The distribution of the spinal terminals (boutons) of the pyramidal tract, determined by experimental degeneration

Abstract

The fact that boutons terminaux in the spinal cord degenerate following section of nerve fibers has been used to determine the endings of the pyramidal tract fibers in the cat and the monkey. The normal synapse in the monkey resembles that of the cat in consisting of small loop-like boutons terminaux and boutons de passage occurring around cell bodies and dendrites of the cord. The cats killed 3 days after removal of the motor cortex showed degenerated boutons in the cord, on the crossed side on practically all cell groups except the ventro-mesial group and Clarke''s column; on the uncrossed side, as widely scattered but less numerous. These degenerated terminals were numerous in all of the cervical and post-thoracic segments, but were rare in the thoracic region. In a monkey in which most of the arm area of the motor cortex of 1 hemisphere was destroyed, degenerated boutons similar to those of the cat were thickly distributed on the crossed side, to both the dorsal and ventral horns of segments C2-C8, but very sparsely on the uncrossed side. These studies are in agreement with Sherrington''s discovery that fibers from the pyramidal tract ramify around ventral horn cells in the cord of the chimpanzee, and confirm histologically the suggestion of Cooper and Denny-Brown that the connection between the motor cortex and the spinal motoneurone is very direct, probably involving only 1 synapse. In addition to the terminals found on the ventral horn cells, many others were seen on cells of the dorsal horn. It is roughly estimated that the number of boutons in the cat''s cord which are terminals of pyramidal tract fibers is about 40,000 in the crossed side, and 7500 in the uncrossed side; in the monkey, the fibers from the arm area of the motor cortex terminate by means of approximately 30,000 boutons in the crossed side and only about 800 in the uncrossed side.