NEURO2020 WEEK 9

SENSORIMOTOR SYSTEM NERVES

• Sensory = afferent 

• Motor = efferent 

FEEDBACK

• Critical for coordination of motor movements 

• Movements that do not require feedback are called ballistic movements 

LEARNING

• Initially, moves are under conscious control, then they become automatic 

 

APRAXIA

• Disorder of voluntary movement 

• Not a motor deficit like paralysis/weakness or lack of motivation/comprehension 

• Typically, a person cannot perform a specific task out of context when requested but can perform the task spontaneously under natural conditions 

• Associated with damage to left parietal cortex 

MIRROR NEURONS

• provide a mechanism for social cognition 

  • Understanding the perceptions, ideas and intentions of others 

• respond to the understanding of the purpose of the action, not just a superficial characteristic 

• respond fully to particular action regardless of how it is detected (auditory/visually) 

ASSOCIATION CORTEX - Posterior Parietal Association Cortex – INPUT 

• Integrates information about environment and about your body 

• Directs movement and attention 

• Damage = deficits in deployment of attention = neglect, apraxia 

ASSOCIATION CORTEX - Dorsolateral Prefrontal Association Cortex – OUTPUT 

• Neurons respond to characteristics and locations of objects 

• Neurons begin to fire before action is executed and continue until it is completed 

• Outputs to secondary + primary motor cortex and frontal eye field  

SECONDARY MOTOR CORTEX

• Three parts: 

  • Supplementary Motor Area 

  • Premotor Cortex 

  • Cingulate Motor Areas  

• Stimulation of these areas elicit complex movements of both sides of the body 

PRIMARY MOTOR CORTEX

• Major point of convergence/depature of sensorimotor signals  

• neurons initiate movements towards specific goals rather than coding for just one direction of movement 

LESIONS IN PRIMARY MOTOR CORTEX

• Typical deficits are loss of ability 

• Lose stereognosis (the ability to identify objects from touch) 

• Reduction in speed/force of movements 

• Voluntary movement not usually lost completely 

CEREBELLUM AND BASAL GANGLIA

• Interconnected within sensorimotor system 

• Not part of the descending pathway to muscles, but interacts with different levels of the hierarchy to coordinate and modulate actions. 

CEREBELLUM

• 10% of brain mass but over 50% of neurons 

• Compares inputs from these systems to correct movements that deviate from intended course 

• Plays a major role in motor learning, particularly those that involve timing 

CEREBELLUM DAMAGE =

• Posture difficult to maintain 

• Can't adapt movement to changing conditions 

• loss of ability to control: 

  • Direction/force/amplitude/velocity of movement 

BASAL GANGLIA

• Cluster of interconnected nuclei 

• Involved with motivation and movement 

• May be responsible for movement vigour 

• Responsible for suppressing motor activity 

SPINAL CORD CIRCUITS

• Both pathways have major tracts, one that descends directly to the spinal cord, another that descends indirectly to the spinal cord 

• Ventromedial neurons project to proximal muscles close to spinal cord 

• Dorsolateral neurons project to distal muscles 

MOTOR UNITS

• Comprises of a single motor neuron and all the skeletal muscles that it innervates.  

• Fingers/face motor units have few fibres for each motor neuron vs legs 

MOTOR END PLATE

• The junction between motor neuron and the muscle fibre 

• The motor neuron releases Acetylcholine at the neuromuscular junction which causes the muscle to contract 

RECEPTOR ORGANS OF TENDONS AND MUSCLES

Golgi Tendon Organs 

• Embedded in the tendons and respond to increase in tension 

• Can inhibit contraction if tension too high 

Muscle spindles 

• Embedded in muscle tissue 

• Signals change in muscle length 

INTRAFUSAL MOTOR NEURONS

• Without intrafusal motor neurons, spindles would become slack and unresponsive to stretch during an extrafusal muscle contraction 

• They function by adjusting the length of intrafusal muscles, maintaining an appropriate degree of tension on muscle spindles

