3 Principles of Sensorimotor
Function
• Hierarchical organization- figure 8.1
–Association cortex at the highest level, muscles at
the lowest
–Parallel structure – signals flow between levels
over multiple paths
• Motor output guided by sensory input
–Sensory feedback (all but ballistic - happen w/o mediation, swing bat etc )
• Learning (experience) changes the nature
and locus of sensorimotor control
–Conscious to automatic
starts @ association cortex down to smaller things
2 Major Areas of Sensorimotor Association Cortex
• Each composed of several different areas
with different functions
• how divide the areas up ?
• Posterior parietal association cortex (also for visual where pathway- good that they're connected so we can see where going)
• Integrates information about
–Body part location
–External objects
• Directs attention
• Receives visual, auditory, and
somatosensory information
• Outputs to motor cortex:
–Dorsolateral prefrontal association cortex,secondary motor cortex, frontal eye fields w/ damage in posterior parietal ass cortex
• Apraxia – disorder of voluntary movement
– problem only evident when instructed to perform an action – usually a consequence of damage to the area on the left - brush teetth in office no toothbrush
• Contralateral neglect – unable to respond to stimuli contralateral to the side of the lesion - usually seen with large lesions on the right-
cooccurs w where they cant see things on left - cant move left arm etc
• Dorsolateral prefrontal association cortex(top sides of frontal)
• Input from posterior parietal cortex
• Output to secondary motor cortex, primary motor cortex, and frontal eye field
• Evaluates external stimuli and initiates voluntary reactions – supported by neuronal responses
• Strongest neuronal firing in anticipation of a movement
Secondary Motor Cortex
• Input mainly from association cortex
• Output mainly to primary motor cortex
• At least 7 different areas
–2 supplementary motor areas
• SMA and preSMA
SMA experiement - look at brain when moving spring, thinking about moving spring, and doing finger movement
–2 premotor areas
• dorsal and ventral
–3 cingulate motor areas
Subject of ongoing research
• May be involved in programming movements
in response to input from dorsolateral
prefrontal cortex
• Many premotor neurons are bimodal –
responding to 2 different types of stimuli
–E.g. visual and somatosensory
Primary Motor Cortex
• Precentral gyrus of the frontal lobe - does lots.
• Major point of convergence of cortical sensorimotor signals
• Major point of departure of signals from cortex
• Somatotopic – more cortex devoted to body parts which make many movements
• Control of hands involves a network of widely distributed neurons - move one part of hand, effect all hand neurons
–Focal Dystonia - when some fingers are so interelated that you forget that they are seperate entities - pinky and ring move a lot w/ middle finger - so middle finger moves and ring and pinky move with it. happens in pianists.
• Stereognosis – recognizing by touch – requires interplay of sensory and motor systems
• Some neurons are direction specific – firing maximally when movement is made in one direction
4:40
subcortical you ask?
Cerebellum and Basal Ganglia
• Interact with different levels of the sensorimotor hierarchy
• Coordinate and modulate
• May permit maintenance of visually guided responses despite cortical damage
Cerebellum
10% of brain mass but has 50% of neurons in brain
• Input from 1° and 2° motor cortex
• Input from brain stem motor nuclei
• Feedback from motor responses
• Involved in fine-tuning and motor learning
–Learning of sequences or movements where timing is critical
• Up with the cerebellum, down with the frontal lobes! - we do better if we dont think about it. want it to be automatic.
–Damage - problems with direction, force, velocity & amplitude of movements, adapting, posture, balance, gait, speech, eye movements
• May also do the same for cognitive responses
- help coordinate to changing stimuli
Basal Ganglia
• A collection of nuclei
• Part of neural loops that receive cortical input and send output back via the thalamus
• Modulate motor output and cognitive functions
–Response learning - learned associations
• Abnormal functioning involved in Tourette’s syndrome (as)- smoothness of movement -
• Substantia Nigra –Loss of nerve cells causes Parkinson’s disease - hyperkenesia - cant stop moving - diskenisia - cant movie. --- cerebellum just working - when he ice skates - no symptoms- video of micheal j fox
• Striatum –Abnormal serotonergic functioning linked to Huntington’s disease
> • chorea- excess of unwanted movements - but these are jerky, not fluid. twitches - video
4 Descending Motor Pathways -
• 2 dorsolateral - figure 8.7
• Most synapse on interneurons of spinal gray matter
–Corticospinal descend through the medullary pyramids, then cross
– Betz cells – synapse on motor neurons projecting to leg muscles
– Wrist, hands, fingers, toes
–Corticorubrospinal synapse at red nucleus and cross before the medulla
– Some control muscles of the face
– Distal muscles of arms and legs
Dorsolateral
• one direct tract, one that synapses in the brain stem • Terminate in one
contralateral spinal segment • Distal muscles • Limb movements
• 2 ventromedial - figure 8.8- take over motor movements if dorso thing fails- but cant do just reaching single limbs out.
