visceral

SPECIAL VISCERAL NERVE

Neuro term. 1. A nerve linked to a facial, jaw, neck, shoulder, or throat muscle that once played a role in eating or breathing. 2. A cranial nerve whose original role in digestion and respiration renders it emotionally responsive today.

Usage: Special visceral nerves mediate those "gut reactive" signs of emotion we unconsciously send through facial expressions, throat-clears, head-tilts, and shoulder-shrugs. Nonverbally these nerves are indeed "special," because the muscle contractions they mediate are less easily (i.e., voluntarily) controlled than are those of the skeletal muscles (which are innervated by somatic nerves).

Evolution. Associated with the pharyngeal arches, special visceral nerves control the branchiomeric muscles that once constricted, or dilated, "gill" pouches of the ancient alimentary canal.

Anatomy I. Special visceral nerves include efferent fibers of a. the trigeminal nerve (cranial V, for biting and chewing); b. the facial nerve (cranial VII, for facial expression); c. the glossopharyngeal nerve (cranial IX, for swallowing); d. the vagus nerve (cranial X, for tone of voice); and e. the accessory nerve (cranial XI, for head-shaking and the shoulder-shrug).

Anatomy II. The paleocircuits of visceral nerves--which originally mediated the muscles for opening (i.e., dilating) or closing (i.e., constricting) parts of the primitive "gill" apparatus in eating and breathing--are today linked to the limbic system.

Emotional signs, pains, and twitches. Special visceral efferent nerves control our facial expressions, cause many of our neck pains, and trigger the occasional twitches we feel in our mammalian-inspired platysma muscle.

Vagus nerve stimulation. "As the nerve is stimulated [by electrical current from an implanted VNS generator device to treat resistant depression], some people may experience a tingling sensation, hoarseness, or the urge to cough" (Cantor 2001).

Neuro-notes I: Efferent (outgoing). The special visceral motor column (in which special visceral nerves are rooted) lies in separate brain-stem and spinal-cord areas from the somatic motor column (which controls skeletal muscles). Overall, the structure of special visceral nerves in mammals is conservative (i.e., is much the same as it used to be in fishes; Walker 1986:223). The most conservative nerve of all (see Walker 1986:213) may be the glossopharyngeal (cranial IX), which renders cues such as the Adam's-apple-jump and throat constriction of the cry so sensitive, trustworthy, and revealing of mood. In fishes, the vagus may have been formed from four separate nerves, each similar to the glossopharyngeal (Walker 1986:213), and may have worked mainly as muscle constrictors. In reptiles, the accessory nerve split off from the vagus: "With the elaboration of the cucullaris to form the trapezius and sternocleidomastoid complex, we find that the special visceral motor fibers that supply these muscles separate from the vagus to form a new cranial nerve, the accessory (XI)" (Walker 1986:223).

Neuro-notes II: Afferent (incoming). Special visceral afferent (incoming) nerves involve the gastrointestinal tract, and include the ability to detect aroma cues and taste cues. They may be found in olfactory (I), facial (VII), glossopharyngeal (IX), and vagus (X) cranial nerves. "The facial nerve receives taste from the anterior two-thirds of the tongue; the glossopharyngeal from the posterior third. SVA fibers in the vagus originate in the larynx and pharynx.[2] The sensory processes, using their primary cell bodies from the inferior ganglion, send projections to the medulla, from which they travel in the tractus solitarius, later terminating at the rostral nucleus solitarius" (Wikipedia, Special visceral afferent fibers [accessed Mar. 24, 2019]).