Cortex Outline
Efferent pathways Muscles When things go wrong
Lecture 11
Motor System and Movement
Keep in mind that the flow of information is opposite to sensory systems
Intro to Motor System
Like sensory systems, hierarchical organization Almost complete complement to somatosensory system Unlike sensory systems, very experiencedependent
Example of an exception:
www.driesen.com/sensorimotor_system.htm
Primary Motor Cortex
Precentralgyrus Forms motor homunculus (somatotopic map) Fingers overlap with each other (not distinct regions)
From loose fist, try lifting ring finger
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Primary Motor Cortex
Neurons have preferred directions
Directional preferences depend on overall direction, not direction of force required NOT center-surround!!! Neurons fire to movements in directions close to preferred
freud.tau.ac.il/~shakhar/neuro/ images/nerveman.gif
Secondary Motor Cortex
Premotor 1 gyrus width anterior to precentral gyrus Supplementary Motor Area (SMA) dorsal to premotor Cingulate Motor Areas ventromedial to SMA Other areas:
Dorsolateral Prefrontal Cortex (DLPFC) anterior to premotor Frontal Eye Fields ventrolateral to anterior portion of DLPFC
Secondary Motor Cortex
Differential functions are a topic of debate, depends on parameters of task. However, some evidence of segregated functions:
Tap one finger M1 Tap fingers in sequence M1 + SMA Imagine tapping in sequence SMA
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DLPFC
Seat of executive functions Evaluation of and response to external (or internal) stimuli Receives projections from posterior parietal, sends signals to frontal eye fields and secondary motor areas
Frontal eye fields control eye movements
Activity before and during movement site of initiation of voluntary movement
Motor signal pathway
Originates in cortex (for voluntary movements) 4 tracts:
Dorsolateral Corticospinal Dorsolateral Corticorubrospinal Ventromedial Corticospinal Ventromedial Cortico-brainstem-spinal
Dorsolateral tracts
Both enter dorsolateral portion of spinal cord and project to distal contralateral limb muscles Corticospinal direct, cortex spinal cord
Decussation at medullary pyramid Wrist, hands, fingers, toes
Muscles controlled are contralateral or ipsilateral with respect to cortical site of signal origination Motor signals exit spinal cord through ventral horn
Corticorubrospinal involves red nucleus
Synapses in red nucleus, decussates Some interstitial synapses in cranial nerve nuclei that control contralateral facial muscles Arms, legs
http://pharyngula.org/~pzmyers/neuro/chap9/index.php?print
Ventromedial tracts
Both enter ventromedial portion of spinal cord and project to proximal trunk and limb muscles on both sides Corticospinal direct
Descends ipsilaterally Sends diffuse projections to both sides
Cortico-brainstem-spinal indirect
Tectum integrate with visual and auditory spatial information
Reticular formation common motor programs Vestibular nucleus balance Descends bilaterally and diffusely
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www.utdallas.edu/~tres/i nt eg/ mot2/motor _2. html
Summary
(Dorso)lateral decussation in brainstem, innervates one side Ventromedial no real decussation (ipsilateral or bilateral), innervates bilaterally and diffusely
Differential functions
Revealed by lesions in monkeys Cut dorsolateral can walk, climb, etc adeptly, but cannot move fingers independently or release grasp
Note: can release grasp as part of climbing DL reaching, fine movements of hands
Muscles
Skeletal muscles (as opposed to smooth muscles) made up of fibers Each motor neuron innervates multiple fibers
Number of fibers innervated dictates amount of selective control Motor unit = motor neuron + all innervated fibers Motor pool = all motor neurons that innervate a single muscle
Cut ventromedial use can elbow, hands, fingers, but cannot sit or walk
VM posture, whole-body movements
Activation of motor end-plate at the NMJ (by Ach) causes contraction
When muscles contract
Each (skeletal) muscle fiber made up of actin (thin) and myosin (thick) filaments
Actin attached to plates marked by z-lines
Activation causes -actinin molecules to jump along myosin filament contraction
Active process (requires ATP-adenosine triphosphate)
Tension builds as sarcomere (distance between zlines) shortens
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When muscles contract
Muscles contract as long as signals come, relax when signals stop 3 types of muscle fibers
Slow twitch slow to contract or relax but can sustain for long time, aerobic, posture Medium twitch able to contract quickly and can sustain for long time, anaerobic and aerobic, walking Fast twitch able to contract quickly but fatigues quickly, anaerobic, sprinting
Types of muscles
Flexor bend joint (biceps) Extensor unbend joint (triceps)
Synergistic multiple muscles that produce same effect
Antagonistic muscles that oppose each other
Inside job
Sensory information can come from the motor system itself Golgi tendon organs embedded in tendons, measure muscle tension
If tension too great, send inhibitory projections to motor neurons
Inside job
Extrafusal muscle outside muscle that actually does work Intrafusal muscle inside muscle that changes length to retain tension in muscle spindle Each has its own motor neuron Muscle spindle usually does not change activity during voluntary movement
Muscle spindle afferents embedded in muscle tissue, measure (unexpected changes in) muscle length Isometric contraction increase tension without changing length Dynamic contraction decrease length without changing tension
Flex and Reflex
Reflexes signal potentially dangerous (tissue damage) interactions with the environment and make appropriate muscle responses Most involve only the spinal cord (no brain) May be monosynaptic or polysynaptic
Polysynaptic reflexes involve interneurons to allow for reciprocal innervation
Some can be mediated by cortical activity
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omega.dawsoncollege.qc.ca/ ray/neuron/reflex.htm
Other Movement Centers
Basal Ganglia (includes...)
Forms reciprocal loop with motor cortex (via thalamus) Modulation of motor output Response learning Inhibition
Prefrontal knowing when to inhibit Basal ganglia generating inhibition signal
Other Movement Centers
Cerebellum
Lots and lots of neurons (as many as the rest of the brain combined) Fine-tuning and learning of motor responses (motor coordination)
May extend to cognitive responses as well
Sensorimotor Integration
Motor output is guided by sensory input
Sensory feedback refines movements both within a movement and over time
Apraxia inability to make specific movements on command (i.e. out of context)
Need sensory feedback and context to make movements
Damage causes loss of control over force, velocity, and amplitude of movements
Also lose control over direction. Given directional coding in M1, why would this be?
Central Programs
Theory suggests that there are a bunch of simple motor programs stored in lower level structures
Simple programs combine to produce complex movements Reduces need for cortical involvement Cortex involved in learning new programs, but eventually stored in lower areas Cerebellum and basal ganglia ensure coordination
Acquisition Theories
Response Chunking
Learn A, learn B, chunk into AB This is why "withe" is common in experienced typists
Shifting control to lower levels
Frees up higher levels for cognitive functions (evaluating output, extraction of information, enjoyment, etc.) Allows for higher efficiency by initiating movements in parallel (or overlapping serial)
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