The Neuroscience of Movement
This is part three of our four-part introduction to neuroscience series. For a review of the five senses, please see our previous article. A review of the foundational concepts in neuroscience may also serve to enhance the reader’s understanding of the forthcoming article. We have included a review of the anatomical terms of location below to assist readers unfamiliar with anatomical terminology.
Anatomical Terms of Location
A quick note regarding neuroanatomical nomenclature: the adjective anterior indicates that a structure is towards the front (face side) of the brain, while posterior indicates that the structure is towards the back. Superior indicates that a structure is towards the top (crown) of the head, while inferior means that the structure is towards the bottom of the brain. In the brain, dorsal can be considered synonymous with superior (like a dorsal fin), and ventral is synonymous with inferior. Medial suggests that a structure is towards the middle of the brain, while lateral indicates that a structure is towards the outside of the brain. A final word of clarification: these descriptors are often used to describe a structure’s relationship to another structure. For example, the brain is superior to the spinal cord.
Our conscious control of our motor actions begins in the primary motor cortex (M1) in the precentral gyrus of the frontal lobe; let’s use M1/motor to remember this cortex.1 M1/motor is arranged somatotopically just like the S1/external sensor, resembling a human draped, head down along the side of the brain. Neurons in M1/motor transmit their signals to motor neurons in the spinal cord, which subsequently innervate different muscles in the body.
M1/motor primarily participates in so-called “skilled” actions (i.e. actions that have not been overlearned as habits).2 The first time we tee off at a golf course, we are employing M1/motor as we perform a “skilled,” cognitive action. Motor actions that are used repeatedly are stored separately in pattern programs. These patterned motor programs are responsible for our ability to run, jump, or hit a golf ball without an overabundance of conscious thought after a great deal of practice. A set of structures in the center of the brain, collectively known as the basal ganglia (BG/pattern-generator), are responsible for storing and executing these overlearned motor programs.1
M1/motor and the BG/pattern-generator work together to allow us to flexibly acquire new motor skills that require a lot of cortical horsepower to learn by way of M1/motor, and then to generate a motor program, courtesy of the BG/pattern-generator, that requires very minimal conscious effort to execute.2 If M1/motor were a tricycle, then the BG/pattern-generator would be a bicycle.
M1/motor receives sensory and instructional information from a wide array of structures in the brain. One such structure, the premotor cortex (PM), lies just in front of M1/motor in the frontal lobe. PM is involved in coordinating the movements of many different muscle groups before the instructional information is sent to the distinct muscle neuronal groups in M1/motor;3 let’s recall the function of the premotor cortex using the mnemonic PM/movement-coordinator. PM/movement-coordinator participates in planning movements that are externally cued,2 meaning that information which allows for the movement’s accurate performance comes from the external environment rather than from internal “memories” of previously performed actions.
The supplementary motor area (SMA) is also located in front of M1/motor, but it is superior to PM/movement-coordinator. The SMA is involved in organizing motor signals into a coordinated and cohesive set of signals just like PM/movement-coordinator. However, the SMA coordinates movements that are internally cue (i.e. from conscious/unconscious memory).1Photo by Anda Ambrosini | Unsplash.com
Let’s use an example to understand our motor system. Imagine that we are asked to extend our arm and pick up a glass of water. Furthermore, let’s imagine that we have never picked up a glass before, and are thus required to perform a skilled action. Because the sensory information that allows us to pick up a glass is gathered from the external environment, our PM/movement-coordinator is the area of our brain that arranges all of the required individual actions into a coordinated script before activating the appropriate M1/motor neuronal groups. The PM/movement-coordinator receives visuospatial information about the distance of the glass, the height of the table, and many other environmental details from the previously discussed occipital and parietal visuospatial networks.
While the PM/movement-coordinator has the entire script, M1/motor neuronal groups are mere actors who only know their own lines. One neuronal group in M1/motor triggers the extension of our shoulder, another group generates the extension at our elbow, while still other separate groups open our hand, close it around the glass, and reverse all of the aforementioned actions to bring the glass toward our body.
Now let’s imagine that we are allowed to memorize the table, glass, and the rest of the environment, committing it to memory. We are then blindfolded, and asked to reach out and pick up the glass again. Now the sensory information that allows us to correctly perform this action is coming from our internal environment, our memory. Instead of the PM/movement-coordinator organizing the required M1/motor neuronal group activation, now the SMA takes over. The SMA is able to draw on all of the visuospatial and kinesthetic memories to execute the same task.
We’re going to fast forward now to the umpteenth time that we are asked to reach out and pick up the glass. Instead of a skilled action, the overlearned motor patterns involved in reaching out and closing our hand on a glass are now stored as motor program “chunks” in the BG/pattern-generator. M1/motor and the prefrontal cortex simply need to send the appropriate initiation signals to the BG/pattern-generator to trigger the assembly of these motor chunks into a meaningful behavior. Thus, the cortex is spared the effort associated with having to consciously process all of the steps required to pick up a glass from a table; thus, our mental space is made available for the learning of our next motor task.Labeled for reuse with modification | Pixabay.com
Now that we discussed how motor commands exit the brain, let’s add one final wrinkle. The previously discussed structure known as the cerebellum sits just posterior to the brainstem/CPU and inferior to the neocortex. The cerebellum contains 69 billion of the approximate 86 billion neurons in the brain.1 And yet, the cerebellum is predominantly involved in motor coordination, balance, and the review of motor input and output signals. There is still a great deal that we don’t know about the cerebellum’s role in motor programming, but no discussion of the motor system would be complete without mentioning the cerebellum.
This completes the third part of our four-part introduction to neuroscience series. In our fourth and final article, we will discuss the neuroscience of emotion, language, and consciousness.
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- Baynes J, Dominiczak MH. Medical Biochemistry.; 2014. http://www.clinicalkey.com/dura/browse/bookChapter/3-s2.0-C20110076986. Accessed July 9, 2016.
- Bruce C, Desimone R, Gross CG. Visual properties of neurons in a polysensory area in superior temporal sulcus of the macaque. J Neurophysiol. 1981;46(2):369-384.
- Gottfried JA, ed. Neurobiology of Sensation and Reward. Boca Raton, FL: CRC Press; 2011.
- Barbey AK, Krueger F, Grafman J. Structured event complexes in the medial prefrontal cortex support counterfactual representations for future planning. Philos Trans R Soc Lond B Biol Sci. 2009;364(1521):1291-1300. doi:10.1098/rstb.2008.0315.
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