Caused by Lesions of Descending Motor Pathways ”) are apparently not caused solely by loss of the corticospinal tract but also reflect damage to additional motor pathways. The
CHAPTER 9 Organization of Motor Function term extrapyramidal is even more problematic. Thus this classification system is not used in this book.
Another way of classifying the motor pathways is based on their sites of termination in the spinal cord and the consequent differences in their roles in the control of movement and posture. The lateral pathways terminate in the lateral portions of the spinal cord’s gray matter (
Fig. 9.13 ). The lateral pathways can excite motor neurons directly, although interneurons are their main target. They influence reflex arcs that control fine movement of the distal ends of limbs, as well as those that activate supporting musculature in the proximal ends of limbs. The medial pathways end in the medial ventral horn on the medial group of interneurons (see
Fig. 9.13 ). These interneurons connect bilaterally with motor neurons that control the axial musculature and thereby contribute to balance and posture. They also contribute to the control of proximal limb muscles. In this book, the terms lateral and medial are used to classify the descending motor pathways. However, even this terminology is not perfect, partly because although motor neuron cell bodies form localized columns, motor neuron dendritic trees are rather large and typically span most of the ventral horn. Thus any motor neuron can potentially receive input from so-called medial or lateral system pathways.
The Lateral System
The corticospinal and corticobulbar tracts originate from a wide region of the cerebral cortex. This region includes the primary motor, premotor, supplementary, and cingulate motor areas of the frontal lobe and the somatosensory cortex of the parietal lobe. The cells of origin of these tracts include both large and small pyramidal cells of layer V of the cortex, including the giant pyramidal cells of Betz. Although Betz cells are a defining feature of the primary motor cortex, they represent a small minority (<5%) of the cells that contribute to these tracts, in part because they are found only in the primary motor cortex, and even there they represent a minority of the cells contributing to the tracts. These tracts leave the cortex and enter the internal capsule, then traverse the midbrain in the cerebral peduncle, pass through the basilar pons, and emerge to form the pyramids on the ventral surface of the medulla (see
Fig. 9.13A). The corticobulbar axons leave the tract as it descends in the brainstem and terminate in the motor nuclei of the various cranial nerves. The corticospinal fibers continue caudally, and in the most caudal region of the medulla, about 90% of them cross to the opposite side. They then descend in the contralateral lateral funiculus as the lateral corticospinal tract. The lateral corticospinal axons terminate at all spinal cord levels, primarily on interneurons, but also on motor neurons. The remaining uncrossed axons continue caudally in the ventral funiculus on the same side as the ventral corticospinal tract, which belongs to the medial system. Many of these fibers ultimately decussate (cross) at the spinal cord level at which they terminate.
•Fig. 9.13 Descending Motor Pathways. Majorpathwaysconnectingthecorticalandbrainstemmotorareastothespinalcordareshown.A, Lateralsystempathways,corticospinal(red) andrubrospinal(blue) pathways.NotethattheventralcorticospinalpathwayispartofthemedialsystembutisshowninA forsimplicity.B, Medialsystempathways,medullary(blue) andpontine(green) reticulospinalandlateralvestibulospinal(red) pathways.C-B,corticobulbar;C-P,corticopontine;C-S,corticospinal;LCST,lateralcorticospinaltract;VCST,ventralcorticospinaltract.