Human root and cord potentials evoked by Achilles tendon tap.

Lumbosacral root potentials and sacral spinal cord segmental responses were recorded epidurally in 24 normal adult subjects by tapping the Achilles Tendon with the electronic hammer (mechanical). The averaged responses were also recorded from the same epidural levels after the electrical stimulation of the Posterior Tibial nerve at the popliteal fossa (electrical). While the dorsal and reflexively evoked ventral root responses were easily obtained by the electrical stimulation; by the mechanical stimulation only a late and small ventral root response with two negative deflections could be evoked at the L4-5 or L5-S1 I.V. (intervertebral) epidural levels. The ventral root responses elicited by mechanical stimulation appeared 4.0 msec later than those elicited by the electrical stimulation. This time interval was exactly the same when the latencies of the Soleus reflexes to both stimuli were compared that for the Achilles tendon reflex being 4.0 msec delayed. When the sums of their afferent and the efferent conduction times (to and from Th12-L1 I.V. levels) were extracted from the total conduction times of the Soleus H and Achilles tendon reflex obtained by the EMG recordings, the sacral cord conduction delay was found 1.8 msec for the H-Reflex and 4.1 msec for the Achilles tendon reflex. It was suggested that the twice longer time delay along the sacral cord to Achilles tendon tap might be due to the oligosynaptic linkage of the Achilles tendon reflex or it may result from highly dispersed afferent volley producing a slowly rising wave or excitation that takes longer to reach threshold for the soleus motoneurone discharges.