THE FUNCTIONAL EFFECTIVENESS OF NECK MUSCLE REFLEXES FOR HEAD-RIGHTING IN RESPONSE TO SUDDEN FALL

Reflex head-righting in normal and labyrinthine-defective (LD) subject s was compared to identify the relative functional effectiveness of ve stibular-collic and cervico-collic myotactic reflexes. To restrict sti muli largely to the head and neck, subjects lay supine, supported up t o the shoulders on a horizontal bed with their head supported in a sli ng over the edge. The head fell freely as the sling was released with an electromagnetic catch. Head drops were delivered with the subjects instructed to relax and accept the fall passively or to actively right the head as fast as possible. With both instructions, righting respon ses in normal subjects commenced with electromyographic (EMG) bursts i n the sternocleidomastoid (SCM) at 24.5 ms latency, which was reflecte d in a deceleration of the downwards head velocity. The latency of the earliest EMG responses in LD subjects was 67.4 ms, accompanied by sim ilar deceleration. It is assumed that the earliest response in normal subjects is vestibular, whereas in LDs the SCM stretch reflex is the e arliest response. These reflexes are followed at circa 100 ms by more intense EMG activity due to voluntary movement, but braking of head fa ll is evident before voluntary activity takes effect. Righting was mor e effective in normal subjects than in LDs, and when ''active'' normal subjects made more vigorous righting responses than when ''passive''; whereas active righting in LDs was no better than passive. The result s demonstrate that reflex responses contribute significantly to head-r ighting. The vestibular contribution gives an advantage over stretch r eflexes alone and also assists in voluntary enhancement of reflex resp onses.