Sympathetically-induced development of tension in jaw muscles: the possible contraction of intrafusal muscle fibres

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Abstract
In rabbits, cats and rats anaesthetized, curarized, with the skull fixed in a stereotaxic apparatus, the peripheral stump of the cervical sympathetic nerve (c.s.n.) was electrically stimulated at frequencies within the physiological range and the isometric tension was recorded at the lower jaw. In a group of experiments the afferent discharges from the jaw elevator muscle spindles was also recorded, in the mesencephalic nucleus of the fifth cranial nerve. Unilateral stimulation of the c.s.n. induced in jaw elevator muscles of rabbits an increase of tension of 5.5±0.5 g (latency: 0.5–2 s, time constant: 2.5–5 s) maintained with little or no decrement until the end of stimulation. This response proved not to be secondary to vasomotor changes since: i) approximately half of it was mediated by the fastest conducting component of the c.s.n. fibres, ii) it was not mimicked by a sudden reduction of blood supply to the muscles, iii) it was unaffected by 10 min bilateral occlusion of both the external and the internal carotid arteries. During c.s.n. stimulation the afferent discharge from spindles belonging to jaw elevator muscles exhibited an increase of firing (often preceded by a transient decrease) lasting throughout the stimulation. Also, the position sensitivity of all the spindle afferents tested was modified by the sympathetic stimulation. The results presented are interpreted to suggest that the sympathetic system may induce an intrafusal muscle fibre contraction in jaw elevator muscles. The possible functional implications are also discussed.