|Satomi Ebara et al. (2017), Scholarpedia, 12(3):32372.||doi:10.4249/scholarpedia.32372||revision #181901 [link to/cite this article]|
Most mammals possess rows of whiskers (i.e., vibrissae) on both sides of the face arranged in an orderly grid on the mystacial pad. The vibrissal shafts of rat and cat range from 0.1 to 0.2 mm in diameter; each vibrissa is emitted from an elaborately structured follicle termed the Follicle Sinus Complex (FSC) by Rice and Munger (1986). Each follicle is surrounded by two large blood sinuses, the ring and cavernous sinuses, hence the term “sinus hair” for the mystacial vibrissae (Fig.1, left panel).
The follicle and sinuses are surrounded by a thick collagenous capsule covered by a layer of skeletal muscle that functions as arrector pili. Particularly germane to this report is the dense innervation of the follicle by a large variety of nerve fibers provided by sensory neurons of the trigeminal ganglia (Fig. 1, inset).
The trigeminal primary sensory neurons within the ganglia are pseudo-unipolar cells; the tips of their peripheral processes terminate within the follicles of the mystacial vibrissae as mechanosensory receptors (Vincent, 1913). Mechanical interactions of the whisker shaft with the environment are encoded by these terminals and conveyed as spike impulses to the brainstem trigeminal nuclei via the centrally projecting processes of the trigeminal primary sensory neurons; the results of brainstem processing are then conveyed to various brain stations.
General Attributes of Sensory Nerve Endings
Sensory nerve endings are synonymous with "sensory receptors." Receptor protein molecules on the neuronal cell membrane should not to be confused with sensory receptors associated with sensory organs such as the follicle sinus complex. Sensory receptors are composed primarily of an “axon terminal” and “terminal Schwann cells.” Opinions have differed as to whether the peripheral branch of the sensory neuron is a dendrite or an axon. The current consensus relies on the use of the term “axon terminal” to designate the peripheral process of pseudo-unipolar neurons. When the parent fiber is myelinated, the term “axon terminal” is applied to the final segment of the unmyelinated part of the axon.
Various kinds of sensory receptors have been characterized according to their branching patterns and/or the presence of enlargements of the axon terminal as well as by the extent and distribution of the terminal Schwann cell sheaths that partly or completely envelop the axon terminals (Takahashi-Iwanaga, 2000). Occasionally, the existence of an outer collagenous capsule is used to differentiate different types of sensory receptors (Malinovsky, 1996), for example, Ruffini endings that possess an outer capsule vs. Ruffini-like endings that do not (see section on Ruffini-like endings, below).
Sensory nerve endings are divided into two groups morphologically: mechanoreceptors and free nerve endings (Fig. 1, inset). The mechanoreceptors originate from thick, myelinated fibers identified electrophysiologically as typical Aβ fibers; the diameters of their axon terminals are larger than the preterminal segments of their axons. Each type of mechanoreceptor has a characteristic morphology (see Merkel or lanceolate endings Figs. 1 and 2). With the application of immunohistochemistry, it is possible to visualize both the intermediate and heavy chain neurofilaments within axon terminals.
Terminal Schwann cells do not possess a myelin sheath, whereas the “non-terminal” axonal segments of common Schwann cells may or may not be myelinated. Terminal Schwann cells envelop individual or bundled axon terminals within a single, thin cell sheath (i.e., a single sheath, rather than multiple wrappings of myelin); however, some terminal Schwan cells envelop axon terminals with several thin layers of cell sheath (e.g., the Pacinian corpuscle). An interesting aspect of terminal Schwan cells is that they are influenced by calcium signaling when the mechanoreceptors are stimulated (Takahashi-Iwanaga, 2012).
Relationship between Morphology and Physiology
In many instances, there are differences in terminology regarding studies of morphology vs. those in which electrophysiological approaches are used (Table 1).
Differences in terminology may or may not reflect differences in function. However, the application of in vivo or ex vivo approaches promises to rectify this deficiency by reconciling the similarities of function expressed by morphological vs. physiological terminologies, as well as by understanding the implications of the differences in morphological vs. electrophysiological terms.
