Interaction between muscles and fascia in the mystacial pad of whisking rodents

Abstract

In whisking rodents, the mystacial pad is supplied with vibrissae and contains a collagenous skeleton that is a part of the snout fascia. The collagenous skeleton is composed of three interconnected layers: superficial, deep spongy mesh and subcapsular fibrous mat. We found that the first two layers contain diverse fascial structures, such as sheets of subcutaneous connective tissue, tendons, ligaments and follicular capsules which transmit muscle efforts to vibrissae and are thus involved in whisking. Subcapsular fibrous mat is built of oriented rostro‐caudal wavy fibrils. It maintains spatial arrangement of whisker follicles, provides a quick response to deformation and connects entire mystacial pad to the skull. To move vibrissae, the forces of intrinsic muscles are applied directly to the capsules of the vibrissa follicles, whereas the forces of extrinsic muscles are applied to other parts of the collagenous skeleton, which transmit the forces to the capsules. According to the spatial distribution and anchoring sites of the muscles and fascia, extrinsic muscles provide vibrissa protraction or retraction by pulling the superficial layer of the collagenous skeleton rostral or caudal, respectively. Vibrissae can be also retracted when the efforts of extrinsic muscles are applied to the subcapsular fibrous mat. When the muscles relax, fascial structures return the vibrissae to their resting position. The deep spongy layer encompasses vibrissal follicles providing a uniform distribution of stresses and strains during whisking. In the mystacial pad, fascia is a dominant type of tissue that maintains the integrity of the vibrissa motor plant, translates muscular momentum to the vibrissae, and plays a role in vibrissae movements.

The follicle‐sinus complex of the bottlenose dolphin (Tursiops truncatus). Functional anatomy and possible evolutional significance of its somato‐sensory innervation

“…Comparisons with other mammals are difficult. Whisking rodents possess the mystacial pad, a thickening of the snout fascia where mimic muscles, sensory receptors, and collagen structures form a highly developed motor‐sensory organ (Haidarliu et al, 2020). In the rat and mouse, a column of neocortical neurons in the whisker somatosensory cortex (wS1, or barrel cortex) corresponds to each FSC, with a highly developed layer IV receiving the thalamic afferent (Bosman et al, 2011; Jeanmonod et al, 1981; Pearson et al, 2006; Rice & Van Der Loos, 1977; Schröder et al, 2020; Van der Loos, 1976; Van der Loos & Woolsey, 1973; Woolsey & Van der Loos, 1970).…”

Functional anatomy of mystacial active sensing in rats

“…All the experimental procedures are in accordance with the Institute Animal Care and Use Committee at the Weizmann Institute of Science. The anatomical methods we employed to describe the muscles, their motor end-plates, and fascia in histological slices of the rat MP have been previously described in detail (Haidarliu et al, 2017(Haidarliu et al, , 2021. Briefly, rats were anesthetized by intraperitoneal injection of urethane (25% [wt/vol]; 0.65 mL/100 g body weight), perfused transcardially with 4% (wt/vol) paraformaldehyde and 5% (wt/vol) sucrose in 0.1 M phosphate buffer, pH 7.4, and then decapitated.…”

“…To scan their environment, rats employ quick rostro-caudal vibrissal sweeps known as whisking (Zucker & Welker, 1969). The muscles and fascial structures composing a vibrissal motor plant mediate whisking (Haidarliu et al, 2010(Haidarliu et al, , 2021. In addition to the predominant rostro-caudal movement, 3D recordings of whisker movements revealed three rotary components: azimuth, elevation, and torsion (Knutsen et al, 2008).…”

“…We were inspired by Wineski (Wineski, 1985), who used the anatomical description of muscles in hamster MP to infer their lines of action and determine their individual roles in the coordinated MP motion. Here, we use detailed histological descriptions of all the rat MP muscles and their spatial organization in situ (Haidarliu et al, 2010(Haidarliu et al, , 2011(Haidarliu et al, , 2021. Considering their shapes, attachment points, force application directions, and supporting fascial structures, we predict the geometrical effects of their contractions on MP motion.…”

What can whiskers tell us about mammalian evolution, behaviour, and ecology?

“…In many rodents, such as mice and rats (Fig. 2c), a quick, continuous, to‐and‐fro cyclic movement, called whisking, is possible (Dörfl 1982, Haidarliu et al 2010, Grant et al 2013a, 2017, Haidarliu et al 2021). It also manifests in other species (Vincent 1912, Grant et al 2018, Muchlinski et al 2020).…”