LFC Corticospinal Tract
THE CORTICOSPINAL TRACT
The larges, best defined motor pathway is a single neuron pathway that extends from the cerebral cortex to the spinal cord called the corticospinal tract. This pathway provides a direct route by which information can travel from the cerebral cortex to the brainstem and spinal cord without an intervening synapse. Its major function is to effect voluntary activity, in particular, skilled movements under conscious control. the corticospinal tract descends from the cerebral cortex through the white matter of the cerebral hemispheres, the pyramids in the brainstem, and the spinal cord to end on ventral horn cells. The axons cross the midline at the junction of the brainstem and spinal cord to end on the opposite side. The effectiveness of this pathway depends on an intact final common pathway to carry information to the muscles.
The corticospinal tract is the route by which the motor areas of the cerebral cortex in each hemisphere control motor neurons in the ventral horn on the opposite side of the spinal cord and in the motor nuclei of the brainstem. The fibers in the corticospinal tract are corticospinal and corticobulbar. those traveling to the spinal cord are called corticospinal or pyramidal, tract. Those ending on brainstem nuclei are corticobulbar fibers. The neurons from which these tracts arise are known as upper motor neurons. The major function of the corticospinal pathway is to initiate and control skilled voluntary activity.
Each corticospinal tract arises primarily from cells in the cortex of the frontal lobe of one hemisphere and descends through the corona radiata into the internal capsule. The tract passes from the internal capsule via the cerebral peduncles to the base of the brainstem where it forms the medullary pyramids. At the junction between the medulla and spinal cord, most of the fibers in each pyramid cross the midline (the corticospinal decussation) to lie in the lateral funiculus of the opposite half of the spinal cord. These crossed fibers form the lateral corticospinal tract of the cord.
The corticospinal tract is formed by axons of neurons located in the primary motor cortex, the lateral premotor cortex, the supplementary motor area (or medial premotor cortex, and the anterior cingulate motor area (on the medial surface of the hemisphere). All these areas are closely interconnected and project to the ventral horn. The primary motor cortex occupies the anterior lip of the central sulcus of Rolando and the adjacent precentral gyrus (area 4). The primary motor cortex integrates input from multiple sources and has a somatotopic organization, with the contralateral body represented upside down: the head area is located above the fissure of Sylvius, the upper extremity next (with the thumb and index finger in proximity to the face), the trunk interposed between the shoulder and hip areas high on the convexity, and the lower limb representation extending on o the paracentral lobule in the longitudinal fissure. the frontal eye field (area 9) contains neurons involved in the generation of spontaneous and visually guided rapid eye movements. Broca's area is immediately ventral to the motor area of the left cerebral hemisphere near where the face is represented. Neurons in Broca's area participate in the motor programming for speech.
Axons from the motor cortex converge in the corona radiata toward the internal capsule, where they are compactly gathered in a topographic localization. The corticobulbar fibers occupy a more anterior location in the posterior limb of the internal capsule than the corticospinal fibers.
The pyramidal fibers remain grouped together as they pass from the internal capsule to the cerebral peduncle in the midbrain. In the midbrain, the corticospinal and corticobulbar fibers occupy the middle two-thirds of the cerebral peduncle, with the corticobulbar fibers being more medial. During their course in the brainstem, the corticobulbar fibers leave the pyramidal pathway at several levels, some crossing the midline and some remaining uncrossed. These fibers synapse in the motor centers and nuclei of the cranial nerves - trigeminal, facial, vagus, spinal accessory and hypoglossal. The fibers in the medulla form the medullary pyramids. At the lower border of the medulla, the main pyramidal decussation occurs with about 80% of the fibers crossing to the opposite side of the spinal cord.
In the spinal cord, the crossed pyramidal fibers occupy the lateral column (the lateral corticospinal tract). Because of the decussation of most of the fibers of the pyramidal tracts, the voluntary movmeents of one side of the body are under the control of the opposite cerebral hemisphere.
More diffuse, extrapyramidal pathways act indirectly on the final common pathway, mediating the enormous number of automatic activities involved in normal motor function. For example, the maintenance of erect posture when sitting or standing requires the corordinated contraction of many muscles This coordination is under subconscious control and is mediated by the reticulospinal, vestibulospinal and rubrospinal tracts.
Damage to the motor pathways result in characteristic clinical patters. There is weakness or paralysis of muscles, especially the distal muscles, especially the distal muscles. The impairment is greatest for fine motor movements, skilled movements and movements under voluntary control. The distribution of the weakness is a function of the site of the lesion. If the lesion os localized in a limited area of cortex, then a single limb or one side of the face only may be involved. If the lesion involves only the pyramidal tract fibers in the pyramids of the medulla, one side of the body below the level of the lesion is affected. Spasticity and hyper-reflexia are the result of the loss of activity of inhibitory interneurons (reticulospinal, the lateral vestibulospinal, and pontine reticulospinal). A lesion at any corticospinal level produces the upper motor neuron syndrome of distal weakness, loss of cutaneous reflexes, and Babinski's sign with increased muscle tone and reflexes
The larges, best defined motor pathway is a single neuron pathway that extends from the cerebral cortex to the spinal cord called the corticospinal tract. This pathway provides a direct route by which information can travel from the cerebral cortex to the brainstem and spinal cord without an intervening synapse. Its major function is to effect voluntary activity, in particular, skilled movements under conscious control. the corticospinal tract descends from the cerebral cortex through the white matter of the cerebral hemispheres, the pyramids in the brainstem, and the spinal cord to end on ventral horn cells. The axons cross the midline at the junction of the brainstem and spinal cord to end on the opposite side. The effectiveness of this pathway depends on an intact final common pathway to carry information to the muscles.
