Functional Hierarchy of the Motor System Flashcards
What controls spinal reflexes and integrates them into higher order reflexes controlling posture and balance?
Brainstem nuclei
What are brainstem nuclei controlled by and receive input from?
Instructing voluntary movements from higher brain centres
- cerebral cortex
- basal ganglia
- cerebellum
Basal ganglia function in terms of movement
Gating proper initiation of movement
Cerebellum function in terms of movement
Sensory motor coordination of ongoing movement
What are the descending systems (Upper motor neurones)?
Motor cortex
Brainstem centers
Function of motor cortex
Planning, initiation and directing voluntary movements
Function of brainstem centres in respect to motor
Basic movements and postural control
What do motor neurone pools require?
Sensory input
What are the 4 systems that control movement?
Descending control pathways
Basal ganglia
Cerebellum
Local spinal cord and brain stem circuits
What directly innervates muscles to initiate reflex and voluntary movements?
Lower motor neurones at spinal levels
What is the final common pathway?
Motor neurone pools + lower motor neurones
How do UMN in the brainstem or cortex regulate motor neurone activity?
Synapse with multiple lower circuit neurones
UMN from the cortex control what?
Complex spatiotemporal skilled movements
Lesions in many posture regulating pathways cause what?
Spastic paralysis
Corticospinal lesions can cause what?
Weakness (paresis) rather than paralysis
Presentation of spinal shock
Immediate sensory and autonomic effects
- loss of sensation
- loss of bowel, bladder and sexual regulation
Loss of supraspinal excitation and reflexes are not evoked for 2-6 weeks - then gradual return of reflexes
Reflexes exaggerated
Extensor reflexes can also be so exaggerated that body weight can be supported briefly
Clonus
Stroking the lateral aspect of the sole causes what?
Plantar flexion - downward curling of the big toe
What has to be in tact and functioning normally for plantar flexion to occur when stroking the lateral aspect of the foot?
Corticospinal tract
What is Babinskis sign?
When strong the lateral aspect of the foot, extension of the foot and the toes fan up and out
When is babinskis sign seen in physiologically? Why?
Children < 1 y/o
Motor system not fully developed yet
Cause of babinskis sign
Damage or disruption to corticospinal tract
In the spatial map of body musculare in the spinal cord, what is the proximal shoulder muscles mapped to?
Medial motorneurones
In the spatial map of the body musculature in the spinal cord, what are the finger muscles mapped to?
Lateral motorneurones
What are the higher brain centres?
Cortex
Basal ganglia
Cerebellum
How does the spinal cord receive descending input?
Via neurones in the brainstem AND
Via direct cortical input via corticospinal/pyramidal tract
If there is damage to sensory inputs at a spinal level, what would this result in?
Paralysis as if the motorneurones themselves have been damaged
What is the simplest segmental reflex?
The stretch reflex
Where is the stretch reflex found?
In every muscle
Examples of a stretch reflex
Patellar tendon / knee jerk reflex
Steps of a stretch reflex
- Afferent impulses from stretch receptor to spinal cord
- Efferent impulses to alpha (a) motor neurones cause contraction of the stretched muscle that resists/reverses the stretch
- Efferent impulses to antagonist muscles are damped (reciprocal inhibition)
How do flexor/withdrawal reflexes work?
Use info from pain receptors (nociceptors) in the skin, muscles and joints
Features of flexor reflex
Polysynaptic
Protective - withdraw the part of the body away from the painful stimulus and in towards the body
Steps of the flexor / withdrawal reflex
- Increased sensory APs from pain receptors - AGamma nociceptor fibres
- increased activity in the flexor muscles of the affected part via a number of excitatory interneurons
- At the same time, via a number of excitatory and inhibitory interneurons, the antagonistic extensors are inhibited
In the flexor / withdrawal reflex, what happens so you do not fall over? How does this work?
Contralateral limb extends
- Several excitatory interneurons which cross the spinal cord and excite the contralateral extensors
- At the same time, via several interneurons, there is inhibition of the contralateral flexors - this helps to maintain and upright posture by extending the limb to bear the body weight
- sensory info acends to the brain in the contralateral spinothalamic tract
Nociceptive sensory fibres vs muscle spindle afferent fibres
Nociceptive sensory fibres
- smaller diameter
- conduct more slowly
Which is slower, the flexor crossed extensor reflex or the stretch reflex?
Flexor crossed extensor reflex
Can reflexes be overridden consciously?
Yes
Example of a standard stretch reflex
Holding an increasing load
The load stretches the muscle and its spindle, causing reflex contraction to restore arm position
When there is an excessive load to be held, what is activated?
The Golgi Tendon Organ (GTO)
How does the GTO work?
- Neurone from Golgi tendon organ fires
- motor neurone is inhibited
- Muscle relaxes
- Load is released
Why would you drop bricks but not drop a child?
If you are holding something heavy but important, descending voluntary excitation of a motor neurone can override the inhibition from the GTOs and maintain contraction
Can the stretch reflex be overridden?
Yes
How can the stretch reflex be overridden?
Strong descending inhibition hyperpolarises a-motor neurones and the stretch reflex cannot be evoked
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What does activity of y-motor neurones entirely depend on?
Descending pathways
What does high y- motor neurone activation of muscle spindles cause?
Muscles to become extremely resistant to stretch and they become “spastic”
What reflex is the biceps jerk?
C6
What reflex is the triceps jerk?
C7
What reflex is the patellar tendon?
L4
What reflex is the achilles tendon reflex?
S1
What is the clinical relevance of the stretch reflex?
Assessing the integrity of the whole spinal cord unit
- afferent nerves
- balance of synaptic inputs to the motorneurones, neuro-muscular junction an muscles
Also allows spinal level localisation of a problem
- reflexes evoked above but NOT below given level which may localise a problem
- stretch reflex muscle spindle is highly localised and affects only a motorneurone of one or two spinal segments
- withdrawal reflex, pain fibre input is diffuse and spreads through several spinal segments
What does a greater power of pain stimulus lead to? What is this called?
The greater the spinal spread and so the larger the response
It is called facilitation
What does facilitation do?
Increases the effects of sensory inputs
How does facilitation occur?
Between similar inputs e.g. pain fibres and between diverse inputs e.g. if burnt hand on hot plate and biceps stretch reflex is evoked, it would be greatly exaggerated
Pain fibre input facilitates the action of the muscle spindles by maintaining the a-motor neurones in a more depolarised state
When would babinskis sign be seen also? Why?
After epileptic seizures
Transient cortical function disruption
