How Does Our Brain Control Movement?

We automatically reach for things we want. We talk. We walk without thinking, even on uneven grounds. We turn our heads in response to a loud sound. How does our brain do this?

The brain receives sensory information from the world around us. An external stimulus, like a cup of tea waiting for us on a counter, can cause us to reach out and grab it. Or we can decide internally that we want to get something or go somewhere.

The motor system is organized hierarchically, with more complex motor tasks initiated at higher levels[1]. Very simple motor commands such as reflexes or scratching are controlled by the spinal cord. Other reflexes such as breathing are controlled at the level of the brain stem, located at the base of your brain. Various regions in the brain stem also control posture. Our cerebral cortex controls the most complicated motor tasks, including goal-directed behavior.

If we voluntarily want to act on the sensory information, then our brain sends signals via the pyramidal motor pathway to our muscles. This pathway begins in the primary motor cortex, or M1, located in the precentral gyrus of the frontal lobe, where there is a little map of our bodies called a motor homunculus, which is Latin for ‘little human.” The homunculus is organized somatotopically, just like a map of our body, with nearby areas adjacent to each other. The size of different body parts corresponds to how important that part is to us. For example, humans have enlarged areas for our hands and fingers. Other animals can have different areas enlarged. For example, mice, rats, and cats, among other animals, have enlarged areas of their whiskers represented in the motor cortex.

Secondary premotor areas (including the supplemental motor area, the posterior parietal cortex, and the premotor cortex) contribute to motor planning, with each premotor area having a different task. Motion initiated internally involves the supplemental motor area. Motion triggered by an external stimulus largely involves the premotor cortex. More complicated motor tasks requiring mental rehearsal, such as playing tennis or ball throwing, involves both the premotor and posterior parietal cortex[2].

Two other parts of the brain help with movement. The basal ganglia, found below the motor cortex, helps organize motor programs for complex movements. If the basal ganglia are damaged, this can prevent an intended movement (as in Parkinson’s disease) or cause involuntary excessive movement (as in Huntington’s disease). The cerebellum helps with the timing and coordination of motor tasks. It also helps integrate sensory and motor information. For example, when you feel textures like velvet or corduroy, you move your fingers over the material at a rate which helps your brain understand the texture. Damage to the cerebellum can cause loss of coordination. These two brain regions receive information from various parts of the brain, and send this information to the motor cortex via neurons in the thalamus, forming the cortico-basal ganglia loop and the cortico-cerebellar loop.

The neurons from the cortex send their axons via the corticospinal tract to the opposite side of the body, and either connect to the appropriate cranial nerves (nerves for movement of our head and neck) or connect to the appropriate spinal nerve in the spinal cord. Neurons in the spinal cord are myelinated (covered with a lipid layer). Myelination helps nerve signals travel much faster than unmyelinated neurons, and lets some of our muscles act in less than a second! The spinal nerves connect to particular muscles or muscle groups which then flex, extend, or rotate, depending on our goal. The nerve signal simultaneously commands a particular muscle action (e.g., flex) while inhibiting the opposite action (e.g., extend).

Our motor cortex learns through experience. That’s why practice improves the accuracy, efficiency, and timing of our movement and muscle action! Changes in our motor cortex occur over weeks, following more rapid changes in the cerebellum, basal ganglia, aond other secondary motor areas that occur within days[3]. Numerous studies have shown that the more we use a particular muscle, or practice a particular task, the larger that part of the brain becomes[4].  Teens who text with their thumbs very likely have a larger thumb area in their motor cortex than their parents!



References

[1] Kandel ER, Shwartz JH, Jessel TM (2000) Principles of Neural Science. 4th edition.McGraw-Hill,NY. 1414pp.

[2] Ibid.

[3] Ungerleider LG, Doyon J, Kami A (2002) “Imaging Brain Plasticity during Motor Skill Learning.” Neurobiology of Learning and Memory 78: 553–564. doi:10.1006/nlme.2002.4091.

[4] Ibid.

 










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