Flexion withdrawal reflex as recorded from single human biceps femoris motor neurones

Reflex responses were elicited in the biceps femoris muscle of patients with cervical spinal cord lesions by stimulating the sural nerve and recording from single motor neurones by single fibre EMG technique. The consecutive responses within the surface-recorded early component displayed a latency jitter between 500 and 600 microsec (S.D.), suggesting an oligosynaptic reflex arc. These responses were rarely seen; they were suppressed by increasing the stimulus strength. The surface-recorded late component consisted in the single fibre EMG recordings of repetitive discharges of individual motor neurones. The latency jitter of the earliest of these discharges was about 1440 microsec, and it progressively increased in the later discharges, reflecting increasingly more variable central conduction time. Ischaemia of the leg made some of the late discharges at a latency of about 0.5 sec remarkably stable and reduced their jitter. They may represent responses to the activity of slowly conducting afferent fibres, normally suppressed by inflow along the fast conducting fibres. Stimulation of dorsal columns through epidurally placed electrodes at T1 and T2 levels resulted in a complete suppression of repetitive discharges, outlasting the actual stimulation. TENS and light stroking of the skin below the level of the spinal cord lesion, as well as subthreshold stimuli to the sural nerve, had a similar effect. These results suggest that the late component of the flexion reflex may share some of the neural mechanisms responsible for feeling pain.