Using direct stimulation of histologically isolated corpuscles in the cat mesentery in vivo, Pacinian corpuscles have been shown to be rapidly adapting mechanoreceptors (Gray and Matthews, 1951). Merkel endings in the skin of the trunk of neonatal mice have been identified as slowly adapting type I low-threshold mechanoreceptors by means of an intracellular ex vivo approach (Woodbury and Koerber, 2007). In patch clamp experiments, individual Merkel cells and their attached axon terminals displayed slowly adapting features (Maksimovic et al., 2014; Woo et al., 2014; Ikeda et al., 2014). Using intra-axonal recording, Furuta et al., (personal communication) also determined that Merkel cells are slowly adapting. Tonomura et al. (2015), employing an intracellular, in vivo approach, showed that club-like endings in rat vibrissal follicles possessed an extremely high rate of firing; using intra-axonal methods, Furuta et al., (personal communication) determined that both club-like and lanceolate endings are rapidly adapting. As yet, not all types of mechanoreceptors have been identified as slowly or rapidly adapting.
Animals with vibrissae use touch to interact with their environment. Passive touching is the incidental contact by the vibrissae with an object. In active touching, the animal actively uses his vibrissae as exploratory tools to determine the nature of his environment.
Currently, experimental approaches using passive touch involve the manipulative contact between vibrissae and an object in an anaesthetized animal. An alternative approach is to effect active touch by stimulation of the facial nerve to move the vibrissae (Zucker and Welker, 1969). Recordings at the TG have shown that there are at least 3 functional groups of follicle receptors: Whisking, i.e., coding whisking regardless of contact, Touch, i.e., coding touch with objects, sub-divided into Contact, Pressure, and Detach [i.e. to cease contact], and Whisking/Touch, i.e., coding both whisking and touch (Szwed et al, 2003). Relating function to morphology in the active mode is, however, an enormously challenging task, which so far has not been addressed adequately.
Although several studies have provided convincing data on stimulus response properties, much additional data are needed in order to establish clearly the types of stimuli that elicit responses characteristic of each type of mechanoreceptor. In conclusion, because numerous mechanoreceptor types associated with the vibrissae are distributed throughout a highly complex environment composed of various types of cells and tissues, it remains difficult to determine at this time which mechanoreceptor(s) respond(s) to a particular mechanical stimulus.
Vibrissal Sensory Innervation
Vibrissal follicles on the face of the rat include two on the eyelid, one on the cheek, and a substantial number on the mystacial pad that contains >30 large vibrissae as well as many smaller ones. The lower jaw contains many small vibrissae. All the mystacial vibrissae are innervated by trigeminal ganglion neurons. In contrast, vibrissal follicles on the wrists are innervated by dorsal root ganglion neurons; the structure of the wrist follicles is identical to that of the mystacial vibrissae.
Each vibrissal follicle is innervated by two sets of nerves, termed deep and superficial. The superficial vibrissal nerves innervate the upper fourth of the vibrissal follicle, whereas the deep nerves innervate the remainder of the follicle; nerve fibers of both types overlap at the level of the inner conical body (Figs. 1-3).
Most deep vibrissal nerves are myelinated, and are gathered into several small bundles at the point of their penetration into the capsule. Typically, the penetration site is oriented towards the infraorbital foramen through which the nerves pass to the trigeminal ganglion (Dörfl, 1986). Taken together, the small bundles consist of approximately 150 thick, myelinated axons that occur within the deep levels of the larger vibrissal follicles in the rat mystacial pad (Rice and Munger, 1986). The bundles are sufficiently large to be observed clearly with the light microscope.
The superficial nerves are mostly of small diameter and unmyelinated, and are therefore difficult to detect within the dense dermal nerve plexus in the upper regions of the follicle. Unmyelinated fibers are distributed abundantly within vibrissal follicles. Most of those fibers do not disappear after sympathectomy or following super cervical ganglionectomy (Waite and Li, 1993), suggesting that these fibers may play a role in sensation.
