Download LECTURE15.VoluntaryMovement

LECTURE 15: VOLUNTARY MOVEMENT
REQUIRED READING: Kandel text, Chapters 33 & 38
Voluntary movement differs from reflexes in several important ways:
1)
Voluntary movement is governed by conscious planning
2)
3)
4)
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
5)
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)
REMAINING HAND REPRESENTATION SPARED
AND MORE CORTEX RECRUITED
(converted from arm/shoulder representation)
LOSS OF GRASPING CAPACITY
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
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