Motor Control Flashcards
What is voluntary brain control of muscles?
Via alpha-motoneurons in spinal cord - final common path
What is reflex control of muscles?
Autonomous and hard wired into motoneuron circuits at each segmental spinal level
How does brainstem nuclei control spinal reflexes?
Integrates them into higher order reflexes that control posture and balance
Receive control inputs about voluntary movements from higher centres - basal ganglia and cerebellum
What are the 4 system which control movement?
Descending control pathways
Basal ganglia
Cerebellum
Local spinal cord/brain stem circuits
Describe the spatial map of your body musculature in spinal cord
Medio-laterally
More medial motoneurons for proximal muscles - shoulder/hip
More lateral motoneurons for distal muscles - finger/toe
Also map along long axis
What does the spinal cord receive descending input from?
Brainstem and direct cortical input via corticospinal tract
Where do sensory inputs come from?
Is crucial and enters at all levels
Spinal cord - proprioceptors, touch, pain
Brainstem - vestibular system for balance
Cortex - movements in response to visual, olfactory, auditory and emotional cues
What happens if there is damage to sensory inputs at spinal level?
Paralysis as if motoneurons themselves have been damaged
Unable to control movement is proprioception is lost - lose sense of body in space
Do dorsal root ganglia regenerate?
No
What is the stretch reflex
Is present in all muscles
Ex. patellar tendon reflex
Uses info from muscle spindles which monitor muscle length
Describe the stretch reflex
Tendon transmits force to muscle fibres and so more able to stretch
Stretch activates 1a afferent sensory nerves in muscle spindle
Increases APs in 1a afferents in dorsal horn
Divide to make 3 connections
What are the 3 connections made by spindle sensory afferents?
Direct contact to alpha-motoneurons in stretched muscle causing rapid contraction of agonist
Indirectly to antagonist muscle causing relaxation
Travels up dorsal columns to thalamus and somatosensory cortex
What type of reflex is the direct contact to alpha-motoneurons in stretched muscle?
Monosynaptic - no interneurons
Describe the reciprocal inhibition
Spindle afferent connections with inhibitory interneurons so decreases activation of alpha-motoneurons to antagonist muscles which relax
What is the inverse stretch reflex?
1b afferent nerves from golgi tendon organs which monitor muscle tension
Muscle contracts and shortens with pulls on tendon and 1b afferent nerve from GTOs increase firing
What does the inverse stretch reflex cause?
Activation of inhibitory interneurons to agonist muscles which decreases contraction strength
Activation of excitatory interneurons to antagonist muscles
Info ascends spinal dorsal column to somatosensory cortex
Why is the inverse stretch reflex sometimes called clasp knife reflex?
Greatly increasing tension in tendon leads to collapse of resistance
Joint extends
Is a protective mechanism to prevent muscle damage
Why is the golgi tendon organ reflex important?
Avoids muscle contracting so hard that the tendon is ripped from bone
Is polysynaptic and protective
Describe the golgi tendon organ reflex
Muscle pulls hard on tendon so GTO afferent fibres increase firing so synapse with inhibitory interneuron to reduce motoneuron firing in agonist muscle - relaxes
Excitatory interneurons increase activation of antagonist motor neuron - contracts
What is the flexor/ withdrawal reflex
Use info from pain receptors in skin, muscle and joints
Polysynaptic and protective
Withdrawal of body from stimuli
Flex affected part
In the flexor reflex, an increase in APs in nociceptors causes what?
Increase activity in flexor muscle - excitatory interneurons
Antagonist extensors are inhibited - excitatory and inhibitory interneuorns
Describe the circuitry of the flexor reflex
Small diameter alpha-delta nociceptor axons triggering pain enter cord
Branch and activate interneurons in several segments of spinal cord above entry point
Activates alpha motoneurons controlling all flexor muscles of affected limb
Explain the crossed extensor reflex
Excitatory interneurons cross spinal cord and excite contralateral extensors
Inhibitory interneurons inhibit contralateral flexors - maintains posture
Sensory info ascends to brain in contralateral spinothalamic tract
Is the flexor reflex faster than stretch reflex?
No, stretch is faster as nociceptor sensory fibres are smaller diameter than muscle spindle fibres so conduct more slowly
Can reflexes be overridden consciously?
Yes,
GTO can be over ridden by voluntary input from CNS
Describe alpha-motoneurons and their synapses
Receive a lot of synapses - many from thalamus and cortex
Cause excitatory and inhibitory postsynaptic potentials
Most are via interneurons
Each motoneuron integrates these signals
How does voluntary control over ride the GTO reflex?
Descending voluntary excitation of alpha motoneurons overrides the inhibition from GTOs and maintains muscle contraction - blocks reflex
How can the stretch reflex be overridden?
Strong descending inhibition hyperpolarises alpha motoneurons which makes the reflex hard to evoke
Jendrassik manoeuvre can be used
Describe the Jendrassik manoeuvre
High activity in upper motor neurons which depolarises lower level motoneurons overcoming descending inhibition
Can withdrawal reflex be overridden?
Yes, can over ride it mentally
Same with anticipating pain can increase the vigour of withdrawal reflex when painful stimulus arrives
What is the clinical relevance of reflexes?
Important for assessing integrity of whole spinal cord circuit
Localisation of spinal level of problem