Mechanoreceptors distributed within the vibrissal follicle are categorized morphologically into several principal types labeled 1 to 6 in the first column of Table 1 and graphically set out in Figure 1. Differences have been noted in the receptor types observed in various animal species, as well as during maturation or aging. In this treatise, we are concerned primarily with the normal structure of the adult rat; however, data derived from the cat or mouse have been included where appropriate. It has been shown that as early as postnatal day three, most types of mechanoreceptors in the rat have regional distributions, densities, and morphologies that are comparable to those in adult mice (Maklad et al, 2010).
Vibrissal follicles have two different types of Merkel endings. One, named a “touch dome” type, is situated at the rete-ridge collar, i.e., the epidermis at the mouth of the follicle opening at the skin surface. A second type of Merkel ending, called “follicle type," is distributed at the level of the ring sinus within the basal layer of the outer root sheath which is the outer layer of the epithelial follicle (Figs. 1-5).
Touch Dome Merkel Endings
Superficial vibrissal nerves innervate the rete-ridge. Several thickly myelinated nerve fibers leave the subcutaneous nerve plexus (Ebara et al., 2002; Fünfschilling et al., 2004), one or more approach the mouth of the follicle, and ramify just below the epithelium of the rete-ridge collar. Each ramification terminates by forming several enlargements called Merkel disks that are attached to the bases of the Merkel cells. The basal-lateral aspects of the disks are covered by terminal Schwann cell sheaths. This arrangement is basically the same as that of touch domes distributed within common hairy skin (Iggo and Muir, 1969; Tachibana, 1998; 2002; Ebara et al., 2008).
The rete-ridge collar is usually not entirely surrounded by Merkel endings; the same is true of touch dome Merkel endings, which are typically distributed within the rostral and caudal parts of the rete-ridge collar (Furuta et al., 2016). Occasionally, vibrissal follicles are situated within deep subcutaneous tissue. This may occur when whiskers are intensively protracted by vibrissal muscles. The rete-ridge collar is also pulled down into deep levels within the follicle, so that the surface of the epithelium and the discoid Merkel cells and their nerve discs become oriented parallel to the whisker shaft. It is likely that during whisking, the Merkel endings at the rete-ridge collar display a variety of orientations. It might be that the orientation of a mechanoreceptor plays an important role in the recognition of the direction of the stimulus (Furuta et al., 2016).
Follicle Type Merkel Endings
A sheet-like distribution of follicle type Merkel cells surrounds the follicle at the level of the ring sinus (see Fig. 2, a whole mounted specimen). Merkel cells are widely distributed at the level of the ring sinus inside the basal layer of the outer root sheath; however, a few Merkel endings occur above the ring sinus at the level of the conical body (Fig. 2, 3). Each follicle type Merkel cell has a thin, discoid shape, measuring roughly 15 x 8 x 2 um (Furuta et al., 2016). The long axes of these cells are arranged nearly vertically with respect to the whisker shaft. Their short axes are tilted downward 30 degrees to the glassy membrane from their upper marginal (long) edge. Merkel cells never abut each other as a few keratinocytes are always situated between them.
Thick myelinated afferents from the deep vibrissal nerves divide into several terminal branches, each of which generates a Merkel disk attached to one Merkel cell. Groups of 5 to 10 Merkel cells associated with a single terminal branch occupy separate territories (Ebara et al., 2002). Thus, a single primary sensory neuron typically innervates about 20 Merkel cells.
Most non-vibrissal hair follicles are innervated by lanceolate endings composed of axon terminals running parallel to each other, giving them the appearance of a palisade (Munger and Halata, 1983). Li et al. (2011) and Li and Ginty (2014) observed that lanceolate endings innervate follicles associated with each of four hair types, termed guard, awl, auchene and zigzag (Chi et al., 2013). In contrast, Merkel endings are associated only with the follicles of the guard hairs.
