Every single body part is represented in the primary motor cortex. However, not every part has equal amounts of brain matter devoted to it. Complex movements that require more precise control take up larger amounts of space in the brain than simple motions do.
For example, a significant portion of the motor cortex is devoted to finger movements and facial expressions, while a smaller portion of the brain is responsible for leg motions. This fact explains why many brain injury and stroke patients struggle with fine motor control or facial paralysis.
Because those movements are controlled by a larger portion of the motor cortex, they have a much higher likelihood of becoming damaged during an injury.
Besides causing general problems with movement, there are some specific signs that indicate damage to the primary motor cortex. These symptoms are known collectively as upper motor neuron syndrome. There are five main signs of upper motor neuron syndrome, which we will examine below.
This refers to weakness on one side of the body. It is most common in stroke victims, but can occur after a traumatic brain injury as well. If a person has damaged their left motor cortex, their right side will be weakened significantly. They may have trouble lifting their right arm, moving the fingers on their right hand, and the entire right side of their face might droop.
These reflexes are often exaggerated in individuals with upper motor neuron syndrome, and can present as clonus or spasticity. There are different types of overactive reflexes, which we will discuss next. They are: clonus, spasticity, and the Babinski sign. Clonus is a condition caused by damage to the primary motor cortex.
It creates involuntary muscle contractions. The contractions are repetitive and rapid, and often occur in a rhythmic pattern. Contractions usually occur in the knees and ankles, but clonus can affect other areas as well, including:. Clonus occurs when the electrical signals sent by the brain to the muscles are interrupted. This can happen after direct damage to the motor cortex or damage along any of the descending motor pathways.
As a result, they end up doing both. Spasticity is similar to clonus, and in fact, it sometimes is a direct result of prolonged clonus. With spasticity, instead of rapidly contracting and relaxing, the muscles are in a constant state of contraction. Neuroplasticity is the fundamental principle in physical rehabilitation , such as physiotherapy for patients following stroke, that allows patients to regain motor function and recover.
Through neuroplasticity, the more a particular movement is performed, the stronger the brain pathways for that movement become and the easier it gets to perform that movement in the future. Harry might have altered patterns of walking due to damage in the leg area of the motor cortex of the right side of his brain.
To help Harry regain efficient walking ability, the physiotherapist helps him perform sequences or patterns of walking by practising activation and control of specific muscle groups in his left leg. At first, Harry will need lots of concentration to use the correct muscles as his brain is laying down new neural pathways to compensate for the damaged areas. But as this practice is repeated and the new pathways are established and strengthened, correct movement becomes easier without much concentration.
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JAMA , — Woodlee, M. The nonprimary motor cortex however is located anterior to the primary motor cortex. The motor cortex is the only motor control centre above the spinal cord which can directly communicate with most of the other motor control structures such as the thalamus, basal ganglia, brain stem, and spinal cord.
This region also contains an inverted representation of the opposite half of the body, known as a motor homunculus. Therefore, the top part of the cortex stimulates movements of the leg, whereas the movements of the face are stimulated by the lowest part of the motor cortex.
Thus, each movement of the body is represented by neuronal activity in different areas of the cortex. This is alike to the sensory homunculus of the sensory cortex, which is situated immediately behind the motor cortex. The primary motor cortex is a region of the motor cortex which is important for initiating motor movements. The areas of the primary motor cortex correspond precisely to specific body parts. The primary motor cortex contains large neurons nerve cells with triangular- shaped bodies call pyramidal neurons.
Pyramidal neurons, also known as upper motor neurons, and the primary output cells of the motor cortex. The axons of these neurons exit the motor cortex, carrying with them information about voluntary movements it wishes to make. To do this, the pyramidal neurons enter one of the tracts of the pyramidal system: either the corticospinal or corticobulbar tracts. When pyramidal neurons travel through the corticospinal tract, it carries with it information to the spinal cord.
Once the information has reached the spinal cord it can then be used to initiate movements. In contrast, the pyramidal neurons which travel down the corticobulbar tract carries motor information to the brain stem at the base of the brain. Once here, cranial nerve nuclei are stimulated to initiate movements of the head, neck, and face.
Both of these tracts therefore carry specific information about voluntary movements down from the brain. To directly supply skeletal muscles to cause movement, the pyramidal or upper neurons form connections with other neurons called lower motor neurons, which are efferent neurons that connect the central nervous system to the muscles.
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