STRETCH REFLEX

• Stretch reflex elicited by sudden external stretching force on a muscle 

• Spindle receptors are stretched and send signal to the spinal cord 

• Automatically stimulates motor neurons in the ventral horn to send APs back to the muscle containing the spinal -> contracts 

RECURRENT COLLATERAL INHIBITION

• Each motor neuron branches just before exiting spinal cord 

• One branch synapses onto a small inhibitory interneuron (Renshaw cell) 

• Renshaw cell inhibits the motor neuron each time it fires, diverting the action to another member of the motor pool 

• Prevents fatigue by giving regular break to motor circuit 

PRACTICE CREATES CENTRAL SENSORIMOTOR PROGRAMS

Response chunking; 

• Practice combines sensorimotor programs that control individual responses into programs that control sequences of behaviour 

Shifting control to lower levels: 

• Practice shifts control of motor programs to lower levels, freeing up resources at higher levels for other things 

BODY REPRESENTATION/SCHEMA 

• Our brain needs to keep track of where our limbs are through time and space 

  • Navigate safely 

  • Interact with objects/people 

  • Communicate and portray emotion non-verbally 

• A vital requirement for maintaining body representation is multisensory integration.

PROPRIOCEPTION

• Sense of the position of body parts relative to one another and their motion 

• What we see and feel the position of our body parts, are integrated 

• unified representation of events, both of the outside world and of our body  

MULTISENSORY INTEGRATION & BODY REPRESENTATIONS

• MSI enables us to update representations of our bodies continuously 

• Automatic - doesn’t require effortful processing 

• MSI more likely to be integrated into a single event if they correspond in time and space 

• Information of each sense is not equally/reliably weighted = greatest weight given to most spatially precise source of information 

• We can exploit these perceptual weightings however 

TEMPORAL PARIETAL JUNCTION

• important for limb representation/updating 

• TPJ damage/disfunction is associated with out of body experiences 

MIRROR HAND ILLUSION PREDICTIONS

• Without mirror there should be no systematic pattern of reach errors related to mirror hand position 

  • It should not matter where left hand is sitting as there is no mismatch between felt and seen hand locations 

  • Control condition 

• Reach errors will be systematic, the more the real and mirror locations differ, the greater the reach error 

  • The pattern of errors will be consistent with the brain representing the hand somewhere between the real and mirror signalled locations 

MIRROR HAND ILLUSION OUTCOMES

• Real hand and mirror signal at same location 

  • Reach is accurate 

• Mirror hand signals a location on the opposite side to real hand 

  • Participants reaches relatively straight  

• Mirror signals a location further left than the real hand 

  • Participants stops further right 

• Mirror signals a location further right than the real hand 

  • Participants stops further left  

MIRROR HAND ILLUSION RESULTS

• Participants reaches showed systematic errors towards the seen location of the hand rather than what they felt 

  • Further the hand in mirror differed from the real hand, the less accurate they were 

• This is consistent with their brain representing their hand as somewhere between its real location and the location signalled in the mirror 

PROPRIOCEPTIVE DRIFT

• The reliability of proprioceptive information is known to degrade over time, with proprioceptive drift occurring over very short timescales 

• Change in perceived position of obscured hand 

• Without fake hand, drift tends to occur towards the middle of the body due to it being the default position 

• With the fake hand, drift tends to occur towards the fake hand because the brain relies on visual input, now that proprioceptive input is degraded 

MIRROR BOX THEORY

For patients with phantom limb syndrome the mirror box illusion can be used to reset the missing limb and alleviate pain 

RUBBER HAND ILLUSION - SYNCHRONOUS STROKING

Temporal correspondence  

• Sensations occur at same time 

Spatial correspondence 

• Stroking occurs in same direction and same location on both hands 

 

• Results in integration of inputs into single event 

RUBBER HAND ILLUSION - OUTCOMES

• Referral of touch – stroking sensation is coming from the rubber hand 

• Proprioceptive drift towards rubber hand  

• Temperature drop in hidden hand 

• These effects weaker in asynchronous than synchronous