In this fifth video, we will discuss movement control in the brain and the corticospinal pathways. The cerebral cortex is very important for producing complex movements, such as talking, walking, and writing. Many areas of the cerebral cortex are involved in movement control, and we will discuss them one by one, beginning with the primary motor cortex.
The primary motor cortex is located in the precentral gyrus. located in the frontal lobe. Axons of neurons in the precentral gyrus are connected to the brainstem and spinal cord, producing the nerve impulses that control muscle activity. Some of these axons also make direct connections to motor neurons, thus allowing us to move with greater dexterity compared to other mammals.
Areas of the primary motor cortex are responsible for controlling certain parts on the contralateral side of the Some areas of the primary motor cortex can overlap with one another, especially the cortical areas that control movements of the fingers. The primary motor cortex is most active when we intend to perform the movement, because the primary motor cortex is responsible for instructing the muscles to perform the intended movements. Movement planning involves the posterior parietal cortex, which monitors our body's position relative to our surrounding environment.
The size of the posterior parietal cortex is proportionately larger in humans than in other primates, which reflects our ability to choose and perform complex and accurate movements. Neurological patients with damage to the posterior parietal cortex often exhibit symptoms like difficulty finding objects and hitting obstacles while walking. The prefrontal cortex responds to light, sound, and other sensory signals that guide movements.
This area is responsible for predicting the outcome of an action and for planning movements according to the probable outcome. Damage to the prefrontal cortex can result in movements that appear disorganized. The premotor cortex is active shortly before we perform a movement.
The function of this area is to receive information about the target of a movement, to integrate information about the body's current position and posture, and to direct the appropriate movement towards the target. While the supplementary motor cortex is active a few seconds after an error in movement occurs, this area is responsible for things like planning and organizing a rapid sequence of movements, inhibiting movements that are inappropriate for achieving the goal of movement, and preventing the future occurrence of incorrect movements. Past research has found that areas of the brain that control movement contain mirror neurons. Mirror neurons are neurons that are active during preparation for a movement and when we're observing other people perform the movement. These neurons may play a special role in our ability to understand, identify, and imitate others.
Researchers also suspect that mirror neurons are important in social behavior. However, it's not currently known whether mirror neurons are the cause or the effect of social behavior. The major efferent pathways for sending messages from the motor cortex to the brainstem and spinal cord are known as the corticospinal pathways or corticospinal tracts. Information that travels along the corticospinal tract is sent to interneurons and motor neurons, which then carry the information to the nucleus.
Axons of the corticospinal tract originates from the pyramidal cells in the fifth layer of the primary motor cortex before descending to the brainstem, where messages are sent to several areas of the brainstem. Under the brainstem, these axons gather to form bundles that are shaped like pyramids, and this is why the corticospinal tracts are also sometimes known as the pyramidal tracts. Some axons from the motor cortex in the left hemisphere cross over to the contralateral side of the brainstem, and the same also goes to axons from the motor cortex on the right hemisphere.
However, some axons do send information ipsilaterally, giving rise to both crossed and uncrossed pathways of the corticospinal The corticospinal pathway can be divided into two divisions, namely the lateral or dorsolateral corticospinal tract and the medial or the ventromedial corticospinal tract. The two pathways contribute to nearly all movements, but certain movements do depend on one tract more than the other. The lateral or dorsolateral corticospinal pathway consists of axons that cross in the brainstem and send information to the contralateral side of the spinal cord. Information along this path is delivered along the lateral side of the spinal cord.
the spinal cord, hence the name lateral corticospinal trance. The main function of this pathway is for sending messages that allow for control of movements in the peripheral or lateral parts of the body, such as the arms, hands, fingers, feet, and toes. As you can see in this picture, the lateral corticospinal pathway brings information from the primary motor cortex, surrounding cortical areas, and the red nucleus, which is a midbrain area that controls certain aspects of movement.
Axons of the lateral tract extend directly from the motor cortex to their target neurons in the spinal cord. Then, the lateral tract crosses to the contralateral side of the spinal cord, at bulges in the medulla called the pyramids. And as mentioned before, this tract controls movements in the peripheral or lateral areas of the body, like the hands and the feet. In this picture, you can see a comparison between the efferent pathway of the lateral corticospinal tract that brings information from the motor cortex to the nucleus, and the afferent spinothalamic pathway that brings touch information from the skin to the somatosensory cortex of the brain. Remember that in one of the previous videos, we've already discussed the pathway that brings somatosensory information from the skin to the brain.
The efferent lateral corticospinal pathway is colored red, while the afferent spinothalamic pathway is colored blue. In this image, you can see how the two pathways cross in the medulla, and how somatosensory information arrives in an area in the somatosensory cortex that is located adjacent to the motor cortex. The side-by-side nature of the somatosensory cortex and the motor cortex makes sense because, after all, somatosensory information is essential for movement. For example, we need to know what our hands are currently sensing to decide what movements of the hands we need to do next.
The ventral or ventromedial corticospinal pathway consists of axons that deliver information to the ipsilateral and contralateral sides of the spinal cord. The difference between the ventral and the lateral pathways is that axons in this ventral pathway deliver information along the ventral or anterior part of the spinal cord. In contrast to the lateral pathway, this particular path is important for controlling movements. on the medial parts of the body, such as movements of the neck, shoulders, and trunk, as well as for controlling movements that involve both sides of the body, like walking, turning, bending, standing up, and sitting down. In this picture, you can see how axons of the ventral corticospinal tract actually originate from various areas of the brain.
including the primary motor cortex and its surrounding areas, the midbrain's tectum, the reticular formation, and the vestibular nucleus, which is a brain area that receives input from the vestibular organ. Axons in the ventral pathway go to both sides of the spinal cord and not just the contralateral side. Also, as previously mentioned, information sent along the ventral corticospinal tract controls muscles on the medial parts of the body, like the neck, shoulders, and trunk, which are typically involved in bilateral movements that require both sides of the body, such as walking, turning, bending, standing up, and sitting down. Feel free to pause your video at this moment to better examine this particular image, which illustrates differences between the lateral and ventral spinothalamic tracts.
It should be easy to remember the difference in their functions too, considering that the lateral tract controls muscles in the lateral parts of the body, like the hands and feet, while the ventral pathway, which is also known as the ventromedial tract, controls the muscles in the medial parts of the body, like the neck, shoulders, and trunk. And finally in this table, you can examine the various types of motor disorders that can result from different types of damage to the neurons of the spinal cord.