Motor systems - Motor control basics Flashcards
what is voluntary brain control of muscles via?
α motoneurones in the spinal cord
how would you describe Reflex control of muscles?
autonomous and hard-wired into motoneurone circuits at each segmental spinal level.
how do Brainstem nuclei control spinal reflexes?
give an example?
integrate them into higher order reflexes that control posture and balance
e.g., descending inputs from vestibulospinal and reticulospinal tracts regulate trunk and limb muscle reflexes.
what do brainstem nuclei receive?
inputs about voluntary movements from higher centres – from the cerebral cortex (motor, premotor and supplementary motor cortex), the basal ganglia and the cerebellum
what are the four systems that control movement?
1) Descending control pathways,
2) Basal ganglia,
3) Cerebellum
4) Local spinal cord/brain stem circuits
The spatial (‘somatotopic’) map of your body musculature in the spinal cord - medio-laterally applies to what?
(applies to arms and legs)
The spatial (‘somatotopic’) map of your body musculature in the spinal cord - proximally applies to what?
Proximal (shoulder/hip) muscle are mapped to medial motoneurones
The spatial (‘somatotopic’) map of your body musculature in the spinal cord - distally applies to what?
Distal (finger/toe) muscles map to more lateral motoneurones
what do muscles do along the long axis of spinal cord?
map spatially
how would you describe higher brain centres?
functionally interdependent and control different aspects of voluntary movements.
where does the spinal cord receive descending input from?
via the brainstem AND direct cortical input via the Corticospinal (Pyramidal) tract.
does sensory input enter at all levels of the spinal cord?
Sensory input is crucial and enters at all levels
describe sensory input of the spinal cord?
Spinal cord - proprioceptors, touch, pain etc
describe sensory input of the brain stem?
vestibular system informs about balance
describe the sensory input of the cortex?
movements in response to visual, olfactory, auditory, emotional, intellectual cues.
what does damage to sensory inputs (at spinal level) result in?
paralysis as if the motoneurones themselves had been damaged.
describe the mechanism of the stretch reflex?
Tendon transmits force to the muscle fibres (more elastic than tendons) and so are more able to stretch.
Stretch activates 1a afferent sensory nerves in the muscle spindle :
Increases the number of action potentials in 1a afferents projecting through the dorsal horn into the spinal cord.
causing rapid contraction of the AGONIST muscle.
Because muscles use agonists and antagonists to move joints - another spindle branch also connects (indirectly) with the ANTAGONIST muscles.
So, as the AGONIST muscle contracts, ANTAGONIST muscle relaxes (enabling agonist joint movement)
how do spindle sensory afferents divide?
Spindle sensory afferents divide to make 3 branches/ connections (two in spinal cord, one with the brain)
what type of reflex is the stretch reflex?
MONOSYNAPTIC REFLEX – one synapse, no interneurones involved
Direct contact to α-motoneurones in the stretched muscle : causing rapid contraction of the AGONIST muscle.
what also makes up the spindle afferetn connection?
inhibitory interneurons
what do inhibitory interneurons allow?
↓ activation of α-motoneurones to the antagonist muscle - which then relaxes
this is called RECIPROCAL INHIBITION.
where does the spindle afferent branch travel to tell the brain about muscle legnth?
Spindle afferent branch also travels up the dorsal columns to thalamus and somatosensory cortex
where does the stretch reflex also exist?
exists in arms as well as legs
Muscle stretch - stimulates muscle spindles
Causes reflex muscle contraction – muscle shortens to previous length
why is inverse stretch reflex also known as the clasp knife reflex?
It is like an automatic knife action (clasp knife, USA)
Greatly increasing tension in tendon leads to a collapse of resistance
So, like a spring-loaded knife opening up, joint extends
- Protective mechanism to prevent muscle damage
describe the mechanism of the golgi tendon organ reflex?
Weightlifting - agonist quadriceps contracted - pulls hard on the tendon - GTO afferents (blue) ↑ firing - synapse with inhibitory interneurone - reduces motoneurone firing (red) – agonist muscle inhibited, so relaxes rapidly
GTO afferents (blue) – also synapse with excitatory interneurone - ↑ activation of antagonist motor neurone (purple axons) – antagonist contracts rapidly
Avoids muscle contracting so hard tendon is ripped from the bone
Reflex is polysynaptic and protective
describe the mechanism of the flexor/withdrawal reflex?
what is the circuitry if the flexor reflex?
if you only withdrew your limb from a tack and nothing else, you fall over (NOT protective!!).
to prevent this, contralateral limb extends via:-
CROSSED EXTENSOR REFLEX (contralateral extension to prevent falling over).
3) excitatory interneurones cross the spinal cord, excite contralateral extensors
4) Other interneurones cross the spinal cord, synapse with inhibitory neurones and inhibit contralateral flexors
Helps maintain upright posture by extending the limb opposite the flexed one - to bear the shift in body weight.
5) Sensory information ascends to the brain in the contralateral spinothalamic tract.
Basic circuitry is similar to that used for walking
Hardwired at spinal cord level.
Spinal cord circuitry for
Flexor & Crossed Extensor reflex
holding an ever increasing load?
holding excessive load?
how can you override the stretch reflex?
how can you override the withdrawal reflex?
