395

In the nervous system, the somatosensory and motor systems are interrelated at all levels. At the level of the spinal cord, sensory information contributes to motor reflexes; in the brainstem, sensory information contributes to complex regulatory movements. At the level of the neocortex, sensory information represents the sizes, shapes, and positions of objects and records just-completed movements.

11-1 Hierarchy of Movement Control

Movement is organized hierarchically, using the entire nervous system (review Figure 11-1). The forebrain plans, organizes, and initiates movements, whereas the brainstem coordinates regulatory functions, such as eating and drinking, and controls neural mechanisms that maintain posture and produce locomotion. Many reflexes are organized at the level of the spinal cord and occur without the brain’s involvement.

11-2 Motor System Organization

Maps produced by stimulating the primary motor cortex show that it is organized topographically as a homunculus with different cortical areas capable of producing different movements. Motor cortex neurons initiate movement, produce movement, control movement force, and indicate movement direction. Disuse of a limb, as might result from a motor cortex injury, results in shrinkage of the limb’s cortical representation. This shrinkage can be prevented, however, if the limb can be somehow forced into use, as in constraint-induced therapy.

Two corticospinal pathways emerge from the motor cortex to the spinal cord. Lateral corticospinal axons project from cortical areas that control arm and hand movements. The lateral tract crosses from the contralateral brain hemisphere to form synapses with spinal interneurons and motor neurons located laterally in the spinal cord, and on the side opposite the brain hemisphere where the lateral tract formed. Anterior corticospinal tract axons project from the cortical motor regions that produce whole-body movements. The anterior tract synapses with interneurons and motor neurons located medially and ipsilaterally in the spinal cord.

Spinal cord interneurons and motor neurons also are topographically organized: lateral motor neurons project to digit, hand, and arm muscles to produce arm and hand movements, and medial motor neurons project to trunk muscles and mediate whole-body movements, including locomotion.

11-3 Basal Ganglia, Cerebellum, and Movement

Movement abnormalities result from damage to the basal ganglia or to the cerebellum. Both structures participate in movement control by modulating the movements initiated by the cortex. The basal ganglia regulate force; the cerebellum maintains accuracy and participates in learning.

11-4 Somatosensory System Receptors and Pathways

Distributed throughout the body, the somatosensory system consists of more than 20 types of specialized sensory neurons and receptors, each sensitive to a particular form of mechanical energy. Located in posterior dorsal root ganglia, each of these neurons carries sensory information into the spinal cord and the brain.

Neurons carrying proprioceptive (location and movement) information and haptic (touch and pressure) information have axons that ascend the spinal cord as the posterior spinothalamic tract. These fibers synapse in the posterior column nuclei at the base of the brain. From there axons cross to the other side of the brainstem to form the medial lemniscus, which ascends to the ventrolateral thalamus. Most of the ventrolateral thalamus cells project to the somatosensory cortex.

Nociceptive (pain, temperature, and itch) posterior root ganglion neurons synapse on entering the spinal cord. Their relay neurons cross the spinal cord to ascend to the thalamus as the anterior spinothalamic tract.

Because the two somatosensory pathways take somewhat different routes, unilateral spinal cord damage impairs proprioception and hapsis ipsilaterally below the site of injury and nociception contralaterally below the site.

11-5 Exploring the Somatosensory Cortex

The somatosensory system is represented topographically in parietal areas 3-1-2. The most sensitive body parts are accorded the largest somatosensory regions, as befits the body parts most capable of fine movements.

A number of sensory homunculi represent various sensory modalities, and these regions are hierarchically organized. If sensory input from anywhere in the body is cut off from the cortex by damage to sensory fibers, adjacent functional sensory cortex can expand into the now-unoccupied region.

Through the dorsal visual stream, the somatosensory cortex contributes to directing hand and body movements to visual targets. The somatosensory cortex also contributes to the ventral visual stream to produce perception of external objects, through which we can imagine the consequences of our movements.