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Neuro B4 - WLB > Posture & Muscle Tone - Ruff > Flashcards

Posture & Muscle Tone - Ruff Flashcards

1
Q

What is the function of postural tone?

A

Keep the center of mass (or center of force) above the support surface.

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2
Q

How is postural tone acheived in general?

A
  • Tonic activity in muscles
    • primarily physiological extensors
  • Mechanism: tonic activity of alpha motor neurons
    • To increase tone –> increase alpha motorneuron activity
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3
Q

How is postural tone increased directly?

A
  • Directly:
    • Increased input from descending tracts
    • Corticospinal, Vestibulospinal, Corticoreticular, Reticulospinal
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4
Q

How is postural tone increased indirectly?

A

Increased gamma motorneuron activity

  • descending projections
  • reflex feedback
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5
Q

How is postural tone acheived through the feedforward system?

A

anticipatory adjustments

(includes corticoreticular & reticulospinal tracts)

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6
Q

How is postural tone acheived through the feedback system?

A

reactive adjustments on the fly

(include vestibulospinal tracts)

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7
Q

How do vestibular reflexes contribute to maintenance of posture?

A
  • Input from the vestibular end organs (SC/otoliths) & the cerebellum
  • Medial Vestibulospinal tract
    • head and neck muscles
  • Lateral Vestibulospinal tract
    • axial & proximal limb muscles
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8
Q

What specific neurons do vestibulospinal tracts contact?

A

Alpha & Gamma Motorneurons

(see when you are off-kilter and adjust muscle tone appropriately)

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9
Q

What other descending systems besides the vestibulospinal system provide central control of posture and adjust reflex sensitivity?

A
  • Corticospinal
  • Rubrospinal
  • Reticulospinal
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10
Q

How does the Corticoreticular & Reticulospinal Tracts affect muscle tone?

A

Coordinate movements of the trunk and proximal limbs.

(feedforward - anticipatory adjustments)

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11
Q

What effect does the Pontine reticular formation have in the Corticoreticular/Reticulospinal Tracts?

A

Excites gamma motorneurons

(is inhibited by the cortex)

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12
Q

What effect does the Medullary reticular formation have on the Corticoreticular/Reticulospinal Tracts?

A

Inhibits gamma motorneurons

(excited by the cortex)

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13
Q

What is the net effect of the Pontine & Medullary reticular formations in the Corticoreticular/Reticulospinal Tracts?

A

Cortex damps down gamma motorneuron activity

(results in less tension)

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14
Q

What does the Rubrospinal tract do?

A
  • Projects from the Red Nucleus to the cervical spinal cord
  • Contributes to flexion of upper limbs
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15
Q

What is the result of a lesion in the pyramidal tract?

A

Loss of fine voluntary movements of the hands

(Corticospinal tract deficits, but most compensated by other descending tracts so this is only problem)

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16
Q

What is the result of a spinal cord transection?

A
  • Lose ALL descending systems (lose all excitatory drive)
  • Flaccidity (no postural tone)
  • No reflexes
17
Q

What occurs about two months after a spinal cord transection? Why?

A
  • Reflexes return and become hyperactive (hyperreflexia)
  • Muscle tone returns and is higher than normal (hypertonia)
  • Why?
    • Because 1a afferents from muscle spindle (intrafusal fiber) are still intact
    • Local axons fill synaptic void (1a) –> More sensitive –> Big response to little input
18
Q

Where does the lesion occur that results in Upper motor neuron syndrome?

A

Motor cortex OR Internal Capsule

19
Q

What are the initial symptoms of an Upper motor neuron syndrome?

A

Weakness (paresis) & hypotonia due to “spinal shock”

20
Q

What are the symptoms of an Upper motor neuron syndrome several days after the lesion occurs?

A
  • Abnormal Babinski (indicative of corticospinal injury)
  • Spasticity
    • increased tone
    • hyperactive deep reflexes
    • clonus
    • clasp-knife rigidity
21
Q

What effect does an Upper motor neuron syndrome have on the Reticulospinal Tracts?

A

Overall increases gamma motor neurons.

  • excitation of gamma motor neurons by the pontine reticular formation is no longer inhibited by the cortex (increasing excitation)
  • inhibition of the gamma motor neurons by the medullary reticular formation is not longer excited by the cortex (decreasing inhibition)
  • Net effect: Increased gamma motorneuron activity –> INCREASED TONE
22
Q

What is Decorticate posture?

A
  • Upper extremities in flexion
  • Lower extremities in extension
  • Brainstem motor centers are intact but are not receiving modulatory input from cortex

(damage is still above the level of the red nucleus)

23
Q

What is Decerebrate posture?

A
  • All limbs in extension
  • Loss of red nucleus (rubrospinal tract)
  • Loss of vestibulo & reticulospinal –> FLACCIDITY
    • medical emergency because this indicates extensive brain damage
    • likely death

Neuro B4 - WLB (17 decks)

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