The Role of the Motor Cortex in Movement - Essay Example

Running Head: The Role of the Motor Cortex in Movement Synopsis In the human body, movement is controlled in the motor cortex, which is in the precentral gyrus of brain cerebellum. This paper discusses the role of the motor cortex in movement. The human body exhibits two main types of movement namely the voluntary movement of skeletal muscles and the involuntary movement of smooth muscles, plus the reflexive movement, which is involuntary for the maintenance of body balance and posture. The motor cortex initiates movement, but the input of several other parts of the brain is vital to ensure the correct movement occurs. The motor cortex plans voluntary actions – it helps in initiating, planning, controlling and maintaining voluntary movement of skeletal muscles, coordinates sequences of movements and relay commands to the appropriate sets of lower motor neurons to execute the desired actions. Diseases or traumas on this part of the brain have adverse effects on the patient. Presently, such diseases include Apraxia, which causes an inability to initiate purposeful learned movement in response to external commands despite the ability and desire to perform the movement, and Parkinson’s and Huntington’s disease – characterized by loss of the ability to initiate movement or stop it once initiated plus the occurrence of uncontrollable movement. Introduction Modern man, Homo sapiens, has an average cranial capacity of 1350cc far more than all his other earlier ancestors. This increased cranial capacity is credited for his higher intellectual ability over other animals and primates. The brain serves many functions in the body, from thought to emotions, waking and sleeping, respiration, memory, controlling actions just to mention but a few and can thus be considered as the central control for the whole body. Like all other living things, man also moves around in search for food, flight from danger or for pleasure. Movement is a function of muscular flexion and extension all which are under the control of the brain – except for reflexive movements, which are controlled by the spinal cord and mainly help maintain posture and balance without involving the conscious mind. The brain is divided into six main parts; the cerebral hemispheres, hypothalamus, thalamus, cerebellum and the brain stem – mid brain, pons and medulla oblongata, each of which has its different functions (Marieb, 2000). The cerebellum contains a region known as the motor cortex in the precentral gyrus that helps in initiating, planning and controlling voluntary movement of skeletal muscles. The motor cortex is further divided into three different areas; the primary motor cortex (M1), premotor area (PMA) and the supplementary motor area (SMA). The latter two receive stimuli from the parietal and lateral lobes and send it to the primary motor cortex that sends out signals to the appropriate motor neurons to control movement (Carlson, 2006). The motor cortex also has somatotopic organization, that is, specific regions of the motor cortex control movement of particular muscles or muscles associated with a particular part of the body. The human body exhibits two main types of movement: the voluntary movement of skeletal muscles and the involuntary movement of smooth muscles. A third involuntary type of movement is the reflexive movement which is mostly not under the control of the brain but occurs involuntarily in order to maintain balance and posture of the body (Carlson, 2006). In order to initiate voluntary movement of skeletal muscles, the brain has to know where, when and how to direct the movement in space in order to achieve the desired result. All this involves the input of several stimuli to enable the calculation of the movement before the motor cortex informs the appropriate muscles to contract and relax (Scott, 2008). Therefore, movement though initiated by the motor cortex, involves the input of several other parts of the brain to ensure the correct movement occurs. The motor cortex, located on the precentral gyrus and on the anterior paracentral lobule on the medial surface of the brain, is the area of the cerebral cortex that plans voluntary actions, coordinates sequences of movements and relay commands to the appropriate sets of lower motor neurons to execute the desired actions (Gerloff, et al., 1998). It consists of three distinct parts namely the primary motor cortex (M1), the premotor area (PMA) and the supplementary motor area (SMA) each of which plays a specific role in initiating, controlling and maintaining movement and has somatotopic organization in muscle control regions (Graziano, et al., 2002). The supplementary motor area located medially between the two hemispheres, anterior to the primary motor cortex in the motor cortex region, is involved in learning and performing sequential behaviors, where performing one activity signals another to occur in a sequence until the action is completed. This region is responsible for planning the sequence in which movement will occur but does not itself initiate the movement but relays the information probably to the M1 to initiate the movement. Studies show that this region anticipates movement and plans on the sequence of actions to be followed to complete the anticipated movement before the action is performed. It gets its stimuli from the parietal lobe, which informs it on the desire to move, and from this, it directs the appropriate sequence of actions to be taken to enable this (Rizzolatti, Luppino & Matelli, 1998). The premotor cortex is located slightly anterior to the primary motor cortex and is involved in learning and execution of complex movements involving sensory functions (Graziano, et al., 2002). It may also be involved in the association function that allows directed movement to result from linking two unrelated stimuli to an action, for example, raising the hand when requested to do so (Carlson, 2006). Speech is a form of arbitrary information that the individual hears, associates with certain actions and decides to carry out the action related to those words. Inactivation of this region results in the difficulty to act to external unrelated stimuli associated with an action fast enough or even at all due to difficulty in associating this stimuli with the action and deciding on the appropriate movement to make (Graziano, et al., 2002). The inferior parietal lobule and the ventral premotor cortex associated with the premotor cortex contain neurons termed as mirror neurons that serve to allow an individual recognize actions of others and possibly copy them too. They have