REQUIRED READING: Kandel text, Chapters 33 & 38 LECTURE 15: VOLUNTARY MOVEMENT REQUIRED READING: Kandel text, Chapters 33 & 38 Voluntary movement differs from reflexes in several important ways: Voluntary movement is governed by conscious planning It is organized around performance of a specific task Sensory stimuli do not dictate the resulting movement, although they guide the specified task Task performance becomes more efficient with experience Voluntary movement can be initiated internally without a sensory stimulus trigger VOLUNTARY MOVEMENT OFTEN REPRESENTS COMPLEX MOTOR TASKS THAT ARE ACCOMPLISHED IN SEEMINGLY EFFORTLESS FASHION, WITH NO THOUGHT GIVEN TO THE MUSCLE GROUPS AND JOINTS THAT PARTICIPATE

SENSORY INPUTS GUIDE VOLUNTARY MOVEMENT THROUGH FEED-BACK AND FEED-FORWARD MECHANISMS

EXAMPLE OF FEEDBACK AND FEEDFORWARD MOVEMENT CONTROL: CATCHING A FALLING BALL Visual input provides feed-forward control of the task enabling us to: 1) Position hand under where ball is anticipated to fall 2) Partially stiffen joints in anticipation of ball’s impact on hand Somatosensory and proprioceptive inputs provide feed-back control used to grasp ball. Some aspects of feedback control involve task-specified programming of spinal reflexes

PROCESSING OF A MOTOR TASK BEGINS WITH AN INTERNAL REPRESENTATION OF THE DESIRED RESULT OF MOVEMENT EXAMPLE 1: HANDWRITING IS SIMILAR STYLE REGARDLESS OF LIMB USED TO WRITE We write text to conform to an internally preimaged style template

PROCESSING OF A MOTOR TASK BEGINS WITH AN INTERNAL REPRESENTATION OF THE DESIRED RESULT OF MOVEMENT EXAMPLE 2: REACHING IS A STRAIGHT-LINE TASK, REGARDLESS OF DIRECTION AND MUSCLES/JOINTS REQUIRED We program the direction and endpoint of task, and use sensory input during task for guidance correction

PROCESSING OF A MOTOR TASK BEGINS WITH AN INTERNAL REPRESENTATION OF THE DESIRED RESULT OF MOVEMENT EXAMPLE 3: SPEED OF REACHING IS PRE-SCALED TO THE DISTANCE OF TARGET The endpoint is built into the premotor program

EFFICIENCY OF EXECUTING MOTOR TASK IMPROVES WITH PRACTICE (LEARNING) Both explicit and implicit memory are components of motor learning Improved efficiency in reaching task is form of implicit learning

CENTRAL PATHWAYS FOR VOLUNTARY MOTOR CONTROL Motor areas of cerebral cortex project directly and indirectly to spinal cord motor neurons and interneurons Motor areas also project to basal ganglia and cerebellum, which project back to cortex via thalamus Cerebellum critical for integrating desired task and sensory inputs into motor planning and execution Cerebellum is a major site for learning within motor circuits Basal ganglia control muscle tone (readiness) and execution of rapid motor tasks

MOTOR CORTEX AND PREMOTOR CORTICES PROJECT TO MOTOR UNITS AND CONSTITUTE SOMATOTOPIC MAPS OF THE BODY Motor cortex axons project to motor neurons both monosynaptically and through brain stem nuclei FOCAL STIMULATION IN MOTOR AREAS INDUCES CONTRACTION OF SPECIFIC MUSCLE OR MUSCLE SET FOCAL LESIONS IN MOTOR AREAS CAUSE LOSS OF SPECIFIC MUSCLE SETS

SOMATOTOPIC MAP IN MOTOR CORTEX CHANGES FOLLOWING FOCAL LESION Remapping of motor cortex following lesion is influenced by experience in the weeks after injury Profound neurological implications for role of physical therapy following brain injury EXPERIMENT-INDUCED FOCAL STROKE AFFECTING MUCH OF HAND/DIGIT REGION OF MOTOR CORTEX NO PHYSICAL THERAPY FOR HAND PHYSICAL THERAPY FOR HAND ~ 1 MONTH REMAINING HAND REPRESENTATION IN MOTOR CORTEX LOST (converted to arm/shoulder representation) LOSS OF GRASPING CAPACITY REMAINING HAND REPRESENTATION SPARED AND MORE CORTEX RECRUITED (converted from arm/shoulder representation) GRASPING CAPACITY RESTORED

MOTOR CORTEX IS ESSENTIAL FOR FINE CONTROL OF THE DIGITS Severing corticospinal tract causes permanent loss of fine digit control Coordinated use of more proximal muscles improves over time, making use of indirect projections from motor cortex through brain stem

FIRING OF MOTOR CORTEX NEURONS DURING VOLUNTARY MOVEMENT DIRECTLY ACTIVATES MOTOR NEURONS IN SPINAL CORD Technique of POST-SPIKE FACILITATION OF MUSCLE ACTIVITY

DIRECTION OF LIMB MOVEMENT IS SUM OF CORTICAL NEURON VECTORS EACH MOTOR CORTEX NEURON ACTIVATES MUTLIPLE MUSCLES TO DIFFERENT DEGREES DIRECTION OF LIMB MOVEMENT IS SUM OF CORTICAL NEURON VECTORS

PREMOTOR AREAS CONTRIBUTE TO MOTOR PLANNING EEG recordings show that medial premotor area is active to performance or mental rehearsal of complex tasks

THE MOTOR CORTEX IS DRIVEN BY DIFFERENT PREMOTOR AREAS IN RESPONSE TO VISUAL CUES VERSUS PERFORMING REHEARSED TASKS

HOW DO WE KNOW THAT MOTOR CORTEX ACTIVITY DETERMINES VOLUNTARY MOTOR FUNCTION? http://ondemand.duke.edu/video/22553/monkeys-thoughts-make-robot-wa