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Neuroscience Exam 2 > Lecture 5: Spinal Cord Reflexes and Locomotion > Flashcards

Lecture 5: Spinal Cord Reflexes and Locomotion Flashcards

1
Q

What is a motor unit

A

single motor neuron and the muscle fibers that it innervates (can inn. multiple muscles)

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2
Q

What is a motor neuron pool

A

all motor neurons inn. a given muscle (ex. biceps)

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3
Q

What are the three different types of motor units with:
* motor unit size
* force
* contraction velocity
* rate of fatigue
* muscle type (ATP synthesis)
* Myoglobin content (oxygen binding protein)
* Common recruitment order

A
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4
Q

What is the size principle

A

Order of motor unit recruitment – First: smaller motor neurons and slow muscle fibers; last: larger motor neurons with fast fatigable muscle fibers.

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5
Q

What is a muscle spindle, what does it contain and how are they arranged?

A
  • A spindle is a connective tissue sheath that contains intrafusal muscle fibers.
  • Spindles are arranged in parallel with extrafusal fibers
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6
Q

What innervates intrafusal muscle fibers?

A

Sensory nerve endings innervate intrafusal muscle fibers

  • Their cell bodies are in the dorsal root ganglion and their axons enter the cord via dorsal roots.
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7
Q

Besides stretching the skeletal muscle, what also stretches? what happens?

A

Stretch of skeletal muscles also stretches primary Ia (faster conduction velocity) and group II (slower) endings and increases their firing rate.

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8
Q

What does Ia afferents measure? II?

A
  • Ia afferents “measure” muscle length and rate of change of length (velocity)
  • Group II measure length

to send signal to SC

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9
Q

What is the intrafusal muscle fibers

A

capsule surounding spindle

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10
Q

intrafusal muscle + capsule =

A

muscle spindle

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11
Q

Compare group Ia and II afferent firings and their importance

A

Ia:

  • when stretch happens, it fires immediately but then goes away immediately
  • initial mvt (thinking about mvt)

II:

  • when stretch happens, it fires slowly but remains longer
  • used to maintain movement
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12
Q

What do you hit in the stretch reflex? and what does it cause (first thing)

A

hitting tendon attached to muscle then casues the muscle to stretch

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13
Q

What happens in the stretch reflex when the muscle is stretched?

A
  • When a muscle is stretched, muscle spindle senses it and sends signal through spindle afferents (Ia and II) to excite alpha motor neurons that innervate the stretched muscle (directly). This is the monosynaptic stretch reflex
  • Ia afferents also inhibit alpha motor neurons that innervate “antagonistic” muscles through inhibitory interneuron (indirectly). This is a di-synaptic reflex for “reciprocal inhibition”: an inhibitory interneuron is interposed between the Ia afferent and the motor neurons (that innervate the un-stretched muscle).
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14
Q

How is the stretch reflex regulates muscle length through neg. feedback

A
  • acts to reduce the length of the muscle by exciting the alpha motor neurons that innervate the muscle with which it is associated
  • This is an expample of direct activation of alpha motor neurons

help us remove hand from hand stove

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15
Q

Presynaptic inhibition normally regulates the_____ or “____ ” of the stretch reflex.

A

Presynaptic inhibition normally regulates the strength or “gain” of the stretch reflex.

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16
Q

What is proprioception?

A

Proprioception is the sense of the relative position of neighboring parts of the body and strength of effort being employed in movement

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17
Q

What does the Ia afferents also provide?

A

proprioceptive information about limb position & joint angle.

  • vibration of a muscle (~100 Hz, 500 micron amplitude) can selectively activate spindle afferents and change the perception of joint angle!!
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18
Q

What does gamma motor neurons selectively innervate?

A

intrafusal muscle fibers

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19
Q

What does the activation of gamma motoneurons by descending signals cause?

A
  • Causes contraction of the two ends of the intrafusal muscle fibers
  • This action stretches the middle and promotes activity in the Ia afferents
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20
Q

What are the two classes of decending routes for alpha motoneuron activation?

A
  • First there is “direct” activation of alpha motoneurons by descending signals (this can be monosynaptic or through interneurons*).
  • The second “route” is “indirect” activation via the gamma loop: descending signals excite gamma -> intrafusal fibers contract -> stretches Ia -> activates alpha motor neuron
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21
Q

What do gamma neurons do?

A

regulate length of spindle at same time as alpha motor neuron that reg. muscle

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22
Q

What is alpha gamma co-activation

A
  • In many instances, movements are produced by alpha - gamma co-activation. This process keeps the intrafusal fibers contracted while extrafusal fibers contract.
  • Thus, the Ia spindle afferents remain active - ready for increase in load!
  • FACILIATES LOAD COMPENSATION
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23
Q

_ afferent nerve endings are found on bundle of small tendon fascicles adjacent to _ junction.

A

Ib afferent nerve endings are found on bundle of small tendon fascicles adjacent to musculotendinous junction.

