Arm movements are influenced by the cerebellum and brainstem neurons. Cerebellum neurons influence upper extremity movements through their ascending cortical connections. Brainstem neurons influence arm movements through their descending pathways to the brachial somatic motor center. These subcortical fibers (from the cerebellum and brainstem) are concerned with regulating the automatic component of movements.

Figure 203.1.— Schematic representation of the motor systems of the face, arms, and legs, and central and peripheral nervous systems structures involved in limb movements. The colored rectangles indicate the location of weakness produced by damage to the different components of the somatic motor system. U: upper motor neurons; V: ventricles; T: thalamus; UQ: upper quadrant; FN: facial nerve; LQ: lower quadrant; L: lower motor neurons; BP: brachial plexus; LSP: lumbosacral plexus.

The brachial somatic motor center is located in the cervical enlargement of the spinal cord. The brachial somatic motor center or brachial center consists of a pair of anterior horn motor neuron columns extending from C5 to T1 spinal segments. The axons of these neurons exit through the ventral surface of the spinal cord forming the ventral roots (Figure 203.2). The ventral roots travel for a short distance to join the dorsal roots. The dorsal roots are made of the central and peripheral axons of the dorsal ganglia neurons. The union of the ventral and dorsal roots form the spinal nerves. The spinal nerves are very short. The spinal nerves exit the spinal canal through the intervertebral foramina and split into dorsal and ventral rami. The dorsal rami innervate the paraspinal muscles and the sensory receptors of the dorsal torso. The ventral rami form the brachial plexus (Figure 203.2).

Figure 203.2.— Schematic representation of the formation of the spinal nerves and their relation to the ventral and dorsal roots and dorsal and ventral rami.