Motor control is a complex system composed of several individual components that together produce movement. These include motor neurons, descending motor pathways and associated cortical areas, basal ganglia, and cerebellum. Pathways that are directly responsible for the voluntary activity of the muscles of the head, neck, and limbs are referred to as the direct activation pathway. The direct motor pathway is monosynaptic. Cortical upper motor neuron (UMN) project to and synapses on a lower motor neuron (LMN) located in the brainstem and spinal cord. Fibers from cortical neurons connect the UMN, LMN, and the skeletal muscle they innervate forming the descending pathways. The corticobulbar tract involves cortical UMNs that project to cranial nerve motor nuclei (LMN) in the brainstem. The corticospinal tract consists of UMN in the cortex projecting to the ventral gray matter of the spinal cord where the LMN cell bodies are located. The axons of LMNs synapse on skeletal muscle to initiate movement. Although not part of the direct motor pathways, the basal ganglia and cerebellum have profound regulatory influence on movement (see Chapter 17). Other descending pathways involved in movement but not originating in the cortex and involving multiple synapses are the rubrospinal tract, the tectospinal tract, reticulospinal tract, and the vestibulospinal tract (see Chapter 17). These four indirect pathways originate in brainstem nuclei. In addition to movement, motor pathways are important components of reflex pathways. Reflexes are involuntary (efferent) responses to sensory (afferent) input.
Stimulation of motor neurons in a specific area of the primary motor cortex results in the contraction of specific muscles. In other words, there is a direct relationship between areas of the motor cortex and regions of the body creating a cortical map, known as a homunculus ().
The proportions of the motor homunculus represent the relative number of motor units involved in control of that particular region. Thus, the homunculus indicates the amount of fine motor control in a given area. For example, the hands, face, and tongue are depicted as being quite large compared to the trunk.
Each a-motor neuron within a motor pool sends an efferent (motor) axon toward the periphery, which branches to form synapses called neuromuscular junctions (NMJs) on specific groups of individual muscle fibers.
Innervation is achieved by release of a neurotransmitter (acetylcholine) that is released from the axon of the LMN. Receptors present in the membrane of the plasma membrane of the muscle fiber bind the acetylcholine, which will facilitate the transmission of the impulse resulting in contraction of the muscle fiber.
The group of individual muscle fibers innervated by a single a-motor neuron and its efferent axon constitutes a motor unit. An action potential generated by a motor neuron results in the simultaneous contraction of all muscle fibers in the motor unit.
Any given muscle contains multiple types of motor units that are interspersed throughout different regions of the muscle and provides a selective level of motor unit activation and control. Difference in motor unit fiber distribution reflects difference in muscle function.
The relative proportion of a specific type of motor unit along with the innervation ratio (motor unit size), the motor unit distribution within a muscle, and the overall number of motor units found in a muscle varies between different muscles and reflects muscle function.
Fig. 16.1 Sensory and motor systems. The sensory system and motor system are so functionally interrelated they may be described as one (sensorimotor system). Cortical areas of the sensorimotor system. Lateral view of the left hemisphere. (Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy. Second Edition. © Thieme 2012. Illustrations by Markus Voll and Karl Wesker.)
Fig. 16.2 (a) Motor homunculus. (b) Sensory homunculus. The relationship between the motor cortex and the rest of the body can be illustrated by the homunculus. This cortical map represents the area of the brain that is responsible for motor function in the rest of the body, as well as the relative number of motor units as seen by the proportions of the structures.
The motor fibers continue to descend through the brainstem (a). As they approach the crus cerebri (the anterior portion of the cerebral peduncle) (b), they reorganize such that the fibers associated with the head are more medial (corticobulbar) and those associated with the legs are more lateral (corticospinal). The fibers that will innervate the upper limbs are in between (corticospinal).
Approximately 85% of the corticospinal fibers will cross as the lateral corticospinal tract. They descend in the lateral funiculus of the spinal cord and will ultimately innervate the more distal musculature.
The remaining 15% of corticospinal fibers that descend uncrossed are called the anterior (ventral) corticospinal tract. These fibers run in the anterior funiculus of the spinal cord and will innervate the more proximal musculature.