Functional hierarchy of the motor system and motor control Flashcards
What does it mean that the motor system is organized as a hierarchy?
Voluntary brain control of muscles is via alpha motorneurones in the spinal cord- they are the “final common path”
But
Reflex control of muscles is autonomous and hard-wired into motorneurone circuit at each segmental spinal level
But
Brainstem nuclei control spinal reflexes and integrate them into higher-order reflexes that control posture and balance
- For example, descending inputs from vestibulospinal and reticulospinal tracts regulate trunk and limb muscle reflexes
But
Brainstem nuclei themselves receive control inputs about voluntary movements from higher centers- from the cerebral cortex (motor, premotor and supplementary motor cortex), the basal ganglia, and the cerebellum
What controls movement?
Four system control movement
1 Descending control pathways
2 Basal ganglia
3 Cerebellum
4 Local spinal cord/ brain stem circuits
Describe the motor system
Explain the spacial map of the bodies musculature
Muscles also map spatially where?
Higher brain centres are
Functionally interdependent and control different aspects of voluntary movements
The spinal cord recieves
Descending input via the brainstem and direct cortical input via the Corticospinal (Pyramidal) tract
Sensory input is crucial and enters at all levels
1 at the spinal cord in the form of proprioceptors, touch, pain etc
2 at the brainstem the vestibular system informs about balance
3 at cortical level, we make movements in response to visual, olfactory, auditory, emotional, intellectual cues
Damage to sensory inputs (at spinal level)
Results in paralysis as if the motorneurones themselves had been damaged
Tell the tale of Ian Waterman
Viral infection caused loss of proprioception therefore unable to move
Autoimmune attack on sensory neurones in the dorsal root ganglia (DRG) causing loss of large myelinated fibres and sensory neuropathy - very rare. DRGs don’t regenerate
So he lost the sens of his body in space and his motor control and was effectively paralysed
Taught himself to mentally visualise the movements he needed to make and the sequence of muscle contractions needed for them
So he used visual system instead of his proprioceptive system to provide feedback on where the parts of his body were
Spinal cord reflexes
Spinal reflexes are simple building blocks for movement
The simplest reflex is the stretch reflex
EG patellar tendon or knee-jerk reflex
Uses information from muscle spindles which monitor muscle length
Describe the reflex arc
Explain how the stretch reflex work
- Force transmitted to the muscle fibres- they are more elastic than tendons and so are more able to stretch
- Stretch activates 1a afferent sensory nerves in the muscle spindle:
- → this increases the number of action potentials in 1a afferents projecting through the dorsal horn into the spinal cord
Spindle sensory afferents divide to make…
3 types of connections
2 spinal cord one brain
- Monosynaptic reflex
- Reciprocal inhibition
- Thalamus and somatosensory cortex
Explain Monosynaptic reflex
many directly contact a-motoneurones in the stretched muscle : causing rapid contraction of the AGONIST muscle.
this is a MONOSYNAPTIC REFLEX – one synapse, no interneurones involved – it is the only such connection known.
What is reciporal inhibition and why is it nesseccary?
Because muscles use agonists and antagonists to move joints - sensory fibres from the stretched spindle also connect indirectly with the antagonist muscles.
So, when the AGONIST muscle contracts,
the ANTAGONIST muscle relaxes (stretches)
This happens because spindle afferents connect with and activate inhibitory interneurones which ¯ activation of a-motoneurones to the antagonist muscle - which then relaxes
this process called RECIPROCAL INHIBITION.
3rd Spindle firing pathway
Spindle afferent firing also travels up the dorsal columns to thalamus and somatosensory cortex to tell the brain about length of muscles
Describe the pathways in a diagram for motor control
Describe the pathways in a diagram for motor control
Stretch reflex in arms
Inverse stretch reflex
Inverse stretch reflex
Why the clasp-knife reflex?
Golgi tendon organ refkex
Flexor/Withdrawal reflex with crossed extension
- Flexor/withdrawal reflexes use information from pain receptors (nocireceptors) in skin, muscles and joints
- They are polysynaptic and protective
- they withdraw part of the body away from the painful stimulus and in towards the body - so flex the affected part
Flexor/Withdrawal reflex with crossed extension
Increased action potentials in nociceptors nerves cause 5 things
- Increased activity in the flexor muscles of the affected part via a number of excitatory interneurones
- At the same time, via a number of excitatory and inhibitory interneurons, the antagonistic extensors are inhibited - Flexor reflex causes ipsalateral flexion in response to pain
- Excitatory interneurones cross the the spinal cord and excite the contralateral extensors
- Other interneurones cross the spinal cord, synapse with inhibitory neurones and they inhibit the contralateral flexors - this helps to maintain an upright posture by extending the limb opposite the flexed one - to bear the shift in body weight
- Sensory information ascends to the brain in the contralateral spinothalamic tract - This basic circuity is similar to that used for walking and is hardwired at spinal cord level
Circuitry of flexor reflex
Spinal cord circuitry for flexor and crossed extensor reflex
Reflexes are “hardwired” building blocks integrated into the
Voluntary control of movement
Can the stretch reflex be overridden
Can the withdrawal reflex be overidden?