Lanceolate endings of vibrissal follicles are oriented both longitudinally and circumferentially (Fig. 3, 5) with respect to the vibrissal shaft; the longitudinal endings are equidistant from each other. In common hair follicles, the longitudinal and circumferential lanceolate endings overlap each other with the circumferential endings situated externally to the longitudinal ones when the latter are present (Munger and Halata, 1983; Kumamoto and Ebara, 2014). In contrast, in vibrissal follicles, the circumferential and longitudinal endings typically are separated from each other. The circumferential endings are situated within the inner conical body above the location of the longitudinal endings that are present at the level of the ring sinus. Occasionally, an overlap of the circumferential and longitudinal endings occurs at the lower level of the inner conical body.
Longitudinal Lanceolate Endings
Deep vibrissal nerves supply longitudinal lanceolate endings in the mesenchymal sheath at the level of the ring sinus (Rice and Munger, 1986; Takahashi-Iwanaga, 2000). The axes of the cross-sectioned axon terminals are perpendicular to the glassy membrane. Numerous spines of the axon terminals project through spaces between the Schwann cell sheaths and attach to the glassy membrane; the spines are considered to be subject to mechanical movement of the outer root sheath (Munger and Ide, 1988).
Longitudinal lanceolate endings display a variety of lengths and thicknesses; some endings are sharply pointed. Lanceolate terminal myelinated axons emit several long and flat thickened lengths of lanceolate axon terminals (Suzuki et al., 2012); these lengths are sandwiched between cell sheaths belonging to terminal Schwann cells (Munger and Ide, 1988; Li and Ginty, 2014, Fig. 5). Those thickened branches are connected to the terminal point by axon segments of narrow diameter. Typically, the longer the longitudinal lanceolate ending, the more likely the ending occurs at higher levels within the follicle. Recently obtained data suggest that even slight morphological differences may be associated with specific firing patterns of lanceolate endings. Rutlin et al. (2014) suggest that lanceolate endings in common hair follicles may respond to hair deflection in a direction-selective fashion. Parameters of directional selectivity of lanceolate endings in vibrissae are not yet well understood.
Longitudinal lanceolate endings are arranged at the level of the ring sinus without wide gaps. These endings do not overlap each other, nor do they overlap with club-like endings (see section Club-like Endings, below); longitudinal and circumferential endings do overlap slightly within the marginal zone (Fig. 3). The longitudinal lanceolate endings situated at the upper two-thirds of the ring sinus occur in close proximity to Merkel endings, but these are separated from each other by the intervening glassy membrane (Ebara et al., 2002; Furuta et al., personal communication).
Circumferential Lanceolate Endings
Superficial vibrissal nerves supply circumferential lanceolate endings within the inner conical body that lies above the ring sinus (Mosconi et al., 1993; Fundin et al., 1997). A few thick myelinated fibers terminate as Merkel endings at the rete-ridge collar, whereas other myelinated fibers descend to the inner conical body terminating as circumferential lanceolate endings. The axon terminals of the circumferential endings are relatively thin compared to the longitudinal endings (Fig. 3). Electron microscopic observation shows a similar cross-sectional structure for both circumferential and longitudinal lanceolate endings. Both lanceolate types are affected by movements of the hair shaft.
Recently, a different type of lanceolate ending, termed the club-like ending, has become the focus of a comprehensive investigation aimed at discerning the functional relationships between the morphological and electrophysiological aspects of these endings (Tonomura et al., 2015) (Fig.1-3, 5-7). Initially, club-like endings were identified either as the cut end of a nerve fiber or as a kind of lanceolate ending (Rice et al., 1986; Rice et al., 1993; Fundin et al., 1997; Maklad et al., 2010), but subsequently, they were recognized as a unique sensory mechanoreceptor (Ebara et al., 2002; Ebara, 2005; Sarko, et al., 2007; Furuta et al., personal communication).
