The Upper Motor Neurons (Motor Systems) Part 1

The process of movement is a highly complex one. Even a supposedly simple event, such as walking, involves complex mechanisms regulating reflex responses and postural and voluntary motor patterns. In using walking as our model, it is important to note the patterns of movements of the legs and arms. When the left leg is extended, the right leg is normally flexed; in addition, the left arm will be extended backwards, while the right arm is flexed somewhat in a forward position. The entire sequence of events requires use of different regions of the central nervous system (CNS). These regions include all levels of the neuraxis, beginning with the spinal cord and extending rostrally through the brainstem, including the cerebellum and forebrain regions of the basal ganglia and cerebral cortex.

The key structures of the CNS that relate to motor functions are illustrated schematically in Figure 19-1. As shown in this illustration, the lowest, and perhaps most simple, level of organization is present within the spinal cord. This level of the CNS provides the mechanisms for reflex motor functions. It also serves as the final common path for neurons that innervate skeletal muscle, which constitute the lower motor neurons for motor responses. It should be further noted that motor neurons of cranial nerves (i.e., special visceral efferent and general somatic efferent neurons) constitute lower motor neurons for motor functions associated with the head region.


The lower motor neurons for both the body and head region are controlled by upper motor neurons. Upper motor neurons are defined as those neurons of the brain that innervate lower motor neurons of the spinal cord and brainstem, either directly or through an interneuron. Therefore, an upper motor neuron may arise from the brainstem or cerebral cortex. A number of pathways arise from various levels of the brainstem and descend to the spinal cord. In doing so, these regions of the brainstem exert different influences on the activity of spinal motor neurons, resulting in directed effects upon flexor and extensor reflexes, including postural mechanisms. The pathways of importance include the medial and lateral vestibulospinal tracts, the medial and lateral reticulospinal tracts, the rubrospinal tract, and the tectospinal tract.

The most significant of all the upper motor neurons are those that arise from the cerebral cortex. As indicated in previous topics, the upper motor neurons that project to the spinal cord are called the corticospinal tracts, and the ones that project to lower motor neurons of the brain-stem are called the corticobulbar tracts. They provide the anatomical substrates for voluntary control of movement. A detailed analysis of the anatomical organization, functional properties, and neurologic disorders associated with these pathways is presented later in this topic.

Organization of the motor systems. Spinal cord reflex mechanisms involve lower motor neurons and are subject to supraspinal control. The brain regions that have direct control over the spinal cord include the brainstem and cerebral cortex. Brainstem pathways that project to the spinal cord include the reticulospinal, vestibulospinal, rubrospinal, and tectospinal tracts. The cerebral cortex gives rise to both corticospinal and corticobulbar fibers. Corticospinal fibers are essential for voluntary control over fine movements, mainly of the distal extremities. Corticobulbar fibers contribute to the control of spinal cord indirectly, by acting on neurons of the brainstem that project to the spinal cord. Other corticobulbar fibers innervate lower motor neurons of the brainstem (cranial nerves) and provide the substrate and mechanism for voluntary movements of the head region. Two other regions, the basal ganglia and cerebellum, play important roles in motor functions. The basal ganglia affect motor systems by acting on neurons in the precentral and premotor regions that comprise the larger part of the corticospinal tract. The cerebellum affects motor function by acting on neurons in both the brainstem and the cerebral cortex that directly control motor functions of the spinal cord.

FIGURE 19-1 Organization of the motor systems. Spinal cord reflex mechanisms involve lower motor neurons and are subject to supraspinal control. The brain regions that have direct control over the spinal cord include the brainstem and cerebral cortex. Brainstem pathways that project to the spinal cord include the reticulospinal, vestibulospinal, rubrospinal, and tectospinal tracts. The cerebral cortex gives rise to both corticospinal and corticobulbar fibers. Corticospinal fibers are essential for voluntary control over fine movements, mainly of the distal extremities. Corticobulbar fibers contribute to the control of spinal cord indirectly, by acting on neurons of the brainstem that project to the spinal cord. Other corticobulbar fibers innervate lower motor neurons of the brainstem (cranial nerves) and provide the substrate and mechanism for voluntary movements of the head region. Two other regions, the basal ganglia and cerebellum, play important roles in motor functions. The basal ganglia affect motor systems by acting on neurons in the precentral and premotor regions that comprise the larger part of the corticospinal tract. The cerebellum affects motor function by acting on neurons in both the brainstem and the cerebral cortex that directly control motor functions of the spinal cord.

