Week 3 - Reflex Neuromechanics PDF

Reflex neuromechanics MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Describe withdrawal and crossed extensor reflexes u Describe how reflexes can be expressed differently in different movement contexts u Identify common automatic postural reactions u Understand how ref...

Reflex neuromechanics MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Describe withdrawal and crossed extensor reflexes u Describe how reflexes can be expressed differently in different movement contexts u Identify common automatic postural reactions u Understand how reflexes and voluntary commands might interact to produce movement u Understand the movement impairments produced by common reflex abnormalities Types of movement u Spinal reflexes u Circuits underlying fast responses to afferent stimuli u Automatic behaviours u More complex reactions to afferent stimuli (e.g. startle) u Involve more complex circuits u Voluntary actions u Generated by the cerebral cortex in response to a desire or need More functional reflexes… MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Describe withdrawal reflex and crossed extensor reflex Withdrawal reflex u The withdrawal reflex is initiated by nociceptor (pain receptor) activation u The afferent impulses are transmitted to an excitatory interneuron in the spinal cord u Motor neurons of (in this case elbow) flexor muscles are activated u Motor neurons of extensor muscles are inhibited (not shown) u The distal segment (hand) is drawn away from the source of pain Crossed extensor reflex u Crossed extensor reflexes are initiated by the same stimulus as withdrawal reflexes and occur simultaneously u Excitatory interneurons activate motor neurons projecting to extensor muscles of the opposite limb u Flexor muscles of opposite limb are inhibited u Limb opposite painful stimulus extends Dealing with instability in the environment MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Describe how reflexes can be expressed differently in different movement contexts Transcortical stretch reflexes u Signals from stretch receptors are transmitted to the sensorimotor cortex u A motor response is transmitted via the corticospinal tract to the same muscle as the spinal stretch reflex u The transcortical reflex arrives slightly later u In people with bilaterally- projecting corticospinal neurons, this reflex can be observed in both arms What can we do if we know the likelihood of instability is high? u Changes to reflex sensitivity (feedback) u Changes in ‘preparatory set’ (feed-forward/feedback) u Early changes in level of co-contraction (feed-forward) u Changes in posture (feed-forward) u In essence, we can respond to postural ‘errors’ early. But are our earliest responses appropriate for stability? What do we know about whether reflexes regulate postural stability? u Experimental setup u Participants seated u Hand attached to robotic arm providing a stable and unstable support and perturbations u EMG recorded from eight arm muscles Perreault et al., 2008 What do we know about whether reflexes regulate postural stability? u Reflex sensitivity is dependent on the amount of stretch encountered u Reflex responses are modulated according to the ability of the muscle to counteract limb disturbances How smart are our reflexes in complex 3D situations? u Can reflexes deal with non-symmetric stability situations (e.g. surfing, tool use)? u Participants’ hands attached to robot that creates high stability in one direction and low stability in the orthogonal direction u The direction of instability changes in each trial u EMG recorded from eight muscles crossing the elbow and shoulder u Hyp: that reflexes would be altered depending on the orientation of instability Krutky et al., 2010 How smart are our reflexes in complex 3D situations? u Reflexes were elicited from all muscles u Reflex amplitude changed depending upon: u The amount of ongoing muscle activity u The direction of the perturbation u The direction of instability provided by the robot u Conclusion: that reflexes (these are probably spinal + cortical) are capable of very smart responses that act to maximise postural stability even in complex 3D situations What is ‘preparatory set’ u The ‘readiness’ of the motor system to generate a desired action or response. u No definitive neural mechanism for this phenomenon has been identified u Pre-movement changes in neural activity within task-specific regions of the motor system have been demonstrated u Thought to reflect increases in activity of task-relevant neural circuits that increase the excitability of those circuits prior to action or a stimulus How might preparatory set assist postural stability? u It might alter the expression of transcortical reflexes to make them task-specific u May also allow the early release of stabilising actions following a sufficiently large stimulus u Hyp: that motor cortex inhibition would eliminate reflex modulation but not StartReact responses between stable and unstable environments Shemmell et al., 2009 How might