ON THE TENDON JERK REFLEX IN THE HUMAN NEONATE (EMG, SPINAL CORD, STRETCH REFLEX)

Tendon jerk reflexes were tested in normal full-term neonates, 1 to 4 days old. Surface EMG recordings were made from the soleus muscle and its antagonist, tibialis anterior. While the neonate was in the supine position, taps were manually applied to the tendons of the soleus, tibialis anterior, and quadriceps muscles, the sole of the foot and the medial malleolus. For each tendon jerk response, 100 milliseconds of EMG data were recorded on a computer, with the state of consciousness of the infant and the force of hammer tap. Reflex EMG latencies and amplitudes were measured for each tapsite. Medians, means, standard deviations and cross-correlations were computed for each study variable.;Reflex-like EMGs were evoked by tapping sites not normally thought to excite muscle spindle afferents to the responding motoneuron pools. For example, the tibialis anterior muscle often responded strongly to Achilles and patellar tendon taps. The simultaneous activation of antagonist muscles by tendon taps in neonates is a novel finding. While other possible physiological and artifactual mechanisms are discussed, it is hypothesized that monosynaptic pathways of the spinal cord other than those classically described may contribute to these findings. Reciprocal excitation, a functional spinal cord pathway, has been proposed to explain these data; it is in contrast with the commonly accepted anatomical pathway of reciprocal inhibition. In the normal neonate, reciprocal excitation may be masked or eliminated in normal development. In perinatal injuries, the reciprocally excitatory pathway may be fostered to persist. Finally, in cervical spondylosis, reciprocal excitation may develop or become unmasked during progressive spinal cord obstruction.;Responses of normal neonates to tendon taps contrasted with the normal adult. Achilles tendon taps consistently evoked almost simultaneous EMG bursts from soleus and tibialis anterior muscles. The amplitudes of the tibialis anterior EMGs were comparable to the soleus responses. The EMG responses from the two antagonist muscles were nearly simultaneous. The latencies were consistent with a monosynaptic conduction delay.