The Motor Cortex and Movement
Movement is a basic human function that many of us consider obvious. However, in reality, executing a simple movement, such as lifting a pen, involves millions of neurons firing in different brain areas.
The area of the brain that is primarily responsible for the planning, control and execution of movement is called the motor cortex.
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To summarize, the motor cortex is subdivided into the premotor cortex (PM), supplementary motor area (SMA) and primary motor cortex, which are all closely related, yet each has a particular function in movement control. The motor cortex is closely associated with posterior parietal cortex.
But how do we know the individual functions of each area? Our knowledge comes from studies designed to determine the functionality of these regions.
Experiments that helped us understand the function of these cortices in movement come from animal studies, particularly using monkeys. Researchers usually record the brain activity or use lesion studies, allowing them to assess the role of particular brain regions [1].
Doing human lesion studies is only possible due to a brain tumour or stroke in a particular area of the brain. However, evidence from humans also exists. We will discuss the studies in great detail as we analyze the different parts of the motor cortex.
Parietal Cortex and the Posterior Parietal Cortex
A part of the parietal cortex called the posterior parietal cortex holds an important function when it comes to voluntary movement. In principle, when we reach for a pen, we need to evaluate the relative position of the pen to the body, the physical properties of the pen, our current posture, and the perspective movement we need to execute in order to reach it. Essentially, the function of the posterior parietal cortex is to integrate the signals from the surroundings into the motor action [2].
The posterior parietal cortex receives strong somatosensory and visual inputs about the body's position and movement. Using this information, the parietal cortex codes for the movement that will be made and sends the information to the premotor cortex and the supplementary motor area [3].
The left and right posterior parietal cortexes have different roles, both contributing to the understanding of our surroundings.
The left posterior parietal cortex is responsible for motor attention, meaning the awareness of our posture and movements needed for the execution of the grasping/reaching activity. Whereas the right posterior parietal cortex is responsible for visual awareness and change detection [4].
Lesions of the right posterior parietal cortex determined in patients resulted in the inability to accurately place their limb according to the spatial location and physical properties of objects [5].
Lesions in the left posterior parietal cortex cause these patients to have difficulties adjusting their reaching or grasping actions an unexpected change in the external environment. However, these lesions can also result in agnosia (difficulty with recognizing objects, people or sounds), agraphia (problems with written communication), and acalculia (loss of the ability to process numbers). Ultimately it can lead to Gerstmann’s Syndrome [6].
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Patients with damage to their posterior parietal cortex show the importance of the parietal cortex in one’s perception of their own body and the external environment [7]. Bilateral damage to the parietal cortex can also cause Balint syndrome, a rare neurodegenerative disorder.
The Premotor Cortex and the Supplementary Motor Area
Originally, there was an assumption that the premotor cortex and supplementary motor area were operating as a single entity, both receiving inputs from the parietal cortex.
However, they vary in their specific functions. Passingham RE 1985 conducted a lesion study of the premotor area and supplementary motor area on monkeys. The monkeys were trained to do a visual conditional motor task and a sequence motor task. The visual task consisted of pulling a handle on a light stimulus and the sequence task consisted of opening a lid by using a certain sequence. In this case, they trained the monkey to twist the lid and then lift it.
Lesions of the premotor cortex in monkeys resulted in impaired execution of the visual task but not the sequence task and lesions of the supplementary motor area resulted in correct execution of the visual task but impaired performance on the sequence task [8].
Furthermore, in 2000 Crammond and Kalaska showed that there is activity in the premotor cortex neurons signalling for the indented movement before the movement has been executed [9].
Both suggested that the premotor cortex is responsible for the planning of visually cued tasks whereas the supplementary motor area is involved in sequence planning of movements.
Premotor Cortex
The premotor cortex is located on the frontal lobe. To summarize, the function of the premotor cortex is motor planning and movement preparation, particularly for visually cued tasks.
To add to the experiments showcasing the function of the premotor cortex Rizzolati et al examined the firing of the neurons in that area. They found out that when the monkey was performing a task, observing another monkey performing the task and observing a human performing the task, neurons in the premotor cortex were firing throughout all of the actions. This again, highlights the importance of the premotor cortex in visually cued tasks[10].
Diseases affecting the premotor cortex lead to various motor dysfunctions. Neurodegenerative diseases can affect the premotor cortex and can cause problems in performing rapid automated movements. An example of such is Parkinson's disease or Mills syndrome [11,12].
Supplementary Motor Area
The supplementary motor area (SMA) is also located in the frontal lobe. However, the function of the supplementary motor area function is rather in sequence planning and movement execution.
Studies have shown that lesions in the supplementary motor area can lead to apraxia, which is a disorder of skilled motor movements. Patients with SMA lesions may experience difficulties in programming complex distal motor acts of the limbs, resulting in apraxia for transitive limb movements [13].
Primary Motor Cortex
The primary motor cortex (M1) communicates with the spinal motor neurons, which are neurons responsible for muscle activation. The communication is either direct with primary motor cortex neurons projecting directly to spinal motor neurons or indirect [14].
Historically, researchers showed that electrical stimulation of different parts of the primary motor cortex resulted in movements of different parts of the body. Thus, the populations of the corticomotor neurons and their location on the cortex are responsible for the activation of different muscles [15].
In addition, studies examining focal lesions of the primary motor cortex result in muscle weakness and slow and imprecise movements. Sometimes, even more drastically, they can cause paralysis of certain body parts, depending on the region of the lesion. This is further evidence that the function of the primary motor cortex is to give signals for movement execution.
Further a study comparing all three different regions of the motor cortex: the primary motor cortex, premotor cortex and supplementary motor area showed that they are distinct in function. They recorded neurons in the M1, PM and SMA regions of monkeys. The monkeys were trained to perform a visually cued task and a sequence task and their neuronal activity was recorded.
The neurons of the M1 were shown to be active during both of the tasks. The neurons from the PM were active only throughout the visually cued task and the neurons from the SMA were engaged only during the sequence task [16].
Conclusion
In conclusion, all these studies and experimental evidence show that during voluntary movements there is a strong connection between the sensory and motor output which contributes to a proper movement execution.
The neurons in the primary motor cortex fire during movement execution and the neurons of the premotor area are active during the planning of the movement.
There are strong connections between the motor and parietal cortex which results in better planning of reaching or grasping movement.
Studying the brain's role in motor control offers valuable insights, aiding our comprehension of how the brain regulates movement and how neurological conditions impact motor functions. Understanding the motor cortex not only contributes to the development of interventions, treatments, or rehabilitation strategies for such disorders. It is also crucial for connecting the knowledge to other cognitive processes such as language, memory, or attention.
Disclaimer
This article does not offer health advice. Always consult a medical professional regarding your condition.