Club-like endings, distributed only within the ringwulst of the vibrissal follicle, are characterized by a unique and relatively simple morphology that renders them the simplest structure observed so far among all types of sensory receptors. Most club-like endings appear at the tips of myelinated axons; typically one, but occasionally two, club-like endings originate from a single TG neuron. They are similar in structure to lanceolate endings but differ markedly from them by virtue of their simplicity and their close association with the ringwulst situated within the lower third of the ring sinus.
The ringwulst is suspended within the ring sinus by collagen fibers attached to a mesenchymal sheath (Vincent, 1913; Patrizi and Munger, 1966; Munger and Rice, 1986; Rice and Munger 1986; Halata, 1993). Although club-like endings associated with the ringwulst approach the glassy membrane, a space of at least 2 μm separates the glassy membrane from the club-like endings (Fig. 6).
It may be that the ringwulst serves to mechanically buffer the club-like endings from movements other than those made by the vibrissal shaft. Alternatively, the close spatial relationship between the club-like endings and the ringwulst suggests that the club-like endings might be more likely to respond to movement of the ringwulst than to that of the vibrissal hair shaft. This also suggests a significant difference between the functional mechanism of club-like endings and that of all other mechanoreceptors that are distributed in close proximity to the glassy membrane. For example, lanceolate endings are usually attached to the glassy membrane (Takahashi-Iwanaga, 2000; Li and Ginty, 2014), whereas at the level of the ring sinus, the axon terminals of Merkel endings pass through the glassy membrane and terminate within the outer root sheath (Ebara et al., 2002).
Tonomura et al. (2015) estimate that in large vibrissae in the rat more than 40 neurons of the trigeminal ganglion surround and innervate a limited belt zone at the inner surface of the neck of the floating ringwulst (see club-like endings Fig. 6 a, b and d rendered in red). According to a previous estimate, about 150 myelinated afferents in a deep vibrissal nerve innervate individual vibrissal follicles (Rice and Munger, 1986). Tonomura et al. (2015) estimated that more than 30% of the 150 myelinated fibers terminate as club-like endings. Additional quantitative investigations of a larger number of vibrissal follicles are necessary in order to refine these estimates.
The axon terminals of Ruffini endings, the endings discovered by Prof. Angelo Ruffini in normal skin, typically are highly branched with abundant enlargements. Ruffini endings are usually enclosed by collagen capsules and are termed corpuscles (Ruffini, 1893; Andres and von During, 1973; Halata and Munger, 1981; Munger and Ide, 1988; Sano, 1995).
In the vibrissal follicle of the rat, Ruffini-like endings are not encapsulated; numerous endings spread broadly on the surface of the glassy membrane present a tree-like appearance. Ruffini-like endings are distributed widely in the mesenchymal sheath at the level of upper cavernous sinus. They are arranged without any space between them with little overlapping, such that they appear as a flattened collection of branches. That arrangement has led some to label Ruffini-like endings in the vibrissal follicle of the rat as “reticular endings.”
For most of the past 20 years, the identification of several mechanoreceptor types has been unclear. For instance Fundin et al. (1997) identified two types of endings within the vibrissa follicle as reticular and Ruffini. Ebara et al. (2002) identified these receptors, respectively, as reticular and spiny endings. In 1985, Byers identified two types of mechanoreceptors in the periodontal ligament: large, complex Ruffini-like receptors and simple Ruffini-like receptors. Maeda et al. (1999) identified two periodontal Ruffini endings, terming them Types 1 and 2. As a result of our recent investigations we have concluded that the two types of mechanoreceptors identified by Maeda et al. as Types 1 and 2 correspond to the reticular/tree-like and spiny endings described in this section.
Within the vibrissal follicle, the area of one terminal myelinated fiber of a Ruffini-like ending is more than 10,000 μm2. The parent afferent often branches once or twice. Sometimes, the branches are distributed far from each other, such that the total territory that an afferent can cover is a wider area than that of all the other types of mechanoreceptors (unpublished data).
