View source for Mechanoreceptors and stochastic resonance

=== Sensory Mechanoreceptors===

For the skin senses, the primary cutaneous receptors include free nerve endings but more so nerve fibers variously encapsulated to afford specificity and/or amplification. The most common receptor, the Meissner corpuscle that lies just beneath the epidermis, is an elongated fiber encapsulated in connective tissue that responds to low-frequency vibrations but adapts rapidly with a decrease in firing rate following stimulus onset. Pacinian corpuscles, located in dermal layers of the skin as well as skeletal joints, are unmyelinated nerve terminals encapsulated in onion-like layers of cells that act as high-pass filters and contribute to a rapidly adapting, high frequency vibration sensitivity.  At the other end of the spectrum, the slowly adapting Merkel’s disk is a branching pressure-sensitive nerve ending contacting the epidermis. Below the skin are mechanoreceptors of the musculoskeletal system, proprioceptors that provide information on body position. Embedded within and parallel to the somatic muscles are muscle spindle organs, specialized muscle fibers with a central non-contractile nuclear bag encircled by sensory neurons that monitor stretch. Sensitivity is maintained by efferent (motor) feedback to the muscle spindle to take up the slack during passive shortening as the main muscle contracts. Sensory nerve terminals embedded serially in tendons, the Golgi tendon organs, monitor muscle tension.  The tendon organs are low-threshold, rapidly-adapting mechanoreceptors important for initiating spinal reflexes that also control muscle contraction.

The primary sensory function in skin and proprioceptive receptors involves mechanoelectric transduction mechanisms resident in the dendritic terminals of afferent nerve fibers that project directly to the central nervous system. In contrast, mechanotransduction in the auditory, vestibular, and lateral line systems resides in epithelial receptor cells that relay signals synaptically to afferent fibers. The receptor cell common to these systems is the sensory hair cell, so named for the fine ciliary processes that arise from the apical surface of the cell membrane ( <figref>Cilia_model.jpg</figref>, adapted from Holt ''et al''., 1997). A stepwise series of parallel stereocilia, increasing in length until they meet a single, longer kinocilium, form an intricate, polarized transducing system. Displacement of these hairs toward the kinocilium results in an excitatory depolarization of the hair cell whereas motion away from the kinocilium results in hyperpolarization. These voltage changes modulate the release of excitatory neurotransmitter onto the postsynaptic afferent fiber. The structural and molecular mechanisms that underlie mechanotransduction have been studied extensively in hair cells beginning with the frog auditory and vestibular systems, providing the following gating-spring model (Hudspeth, 1985; see also Eatock ''et al''., 2006). The tips of adjacent kinocilia are connected by a filamentous ‘tip link’ such that translation of the ciliary bundle toward the stereocilium imparts a mechanical force that activates cation channels near their tips ( <figref>Cilia_model.jpg</figref>). 
[[Image:cilia model.jpg|thumb|300px|right|f1| ]]Since the apical ciliary surface of both auditory and vestibular hair cells is bathed in a high-potassium concentration endolymph, inward potassium current depolarizes the hair cell and in turn activates voltage-gated calcium channels in the cell soma. The resulting influx of calcium ions facilitates the release of neurotransmitter. This mechanism imparts great sensitivity, with threshold movements of ~ 0.3 nm, and response times on the order of tens of microseconds for receptor potentials. Such rapid responses are characteristic of mechanoelectric transduction in contrast to the slower response of photo- and chemotransduction whose receptors rely heavily on indirect G-protein activated second messenger systems.

The mechanism of hair cell transduction as the basis of multiple sensory organs is therefore a function of mechanical coupling to environmental forces. In the cochlea of the inner ear, hair cells are supported by the basilar membrane with their ciliary bundles bathed in a viscous endolymphatic fluid or in contact with the tectorial membrane. The differential movement of the cochlear membranes, or fluid motions, mediated by the middle ear bones results in shearing forces that bend the cilia. Similarly, within the vestibular labyrinth (see Highstein ''et al''., 2004), hair cells of the utricle and saccule macula (the sensory epithelium) are overlain by an otolithic membrane embedded with calcium carbonate crystals. The greater density of these otoliths imparts a similar shearing force onto the ciliary bundles whenever the head alters its position with respect to gravity or experiences acceleration. In the semicircular canals the hair bundles of the sensory epithelium extend into a compliant gelatinous cupula that occludes the canal. Thus, movements of the endolymphatic fluids deflect the cupula in response to angular acceleration of the head. An equivalent system is found in the lateral line of fish and some amphibians, a fluid-filled skin canal with numerous pores opening to the outside. Here, neuromasts lining the canals of the trunk and head feature ciliary tufts embedded in a gelatinous cupula that extends into the canal. Fluid movements inside the canals deflect the cupula as a result of pressure differences alongside the fish. Additional, superficial neuromasts are scattered over the skin surface of fish and amphibians, as illustrated in a superficial neuromast of the bristlenose catfish, ''Ancistrus'' ( <figref>Neuromast.jpg</figref>, M.H. Hofmann, unpublished; k, kinocilia; s, stereocilia).
[[Image:Neuromast.jpg|thumb|300px|left|f2| ]]
Hair cell morphology highlights a conservative evolutionary feature of cell biology as it relates to sensory systems. Although the mechanically-gated ion channels are located in the kinocilia, somewhat of a misnomer since they lack the complete axoneme of 9 + 2 microtubules characteristic of cilia and flagella, the presence of a kinocilium with its ciliary axoneme and polarizing role in directional sensitivity invites comparison with other sensory receptors that feature non-motile cilia. For example, vertebrate rods and cones arise during development as extensions of a ciliary membrane whose invaginations and vesicles form the outer segments containing photopigments. Olfactory cells also feature cilia, the membranes of which contain the odorant binding receptors and G-proteins that initiate sensory transduction. Likewise, modified ciliary structures are characteristic of many primary mechanosensory neurons where dendrites contain the 9 pairs of ciliary fibrils extending from a ciliary base. Indeed, the cilium arose early on in eukaryotic organisms as a cellular compartment for sensing the environment, as in ciliate protozoans where it functions simultaneously as a sensory antenna and locomotory organelle. For example, a mechanical stimulus applied to the anterior cilia of ''Paramecium'' triggers a spike-like membrane potential and calcium current that reverses the effective stroke of ciliary beating resulting in reversed swimming.