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). When the parent fiber is myelinated, the term “axon terminal” is applied to the final segment of the unmyelinated part of the axon. The current consensus relies on the use of the term “axon terminal” to designate the peripheral process of pseudo-unipolar neurons. 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. 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. General Attributes of Sensory Nerve Endings 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. 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). 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. The follicle and sinuses are surrounded by a thick collagenous capsule covered by a layer of skeletal muscle that functions as arrector pili. 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 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). Most mammals possess rows of whiskers (i.e., vibrissae) on both sides of the face arranged in an orderly grid on the mystacial pad.
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