The morphology of Ruffini-like endings observed in the rat is different from that of the cat. Rat Ruffini-like endings are arranged similarly to the fingers of an open hand whose palm is pressed against a flat surface. The finger-like projections of these endings are splayed out over the surface of the glassy membrane. In contrast, Ruffini-like endings in the cat resemble a partially closed hand replete with curved fingers. In the cat, only the tips of the finger-like projections of Ruffini-like endings are applied to the glassy membrane. Perhaps the differences in the shapes of Ruffini-like endings in whisking vs. non-whisking animals correlate with the physiological properties of these endings, but confirmation of this awaits further investigation.
Free Nerve Endings
Free nerve endings in the dermis, termed penicillate endings by Cauna (1973), are fine-caliber axon terminals that originate primarily from unmyelinated nerve fibers but occasionally stem from small diameter, myelinated fibers. Numerous free nerve endings also occur within the epidermis (Fig. 3, Cauna, 1980). In our preparations using immunohistochemistry, the parent afferents are observed to give rise to multiple branches composed of fine, varicose fibers. Their axon terminals lack envelopment by terminal Schwann cell sheaths (Cauna, 1973); instead, they are surrounded by keratinocytes (Fig. 1, inset).
Mechanoreceptors respond to mechanical stimulation, whereas free nerve endings typically respond to chemical stimulation as in pain or itching, or to temperature. Recent studies of the primary sensory neurons, using a combination of gene targeting and immunostaining of stretch receptor molecules such as TRPV2 and TRPV4, suggest strongly that some free nerve endings are low-threshold mechanoreceptors (Lawson et al., 2008; Suzuki et al., 2003; Li et al., 2011; Li and Ginty, 2014). The axons of free nerve endings are considerably smaller in diameter than those of the typical mechanoreceptors described in sections on other types of endings.
With respect to the vibrissae, free nerve endings are distributed widely throughout the entire depth of the follicle with the exception of the outer root sheath. The densest distribution of unmyelinated fibers, including many free nerve endings, is seen within the inner conical body. Abundant unmyelinated nerve fibers surround the follicle at that level, with a few myelinated nerve fibers providing circumferential lanceolate endings (Fig. 3). These endings are of fine caliber, lose their myelin at the terminal point (Fig. 1), and intermingle with free nerve endings, such that it is extremely difficult to differentiate one from the other. A dense innervation of free nerve endings is also observed in the intervibrissal skin (Fig. 3).
Because free nerve endings originate from either unmyelinated (C fibers) or myelinated (Aδ) fibers, at present the application of intracellular recording and labeling with neuronal tracers offers the best chance to identify the precise origins of these endings. Other types of mechanoreceptors intermingle with free nerve endings (Fig. 3); the extent to which these receptor types functionally interact at the level of the follicle is not clear.
Encapsulated, Lamellated Endings
Endings encapsulated by a collagenous capsule are termed corpuscles. Axon terminals of laminar corpuscles have multiple lamellae of terminal Schwann cell sheath, e.g., Pacinian corpuscles, simple corpuscles that resemble the inner core of Pacinian corpuscles, and Meissner corpuscles. Murine or canine vibrissal follicles have simple corpuscles at the level of the cavernous sinus; the rat vibrissal follicle, however, does not have corpuscles (Halata et al., 1993; Ebara et al., 2002). Ruffini-like endings within vibrissal follicles are not encapsulated, but those in the skin are occasionally lamellated and/or encapsulated.
Additional types of endings
A variety of other types of endings that do not occur within vibrissal follicles are not mentioned in this report, because as yet, there is little that can be said about the details of their specific morphologies and functional attributes. When sufficient knowledge is provided about these mechanoreceptor types, it is expected that they will form the subject of a later entry.
Central Processes of Primary Sensory Neurons
Tonomura et al. (2015) suggest that no major differences exist between the central processes of different types of mechanoreceptors, i.e., from the most simple shape of club-like endings to the highly branched Ruffini-like endings. A large number of collaterals emitted by central processes may be the major factor in determining the tactile stimulation that is encoded even by the simplest mechanoreceptor, i.e., the club-like ending. See also the chapter in this volume by Prof. Martin Deschenes.
