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Neuro B4 - WLB > Motor Units & Spinal Reflexes - Fitz > Flashcards

Motor Units & Spinal Reflexes - Fitz Flashcards

1
Q

What is the final common pathway in the motor system?

A

Alpha motorneuron

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2
Q

When are the actions of spinal reflexes often revealed?

A

When there are upper motor neuron lesions.

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3
Q

High centers (including primary motor cortex) have what kind of actions on alpha motorneurons through descending pathways?

A

tonic

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4
Q

Collectively, the action of the descending control systems is usually inhibitory OR excitatory (which one)?

A

Inhibitory

(This is why an upper motor neuron lesion results in hyperactive spinal reflexes.)

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5
Q

Groups of neurons whose activation results in stereotyped movements are called what?

A

“Fixed action patterns”

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6
Q

In fixed action patterns, muscle groups can be controlled by what?

A

Central Pattern Generators

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7
Q

How are the muscle spindles arranged in relationship to the contractile extrafusal muscle fibers?

A

In parallel

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8
Q

What are the contractile ends of muscle spindles called?

A

Intrafusal Muscle Fibers

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9
Q

What innervates the contractile ends of the muscle spindles?

A

Gamma motor neurons

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10
Q

What innervates the noncontractile central region of muscle spindles?

A

Primary (Ia) fibers & Secondary (II) fibers

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11
Q

Where are the Golgie Tendon Organs (GTO) found?

A

At the junction of muscle fibers & tendons.

In series with the contractile components.

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12
Q

Physiologically, what type of receptors are muscle spindles? What do they signal?

A

MECHANORECEPTORS

-signal stretch

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13
Q

When the muscle is at rest, what kind of activity does the spindle have?

A

Low level of tonic activity.

(maintains basal level of tension in the muscle)

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14
Q

Muscle stretch deforms the nerve endings on the muscle spindle causing what?

A

Depolarization!

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15
Q

Primary (Ia) endings are ________, they respond to the velocity of the change.

A

Dynamic

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16
Q

Secondary (II) endings are ________, they signal the amplitude of the change.

A

Static

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17
Q

In the stretch reflex, increased activity in the spindle afferents causes what, which results in contraction of the extrafusal muscle fibers.

A

Depolarization of the alpha motor neurons

(i.e. monosynaptic reflex)

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18
Q

What are the five steps in the stretch reflex sequence?

A
  1. Muscle stretch
  2. Depolarization and generation of action potentials in the spindle afferent
  3. Activation of alpha and gamma motor neurons
  4. Muscle contraction (BOTH extra/intrafusal fibers)
  5. Decreased firing of spindle afferent (negative feedback)
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19
Q

What is resistance of a muscle to stretch called?

A

Tone

20
Q

Co-activation of both the alpha and gamma motor neurons (step 3 in stretch reflex sequence) prevents what?

A

Prevents the spindle afferents from becoming completely silent.

21
Q

What effect does activation of the gamma motor neurons have on the intrafusal fibers and the central region of the spindle?

A

Contracts the intrafusal fibers

and

lengthens the central region of the spindle

= MAINTAINING THE STRETCH

(keeps receptor in the middle of its dynamic range)

22
Q

Severing the motor nerve to a muscle or causing a lower motor lesion eliminates resistance, causing what?

A

FLACCID Paralysis

23
Q

When there is a high resistance to stretch because the muscle spindles are overactive (usually due to high rate of gamma motor neuron discharge) or there is an upper motor neuron lesion, what condition is the result?

A

HYPERTONIC (Spastic) Paralysis

24
Q

What changes do Golgi Tendon Organs react to?

A

Changes in muscle force.

25
Q

How do golgi tendon organs function in the Inverse Stretch Reflex?

A

GTO afferents excite interneurons that inhibit the alpha motor neurons, causing relaxation of the muscle.

26
Q

ALL motor control signals ULTIMATELY synapse on what?

A

Alpha Motorneurons

(“final common pathway” for CNS control of behavior)

27
Q

What makes up the “motor unit”?

A

Combination of an alpha motor neuron and the muscle fibers it innervates.

28
Q

Why must there be a continuous sensory feedback provided to the CNS?

A
  • Skeletal muscles are nonlinear force generators
  • Adjustments need to be made in the force being produced
29
Q

What does the basic unit of integrated reflex activity (i.e. reflex arc) consist of?

(5 parts)

A
  1. Sensory receptor
  2. Afferent neuron
  3. 1+ synapses in the CNS
  4. Efferent neuron
  5. Muscle
30
Q

What is a monosynaptic reflex?

A

Afferent neuron synapses directly on the efferent neuron.

31
Q

What is a polysynaptic reflex?

A

Paths branch in a complex way.

  • synaptic delays in longer pathways
  • single stimulus –> “waves” of response
  • recurrent inhibition
32
Q

What do you call a precise stimulus that evokes a reflex?

A

Adequate Stimulus

(fleas jumping = nonadequate stimulus)

33
Q

What two modifications can be made in reflex responses that are stereotyped?

A
  • Habituate
    • decline in amplitude in response to repeated stimuli
  • Sensitized
    • augmented post-synaptic responses after a habituated stimulus is paired with a noxious stimulus
34
Q

What is the combination of the Stretch and Inverse Stretch Reflexes called?

A

Lengthening reaction

35
Q

How does the lengthening reaction respond under moderate stretch conditions (e.g. light weight)?

A

Contraction

36
Q

How does the lengthening reaction respond to strong stretch (e.g. heavy weight)?

A

Relaxation

37
Q

What is the clasp-knife effect?

A

Lengthening reaction is exaggerated in a patient with a upper motor neuron lesion.

Passive rapid motion –> Catch (stretch reflexes), followed by Sudden Relaxation (inverse stretch reflex).

38
Q

What polysynaptic reflex occurs in response to noxious stimulation (usually painful)?

A

Withdrawal reflex

39
Q

What occurs in reciprocal inhibition?

A

Collateral branches of the 1a afferent fibers synapse on inhibitory interneurons that suppress the activity of alpha motor neurons that innervate antagonist muscles.

40
Q

In the “crossed extensor response,” if the stimulated part is a limb, the response includes what action?

A

Extension of the opposite limb

41
Q

What kind of lesion produces accentuated responses (spasticity) or a loss of differentiation of the reaction (i.e. the specific pattern no longer fits into an overall scheme)?

A

Upper motor lesions (specifically corticospinal tract)

OR

Transection of the neuraxis

(“releases” activities integrated below the site of the section from the control of higher brain centers)

42
Q

What do changes in descending control usually result in?

A

Increased muscle tone

(Upper motor neuron lesion removes inhibition –> resulting in increased muscle tone)

43
Q

What is Alpha Rigidity?

A

Increase in excitatory signals OR a decrease in inhibitory signals (loss of vestibulospinal input) that contact alpha motor neurons directly

44
Q

What does Alpha Rigidity result in?

A

Contraction of extrafusal fibers

and

generation of force

(independent of stretch reflex pathway)

45
Q

What is Gamma Rigidity?

A

Increase in excitatory signals OR a decrease in inhibitory signals that contact gamma motor neurons.

46
Q

What does Gamma Rigidity result in?

A

Contraction of intrafusal fibers.

(dependent upon stretch reflex pathways)

Neuro B4 - WLB (17 decks)

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