Diurnal rhythms in primate spinal reflexes and accompanying cortical somatosensory evoked potentials

We recorded spinal reflexes and cortical somatosensory evoked potentials (SEPs), elicited by stretching the biceps or the triceps muscle or by electrically stimulating the posterior tibial nerve, from monkeys throughout the day. Amplitudes of the spinal stretch reflex (SSR) and of its electrically evoked analogue, the H-reflex, varied diurnally: both were greatest midway through the lights-off period and smallest during the lights-on period. Stretch-evoked and electrically evoked SEP amplitudes also varied diurnally, but were out of phase with the spinal reflex rhythms. The H-reflex is elicited by direct stimulation of the nerve and thus, unlike the SSR, bypasses the muscle spindle. The H-reflex diurnal rhythm and the phase difference between the spinal reflex and SEP diurnal rhythms indicate that these rhythms are mediated at least in part by central mechanisms. Furthermore, both the spinal reflex and SEP diurnal rhythms appeared to be entrained by the light-dark cycle, which suggests that they may be coupled to the same oscillator. Besides their theoretical importance, these rhythms have important implications for experimental and clinical studies of spinal reflexes and SEPs. These rhythms are especially pertinent to the interpretation of long-term monitoring studies, as are often carried out in the Intensive Care Unit and during lengthy neurosurgical procedures.