“…They have shown that, in curarized animals, sympathetic stimulation can induce an increase in muscle spindle activity, often preceded by a transient decrease of the firing rate, which is accompanied by a small force development (a few g for all jaw elevator muscles) attributed to sympathetically induced contraction of intrafusal and/or a few extrafusal muscle fibres (Passatore & Filippi, 1981, 1982Passatore, Grassi & Filippi, 1985;Passatore & Grassi, 1989. A reduction of spindle sensitivity to sinusoidal muscle stretch was also reported, which was thought to be responsible for the marked spindle afferents and EMG activity was reduction of both the jaw jerk reflex and the tonic vibration reflex induced by sympathetic stimulation in decerebrate rabbits (Passatore & Grassi, 1989;Grassi, Deriu, Artusio & Passatore, 1993a;Grassi, Deriu & Passatore, 1993b). The jaw jerk reflex is a monosynaptic brainstem reflex; the sensory neurones of muscle spindles (the mesencephalic trigeminal nucleus) make excitatory connections with the motoneurones of the jaw closing muscles (the trigeminal motor nucleus).…”