been observed to fire in a similar way when the individual actually performs an action or when stimuli like sound associated with that action is perceived (Rizzolatti, Luppino & Matelli, 1998). The primary motor cortex, located in the posterior portion of the frontal lobe, is thought to be the main initiator of movement in the brain. It incorporates stimuli from the above two parts and through specific neural pathways directs the appropriate muscles to contract initiating the required movement. This is achieved through two neural pathways/tracts; the lateral group and the ventromedial group, which control the limbs and body trunk ensuring balance and posture during movement (Rizzolatti, Luppino & Matelli, 1998). Each of these neural groups serves specific functions and is further subdivided into tracts depending on the regions it controls. The lateral group is composed of the corticospinal tract, the corticobulbar tract and the rubrospinal tract while the ventromedial group is composed of the vestibulospinal tract, the tectospinal tract, the reticulospinal tract and the ventral corticospinal tracts. Control by these tracts is achieved as follows: the lateral group’s corticalspinal tract that originates from the region of the primary motor cortex and the supplementary motor area responsible for controlling the distal parts of limbs controls muscles of hands, fingers, toes- through synapses directly with motor neurons or via interneurons. The corticobulbar tract terminates in the motor nuclei of cranial nerves and serves to control facial, neck, tongue and extraoccular eye muscles thus controls facial movement. The rubrospinal tract originates in the red nucleus of midbrain receiving inputs from the motor cortex and is responsible for controlling the fore and hind limbs but not their digits. The ventromedial group controls and coordinates posture-vestibulospinal tract, head and trunk movement with the eye- tectospinal tract, involuntary action such as respiration and muscle tone-reticulospinal tract and muscles of the upper leg and trunk-ventralcorticospinal tract. Thus, as can be seen, the lateral group is mainly involved in controlling the actual movement while the ventromedial group controls and maintains posture and balance to ensure proper coordination and execution of movement (Carlson, 2006). What effect would disease or trauma on this part of the brain have on a person? One might ask. Presently two common diseases are known, Parkinson’s and Huntington’s disease, whose effects include loss of the ability to initiate movement or stop it once initiated and also the occurrence of uncontrollable movement due to damage of neurons in the motor cortex (Turner, et al., 1998). Another form of disorder is apraxia, which causes an inability to initiate purposeful learned movement in response to external commands even though the ability and desire to perform the movement exists. This disorder results from the damage of neurons in the parietal and frontal lobes and the corpus collasum and results in the inability to plan movement even though motor neurons and muscles are okay. The patient is observed trying to perform the requested action but ends up not accomplishing it or not performing it appropriately as expected. For example, without a toothbrush, the patient cannot demonstrate the action of brushing teeth or has to put out a finger simulating the toothbrush in order to demonstrate without which incoherent movement would be demonstrated if it does occur at all (Carlson, 2006). Several forms of apraxia have been observed and they include; limb apraxia – inability to move limbs, hands and/or fingers, oral apraxia –inability to move the tongue thus impaired speech, apraxia agraphia – a form of writing disability and constructional apraxia that is an inability to draw or construct objects. Of special note is the role played by the cerebellum. Though motor cortex is involved in planning and initiating movement, it lacks the ability to collect real time feedback on the ongoing actions and provide corrective measures where needed. This is where the cerebellum comes in to play an important role of collecting and organizing feedback before passing it on to the motor cortex for appropriate corrective signals to be transmitted (Turner, et al., 1998). It allows for smooth simultaneous execution of movement by collecting and controlling the timing when each action begins and ends. Damage to the cerebellum even though it would not prevent an individual from performing movements, would cause jerky, erratic and uncoordinated movements, especially where a sequence of movements would be involved in achieving an action (Carlson, 2006). Conclusion The motor cortex can be taken to be the movement controller of the body involved in coordinating complex inputs from various inputs to produce appropriate productive movement as required in day-to-day life. Though considered responsible for actual movement in the body, it serves to communicate with appropriate motor neurons to initiate movement, but the actual intent and planning of the movement originates from other parts of the brain and is then relayed to the primary motor cortex for execution. Also in currently occurring actions, feedback stimuli obtained by other parts of the brain, either from sight, sound or relative position in space, permit the primary motor cortex to send corrective stimuli to appropriate muscles to ensure that the right action occurs in the right way and at the appropriate timing. It is important to note the fact that although the motor cortex plays a major role in movement, it constitutes only a part of a complex neural system that serves to ensure that movement is coordinated and balanced. References Carlson, N. R., (2006): Physiology of Behavior. Boston, MA: Pearson Allyn and Bacon. Gerloff, et al., (1998): The Role of the Human Motor Cortex in the Control of Complex and Simple Finger Movement Sequences. No.121, p1695–1709. Graziano, et al., (2002): The Cortical Control of Movement Revisited. Neuron, Vol. 36, p349–362. Marieb, E. N., (2000): Human Anatomy and Physiology. Michigan, MI: Benjamin-Cummings Publishing Company. Rizzolatti, G., Luppino, G. & Matelli, M., (1998): The organization of the cortical motor system: new concepts. Electroencephalography and clinical Neurophysiology. No. 106, p283–296. Scott, S. H., (2008): Symposium report: Inconvenient Truths about Neural Processing in Primary Motor Cortex. Journal of Physiology, No. 586.5, p1217–1224. Turner, et al., (1998): Motor Subcircuits Mediating the Control of Movement Velocity: a PET study. Journal of Neurophysiology, No.80, p2162–2176. Read More