column pathway) or the spinothalamic tract. The dorsal column pathway relays fine touch sensations and the spinothalamic tract relays crude touch and pain information. Together, they relay somatosensory information to the brain.
Free Nerve Endings
Free nerve endings are the most copious mechanoreceptors found in the epidermis. They are made up of branched termini of sensory fibers. They typically have minimal or no Schwann cells around their fibers. Free nerve endings are unspecialized and can function as nocireceptors (respond to pain), mechanoreceptors (respond to displacement), or thermoreceptors (respond to temperature). Free nerve endings can function independently in the skin or may be accompanied by accessory structures to form specialized receptors which will be described in more detail below.
Adaptation is the tendency for a receptor to respond less strongly to a sustained stimulus. The rate of adaptation is a characteristic of the unique structural qualities of different sensory receptors. Slowly adapting receptors, those that adapt little to a sustained stimulus, are suited to perceiving an unchanging stimulus such as pressure and stretch. Conversely, rapidly adapting receptors adapt quickly and respond to movement or change of a stimulus. Rapidly adapting receptors perceive vibrations or quick touches. Both will be discussed further in the following paragraphs.
Slowly Adapting Receptors
Merkel Nerve Ending (Merkel’s Discs)
Merkel cells (epithelial tactile cells) are located in the stratum basale of the epidermis in regions of the body requiring highly acute sensitivity. For example, Merkel cells are common in the tips of the fingers. Their small receptive fields make them well suited for responding to fine touch. Desmosomes bind Merkel cells to neighboring keratinocytes. Neurosecretory granules provide an identifying factor when viewed under a microscope. Merkel cells are tightly associated with free nerve endings in the basal lamina. The combination of a Merkel cell and a free nerve ending is referred to as a Merkel nerve ending or disc.
The Merkel disc is sensitive to displacement of the skin; perceived as pressure. Recent research has clarified the function of Merkel cells in this process. Displacement of the skin simultaneously triggers an action potential in the free nerve ending of the disc and the opening of specific membrane channels of the Merkel cell. Neurotransmitters released through the open channels trigger sustained firing of the nerve ending. Without a Merkel cell, displacement of the skin would be perceived as light and rapid tapping rather than pressure.
Ruffini Endings or Corpuscle (Bulbous Corpuscle)
Ruffini endings are small, spindle-shaped, slowly adapting receptors found throughout the dermis, subcutaneous tissue, and some connective tissues. The structure of a Ruffini ending is made up of a single, branching sensory fiber in a thin capsule surrounded by collagen fibers. The collagenous connective tissue crosses the capsule longitudinally. When tension is applied to the ends of the capsule the nerve fiber branches are compressed between the collagen fibers. The inelastic properties of collagen cause the compression of the nerve fibers to be maintained as long as the tension is applied to the capsule. Nerve impulses from these receptors are perceived as stretching.
Rapidly Adapting Receptors
Hair Follicle Receptors
Some free nerve endings surround hair follicles. These are referred to as hair follicle receptors and they sense fine movements of hair on the skin. It is thought that bending the hair distorts the receptor ending wrapped around the hair follicle and generates a potential. Hair follicle receptors are rapidly adapting receptors that respond better to something brushing across the skin rather than pressure. They show variable complexity depending on location. For example, the receptors surrounding the whiskers of a cat are significantly more sophisticated than those on human skin.
Pacinian Corpuscles (Lamellar Corpuscles)
Pacinian corpuscles are large, widespread mechanoreceptors sensitive to mechanical pressure and vibrations. They exist in the dermis (skin) all over the body and within joints, periosteum, connective tissue, and internal organs. Structurally, they consist of a single nerve ending encapsulated in a series of concentric lamellae extending from a Schwann cell. The lamellae of a Pacinian corpuscle are separated by a small space filled with lymph-like fluid. The space may also contain collagen fibrils and some capillaries.
Displacement of the skin compresses the corpuscle and triggers an action potential in the nerve ending. Pacinian corpuscles are rapidly adapting receptors and respond to acceleration, or the onset/cessation of displacement of the skin. This makes them optimized to process brief touches and vibrations. Pacinian corpuscles are incredibly sensitive and can respond to a skin displacement as small as 1 µm.
Meissner’s Corpuscles (Tactile Corpuscles)
Meissner’s corpuscles are found in the dermal papillae of hairless skin. They are characterized by encapsulated nerve endings sensitive to light pressure. Meissner’s corpuscles are thought to be responsible for our ability to manipulate fine objects with precision. For example, they are are found in the finger pads.
Structurally, Meissner’s corpuscles are elongated lamellae with the long axis oriented perpendicular to the skin surface. Once again, the capsule is composed of thin epithelial cells. However, unlike the Pacinian corpuscle that has concentric lamellae, the capsule of a Meissner’s corpuscle consists of a thin layer of epithelial cells surrounding a stack of Schwann cells. One or more nerve fibers reaches the base of the corpuscle and then winds its way between the stacks. When pressure is applied to the skin, the stacks of the corpuscle are compressed and depolarize the nerve ending.
Location of Receptors
Human skin is said to be either hairy or glabrous. Glabrous skin, for example skin on the sole of the foot, is devoid of hairs. Although many mechanoreceptors are found in both glabrous and hairy skin, some are limited to one type. Free nerve endings, being unspecialized, are found throughout the body in both hairy and glabrous skin.
Ruffini endings, Pacinian corpuscles, and Merkel nerve endings are not as involved with fine touch and are also found throughout the body in both skin types. Meissner's corpuscles however respond to light touches and are only found in the glabrous skin in areas of the body that require highly acute sensation such as the eyelids and fingertips. Hair follicle receptors are associated with hair follicles and therefore can only be found in hairy skin. Including the concept that some receptors are only located in glabrous skin vs hairy skin (or both) would also help in terms of knowing their functions.