The brain has localised areas of function. In the early days of psychological research, many psychologists liked to prod and poke certain areas of the brain to see what effect it would have on an animal (or a person). Gustav Fritsch and Eduard Hitzig did just that in 1870. They experimented on a dog and found that electrically stimulating a specific area in the brain would cause the dog to move its muscles involuntarily. They also found that different muscles would move if they stimulated various spots of these specific areas, now dubbed the motor cortex, the motor area of the brain.
Although their experiment would be deemed highly barbaric in the modern age (considering that no form of pain relief was given to the dog at the time of the experiment), it led to significant advances in our understanding of the brain and the localisation of function.
- We will start by looking at a motor area of the brain summary to help you identify where the motor cortex is.
- Next, we will cover the motor area of brain function to help you understand what role the motor cortex performs.
- Then, we will look at and learn more about some specific structures within the motor cortex, such as the primary motor of the brain and the nonprimary motor cortex.
Motor Area of Brain: Summary
Following the discovery made by Fritsch and Hitzig, the motor cortex was identified as one of the first major insights into modern neurophysiology.
The motor cortex is the area of the brain that's most involved in controlling your voluntary movements (from planning to executing them). It is the primary motor area of the brain.
The motor cortex itself is situated in the frontal lobe, located in front of a large groove known as the central sulcus.
Motor Area of Brain: Function
The motor area of brain function is further divided into two regions:
- The primary motor cortex
- The nonprimary motor cortex
They can also be referred to as Brodmann's area 4 (the primary motor cortex) and Brodmann's area 6 (the nonprimary motor cortex).
The primary motor cortex is the first thin section that runs along the central sulcus (essentially found in the precentral gyrus). The nonprimary motor cortex is the next strip that sits in front of the primary motor cortex and is a little bit wider.
The nonprimary motor cortex can then be even further subdivided into two regions:
- The premotor cortex
- The supplementary motor cortex.
Dr Penfield developed what is known as the motor homunculus (a somatotopic, topographic organisation of the brain) from his neuro experiments stimulating various areas of the motor cortex. This is the famous image that transposes images of various body parts across the motor cortex.
Through improved techniques (such as intracortical microstimulation), we now know that the basis of movement and the motor cortex topography is much more complex than the motor homunculus would have us believe. However, it is still somewhat representative of mapped areas.
Areas of the motor cortex do relate to specific parts of the body. Still, we have found through recent neurophysiological research that different cell columns exist within these subdivisions controlling specific areas, and these offer finer control of muscles within that body part.
Motor Control Area of Brain: The Primary Motor Area of Brain
The primary motor cortex is the major motor area of the brain that requires the least amount of electrical stimulation to induce some form of movement (it is the most sensitive to it).
What we can say is that the primary motor cortex:
‘Synthesises’ movement commands but is not where the commands originate.1
It has large pyramidal neurons that send signals down their axons that extend down separate tracts of the pyramidal system, such as corticospinal and corticobulbar tracts, to the motor neurons in the spinal cord or down the brainstem. This initiates movement of the body and the head (including the neck and face).
These neurons can also be referred to as upper motor neurons (UMN), connecting to lower motor neurons (LMN). The UMN sends information, and the LMN stimulates the muscles to contract.
Fig. 2 - A pyramidal cell labelled. Soma and dendrites are labelled in red, axon arbor in blue³.
Stimulation to the primary motor cortex will produce localised muscle contraction in the areas associated with it, on the opposite side of the body (contralateral).
Interestingly, a lot of the primary motor cortex is dedicated to controlling muscles in the hands and controlling muscles in speech. When one of these areas is stimulated, it often results in a specific muscle contracting instead of a group of muscles contracting.
This could be related to the complexities of human speech and how much importance humans place on facial expressions.
What is the Role of the Motor Area: Brain?
The main function of the motor cortex is to send signals to allow movement of the body.
It is situated in the frontal lobe.
Other structures within this brain region include the primary motor cortex, premotor cortex, and supplementary motor area.
