Brainstem influences on biceps reflex activity and muscle tone in the anaesthetized rat

This study analyzes the effect of electrical stimulation of the locus coeruleus (LC) and adjacent brainstem structures on the tonic reflex (TVR), the tonic stretch reflex (TSR) and on muscle tone (MT) in anaesthetized rat. Increases in TVR. TSR and MT of the m. biceps were evoked from regions rostrally and ventrally of LC, the caudal pontine reticular nucleus, the cuneiform nucleus and from the ventral parts of the colliculus inferior. Stimulation of the LC did not influence biceps EMG activity. The results indicate that the observed facilitation of muscle activity is due to stimulation of parts of the mesencephalic locomotor region. It is discussed that the recorded increase in TVR, TSR and MT possibly is due to an excitatory action on α motoneurones on one hand and to an enhanced fusimotor drive on the other.