Character of activation of medullary reticulospinal neurons by collaterals of pyramidal fibers

Abstract

The character of activation of medullary reticulospinal neurons by collaterals of pyramidal fibers was investigated in cats anesthetized with pentobarbital (40 mg/kg) or a mixture of chloralose (45 mg/kg) and pentobarbital (15 mg/kg). The experiments were carried out on animals after preliminary destruction of the contralateral red nucleus and division of the ipsilateral dorsolateral fasciculus in segment C4. A conditioning technique showed that pre- and postsynaptic effects arising in the medullary gigantocellular nucleus to stimulation of the cortex and of the isolated dorsolateral funiculus are due to activation of collaterals of pyramidal fibers projecting into the brain stem. In most reticulospinal neurons tested, stimulation of the fasciculus induced monosynaptic EPSPs. Their generation was due to influences transmitted via fast- and slow-conducting pyramidal fibers. Pyramidal fibers with different conduction velocities are distributed irregularly in the pyramidal tract in the cervical region of the spinal cord. Mainly slowly-conducting fibers are found in its medial zones and fast-conducting pyramidal fibers in its lateral zones. The results are evidence that in cats fibers of the pyramidal tract, running into the spinal cord, can activate medullary reticulospinal neurons directly.