The Nonprimary Motor Cortex
As we mentioned above, the nonprimary motor cortex is divided into two regions: the premotor cortex and the supplementary motor cortex.
Information provided by other parts of your brain that tell you where your body is, where your limbs are, and other information about the environment is integrated within the premotor cortex. It uses this integrated information to help guide muscle control.
When planning complex movements and then coordinating these movements involves the supplementary cortex. A sequence of movements that grows increasingly complex or requires more dexterity than normal is thought to be controlled here (although we're not 100% sure on this, more research is needed here).
Electrical stimulation of these areas requires a higher current to produce results similar to that of the primary motor cortex, and longer, higher stimulation results in complex movements.
Consider the following study by Graziano et al. (2002):
- They electrically microstimulated the primary and premotor cortexes in monkeys.
- Each stimulation lasted for around 500ms, based on the general time scale one would expect for reaching and grasping movements and the neuronal activities that coincide with this.
- This stimulation resulted in coordinated, complex postures. Specifically, when they stimulated one site, they found that the mouth would open, the monkey would change their hand shape to a gripping posture, and then they would move their hand to their open mouth.
- Regardless of where the limbs started, stimulating this site would always result in the final position the joints would settle in. It would bring the limbs up and into this final posture, no matter what.
- Stimulation of different sites would result in different postures.
What this means is that these regions work together. It also offers insight into the areas associated with complex movements that involve a series of events.
Fig. 3 - Graziano et al. (2002) studied motor centres in monkeys.
The Environment and the Major Motor Areas of the Brain
As you can imagine, your brain is constantly being bombarded with sensory information from all avenues of life.
Consider something as simple as crossing the road. Your brain will:
- See the distance between one path and the next.
- Begin movement for you to step off and down the curb whilst also keeping every other muscle contracted just right to keep you in an upright and walking position
- Determine how quickly you need to walk across the road and how much the muscles in your body need to contract to allow you to do so
- Acknowledge all of the stimuli occurring around you, such as any oncoming cars, how fast they're going, how quickly you can cross in relation to how quickly they're approaching, as well as people around you, the sounds they make, amongst a whole host of other events
- Navigate any obstacles you may encounter on the road, such as potholes and other pedestrians
- Listen for sounds and look out for any visual aids.
Something as simple as standing upright and balancing without falling over required years of evolutionary leaps to reach the point where we're at today.
Whilst most things may seem like a simple process, a lot of tiny calculations are happening in the brain at any given time to make sure the fine movements you've grown so used to can be completed all within the reality of the world. Any instantaneous changes that can happen at any given moment are acknowledged and accounted for with appropriate adjustments.
This is somewhat down to your nervous system and the cerebellum, specifically proprioceptors.
Proprioceptors are sensory receptors located within your body's muscles, tendons and joints that perceive stimuli and relate them to the body's position, posture, equilibrium, and external/internal situations. It's essentially the perception of the self, and these sensory neurons can detect movement/motion in relation to the body.
Motor Centres - Key takeaways
- The motor cortex is the area of the brain that's most involved in controlling your voluntary movements (from planning to executing them). It is the primary motor area of the brain.
- The motor cortex is situated in the frontal lobe, sitting in front of a large groove known as the central sulcus. It is divided into the primary and the nonprimary motor cortex.
- The nonprimary motor cortex is divided into two regions: the premotor and the supplementary motor cortex.
- The premotor cortex uses integrated information from other brain parts to help guide the body in planning and executing movement. The supplementary motor cortex is involved in planning complex movements and then coordinating these movements.
- The nervous system as a whole, the cerebellum, and proprioceptors in your muscles, tendons, and joints help you navigate the space around you relative to your body's position.
References
- 1. Mtui, E., Gruener, G., & Dockery, P. (2020). Fitzgerald's Clinical Neuroanatomy and Neuroscience E-Book. Elsevier Health Sciences.
- Fig. 2 - Piramidal cell (https://commons.wikimedia.org/wiki/File:Piramidal_cell.svg) by Fabuio is licensed by CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en)
- Fig. 2 - Pyramidal cell by Fabuio, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons
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