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Neuroscience > Lecture 8: Reflexes > Flashcards

Lecture 8: Reflexes Flashcards

1
Q

what is the main role of upper motor neurons

A

Integrates excitatory and inhibitory signals from the cortex and translates them into a signal to modulate LMN activity

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2
Q

what is the main role of lower motor neurons

A

Receives the signal from UMN and transmits it to effector muscle to perform a movement

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3
Q

what do muscle spindles do

A

Detects muscle length and velocity

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4
Q

what are the 2 types of sensory axons in muscle spindles

A

group 1a axons

group 2 axons

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5
Q

what are group 1a axons

A

Changes in length and velocity

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6
Q

what are group 2 axons

A

measures length

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7
Q

when is there a decrease in muscle spindle activity?

A

when there’s stimulation of the alpha motor neurone only, not actively contracting the muscle voluntarily

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8
Q

do gamma motor neurons directly cause muscle contraction

A

no

they Innervate muscle spindles and dictate their sensitivity

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9
Q

do alpha motor neurons or gamma motor neurons have a lower threshold for activation

A

gamma motor neurons

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10
Q

how can gamma motor neurons be modulated

A

ascending pathways

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11
Q

what is muscle spindle alpha-gamma co-activation

A

during active muscle contraction, gamma motor neurons cause intrafusal fibres to contract at the same time as extrafusal fibres

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12
Q

why is Alpha-Gamma Co-activation important

A

Maintains sensitivity of the muscle spindles during different muscle lengths

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13
Q

does Alpha-Gamma Co-activation occur during reflexes

A

no.

alpha-gamma co-activation only occurs in movements initiated by the CNS

only alpha motor neurons are activated during reflex

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14
Q

what are Golgi tendon organs

A

Group 1b afferent

Designed to measure muscle tension/force

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15
Q

does muscle stretch affect afferent activity in Golgi tendon organs

A

no

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16
Q

does active muscle contraction affect afferent activity in Golgi tendon organs

A

yes activity increases

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17
Q

what is the simplest level of movement

A

reflexes

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18
Q

what is the stretch reflex

A

homonymous - same muscle that is stretched is excited and contracts

Spindle sensitivity modulated by UMNs

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19
Q

what is the purpose of the stretch reflex

A

Maintain muscle length

compensate for limb
during movement

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20
Q

what is reciprocal inhibition of the stretch reflex

A

Automatic inhibition of the antagonist alpha motor neuron (biceps) caused by contraction of the agonist muscle (triceps)

  • Occurs in conjunction with stretch reflex
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21
Q

what is the purpose of reciprocal inhibition

A

to allow agonist to contract

22
Q

what is the Golgi Tendon Organ (GTO) Reflex

A

Excitation of the inhibitory interneuron
* Inhibit “agonist” homonymous muscle LMN pool (biceps)

  • Excitation of the excitatory interneuron of antagonist
23
Q

what is the function of the Golgi Tendon Organ (GTO) Reflex

A

Release load to prevent muscle tearing and
maintains muscle tone during muscle fatigue

24
Q

what is the flexor withdrawal reflex?

A

Ipsilateral limb (painful stimulus)
* Interneuron to excite flexors
* Inhibitory interneuron to
inhibit extensors

25
Q

what is the function of the flexor withdrawal reflex

A

Remove limb from noxious stimulus

26
Q

what is the crossed extensor reflex

A

Occurs with flexor withdrawal reflex

Input: Noxious stimulus detected
* Synapse on long distance interneuron networks
* Contralateral limb (cross-extensor)
* Excite Extensors (postural)
* Inhibit Flexors

27
Q

what is the purpose of the crossed extensor reflex?

