The corticospinal tract is the primary voluntary motor control system of the central nervous system.
Their axons travel from the cerebral cortex to the spinal cord, and are partly responsible for our ability to move limbs and the trunk and to perform, along with other nerve pathways, finer and more precise movements.
In this article, we explain what the corticospinal tract is, what are its main features and functions it performs, as well as the clinical signs and symptoms that occur due to the injury of this nervous tract.
The corticospinal tract: definition and characteristics
The central nervous system is a complex network of components that allow an organism to interact with its environment. It consists of several parts which perform different functions. The upper motor neurons are located in the cerebral cortex, which send movement signals to the lower motor neurons that tell the muscles whether to contract or relax.
The corticospinal tract it is formed by axons of motor neurons that travel from the motor cortex (primary motor cortex, supplementary motor area, and premotor cortex) to the spinal cord. These neurons control the voluntary movements of the limbs and trunk. The small nuclei of neurons also originate from areas of the parietal lobe (ascending parietal gyrus and superior parietal cortex).
This motor system is one of the last to develop, as the fibers of the corticospinal tract have just been myelinated about 2 years after birth. One of the characteristic aspects of this axon bundle is the so-called pyramidal decussation: This means that a large part of the corticospinal fibers (about 75-80%) crosses the contralateral side of the medulla oblongata, and the nerve fibers on the left side of the brain pass to the right hemibody, and vice versa.
The pyramidal decussation leads to an obvious conclusion, namely that the areas of the brain which control the right side of the body are located in the left hemisphere, and those which control the left side are in the right hemisphere. This can be checked when an injury occurs in one of the hemispheres; for example, some patients who have suffered damage to the left hemisphere may experience paralysis on the right side of their body.
The corticospinal tract it comes from various areas of the cerebral cortex, Mainly in the primary motor cortex (Brodmann zone 4) and in the premotor zones (Brodmann zone 6). However, they can also originate from the somatosensory cortex, cingulate gyrus, and parietal lobe. This pathway connects these areas of the brain to the gray matter of the spinal cord.
The bundle of axons from the corticospinal tract travel from the cortex, through the deep white matter, to the brainstem. Most of them they extend from side to side in the lower brainstem and descend into the contralateral white matter of the cord, Which is called the lateral corticospinal tract.
About 15% of axons do not decussate and descend like the ventral corticospinal tract. In addition to the corticospinal tract, this system contains indirect pathways that project first to the motor nuclei in the brainstem and then to the spinal cord.
The gray matter of the spinal cord is the target of this axon bundle. Corticospinal projections from the primary motor and premotor cortical areas are directed to the motor regions of the spine, which are made up of the deeper lamellae of the dorsal shaft, middle area, and dorsal shaft. The corticospinal system too it projects from the somatosensory cortex to the sensory processing centers of the dorsal shaft and brainstem to regulate proprioceptive information generated during movement.
The corticospinal tract conforms an essential function in the control of limb and trunk movements, Both in competence and precision in their implementation. It is also important in performing finer movements (like the fingers), although in this case it needs other fibers for its initiation.
It has been suggested that the corticospinal tract is also responsible for modulating the body’s sensory information, due to the connections it has with the somatosensory cortex. As we have already mentioned, the decussation of fibers crossing the midline implies that each cerebral hemisphere performs the function of controlling the muscles of the extremities on the opposite side of the body, whereas the muscles of the trunk do not.
The corticospinal tract contains pyramidal neurons (Betz cells), from which large axons originate that mainly innervate the legs; the particular characteristics of this type of neurons allow them to conduct nerve impulses at high speed.
Injuries in this part of the nervous system
Lesions of the corticospinal tract produce a number of clinical signs and symptoms which constitute the so-called pyramidal syndrome. Then let’s see what they consist of.
1. Assignment of voluntary movements
A characteristic effect of corticospinal injury is muscle weakness, Either total (pléia) or partial (paresis), as well as the awkwardness of the fine movements of the hemicuerpo of the same band in which the spinal cord injury takes place. This weakness mainly affects the extensor muscles of the upper extremities and the flexors of the lower extremities.
2. Increased muscle tone
It is common after injury to this nervous tract to occur increased muscle tone or hypertension, as well as spasticity in the extremities, Because the fibers of the corticorrhoidal pathway that descends with the pyramidal tract are usually affected.
3. Presence of pathological reflexes
Damage to the corticospinal tract can lead to the presence of pathological reflexes, which are those that can only be triggered under abnormal conditions, which involves an alteration of the central nervous system (eg Babinski’s sign).
4. Increased deep reflexes
Another of the clinical signs that causes damage to corticospinal fibers is increased deep reflexes. If the corticoreticular fibers are affected, hyperreflexia may occur, An increase in the area in which the reflex occurs if triggered by percussion beyond the provocation zone.
Diffusion of the reflexes can also occur if the response affects other muscles, apart from that corresponding to the struck tendon, or a more violent movement if the response is multiple.
5. Other signs and symptoms
For example, the absence of superficial reflexes and muscle atrophy. The latter is usually mild and is usually caused by a lack of muscle use due to motor weakness.
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