Physiological and Morphological Correlations
Conduction velocity and Nerve fiber diameter
A useful approach to classify sensory receptors relies on a combination of morphological and physiological characteristics. For example, it is well known that conduction velocity correlates with the diameter of nerve fibers: Thicker myelinated fibers convey action potentials more quickly than thinner fibers. In 1937, Erlanger and Gasser proposed a system of classification that relied on physiological characteristics alone. They categorized nerve fibers into three main groups: A (Aα, Aβ, Aδ), B and C, according to their conduction velocities (cf. Sano, 1995; Gardner and Johnson, 2013). Aβ fibers in vibrissal follicles have been termed Aαβ (Rice et al., 1993) because the Aβ fibers were determined to be collaterals of Aα and fusimotor fibers. Physiological classifications provide useful descriptions of the functional aspects of sensory nerve fibers. However, a classification system that relies only on fiber diameter does not necessarily correlate with conduction velocity because the diameters of the nerve fibers vary along their lengths (Sano, 1995 and unpublished observations).
With the exception of free nerve endings, Aβ fibers supply most kinds of mechanoreceptors, whereas Aδ and occasionally C fibers supply mechanoreceptive free nerve endings. Using intracellular recording, Tonomura et al. (2015) observed that all types of mechanoreceptors, excluding free nerve endings, originate from thickly myelinated fibers, such as an Aβ fiber. It should be noted that axons belonging to a particular type of mechanoreceptor do not always have fibers of the same diameter.
Threshold, Adaptation, and Receptive Field
Traditionally, mechanoreceptors in primates have been categorized by their times of adaptation into rapidly/fast adapting (RA/FA) receptors and slowly adapting (SA) receptors (Talbot et al., 1968, and see Table 1). Both types of mechanoreceptors are distributed within the vibrissal follicles as well as in other areas of the skin that lack vibrissae. Gardner and Johnson (2013) pointed out that these receptors respond to discrete stimuli and in doing so, reflect changes in temporal-spatial patterns of stimulation. A number of researchers have determined that adaptation responses of mechanoreceptors can vary under certain conditions (Zucker and Welker, 1968; Cash and Liden, 1982; Leiser and Moxon, 2006; Abraia and Ginty, 2014). Recently, Furuta et al. (2016) demonstrated clearly that Merkel endings in vibrissal follicles are slowly adapting when vibrissae are stimulated in a preferred direction, but evoke rapid adaptation in non-preferred directions.
A receptive field is defined as a limited region on the skin surface which, through the mediation of mechanoreceptors, can evoke a response in a primary sensory neuron. Zucker and Welker (1969) demonstrated that each “receptive field” on the skin of the mystacial pad is limited to a single vibrissal shaft.
The standard classifications are no longer necessarily applicable to mechanoreceptors associated with the vibrissae. One reason is that each whisker has a different intrinsic curvature and emerges from the mystacial pad at a different angle (Towal et al., 2011), such that dynamic and minute movements of those whiskers during stimulation complicate the identification and analysis of the functional aspects of mechanoreceptors (Furuta et al., personal communication). Making detailed 3-dimensional analyses of the environmental structures and their movements in relation to the receptors, as well as quantifying the areas innervated by single neurons, is necessary to determine the sizes of receptive fields (Tonomura et al., 2015; Furuta et al., 2016). Increasing the relevancy of both previous and newer classifications applied to the various aspects of mechanoreceptors requires a much closer interaction between physiological and morphological approaches than hitherto employed.
Recent Findings Regarding Vibrissal Innervation
Within a vibrissal follicle, multiple innervations of diverse mechanoreceptors that provide a variety of responses to stimulation are considered to play a key role in the highly sophisticated functions of the whisker sensory system.
Furuta et al. (2016) determined that follicle type Merkel endings evoke slowly adapting action potentials to stimulation from particular directions that are identical to their orientation. In contrast to the Merkel endings, the club-like endings evoke rapidly adapting firing and occasionally evoke continuous firing to stimulation from most directions regardless of their location and orientation (Furuta et al., personal communication).
