Curious About The Receptors In Your Hands, Fingertips And Their Roles?
The active touching, or haptics, helps us interpret complex spatiotemporal patterns of stimuli that are likely to activate many classes of mechanoreceptors. Stereognosis, is the process of manipulating an object with the hand, enough to provide information to identify the object.
Merkel cell afferents are slowly adapting fibers and account for 25% of the mechanosensory afferents in the hand. Particularly enriched in the fingertips, and are the only afferents to sample information from the receptor cells located in the epidermis. Merkel cell–neurite complexes are located in the tips of the primary epidermis ridges. These cells sense mechanical stimuli, pressure. Individual Merkel afferents can resolve spatial details of 0.5 mm. Thus, highly sensitive to points, edges, and curvature. In conclusion these cells are ideally suited for processing information about shape and texture.
Meissner afferents are rapidly adapting fibers that innervate the skin more densely than Merkel afferents and thus accounting for about 40% of mechanosensory innervation in the human hand. Known to be more than four times as sensitive to skin deformation as Merkel afferents. However, their receptive fields are larger than those of Merkel afferents leading to their transmitted signals having reduced spatial resolution. Meissner corpuscles are quite efficient in transducing information between 3-40 Hz, which is a low frequency that occurs when textured objects are moved across the skin.
It has also been suggested that Meissner afferents are responsible for detecting slippage between the skin and an object, thus giving the essential feedback information for proper grip in response to the situation.
Pacinian afferents are rapidly adapting fibers that make up 10-15% of the mechanosensory innervation of the hand, located deep in the dermis or in the subcutaneous tissue. Detecting frequencies between 250-350 Hz, adapt more rapidly than Meissner corpuscles and have a lower response threshold. The Pacinian afferents generate action potentials from displacement occurring as small as 10 nanometers. With large receptive fields, they are well suited to detect vibrations transmitted through objects, which is an important property needed when using a wrench, knife or when writing.
Ruffini afferents are slowly adapting fibres and are the least understood. The Ruffini corpuscles are particularly sensitive to cutaneous stretching produced by movements i.e limbs and account for 20% of the mechanoreceptors in the hand. Generally thought to be responsive to skin stretches i.e providing accurate information in regards to the confirmation of the hand and finger positions.
Each of these afferent types are activated by stimulations, but the information supplied by each are different. As a summary, the Merkel afferents recognize the detailes of a texture, Meissner afferents supply a slightly coarser version of this pattern. The details are lost in the response of Pacinian and Ruffini afferents, since they have more to do with tracking movement and position of the finger than with the specific identity of an object. . Microgeometries of different textures produce different patterns of vibrations on the skin and thus are best detected by rapidly adapting afferents.
Moving on to the afferents that innervate the hair follicles in hairy skin, include Merkel cell afferents having touch domes associated with the apical collars of hair follicles, circumferential endings, and longitudinal lanceolate derived from Aβ, Aδ, or C fibers forming rapidly adapting and low threshold mechanoreceptors. Longitudinal lanceolate endings, shape a palisade around the follicle, sensitive to anything causing a deflection to the hair follicles. Lanceolate endings are proven to be important for responding to sensual touch i.e caress.
The Book; Neuroscience Sixth Edition, Dales Purves, George J. Augustine
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