Motor CortexOverall organization of neural structures that control movementQuestions① What is the function of the motor cortex in motor control?② Does the primary motor cortex encode for muscle or for movement?③ How does the motor cortex encode precise movement patterns such as reaching movements?④ What motor functions are lost when the motor cortex is damaged?The primary motor cortex and the premotor areaMovement executionMovement coordination and planningMore monosynaptic connections to alpha MNs controlling upper limbsQuestions① What is the function of the motor cortex in motor control?② How does the motor cortex encode for muscle or for movement?③ How does the motor cortex encode precise movement patterns such as reaching movements?④ What motor functions are lost when the motor cortex is damaged?Motor cortex – functional organization• Somatotopic organization of the contralateral body = motor maps• Large areas reflect larger number of neurons needed for fine motor control (fingers, face, mouth)What do motor maps represent?Evidence from cortical electrical stimulation studies:• Motor cortex contains a spatial map of the contralateral body• Direct pathway to brainstem and spinal cord• low current stimulation to motor cortex activates different muscles (low threshold=monosynaptic connection)• Each muscle can be activated by stimulating many widely dispersed sites (2-3 mm) of the motor cortex• Movements are encoded instead of muscles (“Muscle field”)• axons from motor cortex terminate on sets of spinal motor neurons that innervate different muscles• Representation of different muscles overlap on the motor cortex• Allows coordinated activation of multiple muscle groups across multiple joints to perform behaviorally useful actionsWhat do motor maps represent?Example: arm/hand controlSites controlling an individual muscle are not located together but are distributed over a wide area of motor cortex. Intracortical microstimulation was used to identify sites in monkey primary motor cortex at which low-threshold stimulation evoked electromyographic activity (indicating monosynaptic connections) in a shoulder abduction muscle (middle head of deltoid muscle) and a wrist extensor muscle (extensor carpi radialis; ECR). Topographic maps of the identified sites, reveal overlap between shoulder and wrist representations. (From Humphrey DR, Tanji J. 1991. In: DR Humphrey, HJ Freund (eds.). Motor Control: Concepts and Issues, pp 413-443. New York: Wiley.)Questions① What is the function of the motor cortex in motor control?② Does the motor cortex encode for muscle or for movement?③ How does the motor cortex encode precise movement patterns such as reaching movements?④ What motor functions are lost when the motor cortex is damaged?Direction of arm movements is predicted by a neuronal population vectorNeuronal population vector• Concurrent discharge of a large population of upper motor neurons prior to movements determines movement direction•Discharge of individual upper motor neurons cannot specify arm movement direction (“broadly tuned” to direction)•Same site in motor cortex (= same UMN population) can encode different movement parameters (trajectory, final position, etc)• Example: cortical recording during visually guided reaching movements of arm/hand in monkey (next slides)Directional tuning of an upper motor neuron in the primary motor cortex (Part 1)Monkey is trained to move a joystick in the direction indicated by a lightDirectional tuning of an upper motor neuron in the primary motor cortex (Part 2)Single neuron activity recorded during arm movements in each directionZero = movement onset; Short black lines = action potentialYellow zone = Increased activity for 90-225º movements Blue zone= decreased activity for 45-315 movementsDirectional tuning of an upper motor neuron in the primary motor cortex (Part 3)Plot showing that the neuron’s “preferred direction”: neuron’s discharge rate was greatest before movements in a particular directionDirectional tuning of an upper motor neuron in the primary motor cortex (Part 4)Black lines: discharge rates of individual upper motor neurons prior to each direction of movement.Red arrows: “population vector” defined by combining the responses of all the neurons; movement direction encoded by simultaneous activity of entire populationQuestions① What is the function of the motor cortex in motor control?② Does the primary motor cortex encode for muscle or for movement?③ How does the motor cortex encode precise movement patterns such as reaching movements?④ What motor functions are lost when the motor cortex is damaged?Overview of descending motor controlCortical damageCortical damage (stroke, Cerebral Palsy -CP):- Spasticity: Increased response to muscle rate of stretch – velocity- Lack of inhibition to spinal cord- Abnormal reflexes (Babinsky)- Loss of fine controlFigure 17.16 The Babinski signA monkey that controls a robot with its