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Distribution of Ia effects onto human hand muscle motoneurones as revealed using an H reflex technique.
Author(s) -
Mazzocchio R,
Rothwell J C,
Rossi A
Publication year - 1995
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1995.sp021048
Subject(s) - reflex , neuroscience , distribution (mathematics) , h reflex , anatomy , chemistry , biology , mathematics , mathematical analysis
1. The possibility of eliciting H reflexes in relaxed hand muscles using a collision between the orthodromic impulses generated by magnetic cortical stimulation and the antidromic motor volley due to a supramaximal (SM) peripheral nerve stimulus was investigated in seven subjects. 2. Magnetic stimuli, applied through a circular coil (outer diameter, 13 cm) centred at the vertex, evoking EMG responses of 3‐5 mV amplitude in the relaxed abductor digit minimi (ADM) muscle, and SM test stimuli to the ulnar nerve at the wrist producing a direct maximal motor response (Mmax) in the ADM muscle, were given either alone or combined. 3. In all subjects, combined cortical and SM ulnar stimulation produced a response after the Mmax with the latency of an H reflex evoked by the ulnar stimulus. This response occurred only within interstimulus intervals (1‐20 ms) compatible with collision in the motor axons. The response behaved like an H reflex being time‐locked to the SM ulnar stimulus, facilitated by voluntary activation of ADM muscle, depressed by vibration (4 s, 100 Hz) of ADM tendon and by a submotor‐threshold ulnar nerve stimulus applied 50 and 80 ms before the combined stimulation, respectively. 4. In some subjects, it was also possible to distinguish an earlier response preceding the H reflex by 3 ms. Evidence is given that this response is probably of cortical origin. 5. Varying the intensity of magnetic stimulation resulted in a non‐linear relationship between the H reflex size and the size of the cortical response. When the latter was between 5‐25% of Mmax, H reflexes were small (2.5‐7.5% of Mmax); with cortical responses between 25‐50% of Mmax, there was a steep increase in H reflex amplitude (10‐30% of Mmax). We suggest that this behaviour is due to an uneven distribution of Ia effects within the motoneurone pool.