Sensorimotor system

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