A

Ground stance limb to allow ipsilateral limb to withdraw from pain

28
Q

which is the only reflex that crosses spinal cord

A

crossed extensor reflex

29
Q

what is the 5 point scale of reflexes

A

0 - No response

1+ - Present but depressed (hyporeflexic; may or may not be normal)

2+ - Normal (average)

3+ - Brisk (increased; possibly abnormal)

4+ - Very brisk, hyperactive, with clonus (abnormal)

5+ - Sustained clonus

30
Q

what do we usually see in terms of the reflex grading scale with upper motor neuron lesions

A

3+ - Brisk (increased; possibly abnormal)

4+ - Very brisk, hyperactive, with clonus (abnormal)

5+ - Sustained clonus

31
Q

where does the Reticulospinal tract originate and terminate

A

originates in reticular formation and terminates bilaterally in spinal cord

32
Q

what is the Dorsal/lateral reticulospinal tract responsible for

A

descending inhibition (hitting the breaks)

33
Q

what is the Medial reticulospinal tract responsible for

A

descending excitation (hitting the gas)

34
Q

what happens with both reticulospinal tracts we have a complete spinal cord injury

A

both are damaged

  • Excessive firing of LMNs BUT
    Inhibitory interneurons unable to inhibit LMNs (driving down a hill with no breaks)
  • Hypertonia is due to a loss of descending control
35
Q

What happens when we have a stroke on the right side of our brain

A

Right lateral corticospinal tract damaged

Reduced cortical drive to dorsal/lateral reticulospinal tract

Medial reticulospinal tract increases descending drive from the contralesional side

  • Results in excessive (medial) reticulospinal drive (all gas no breaks)
36
Q

In UMN lesions, there is excessive neural signaling due to a loss of descending control, which contributes to

A

Hypersensitivity of the muscle spindle

Excitable  motor neuron

37
Q

what happens when we do a tendon tap of the patella

A

muscle stretches causing increase activation of 1a afferent

shortly after, alpha motor neurons are recruited, causing shortening of muscles due to stretch reflex

38
Q

what happens in clonus

A

muscle stretches causing increase activation of 1a afferent

shortly after, alpha motor neurons are recruited, causing shortening of muscles due to stretch reflex

there is repeated activity of 1 a afferents, and the cycle repeats

39
Q

what is the babinski test

A

stroke the lateral side of foot across base of toes, extension of big toes, other toes fan out

normal in infants and not in adults

40
Q

what is Hoffman’s Reflex

A

flick middle finger tip down

Positive if tip flicks back thumb flexes and adducts
More likely to have a false positive than Babinski

41
Q

what do we usually see in terms of the reflex grading scale with lower motor neuron lesions

A

0 - No response (areflexia)

1+ - Present but depressed (hyporeflexic; may or may not be normal)

42
Q

when would we see hyporeflexia

A

Lower motor neuron lesions

43
Q

What happens when a muscle is stretched?

There is a decrease in firing of the Golgi Tendon Organ afferent

The afferent activity in the Golgi Tendon Organ changes very little

There is an increase in firing of the Golgi Tendon Organ afferent

There is a decrease in firing of the muscle spindle afferent

A

The afferent activity in the Golgi Tendon Organ changes very little

44
Q

If there is an increase in cortical drive to the dorsal/lateral reticulospinal tract, what would you expect to observe?

A

A reduction in muscle tone

45
Q

If there were an injury to the anterior horn of the spinal cord only, what type of presentation would you expect to observe?

Normal reflexes

Clonus

Hyporeflexia

Hyperreflexia

A

Hyporeflexia

46
Q

During active muscle contraction:

None of the response options are correct

Only the intrafusal fibres contract

The intrafusal fibres contract at the same time as extrafusal fibres

Only the extrafusal fibres contract

A

The intrafusal fibres contract at the same time as extrafusal fibres

47
Q

What is the most likely contributor to hypertonicity in stroke?

A

Increased medial reticulospinal tract drive

48
Q

Which of the following responses is true regarding the gamma motor neuron?

It modulates the sensitivity of the muscle spindles

It innervates the muscle spindles

All of the response options are correct

It can be modulated by descending inputs

A

All of the response options are correct

49
Q

What is the most likely contributor to hypertonicity following spinal cord injury?

Increased medial reticulospinal tract drive

Decreased lateral reticulospinal tract drive

Insufficient firing of alpha motor neurons

A lack of inhibition from interneurons to lower motor neurons

A

A lack of inhibition from interneurons to lower motor neurons

50
Q

The muscle spindle is responsible for detecting changes in:

A

Muscle length and velocity

Neuroscience (20 decks)

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