Porth's Essentials of Pathophysiology, 4e

881

Disorders of Neuromuscular Function

C h a p t e r 3 6

lowest level of the hierarchy occurs at the spinal cord, which contains the basic reflex circuitry needed to coordinate the function of the motor units. Above the spinal cord is the brain stem, and above the brain stem are the cerebellum and basal ganglia, structures that modulate the actions of the brain stem systems. Overseeing these supraspinal structures are the motor centers in the cerebral cortex. The highest level of func- tion, which occurs at the level of the frontal cortex, is concerned with the purpose and planning of motor movements. The efficiency of movement depends on input from sensory systems that operate in parallel with the motor systems. The Spinal Cord The spinal cord is the lowest level of motor hierarchi- cal organization. 2 It contains the neuronal circuits that mediate a variety of reflexes and automatic rhythmic movements. Similar circuits governing reflex movements of the face and mouth are located in the brain stem. The simplest circuits are monosynaptic, containing only a primary motor neuron. However, most reflexes are polysynaptic, involving more than one interposed inter- neurons. Interneurons and motor neurons also receive input from axons descending from higher centers. These supraspinal signals can modify reflex responses to peripheral stimuli by facilitating or inhibiting differ- ent populations of interneurons. They also coordinate movements through these interneurons. The Brain Stem The next level of motor hierarchy is in the brain stem. The brain stem contains many groups of neurons that project from the spinal gray matter. These projections are grouped into two main systems—the medial and lateral brain stem systems—which receive input from the cerebral cortex and subcortical nuclei and proj- ect to the spinal cord. 2 The medial descending systems contribute to the control of posture by integrating visual, vestibular, and somatosensory information. The lateral descending systems control more distal limb muscles and are thus more concerned with goal- directed movements, especially of the arm and hand. Brain stem circuits also control movements of the eyes and head. The Motor Cortex The motor cortex represents the highest level of motor function. Precise, skillful, and intentional movements of the distal and especially flexor muscles of the limbs and speech apparatus are initiated and controlled by the primary, premotor, and supplementary motor cor- tices located in the posterior part of the frontal lobe 1,3 (Fig. 36-1). These motor areas receive information from the thalamus and somatosensory cortex and, indirectly, from the cerebellum and basal ganglia. The primary motor cortex (Brodman area 4) is located on the medial surface of the brain, and adjacent portions of the central sulcus. It controls specific muscle

Supplementary (6,8)

Motor (4)

Premotor (6)

Somatosensory (3,1,2)

Primary visual (17)

Premotor cortex (8) (6)

Motor cortex (4)

Somatosensory cortex (3,1,2)

Frontal eye fields

(part of area 8)

Broca area (45,44)

Primary visual cortex (17)

Primary auditory cortex (44)

Vestibular cortex

movement sequences and is the first level of descending control for precise motor movements. The neurons in the primary motor cortex are arranged in a somatotopic array or distorted map of the body called the motor homunculus 4 (Fig. 36-2). The areas of the body that require the greatest dexterity have the largest cortical areas devoted to them. More than half of the primary motor cortex is concerned with controlling the muscles of the hands, of facial expression, and of speech. The premotor cortex (areas 6 and 8), which is located just anterior to the primary motor cortex, sends some fibers into the corticospinal tract but mainly innervates the primary motor cortex. Nerve signals generated by the premotor cortex produce much more complex “pat- terns” of movement than the discrete patterns generated by the primary motor cortex. For example, the move- ment pattern to accomplish a particular objective, such as throwing a ball or picking up a fork, is programmed by the prefrontal association cortex and associated tha- lamic nuclei. The supplementary motor cortex, which contains representations of all parts of the body, is located on the FIGURE 36-1. Primary motor cortex. ( Top ) The location of the primary, premotor, and supplementary cortex on the medial surface of the brain. (Bottom)The location of the primary and premotor cortex on the lateral surface of the brain.