Functional Hierarchy of the motor System Flashcards

1
Q

What type of neuron supplys skeletal muscles?

A

Alpha-motor neurons (A type of LMN)

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2
Q

What structures are involved in controlling movement?

A
  • Descending control pathways from the cerebral cortex (e.g. corticospinal tract) (planning movements)
  • Basal Ganglia (Initiation/termination)
  • Cerebellum
  • Local Spinal Cord & brain stem circuits
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3
Q

Where do UMNs sit and what would be manifestions of a lesion?

A

They start in the primary motor cortex or brainstem.

Spasticity (Increased Tone)
Weakness
Wasting in chronic lesions due to lack of use
Hyperreflexia

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4
Q

Where do LMNs sit and what would be the manifestations of a lesion?

A

In the cranial nerve nuclei or grey matter of the spinal cord

  • Flaccid Paralysis
  • Hyporeflexia
  • Fasciculation
  • Muscle Atrophy
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5
Q

There is a spatial map of the bodies muscles in the spinal cord how is it arranged?

A

Medial -> Lateral

So the more medial the grey matter the more medial muscles it supplies

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6
Q

How does the stretch reflex work?

A
  • Stretch the tendon
  • Pulls on the Muscle Spindle Fibres
  • Trigger 1a afferent fibres which enter spinal cord through the Dorsal Root Ganglion

The 1a afferent fibres make two connections:

  • One is a monosynaptic link to an alpha-motor neuron which returns through the ventral root to the agonist triggering contraction
  • Second is a polysynaptic link through an inhibitory interneuron to an alpha-motor neuron and to the antagonistic muscle causing relaxation
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7
Q

What stretch reflexes can we test and what vertebral level do they originate?

A

Biceps Jerk - C6
Triceps Jerk - C7
Patellar Tendon - L4
Achilles Tendon - S1

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8
Q

Which is faster the flexor or stretch reflex?

A

The stretch reflex because the nociceptive sensory fibres involved in the flexor reflex have a smaller diameter and therefore a slower conduction than 1a afferent fibres

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9
Q

How does the Flexor (withdrawal) reflex occur?

A
  • Nociceptor triggered
  • Increased APs in cutaneous afferent fibres
  • Afferent fibres pass up and down the spinal cord to reach all the flexors of the effected limb
  • Through a polysynaptic excitatory link they trigger contraction in all the limb’s flexors
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10
Q

How do we stop ourselves from falling over when our lower limb uses the flexor reflex?

A

Contralateral limb extension

Some interneurons cross the spinal cord to excite the contralateral extensors and inhibit contralateral flexors

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11
Q

How does pain facilitation work?

A

When multiple impulses come along one after another they increase in strength.

This occurs between the same inputs e.g. nociceptors
And between different inputs e.g. pain coupled with a stretch reflex can cause it to be very exaggerated

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12
Q

What is the Golgi tendon organ reflex?

A

GTO reflex

  • When there’s an excessive load on a muscle
  • GTO is stretched (indicating risk of damaging tendon/muscle)
  • Afferent fibres –> Inhibitory motorneuron –> Alpha-motor neuron –> Muscle relaxes
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13
Q

How is it possible to override a reflex?

A
  • Every alpha-motorneuron receives >10000 synapses
  • Some of these come from descending cortical inputs
  • Descending impulses acting on the alpha-motorneurons will outdo those of the reflex
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14
Q

What do gamma-motorneurons do?

A

They come on descending pathways from the brain and innervate muscle spindle fibres

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15
Q

How do gamma-motorneurons affect muscle tone?

A

Excess stimulation of muscle spindles

  • -> Excess stimulation of 1a afferent fibres
  • -> Excess muscle activity
  • -> Spasticity

And vice versa

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16
Q

Define Spinal Shock?

A

A result of spinal injury:

  • Hyporeflexia
  • Clonus
  • Autonomic Dysfunction

The reflexes gradually return over a few weeks and autonomic dysfunction takes longer to resolve, possibly it never properly does