Functional Hierarchy of the Motor System Flashcards
What is direct control of muscles via?
a motoneurones in the spinal cord
Where do reflexes occur autonomously?
Reflexes occur autonomously at each segmental spinal levels
What do brainstem nuclei control?
Brainstem nuclei control spinal reflexes and integrate them into higher order reflexes controlling posture and balance
Where do the trunk and limb muscles receive descending input from?
Vetsibulospinal and reticulospinal tracts
What are brainstem nuclei controlled by and receive input from?
Brainstem nuclei are controlled by and receive input instructing voluntary movements from higher brain centres : including Cerebral cortex (Motor cortex and premotor and supplementary motor areas), basal ganglia and cerebellum
What 4 systems control movement?
- Descending control pathways
- Basal ganglia
- Cerebellum
- Local spinal cord and brain stem circuits
Why do LMN lesions cause flaccid paralysis and muscle atrophy?
Lower Motor Neurones at spinal levels directly innervate muscles to initiate reflex and voluntary movements
What do UMN in the brainstem or cortex synapse with?
Synapse with lower circuit neurones to regulate a motorneurone activity
What do UMN in the cortex control?
Complex spatiotemporal skilled movement
What do UMN lesion cause?
Spasticity, some paralysis, may be transuent
What do lesions in many posture regulating pathways cause?
Spastic paralysis
What do corticospinal lesions cause?
Weakness (paresis), rather than paralysis
How is the spatial map of the body musculature in the spinal cord arranged?
- Mediolaterally (applies to arms and legs)
- Proximal shoulder muscle are mapped to medial motorneurones
- Finger muscles are mapped to much more lateral motorneurones
What axis of the spinal cord are muscles also mapped spatially along?
Long axis
What are higher brain centres (cortex, basal ganglia, cerebellum) functionally?
Interdependent and control different aspects of voluntary movements.
What does spinal cord receive descending input from?
Via neurons in the brainstem
What does spinal cord receive direct input via?
Cortiocospinal/pyramidal tract
What sensory input enters at the level of the spinal cord?
- Proprioceptors
- Touch
- Pain
What sensory input enters at the level of the brainstem?
Vestibular system informs about balance
What sensory input enters at the level of the cortex?
We make movements in response to visual, olfactory, auditory, emotional, intellectual cues
What does damage to the sensory inputs at the spinal level result in?
Paralysis as if the motoneurones themselves had been damaged.
What can attack (viral/autoimmune etc.) on the DRG result in?
Loss of large myelinated fibres and sensory neuronopathy
What is the simplest segmental reflex?
Stretch reflex
Where is the stretch reflex found?
In every muscle
Give examples of the stretch reflex?
Patellar tendon/knee jerk reflex
Briefly describe the stretch reflex.
- Afferent impulses from stretch receptor to spinal cord
- Efferent impulses to alpha motor neurones cause contraction of the stretched muscle that resists/reverses the stretch
- Efferent impulses to antagonist muscle are dampened (reciprocal inhibition)
What is the crucial cord segment for testing the biceps jerk?
C6
What is the crucial cord segment for testing the triceps jerk?
C7
What is the crucial cord segment for testing the patellar tendon reflex?
L4
What is the crucial cord segment for testing the Achilles tendon reflex?
S1
What is the significance of testing reflexes?
- Helps to detect level of spinal cord damage. Can evoke reflexes above but not below level of damage.
- Impaired reflexes may indicate areas of nerve damage - in diabetes, patients may lose their ankle jerk reflexes.
- Patients must be distracted to avoid voluntary influencing of responses.
What does the flexor(withdrawal) reflex use information from?
Pain receptors (nociceptors) in skin, muscles and joints
Why is the withdrawal response referred to as the flexor response?
They withdraw part of the body away from the painful stimulus and in towards the body - so flex the affected part.
In the withdrawal reflex what does increased sensory APs from pain receptor cause?
- 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 interneurones, the antagonistic extensors are inhibited
What is the flexor reflex?
Ipsilateral flexion in response to pain
What fibres triggering pain enter the spinal cord in the flexor withdrawal reflex?
Small diameter A delta nociceptive
Why are interneurones in several spinal segments activated in the flexor withdrawal reflex?