Tonomura et al. (2015) suggest that there is a more dense innervation within the confines of the ringwulst than that observed in other regions of the vibrissal follicle (Fig. 7). Their results also suggest that neurons having club-like endings can respond adaptably to even delicate vibrations of the hair shaft, evoking high-frequency action potentials. In contrast to the club-like endings, Ruffini-like endings display voluminous branching within their receptive fields (Fig. 2, and cf. 6) and respond to a long-lasting deformation of the follicle at the cavernous sinus, thereby evoking continuous firing with regular interspike intervals (Tonomura et al., 2015; personal communication; and see Table 1). Their findings and those of Gottschaldt et al. (1973) represent an important key that has enabled us to relate receptor adaptation and frequency of firing to identified mechanoreceptor endings.
Club-like endings associated with the ringwulst represent an ideal model to investigate various mechanisms of mechanoreception regarding how movements of the ringwulst are related to whisker stimulation and the character of their central neurotransmission. An interesting finding by Furuta et al. (personal communication) showed that dynamic mechanoreceptor responses in vibrissal follicles contrast markedly with the responses predicted by computational models of lanceolate, club-like, and the two types of Merkel endings.
Different types of mechanoreceptors are considered to support different perceptual functions, such as the monitoring of whisker kinematics (Wallach et al., 2016) and the coding of various features of the contacted objects (Arabzadeh et al., 2005; Lottem and Azouz, 2009; Szwed et al., 2003). Intensive microscopic and electrophysiological examination and analysis can be expected to elucidate correlations of structure and function that will pave the way for a more complete understanding of mechanoreception, thereby greatly facilitating our understanding of touch.
Vision for the Future
The visual, auditory, gustatory and olfactory senses are each stimulated by a single element, for example, the retina responds to light, the cochlea to sounds, taste buds to molecules and olfactory receptors to odorants. In stark contrast, skin sensation such as touch, pain, pressure, temperature etc. is mediated by a wealth of different receptors, each of which possesses a specific morphology and a particular set of functional properties.
The vibrissal follicle is replete with various skin sensory mechanoreceptors concentrated within a very small volume (i.e., a cylinder smaller than 2 x 5 mm). Its contents are embedded within a matrix of collagen fibers and are limited by a surrounding collagenous capsule. As indicated in the section on “Characteristics of Mechanoreceptors,” each type of receptor occurs, for the most part, primarily at a specific level of the follicle where each receptor type is distributed from the inner periphery of the follicle to regions closely surrounding the vibrissal shaft. The result is a dense concentration of mechanoreceptors throughout the length and breadth of the follicle.
The follicle, having a single, central vibrissa surrounded by mechanoreceptors, can be conceived of as akin to a symphony orchestra in which the different groups of instruments (the mechanoreceptors) respond to the direction of the conductor (the stimuli provided by the vibrissa). Just as the different instruments played in concert produce a symphony, it is possible that the entire complement of mechanoreceptors within the follicle, all of which are situated within a complex environment of various tissues, can interact in some way with each other. For instance, movement of the vibrisal matrix and/or the mechanoreceptors within the follicle might facilitate the evoking of responses by other receptors that are not directly involved with the vibrissa. Thus responses of mechanoreceptors may vary depending not only on vibrissal movements but also on the movements of other structures within the follicle.
Because of its small size and compactness the vibrissa follicle represents a unique opportunity to reconstruct an entire follicle using automatic serial block-face scanning electron microscopy. Such analyses would provide a 3-dimensional panoramic view of the follicle that would manifest the detailed structure of the environmental surround for each receptor enabling insight into the functional relationships between the mechanoreceptors and the matrix of collagen fibers within which they are embedded.
We are exceedingly pleased to acknowledge Dr. Sotatsu Tonomura for kindly providing us with unpublished data and for his efforts to instruct us in the art of intracellular recording. We are especially grateful to Prof. Edward L. White for his expert assistance with editing the manuscript.
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