preparatory set assist postural stability? u EMG recorded from biceps brachii u TMS inhibited motor cortex for ~100 ms u Long latency stretch reflexes were larger in an unstable (compliant) situation without TMS but not with TMS u Stabilising StartReact responses were not suppressed by TMS u Conclusion: that preparatory set can influence both cortical reflexes and subcortical StartReact responses How important are changes in co- contraction to balance? u Experimental setup: u Participants seated u Foot attached to a motor acting as an inverted pendulum u EMG recorded from Sol, MGas, LGas, TAnt u Hypothesis: that co-contraction levels and stretch reflex sensitivity increase as stability is reduced? Finley et al., 2010 How important are changes in co- contraction to balance? u Stretch reflex sensitivity decreased in unstable situations How important are changes in co- contraction to balance? u Co-contraction increased as environmental stability is reduced n So, at the ankle increases in intrinsic stability are dependent upon voluntary (feed-forward) mechanisms. n In the upper limbs, reflexes appear to play a larger role. Main points (transcortical reflexes) u Stretch reflexes can reach the cortex u Transcortical reflexes are slower than their spinal equivalents, but can be flexibly tuned to the stability requirements of a task u Stability can also be provided by co-contraction – this seems particularly true in the legs Automatic postural reactions MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Identify common automatic postural reactions Controlling postural perturbations u Three coordinative strategies used for correcting standing posture a) Ankle strategy: used for small perturbations when surface is firm b) Knee strategy: used for longer or faster perturbations or when surface is compliant or small c) Stepping strategy: used to enlarge BoS when ankle and hip strategies are insufficient Reactive postural control strategies u These responses involve complex patterns of muscle activity u They occur too late to be reflexive u Likely to be generated in the brainstem (MEDI330) u The size of these responses scales with the size of the postural perturbation u It remains unclear whether they are driven by a feedback or feedforward mechanism Anticipatory postural adjustments (APAs) u Anticipating perturbations caused by self-movement enables us to move our limbs independent of our body u An example of feed-forward motor control u Anticipation causes: u Early activation of postural muscles (sometimes before event) u Smaller perturbation Anticipation of arm unloading Hugon, Massion & Wiesendanger (1982) Main points (automatic reactions) u Automatic reactions: u Occur later than reflex responses u Generally involve more muscles u Are modified based on the context u Are related to functional needs u Often mediated by brainstem circuits (faster than voluntary control from cortex) Interactions between reflexes and voluntary movement MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Understand how reflexes and voluntary commands might interact to produce movement The cortex can regulate all lower motor circuits Reflex inhibition during voluntary actions u The stretch reflex circuit can demonstrate cortical/spinal interactions u Descending input to ankle dorsiflexors (voluntary contraction) reduces the size of stretch (or H-) reflexes generated in the plantarflexors But how does the cortex decide how to modulate reflexes? Muscle synergies u “a group of muscles activated in synchrony with fixed relative gain” (Torres-Oviedo & Ting, 2007) u Could simplify the degrees of freedom problem BUT, does the CNS work this way? Joint torques Main points (reflex/voluntary interactions) u Spinal reflexes are constantly regulated by descending motor commands u Direct cortical activation of a muscle is often associated with indirect inhibition of other muscles via reflex circuit activation u It has been suggested that almost our entire movement repertoire could be produced by a set of spinal reflex circuits, which are triggered by simple descending commands u It is still unclear if the CNS operates in this way Reflex abnormalities MEDI258: HUMAN NEUROMECHANICS Lecture learning outcomes u Understand the movement impairments produced by common reflex abnormalities Spasticity u A condition involving an involuntary increase in muscle tone that leads to resistance against normal movements of the body and may cause pain in some individuals u becomes evident after stroke (in some cases) How do reflexes play a role in spasticity? u Reduced input from motor centres in the brain to spinal motor neurons u Normal inhibition of stretch reflexes reduced u Stretch reflex becomes hyper- reactive u Produces a force resisting movement that is velocity- dependent u Why velocity-dependent? Main points (reflex abnormalities) u In adulthood, spinal reflexes are regulated by descending signals from the motor areas of the brain u Removal of these descending signals can remove ongoing inhibition, resulting in the over-expression of reflexes