The nociceptive fibres branch a lot
What do the interneurones activate in the flexor withdrawal reflex?
a motorneurones controlling all the flexor muscles of the affected limb
What occurs alongside the flexor withdrawal reflex as a protective mechanism?
Contralateral limb extends
What happens in order for contralateral limb extension to take place in the flexor reflex?
- Several excitatory interneurones which cross the spinal cord and excite the contralateral extensors
- At the same time, via several interneurones, there is inhibition of the contralateral flexors
- This helps to maintain an upright posture by extending the limb to bear the body weight
- Sensory information ascend to the brain in the contralateral spinothalamic tract
Why is the flexor-crossed extensor reflex is far slower than the stretch reflex?
- One leg flexes to withdraw from the painful stimulus while the other leg extends to maintain balance because of shifted weight
- There are several interneurons in the pathway each with a small synaptic delay
- Nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly
Give an example of when a reflex can be overridden?
- When holding a load, the load stretches the muscle and its spindles, causing reflex contraction to restore arm position
- With an excessive load- the Golgi tendon organ reflex is activated
- However, if the load was a child you would override this
What are the basic steps in the Golgi tendon organ reflex?
- Neuron from Golgi tendon organ fires
- Motor neuron is inhibited
- Muscle relaxes
- Load is relaxed
How many synapses does one a motornerone receive?
> 10,000 synapses
Where are many synapses a motorneurones receive from?
Many are from descending cortical excitatory and inhibitory inputs, with continual integration of EPSPs and IPSPs
How can reflexes be overridden?
Descending voluntary excitation of motoneurones can override the inhibition from the GTOs and maintain contraction.
Why are subjects distracted when testing reflexes?
- Strong descending inhibition hyperpolarizes α-motoneurones and the stretch reflex can not be evoked.
- So when testing reflexes, subjects are distracted to prevent voluntary effects.
- So the absence of reflex responses in an uncooperative patient may not be evidence of peripheral nerve damage.
What does activity of y-motorneurones depend on?
Depend entirely on descending pathways
What does high y-motorneurone activation of muscle spindles cause?
Muscles to become extremely resistant to stretch and they becomes spastic
What do many neurological disorders associated with altered y-motorneurone discharge to?
Spindles
What is the stretch reflex important in assessing?
Important in assessing integrity of whole spinal cord circuit:
- Afferent nerves
- Balance of synaptic inputs to the motorneurones
- Motorneurones
- Neuro-muscular junction and muscles
When can reflexes help localise a problem?
Reflexes evoked above but NOT below
How does input differ in the stretch and withdrawal reflexes?
- In the stretch reflex, muscle spindle input is highly localised and affects only motoneurones of one or two spinal segments
- In the withdrawal reflex, pain fibre input is diffuse and spreads through several spinal segments.
Why is the response larger with a more powerful pain stimulus?
The greater the spinal spread
What is facilitation?
A mechanism which increases the effects of sensory inputs
Where does facilitation occur?
- Occurs between similar inputs e.g. pain fibre inputs
- And between diverse inputs eg. if burnt hand on a hotplate and biceps stretch reflex is evoked, it would be greatly exaggerated
How do pain fibres input facilitate the action of the muscle spindles?
By maintaining the a-motorneurones in a more depolarised state
What does stroking the lateral aspect of the sole usually cause?
Plantar flexion, downward curling of the big toe
When will the plantar flexor reflex occur?
This is a balance between flexor and extensor reflexes and only works if the corticospinal tract is intact and functioning normally
What is Babinksi sign?
Extension (not flexion), toes fanning up and out due to damage or disruption to CS tract
Who will show Babinski sign?
- Damage to the CS tract
- Children <1 year
- Post ictal
Where is the importance of higher centres in maintaining normal reflexes seen?
Spinal transection
What happens in spinal transection?
Immediate sensory and autonomic effects
- Loss of sensation
- Loss of bowel, bladder and sexual regulation
What happens in spinal shock?
In spinal shock, there is loss of supraspinal excitation and reflexes are not evoked for 2-6 weeks - then gradual return of reflexes
Clonus
Stretch causes oscillatory muscle contraction/relaxation
How exaggerated can reflexes in spinal shock become?
- Often reflexes are exaggerated - light touch evokes powerful withdrawal reflex of the whole limb
- Extensor reflexes can be so exaggerated that body weight can be supported briefly
