The brain is key to our existence, but there’s a long way to go before neuroscience can truly capture its staggering capacity. For now though, our Brain Control series explores what we do know about the brain’s command of six central functions: language, mood, memory, vision, personality and motor skills – and what happens when things go wrong.
Having voluntary control over body movements is the only way we can interact with people, objects and our environment. Body movement is not just about controlling arms and legs; it’s also for our head and eyes to visually explore the world, for our facial expressions to show emotion, and for articulation of our lips, tongue and mouth to communicate.
Further reading: What brain regions control our language? And how do we know this?
The devastating effects of the brain losing its ability to control body movements are seen in motor neuron disease – where progressive degeneration and muscle wasting leads to some patients becoming “locked-in”, meaning they can’t move or communicate in any way.
The motor system and primary motor cortex
The brain’s motor system is contained mostly in the frontal lobes. It starts with premotor areas, for planning and coordinating complex movements, and ends with the primary motor cortex, where the final output is sent down the spinal cord to cause contraction and movement of specific muscles.
The primary motor cortex on the left side of the brain controls movement of the right side of the body, and vice-versa, the right motor cortex controls movement of the left side of the body.
Different areas of the primary motor cortex connect to, and control, movement of different parts of the body, forming a kind of body map known as the homunculus.
The size of the area on the homunculus determines the level of fine movement control we have with that part of the body. So, for instance, a large proportion of the motor cortex is devoted to our thumb, fingers, mouth and lips, as they are vital for manipulating objects and speech articulation.
The connection from the primary motor cortex to muscles of the body is so important that any damage leads to an impaired ability to move. If someone suffers a stroke, for instance, that causes damage to the primary motor cortex on one side of their brain, they will develop an impaired ability to move on the opposite side of their body.
If the area of damage is specific to only part of the primary motor cortex, such as the hand area of the homunculus, it will affect movements only of the corresponding part of their body, for example, the hand.
Neuroplasticity and movement rehabilitation
As with other parts of the brain, when neurons of the primary motor cortex are damaged they will never regrow or repair. However, the brain can heal itself and regain some lost function through neuroplasticity. This means undamaged parts can change their connections and remap to other areas of the body to take over function, compensating for damaged parts of the motor cortex.
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.
Let’s look at an example of a stroke patient, Harry, who has problems with movement in his left leg. 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.
This same principle of neuroplasticity also applies for learning in the healthy brain. Anytime we acquire a new skill such as learning to ride a bike, writing our signature or dancing the tango, it’s our brain’s ability to strengthen or make new connections to adapt and change that allows us to learn.
So if you are a ballet dancer or a gymnast, a swimmer or a soccer player, a watch-maker or micro-surgeon, your brain connections in your motor system will be different depending on the practice and skill you have with fine movement of different parts of your body.
This article was co-written with Zita Arends, who is a physiotherapist in stroke rehabilitation and aged care.