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Systems: Neurology AB > Functional Hierarchy of the Motor System > Flashcards

Functional Hierarchy of the Motor System Flashcards

1
Q

What is direct control of muscles via?

A

a motoneurones in the spinal cord

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2
Q

Where do reflexes occur autonomously?

A

Reflexes occur autonomously at each segmental spinal levels

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3
Q

What do brainstem nuclei control?

A

Brainstem nuclei control spinal reflexes and integrate them into higher order reflexes controlling posture and balance

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4
Q

Where do the trunk and limb muscles receive descending input from?

A

Vetsibulospinal and reticulospinal tracts

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5
Q

What are brainstem nuclei controlled by and receive input from?

A

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

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6
Q

What 4 systems control movement?

A
  • Descending control pathways
  • Basal ganglia
  • Cerebellum
  • Local spinal cord and brain stem circuits
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7
Q

Why do LMN lesions cause flaccid paralysis and muscle atrophy?

A

Lower Motor Neurones at spinal levels directly innervate muscles to initiate reflex and voluntary movements

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8
Q

What do UMN in the brainstem or cortex synapse with?

A

Synapse with lower circuit neurones to regulate a motorneurone activity

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9
Q

What do UMN in the cortex control?

A

Complex spatiotemporal skilled movement

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10
Q

What do UMN lesion cause?

A

Spasticity, some paralysis, may be transuent

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11
Q

What do lesions in many posture regulating pathways cause?

A

Spastic paralysis

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12
Q

What do corticospinal lesions cause?

A

Weakness (paresis), rather than paralysis

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13
Q

How is the spatial map of the body musculature in the spinal cord arranged?

A
  • Mediolaterally (applies to arms and legs)
  • Proximal shoulder muscle are mapped to medial motorneurones
  • Finger muscles are mapped to much more lateral motorneurones
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14
Q

What axis of the spinal cord are muscles also mapped spatially along?

A

Long axis

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15
Q

What are higher brain centres (cortex, basal ganglia, cerebellum) functionally?

A

Interdependent and control different aspects of voluntary movements.

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16
Q

What does spinal cord receive descending input from?

A

Via neurons in the brainstem

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17
Q

What does spinal cord receive direct input via?

A

Cortiocospinal/pyramidal tract

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18
Q

What sensory input enters at the level of the spinal cord?

A
  • Proprioceptors
  • Touch
  • Pain
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19
Q

What sensory input enters at the level of the brainstem?

A

Vestibular system informs about balance

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20
Q

What sensory input enters at the level of the cortex?

A

We make movements in response to visual, olfactory, auditory, emotional, intellectual cues

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21
Q

What does damage to the sensory inputs at the spinal level result in?

A

Paralysis as if the motoneurones themselves had been damaged.

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22
Q

What can attack (viral/autoimmune etc.) on the DRG result in?

A

Loss of large myelinated fibres and sensory neuronopathy

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23
Q

What is the simplest segmental reflex?

A

Stretch reflex

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24
Q

Where is the stretch reflex found?

A

In every muscle

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25
Q

Give examples of the stretch reflex?

A

Patellar tendon/knee jerk reflex

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26
Q

Briefly describe the stretch reflex.

A
  • 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)
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27
Q

What is the crucial cord segment for testing the biceps jerk?

A

C6

28
Q

What is the crucial cord segment for testing the triceps jerk?

A

C7

29
Q

What is the crucial cord segment for testing the patellar tendon reflex?

A

L4

30
Q

What is the crucial cord segment for testing the Achilles tendon reflex?

A

S1

31
Q

What is the significance of testing reflexes?

A
  • 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.
32
Q

What does the flexor(withdrawal) reflex use information from?

A

Pain receptors (nociceptors) in skin, muscles and joints

33
Q

Why is the withdrawal response referred to as the flexor response?

A

They withdraw part of the body away from the painful stimulus and in towards the body - so flex the affected part.

