Lesions of Descending Motor Pathways (commonly known as "Caused by Lesions of Descending Motor Pathways") do not solely result from the loss of the corticospinal tract, but also indicate damage to other motor pathways. In this book, the classification system based on the term "extrapyramidal" is not used. Instead, the motor pathways can be classified based on their termination sites in the spinal cord and their respective roles in movement and posture control. The lateral pathways terminate in the lateral portions of the gray matter of the spinal cord, primarily targeting interneurons. These pathways influence reflex arcs involved in precise movements of distal limb ends and activation of supporting muscles in proximal limb ends. On the other hand, the medial pathways terminate in the medial ventral horn, connecting with interneurons that control axial musculature, contributing to balance, posture, and the control of proximal limb muscles. Although the terms "lateral" and "medial" are used to classify the descending motor pathways in this book, it should be noted that motor neuron dendritic trees are extensive and can potentially receive input from both medial and lateral system pathways. The corticospinal and corticobulbar tracts originate from various regions of the cerebral cortex, including the primary motor cortex, premotor cortex, supplementary motor area, cingulate motor area, and the somatosensory cortex. These tracts comprise pyramidal cells of different sizes from layer V of the cortex, including the Betz cells. However, Betz cells represent only a small percentage of the cells contributing to these tracts due to their exclusive presence in the primary motor cortex. Leaving the cortex, these tracts pass through the internal capsule, traverse the midbrain via the cerebral peduncle, travel through the basilar pons, and emerge as pyramids on the ventral surface of the medulla. While corticobulbar axons terminate in motor nuclei of cranial nerves in the brainstem, corticospinal fibers continue caudally. In the most caudal region of the medulla, approximately 90% of these fibers cross to the opposite side and descend as the lateral corticospinal tract in the contralateral lateral funiculus of the spinal cord. The lateral corticospinal axons primarily terminate on interneurons, as well as motor neurons, at all levels of the spinal cord. The remaining uncrossed axons continue caudally in the ventral funiculus on the same side, forming the ventral corticospinal tract of the medial system. Many of these fibers decussate at the spinal cord level at which they terminate.
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.