Facial morphology and vibrissal movement in the golden hamster

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Abstract
The major cranial vibrissae in the golden hamster can be moved in complex ways that suggest they are served by a finely controlled motor system. Movements are hypothesized to be the products of (1) differential blood flow and pressure regulation in the sinus surrounding each vibrissal follicle, (2) contractions of the striated facial muscles, and (3) elastic rebound in the connective tissues. The vasculature contributes hydrostatic forces that (a) erect the vibrissae slightly and distort their connective tissue bedding, (b) rigidify the vibrissal capsules, thus forming firm bases of attachment for certain facial muscles, and (c) theoretically provide a pressure plate around the follicle, important in lowering the firing thresholds of receptor endings. The facial muscles supply the major forces in erection and protraction of the vibrissae by acting on both the capsules and the connective tissue bedding. The connective tissues are organized into capsular and extracapsular systems that serve to stabilize the vibrissae and return them to initial rest positions. The slight movements of the genal vibrissa are the effects of vascular and connective tissue dynamics, the musculature being uninvolved. Wide angle movements of the supraorbital vibrissae are products of the vasculature and connective tissues, plus contractions of the Mm. orbicularis oculi and frontalis. Mystacial vibrissal movement is quite complex. The vasculature supplies a small degree of capsular erection and mystacial pad distortion, but primarily rigidifies the capsules. The bulk of erection and protraction is produced by the M. nasolabialis profundus (NLP) and the vibrissal capsular muscles (VCM). The NLP distorts the mystacial pad; the VCM tilt the capsules relative to the pad. Retraction is mainly accomplished by elastic rebound in the pad, this being aided in its extreme degrees by the Mm. nasolabialis and maxillolabialis. The Mm. nasolabialis superficialis and buccinator pars orbicularis oris help to spread the vibrissae into a dorsoventral fan and stabilize the mystacial pad during whisking.