34
Q

In the withdrawal reflex what does increased sensory APs from pain receptor cause?

A
  • 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
35
Q

What is the flexor reflex?

A

Ipsilateral flexion in response to pain

36
Q

What fibres triggering pain enter the spinal cord in the flexor withdrawal reflex?

A

Small diameter A delta nociceptive

37
Q

Why are interneurones in several spinal segments activated in the flexor withdrawal reflex?

A

The nociceptive fibres branch a lot

38
Q

What do the interneurones activate in the flexor withdrawal reflex?

A

a motorneurones controlling all the flexor muscles of the affected limb

39
Q

What occurs alongside the flexor withdrawal reflex as a protective mechanism?

A

Contralateral limb extends

40
Q

What happens in order for contralateral limb extension to take place in the flexor reflex?

A
  • 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
41
Q

Why is the flexor-crossed extensor reflex is far slower than the stretch reflex?

A
  • 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
42
Q

Give an example of when a reflex can be overridden?

A
  • 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
43
Q

What are the basic steps in the Golgi tendon organ reflex?

A
  • Neuron from Golgi tendon organ fires
  • Motor neuron is inhibited
  • Muscle relaxes
  • Load is relaxed
44
Q

How many synapses does one a motornerone receive?

A

> 10,000 synapses

45
Q

Where are many synapses a motorneurones receive from?

A

Many are from descending cortical excitatory and inhibitory inputs, with continual integration of EPSPs and IPSPs

46
Q

How can reflexes be overridden?

A

Descending voluntary excitation of  motoneurones can override the inhibition from the GTOs and maintain contraction.

47
Q

Why are subjects distracted when testing reflexes?

A
  • 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.
48
Q

What does activity of y-motorneurones depend on?

A

Depend entirely on descending pathways

49
Q

What does high y-motorneurone activation of muscle spindles cause?

A

Muscles to become extremely resistant to stretch and they becomes spastic

50
Q

What do many neurological disorders associated with altered y-motorneurone discharge to?

A

Spindles

51
Q

What is the stretch reflex important in assessing?

A

Important in assessing integrity of whole spinal cord circuit:

  • Afferent nerves
  • Balance of synaptic inputs to the motorneurones
  • Motorneurones
  • Neuro-muscular junction and muscles
52
Q

When can reflexes help localise a problem?

A

Reflexes evoked above but NOT below

53
Q

How does input differ in the stretch and withdrawal reflexes?

A
  • 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.
54
Q

Why is the response larger with a more powerful pain stimulus?

A

The greater the spinal spread

55
Q

What is facilitation?

A

A mechanism which increases the effects of sensory inputs

56
Q

Where does facilitation occur?

A
  • 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
57
Q

How do pain fibres input facilitate the action of the muscle spindles?

A

By maintaining the a-motorneurones in a more depolarised state

58
Q

What does stroking the lateral aspect of the sole usually cause?

A

Plantar flexion, downward curling of the big toe

59
Q

When will the plantar flexor reflex occur?

A

This is a balance between flexor and extensor reflexes and only works if the corticospinal tract is intact and functioning normally

60
Q

What is Babinksi sign?

A

Extension (not flexion), toes fanning up and out due to damage or disruption to CS tract

61
Q

Who will show Babinski sign?

A
  • Damage to the CS tract
  • Children <1 year
  • Post ictal
62
Q

Where is the importance of higher centres in maintaining normal reflexes seen?

A

Spinal transection

63
Q

What happens in spinal transection?

A

Immediate sensory and autonomic effects

  • Loss of sensation
  • Loss of bowel, bladder and sexual regulation
64
Q

What happens in spinal shock?

A

In spinal shock, there is loss of supraspinal excitation and reflexes are not evoked for 2-6 weeks - then gradual return of reflexes

65
Q

Clonus

A

Stretch causes oscillatory muscle contraction/relaxation

66
Q

How exaggerated can reflexes in spinal shock become?

A
  • 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

Systems: Neurology AB (51 decks)

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