–Corticospinal
– Descends ipsilaterally
– Axons branch and innervate interneuron circuits bilaterally
in multiple spinal segments
–Cortico-brainstem-spinal tract
– Interacts with various brain stem structures and descends
bilaterally carrying information from both hemispheres
– Synapse on interneurons of multiple spinal segments
controlling proximal trunk and limb muscles
Ventromedial
• Both corticospinal tracts are direct
• one direct tract, one that synapses in the brain stem • More diffuse • Bilateral innervation •Proximal muscles • Posture and whole body movement
Motor Units and Muscles
• Motor units – a motor neuron + muscle
fibers, all fibers contract when motor neuron fires (contraction message)
• Number of fibers per unit varies – fine control(1-1 ratio), fewer fibers/neuron
• Muscle – muscle fibers bound togetherby a tendon
• Acetylcholine (curare and botox are antagonists of acetyocholine) released by motor neurons at the neuromuscular junction causes contraction
• Motor pool – all motor neurons innervating the fibers of a single muscle
• Fast muscle fibers – fatigue quickly - they work quickly when you need rxn but they dont have a great supply of oxygen or blood - sprinting
• Slow muscle fibers – capable of sustained contraction due to vascularization - capable of sustained contraction - swimming vs running - have good blood and oxy flow
• all Muscles are a mix of slow and fast
• Flexors – bend or flex a joint
• Extensors – straighten or extend
• Synergistic muscles – any 2 muscles whose contraction produces the same movement
• Antagonistic muscles – any 2 muscles that act in opposition
FIGURE 8.11
MUSCLE ORGANs
• Golgi tendon organs
–Embedded in tendons
–Tendons connect muscle to bone
–Detect muscle tension
• Muscle spindles
–Embedded in muscle tissue
–Detect changes in muscle length
Reflexes FIGURE 8.13 etc
• Stretch reflex – monosynaptic, serves to maintain limb stability
• Withdrawal reflex – multisynaptic
• Reciprocal innervation – antagonistic(that do opp move w/ joint) muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc.-
Recurrent collateral inhibition - each time a motor neuron fires, it momentarily inhibits itself via Renshaw cells- cant fire twice real quick - so it doesnt hurt itself - take turns
back to more general...
Central Sensorimotor Programs
• Perhaps all but the highest levels of the sensorimotor system have patterns of
activity programmed into them and complex movements are produced by activating these programs
• Cerebellum and basal ganglia then serve to coordinate the various programs
Motor equivalence
• A given movement can be accomplished various ways, using different muscles
• Central sensorimotor programs must be stored at a level higher than the muscle (as different muscles can do the same task)
• Sensorimotor programs may be stored in secondary motor cortex
–Signing name
The Development of Central Sensorimotor Programs
• Perception & sensorimotor programs (figure 8.17 - the moon! )
• Programs for many species-specific
behaviors established without practice
–Fentress (1973) – mice without forelimbs still make coordinated grooming motions
• Practice can also generate and modify programs
–Response chunking –Practice combines the central programs controlling individual response • E.g. typing (hunt and peck v touch typing )
–Shifting control to lower levels–Frees up higher levels to do more complex tasks –Permits greater speed
Motor cortex-controlled robots- vid
Summary
• The motor cortex is organized much like the sensorimotor cortex, information just flows in the opposite direction.
• The brain strives to perfect movements through feedback and move them from upper to lower levels.
• Movement can happen at the level of the motor unit, usually to enhance survival.
test on tuesday - read 5, 6, skim 7. read 8, take notes.
study notes
