Indentation or lateral stretch of the skin is believed to excite mechanoreceptors by direct gating of cation channels in the sensory nerve ending. Mechanical stimulation deforms the receptor protein, thus opening stretch-sensitive ion channels, and increasing Na+ and Ca2+ conductance. The resulting inward current through these channels produces local depolarisation of the nerve or receptor cell called the receptor potential. The amplitude of the receptor potential is proportional to the amount of pressure exerted by the object and how fast it is applied. Removal of the pressure stimulus relieves mechanical stretch on the receptor and allows stretch-sensitive channels to close. Direct activation of mechanoreceptive ion channels permits rapid activation and inactivation as forces are applied to the skin.
The molecular biology of mechanoreception in the mammalian skin is not well understood, primarily because it is difficult to isolate the receptors from other cell types in this tissue. Various mechanisms for mechanotransduction have been proposed in other sensory systems. The most widely accepted hypothesis for mechanically gated channels involves structural proteins that link the channel to the surrounding tissue of the skin and to the cytoskeleton.
The channel subunits are tied to extracellular matrix elements such as collagen or other filamentous proteins that are stretched by mechanical deformation; these links are often represented as a spring. The intracellular portion of the channel is anchored to the cytoskeleton most likely by actin filaments. Other models propose indirect activation of the ion channel through second-messenger pathways. In the indirect model, the force sensor is a protein in the receptor cell’s membrane distinct from the ion channel. Stimulation of these receptors by a mechanical stimulus is conveyed to the ion channel by a variety of intracellular messengers that cause the channel to open or close.
Studies of touch receptors in Caenorhabditis elegans suggest that the transduction proteins in mammals may belong to the degenerin/epithelial Na+ channel (DEG/ENaC) superfamily. Other studies have implicated various transient receptor potential (TRP) cation channels in mechanotransduction. Mechanosensory channels in individual receptors differ somewhat in their structural linkage to the matrix elements, and these molecular properties modify the channel open times. Thus some mechanoreceptors yield slowly adapting, sustained depolarization to pressure, whereas others inactivate rapidly. The molecular diversity of gating and adaptation mechanisms is also expressed in the morphology of the capsules surrounding the sensory nerve terminal.
PACINIAN CORPUSCLE IS DESIGNED TO DETECT VIBRATION
The role of the non-neural capsule in sensory transduction has been studied extensively in the Pacinian corpuscle, a mechanoreceptor located in the subcutaneous tissue and in the mesentery of the abdominal wall. The Pacinian corpuscle consists of a 1-mm long, multilamellar, fluid-filled capsule that encloses the sensory ending of a primary afferent fiber. The nerve loses its myelin sheath inside the capsule; its naked endings contain mechanosensory channels sensitive to compression.
When the skin is touched, or a probe is applied experimentally to the capsule, the lamellar structure filters the stimulus so that only rapid displacements are transmitted to the nerve terminal. If the probe is vibrated at frequencies of 200 Hz, the capsule is sufficiently stiff that it cannot change shape as fast as the probe moves. Therefore, all of the lamellae move up and down together in phase with the probe, compressing and decompressing the nerve. Each vibratory cycle evokes a brief depolarizing response in the sensory nerve that is sufficiently intense to generate an action potential.
However, if the vibratory frequency is slowed to 20 Hz, the displacement is so slow that the upper layers of the capsule are squeezed together, displacing the fluid laterally within the capsule, whereas the bottom layers close to the nerve remain rigid. The nerve is unresponsive to the low-frequency stimulus because the energy is not transmitted by the capsule to the mechanosensory channels.
This physical structure endows the Pacinian corpuscle with exquisite sensitivity to vibration in the range of 100–400 Hz. It is the most sensitive mechanoreceptor in the body and can capture signals from a wide area of skin because of its large size. Humans are able to detect vibrations as weak as 1 mm in amplitude when tested at 250 Hz.
The hum of motors on a computer disc drive that one perceives with the hand, or the vibrations felt in the concert hall during forte passages played by a symphony orchestra, are detected by the Pacinian corpuscles. These receptors can also sense the weak shock waves transmitted to a grasped object when it is placed on a rigid surface and released. This type of sensory feedback is particularly useful in controlling the actions of the hand during skilled movements and when using tools.
RECEPTIVE FIELDS OF MECHANORECEPTORS
Individual Meissner corpuscle and Merkel disc receptors are smaller than a fingerprint ridge. Their morphology enables them to detect displacements localized to the ridge in which they reside. However, the primary afferent fiber that transmits this information to the brain detects touch over a larger region of skin, called its receptive field, because it has 10–25 terminals, each enclosed by a Meissner corpuscle or Merkel cell. This arrangement allows the nerve to sample the activity of multiple receptors of the same class, while also resolving fine details.
The receptive field of mechanosensory neurons labels the tactile information conveyed to the brain with a specific topographic location (Figure). Thus the brain knows where touch occurs by determining which receptors are activated. The regions of the body that are used most extensively to touch other persons or things – the fingertips and lips – have the largest number of receptor organs in the skin and the smallest receptive fields; the ability to localize touch is highest here. The more proximal regions of the body – the arm, leg, and trunk – are less densely innervated and have fewer receptors. These areas have large receptive fields and do not resolve fine spatial details.
The specificity of the spatial information transmitted from each receptor is also essential for perceiving the size, shape, and texture of the object. Mechanoreceptors subdivide the object that is touched into small regions and analyze the local curvature profile. The collective activity in the population of stimulated receptors indicates the total area of skin that is touched, and the profile of skin indentation defines the object’s shape. The brain must reassemble the individual parts to construct a unified percept of the object.