Two other systems play important roles in the regulation of motor functions. These include the basal ganglia and cerebellum. The basal ganglia participate in the control of movement by receiving significant inputs from the cerebral cortex and feeding back signals to different regions of the frontal cortex involved in the initiation of movement. In this manner, the basal ganglia serve to modulate the activity of neurons of the motor regions of the cortex. The cerebellum receives inputs from most parts of the CNS that contribute to motor functions. In turn, it sends back messages to each of these regions. The presence of these feedback circuits enables the cerebellum to serve as a principal integrator of motor function by synchronizing the output messages distributed to each of these regions at any given point in time.

The CORTiCOSPINAL TrACT

As noted earlier, the corticospinal tract is crucial for the expression of precise, voluntary movements. In attempting to develop an understanding of the nature of the corticospi-nal tract, it is important to answer the following questions:

1. From where does this tract originate?

2. What is the anatomical organization of the tract as it passes caudally through the brain to the spinal cord?

3. How are the fibers distributed within the spinal cord?

4. What are the important sources of inputs that corticos-pinal neurons must receive in order to function properly?

5. What are the differential contributions of the descending components of the corticospinal tact?

6. What are the clinical manifestations of lesions that affect the corticospinal tract, and how can these be understood in terms of the basic anatomical and physiological properties of this pathway?

Origin of the Corticospinal Tract

The corticospinal (or pyramidal) tract arises from three different regions of the cortex (Fig. 19-2). Approximately 30% of the fibers arise from the precentral gyrus (area 4, which is referred to as the primary motor cortex called "MI" [where I is roman numeral I]). Forty percent of the fibers arise from the postcentral gyrus (which is referred to as the primary somatosensory cortex [S-1] and includes areas 3, 1, and 2). The remaining 30% of the fibers originate from the region immediately rostral to the precentral gyrus (area 6, called the supplemental motor area [SMA] and the premotor cortex [PMC]). While both regions of area 6 contribute fibers to the cor-ticospinal tract, the larger majority arises from the supplemental motor area.

Both the precentral and postcentral gyri are somato-topically organized. Electrical stimulation of the dorsal and medial aspect of the precentral gyrus in humans produces movements associated with the lower limb, while stimulation of more lateral aspects of the motor cortex produces movements of the upper limb. Moreover, stimulation of the far lateral aspect of the precentral gyrus produces movements of the face and tongue. This functional representation of the pre-central gyrus is referred to as a motor homunculus (Fig. 19-3). A similar homunculus is also present for the primary somatosensory cortex.

The regions of the cerebral cortex that give rise to the corticospinal tract. MI = primary motor cortex; PMC = premotor cortex; PPC = posterior parietal cortex; SI = primary somatosensory receiving area; SMA = supplementary motor area. Note that the PPC does not contribute to the corticospinal tract but does modulate its activity.


FIGURE 19-2 The regions of the cerebral cortex that give rise to the corticospinal tract. MI = primary motor cortex; PMC = premotor cortex; PPC = posterior parietal cortex; SI = primary somatosensory receiving area; SMA = supplementary motor area. Note that the PPC does not contribute to the corticospinal tract but does modulate its activity.

The relative homuncular representation of the primary motor cortex reveals the relative sizes of the regions of the primary motor cortex, which represent different parts of the body as determined by electrical stimulation experiments.

FIGURE 19-3 The relative homuncular representation of the primary motor cortex reveals the relative sizes of the regions of the primary motor cortex, which represent different parts of the body as determined by electrical stimulation experiments.

Histology of the Motor Cortex

Based on the histological appearance of the gray matter, the cerebral cortex typically has six layers.In brief, there are generally two layers of granule cells (an external and internal granule cell layer), which receive information mainly from the thalamus and other regions of the cortex, and two layers of pyramidal cells (an external and internal pyramidal cell layer), which serve as the origins of the efferent pathways of the cortex. In motor regions of the cortex, the pyramidal cell layers are of much greater size than the granule cell layers, and the reverse holds true for sensory regions. The corti-cospinal tracts arise from the internal pyramidal cell layer situated mainly in layer V (Fig. 19-4). Pyramidal cells lying in other cortical layers, as well as layer V, project to different areas of the CNS. For example, the cortical pyramidal cells of layer III project to both the ipsilateral and the contralateral cortex (as axons of the corpus cal-losum), while pyramidal cells of layers V to VI give rise to descending fibers that reach the spinal cord, brainstem, and thalamus.

