Primary changes in the membrane potential (i.e. For example, the excitability of the H-reflex and the duration of the silent period in a limb muscle both reflect the actions of various mechanisms affecting motoneuronal excitability. Several physiological mechanisms influence spinal motoneuronal excitability in vivo and the net contribution of each individual mechanism is often difficult to estimate (for references, see Rothwell, 1994).
They also differ from them in several ways: for example, they receive strong inhibitory inputs from mechanoreceptors through Aβ and possibly from Aδ afferents ( Nakamura, 1980 Miles & Turker, 1987 Cruccu & Ongerboer de Visser, 1999), and have neither reciprocal nor recurrent inhibition ( Lorente De Nò, 1933 Nakamura, 1980). Trigeminal motoneurones have been studied less extensively than spinal motoneurones. 1989) and thereby leading to a transient suppression of the voluntary ongoing EMG activity in the masseter muscle, namely the silent period ( Merton, 1951). Stimulation of the masseteric nerve also evokes an antidromic volley along motor axons thus invading the homonymous masseteric motoneurones ( Cruccu et al.
The activation of heteronymous Ia projections arising from the masseter muscle and directed to temporalis motoneurones elicits the pure reflex excitation of temporalis motoneurones with no antidromic influences on their excitability.
1989) and temporalis muscles ( Macaluso & De Laat, 1995). Weak electrical stimuli delivered to the motor nerve supplying the masseter muscle (masseteric nerve) during voluntary contraction of jaw-closing muscles elicit, in humans, an H-reflex in the masseter ( Godaux & Desmedt, 1975 a Cruccu et al.
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