S7-1.4.2 Jan. 6 An Inhibitory Spinal Reflex Between Elbow Flexor Synergists BARRY, BK*; ENOKA, RM; Univ. of Colorado, Boulder; Univ. of Colorado, Boulder Benjamin.Barry@colorado.edu
Decades of work on spinal reflexes in humans based on measuring Hoffmann (H) reflexes in muscles of the forearm and lower leg has provided a relatively limited understanding of the modulation of afferent pathways during functional tasks (Pierrot-Deseilligny & Burke, 2005). An alternative approach is to use the spike-triggered stimulation technique (Stephens et al. 1976), which allows investigation of reflex pathways in muscles from which the H-reflex may not readily be recorded, and it avoids some of the interpretation issues associated with the H-reflex. Such an approach is being used in our laboratory to investigate an inhibitory reflex between the brachioradialis and biceps brachii muscles. An electrical stimulus is applied at a submotor threshold to the brachioradialis branch of the radial nerve and its effect on the discharge of a tonically active motor unit in biceps brachii is determined. Post-stimulus time histograms constructed from the interspike intervals of the isolated motor unit indicate that the stimulus consistently delays the occurrence of the subsequent discharge of a biceps brachii motor neuron. Presumably, the inhibitory reflex connection between these two synergist muscles for an elbow flexion torque, aids in the control of the substantial supination torque exerted by biceps brachii. In support of this, the magnitude of the inhibition recorded from the same biceps brachii single motor unit (n = 18) varies systematically when the forearm posture is shifted between pronated and supinated positions. The reflex pathway likely involves either type I non-reciprocal inhibition or presynaptic inhibition mediated by primary afferent depolarization interneurons. This posture-dependent spinal reflex may contribute to the durations that various submaximal contractions can be sustained with the elbow flexor muscles. Supported by NINDS NS043275.