Course of the Corticospinal Tract

Pyramidal cell axons that exit the gray matter of cortex enter the white matter and internal capsule first. Within the internal capsule and crus cerebri, pyramidal tract fibers are somatotopically organized (see Fig. 9-12). Corti-cospinal fibers are contained within the posterior limb, with those fibers associated with the arm region of the cortex located slightly closer to the genu of the internal capsule than those associated with the leg. Note that cor-ticobulbar fibers are located within the region of the genu. As the fibers reach the crus cerebri of the midbrain, they become reorganized in a slightly different way.Collectively, corticobulbar and corticospinal fibers are situated within the middle three fifths of the crus cerebri. Fibers associated with the leg region are located in a more lateral position than those associated with the arm, whereas fibers associated with the head region (i.e., corticobulbar fibers) are located medial to the corticospinal fibers. As the fibers reach the lower brain-stem, they enter the pyramid and continue to pass cau-dally to the spinal cord-medulla junction. At that level, 90% of the fibers cross over to the opposite side in the pyramidal decussation and descend through the lateral funiculus of the contralateral spinal cord, largely as the lateral corticospinal tract. The remaining 10% of the fibers, which remain uncrossed, descend into the spinal cord, 8% as the anterior corticospinal tract, and 2% as the uncrossed lateral corticospinal tract. Of these 10% of fibers, 8% (anterior corticospinal tract) cross over to the contralateral side. Ultimately, 98% of the corticospinal tract fibers project to the contralateral spinal cord. Thus, a scant 2% of the lateral corticospinal tract remains ipsi-lateral over its entire course.

Histological appearance of the motor cortex displaying the specific layers which give rise to the varied efferent projections of this region of cortex. Afferent fibers to these layers are also shown.

FIGURE 19-4 Histological appearance of the motor cortex displaying the specific layers which give rise to the varied efferent projections of this region of cortex. Afferent fibers to these layers are also shown.

The distribution of axon terminals in the spinal cord of the monkey (shown as dots in spinal cord) as determined by autoradiographic tracing procedures. Depicted also are the sites of origin of the pathway in the motor and somatosensory cortices (top). Most fibers, which are uncrossed (2% of the total corticospinal tract), pass in the anterior corticospinal tract and terminate mainly in the gray matter in the medial aspect of the ventral horn, contacting neurons that innervate axial and proximal muscles, and also in the dorsal horn, contacting somatosensory neurons (bottom). Crossed fibers also supply both the dorsal and ventral horn. Fibers that issue from the postcentral gyrus (depicted in purple) supply the dorsal horn (also shown in purple), whereas those that arise from the motor cortex (depicted in red) supply the ventral horn (also depicted in red).

FIGURE 19-5 The distribution of axon terminals in the spinal cord of the monkey (shown as dots in spinal cord) as determined by autoradiographic tracing procedures. Depicted also are the sites of origin of the pathway in the motor and somatosensory cortices (top). Most fibers, which are uncrossed (2% of the total corticospinal tract), pass in the anterior corticospinal tract and terminate mainly in the gray matter in the medial aspect of the ventral horn, contacting neurons that innervate axial and proximal muscles, and also in the dorsal horn, contacting somatosensory neurons (bottom). Crossed fibers also supply both the dorsal and ventral horn. Fibers that issue from the postcentral gyrus (depicted in purple) supply the dorsal horn (also shown in purple), whereas those that arise from the motor cortex (depicted in red) supply the ventral horn (also depicted in red).

Distribution of the Corticospinal Fibers Within the Spinal Cord

Fibers contained within the corticospinal tract are distributed throughout the entire rostrocaudal extent of the spinal cord. The largest components of these fibers terminate at lower cervical and lumbar levels of the cord. Within a given level of the spinal cord, corticospinal fibers terminate within both the dorsal and ventral horns as well as medial and lateral cell groups (Fig. 19-5). Anatomical and physiological studies have shown that fibers that originate from the primary motor, supplemental, or premotor cortices project primarily to interneurons of the ventral horn of the spinal cord, which then synapse upon motor horn cells, whereas fibers that arise from the primary somatosensory region of cortex project primarily to the dorsal horn of the spinal cord. Moreover, frontal lobe fibers that form the anterior corticospinal tract innervate more medial regions of the ventral gray matter, whereas the lateral corticospinal tract innervates more lateral cell groups of the ventral horn.

Functions

The functional significance of these projections can be briefly summarized as follows. Recall that the ventral horn is organized in such a manner that neurons located medially innervate the axial musculature, whereas neurons located more laterally innervate the distal musculature.Therefore, lateral corticospinal fibers that innervate the cervical and lumbar cord serve to control fine movements of the extremities, whereas axons from the anterior corticospinal tract (and elsewhere) that innervate the medial aspect of the ventral horn serve to regulate postural mechanisms. Furthermore, corticospinal neurons that innervate the dorsal horn serve to modulate primary sensory afferent information to the cerebral cortex rather than to produce movement.

Knowledge of the afferent supply to the regions of cortex that give rise to the corticospinal tract provides clues concerning the possible functions of each of these components. As such, the discussion that follows considers such inputs to provide a better understanding of the functions associated with the motor regions of the cortex.

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