Corticospinal tract: characteristics and functions.

This bundle of neurons is connected to several areas of the brain and the spinal cord.

The corticospinal tract is the main voluntary motor control system of the central nervous system..

Its axons travel from the cerebral cortex to the spinal cord, and are partly responsible for enabling us to move the limbs and trunk, and to carry out, together with other nerve tracts, finer and more precise movements.

In this article we explain what the corticospinal tract is, what are its main characteristics and the functions it performs, as well as the clinical signs and symptoms that are produced by the lesion of this nerve 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 is composed of multiple parts that perform different functions. In the cerebral cortex are located the upper motor neurons, which send movement signals to the lower motor neurons that tell the muscles whether to contract or relax.

The corticospinal pathway is made up of the axons of motor neurons that travel from the motor that travel from the motor cortexes (primary motor cortex, supplementary motor area and premotor cortex) to the spinal cord. These neurons control voluntary movements of the limbs and trunk. Small nuclei of neurons also originate in areas of the parietal lobe (ascending parietal gyrus and superior parietal cortex).

This motor system is one of the last to develop, since the fibers of the corticospinal pathway finish myelinating approximately 2 years after birth. One of the characteristic aspects of this axon bundle is the so-called pyramidal decussation the so-called pyramidal decussationThis means that a large part of the corticospinal fibers (about 75-80%) cross over to the contralateral side of the medulla oblongata, and nerve fibers from the left side of the brain pass to the right hemibody, and vice versa.

Pyramidal decussation leads to an obvious conclusion, which is that the areas of the brain that control the right side of the body are located in the left hemisphere, and those that control the left side are in the right hemisphere. This can be verified when there is an injury to one of the hemispheres; for example, some patients who have suffered damage to the left hemisphere may suffer from paralysis of the right side of their body.

Neuroanatomical organization

The corticospinal pathway originates in several areas of the cerebral cortex, mainly in the motor cortex.mainly in the primary motor cortex (Brodmann's area 4) and in premotor areas (Brodmann's area 6). However, they can also originate in the somatosensory cortex, cingulate gyrus and parietal lobe. This pathway connects these brain areas with the gray matter of the spinal cord.

The axon bundle of the corticospinal tract travels from the cortex, through the deep white matter, to the brain stem. Most of them decussate back and forth in the lower brainstem and descend into the contralateral white matter of the cord, in what is called the lateral corticospinal pathway.The lateral corticospinal pathway is called the lateral corticospinal pathway.

About 15% of the axons do not perform pyramidal decussation and descend as the ventral corticospinal tract. In addition to the corticospinal tract, this system contains indirect pathways that project first to the motor nuclei of the brainstem, and from there 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 spinal motor regions, which is composed of the deeper laminae of the dorsal horn, the intermediate zone and the dorsal horn. The corticospinal system also projects from the somatosensory cortex to sensory processing centers in the dorsal horn and brainstem. to regulate the proprioceptive information generated during movement.

Functions

The corticospinal pathway plays an essential function in the control of limb and trunk movements, both in the ability and accuracyThe corticospinal pathway plays an essential role in the control of limb and trunk movements, both in the ability and precision to perform them. It is also important in the execution of finer movements (such as finger movements), although, in that case, it requires other fibers for its initiation.

It has been suggested that the corticospinal tract is also responsible for modulating the sensory information of the organism, due to its connections with the somatosensory cortex. As we have already mentioned, the decussation of the fibers that cross the midline implies that each cerebral hemisphere fulfills the function of controlling the muscles of the extremities on the opposite side of the body, which does not occur with the muscles of the trunk.

The corticospinal pathway contains pyramidal neurons (Betz cells), from which arise large axons that innervate mainly the legs; The special characteristics of this type of neurons allow them to conduct nerve impulses at high speed..

Lesions in this part of the nervous system

Lesions in the corticospinal pathway produce a series of clinical signs and symptoms that make up the so-called pyramidal syndrome. Next, let's see what they consist of.

1. Impairment of voluntary movements

A characteristic effect of the lesion of the corticospinal pathway is Muscle weakness, either total (plegia) or total (plegia).This weakness mainly affects the muscles, either total (plegia) or partial (paresis), as well as the clumsiness of fine movements of the hemibody on the same side on which the spinal cord damage occurs. This weakness mainly affects the extensor muscles of the upper limbs and the flexor muscles of the lower limbs.

2. Increased muscle tone

It is common that, after the lesion of this nervous tract, there is an increase in muscle tone or hypertonia, as well as spasticity in the extremities. increased muscle tone or hypertonia, as well as spasticity in the extremities, due to the fact that the corticoreticular fibers that descend along the pyramidal bundle are usually affected.This is due to the fact that the fibers of the corticorecticular pathway, which descends together with the pyramidal bundle, are usually affected.

3. Presence of pathological reflexes

Lesions of the corticospinal tract may cause the presence of pathological reflexes, which are those that can only be elicited under abnormal conditions, implying an alteration of the central nervous system (e.g. Babinski's sign).

4. Increased deep reflexes

Another clinical sign caused by a lesion in the corticospinal fibers is an increase in deep reflexes. If the corticoretinal fibers are affected, hyperreflexia may occurIn addition, an increase in the area in which the reflex is produced if it is triggered by percussion beyond the area of provocation.

There may also be a diffusion of reflexes if the response affects muscles other than the one corresponding to the tendon that has been percussed, or a more violent movement if the response is multiple.

5. Other signs and symptoms

For example, absence of superficial reflexes and muscle atrophy.. The latter is usually of a mild type, and is usually caused by a lack of muscle use due to motor weakness.

Bibliographical references:

• De Silva, R. (2002). Neuroanatomical Basis of Clinical Neurology.
• Eyre, J. A. (2007). Corticospinal tract development and its plasticity after perinatal injury. Neuroscience & Biobehavioral Reviews, 31(8), 1136-1149.
• Preston DC, Saphiro BE, Brooke MH (2004). Proximal, Distal and Generalized Weakness. En: Bradley WG, Daroff RB, Fenichel GM, Jankovic J (Eds), Neurology in Clinical Practice, (pp 367-386). Philadelphia: Butterworth Heinemann.