The corticospinal tract is the route by which the motor areas of the cerebral cortex in each hemisphere control motor neurons in the ventral horn on the opposite side of the spinal cord and in the motor nuclei of the brainstem. The fibers in the corticospinal tract are corticospinal and corticobulbar. those traveling to the spinal cord are called corticospinal or pyramidal, tract. Those ending on brainstem nuclei are corticobulbar fibers. The neurons from which these tracts arise are known as upper motor neurons. The major function of the corticospinal pathway is to initiate and control skilled voluntary activity.
Each corticospinal tract arises primarily from cells in the cortex of the frontal lobe of one hemisphere and descends through the corona radiata into the internal capsule. The tract passes from the internal capsule via the cerebral peduncles to the base of the brainstem where it forms the medullary pyramids. At the junction between the medulla and spinal cord, most of the fibers in each pyramid cross the midline (the corticospinal decussation) to lie in the lateral funiculus of the opposite half of the spinal cord. These crossed fibers form the lateral corticospinal tract of the cord.
The corticospinal tract is formed by axons of neurons located in the primary motor cortex, the lateral premotor cortex, the supplementary motor area (or medial premotor cortex, and the anterior cingulate motor area (on the medial surface of the hemisphere). All these areas are closely interconnected and project to the ventral horn. The primary motor cortex occupies the anterior lip of the central sulcus of Rolando and the adjacent precentral gyrus (area 4). The primary motor cortex integrates input from multiple sources and has a somatotopic organization, with the contralateral body represented upside down: the head area is located above the fissure of Sylvius, the upper extremity next (with the thumb and index finger in proximity to the face), the trunk interposed between the shoulder and hip areas high on the convexity, and the lower limb representation extending on o the paracentral lobule in the longitudinal fissure. the frontal eye field (area 9) contains neurons involved in the generation of spontaneous and visually guided rapid eye movements. Broca's area is immediately ventral to the motor area of the left cerebral hemisphere near where the face is represented. Neurons in Broca's area participate in the motor programming for speech.
Axons from the motor cortex converge in the corona radiata toward the internal capsule, where they are compactly gathered in a topographic localization. The corticobulbar fibers occupy a more anterior location in the posterior limb of the internal capsule than the corticospinal fibers.
The pyramidal fibers remain grouped together as they pass from the internal capsule to the cerebral peduncle in the midbrain. In the midbrain, the corticospinal and corticobulbar fibers occupy the middle two-thirds of the cerebral peduncle, with the corticobulbar fibers being more medial. During their course in the brainstem, the corticobulbar fibers leave the pyramidal pathway at several levels, some crossing the midline and some remaining uncrossed. These fibers synapse in the motor centers and nuclei of the cranial nerves - trigeminal, facial, vagus, spinal accessory and hypoglossal. The fibers in the medulla form the medullary pyramids. At the lower border of the medulla, the main pyramidal decussation occurs with about 80% of the fibers crossing to the opposite side of the spinal cord.
In the spinal cord, the crossed pyramidal fibers occupy the lateral column (the lateral corticospinal tract). Because of the decussation of most of the fibers of the pyramidal tracts, the voluntary movmeents of one side of the body are under the control of the opposite cerebral hemisphere.
More diffuse, extrapyramidal pathways act indirectly on the final common pathway, mediating the enormous number of automatic activities involved in normal motor function. For example, the maintenance of erect posture when sitting or standing requires the corordinated contraction of many muscles This coordination is under subconscious control and is mediated by the reticulospinal, vestibulospinal and rubrospinal tracts.
Damage to the motor pathways result in characteristic clinical patters. There is weakness or paralysis of muscles, especially the distal muscles, especially the distal muscles. The impairment is greatest for fine motor movements, skilled movements and movements under voluntary control. The distribution of the weakness is a function of the site of the lesion. If the lesion os localized in a limited area of cortex, then a single limb or one side of the face only may be involved. If the lesion involves only the pyramidal tract fibers in the pyramids of the medulla, one side of the body below the level of the lesion is affected. Spasticity and hyper-reflexia are the result of the loss of activity of inhibitory interneurons (reticulospinal, the lateral vestibulospinal, and pontine reticulospinal). A lesion at any corticospinal level produces the upper motor neuron syndrome of distal weakness, loss of cutaneous reflexes, and Babinski's sign with increased muscle tone and reflexes