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24
Q

What is Ib afferent activated by?

A

tension produced by muscle contraction

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25
Q

What can the tendon organ reflex, also be called?

A

inverse myotatic reflex

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26
Q

tendon organs are in _ and measure what?

A
  • in series
  • muscle force
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27
Q

the tendon organ (inverse myotatic) reflex is what type of synaptic reflex

A

a di- (two) synaptic reflex arc (negative feedback)

  • motor neurons to synergistic muscles are inhibited via inhibitory interneurons
  • Motor neurons to antagonistic muscles are excited via excitatory neurons
28
Q

how does tendon organ reflex regulate muscle force through negative feedback

A

acts to reduce force

29
Q

how does the inverse myotatic reflex act on muscles

A

you relax the same muscle becasue neg feedback
* motor neurons to synergictic muscles are inhibited are inhibited via inhibitory interneuron
* motor neurons to antagonistic muscles are excited via excitartory interneuron

30
Q

how does the inverse myotatic reflex act on muscles

A

you relax the same muscle becasue neg feedback

31
Q

What does stretch and tendon organ reflexes contribute to?

A

generation of appropritate stiffness for support and shock absorber functions

32
Q

What is muscle stiffness equation

A

change in force/change in length

33
Q

How does the stretch reflex and tendon organ reflex affect muscle stiffness

A
  • Stretch reflex tends to increase muscle stiffness
  • Tendon organ reflex tends to decrease muscle stiffness
34
Q

polysynaptic reflex (flexor reflex) responds to what?

A

noxious stimulation of the skin or muscle.

35
Q

What happens in the polysynaptic reflex?

A

Cutaneous nociceptors evoke a sensory signal to the spinal cord to cause poly synaptic spinal reflex for withdrawal and support

  • inhibitory interneuron stops the extensor on the stimulated leg and the interneuron activates the extensor on the other leg.
  • interneuron activates the flexor on the effected leg and inhibitory interneuron stops the flexor on the other leg
36
Q

What is an upper motor neuron

A

is a clinical term for neurons in the brain that project to
spinal motor circuits & motor neurons that directly innervate muscles

37
Q

what does upper motor neuron damage

A

changes in the resting level of tension in muscle (motor tone)

38
Q

With upper motor neuron damage, what happens first and what is followed?

A

First hypotonia (spinal shock) followed by hypertonia (“spasticity”)

39
Q

What happens with damage with LMN? UMN?

A

LMN-> paralysis
UMN-> spasmtic (contractions)

40
Q

What are inhibitory NT for the UMN and LMN

A

gaba and glycine-> neurons must have low levels of cl- inside

41
Q

What does GABA a and glycine inhibition of motor neurons depend on what? What is it maintained by?

A
  • Depends upon low intracellular chloride concentration
  • Maintained by the potassium-chloride cotransporter KCC2
42
Q

What happens with the cotransporter, KCC2, after spinal cord injury

A

cotransporter is downregulated in motorneurons after spinal cord injury in rats -> depolarizing the chloride equilibrium potential and reducing the strength of postsynaptic inhibition-> contributing to hyper-excitability (SPASTICITY)

43
Q

What is something that upregulates the transport after spinal cord injury

A

neurotrohic factor BDNF

44
Q

what normally does the activation of GABAa and glycine receptors that inhibit motor neurons result in?

A

Result in low intracellular chloride concentration maintained by the potassium-chloride cotransporter KCC2.

45
Q

Define Central Pattern Generator (CPG)?

A

An assembly of neurons that, by virtue of their intrinsic properties and synaptic interactions is capable of generating and controlling the spatial and temporal activity of motor neurons

46
Q

Where is Central Pattern Generator (CPG) for locomotion located?

A

In the spinal cord

47
Q

Reflex feedback is not necessary for rhythm generation …

A

but reflex feedback does influence phase durations & pattern. Phase dependent reflex reversal

48
Q

Explain the Central Mechanisms for generating stepping rhythm in the spinal cord?

A
  • “Spinal” animals with deafferented hind limbs can walk on tread mill
  • Temporal sequence of muscles activated not altered
  • Central pattern generator sets basic rhythm & order of activation of different muscles
49
Q

Is Reflex feedback necessary for rhythm generation?

A

NO

50
Q

What does reflex feedback influence?

A

phase durations & pattern

51
Q

Explain Phase Dependent Reflex Reversal

A
  • In the swing phase, stimulating the dorsal surface of the foot can produce additional flexion.
  • During the stance phase, on the other hand, the most effective way for the animal to get over a moving object touching its paw would be to perform an increased rapid extension followed by flexion
52
Q

What are the two classes of mechanisms for CPG?

A
  1. Pacemaker cells have intrinsic membrane properties that cause periodic “bursts” of action potentials. (For example, alternating inward (Ca++) and outward (K+) currents)
  2. “Emergent” network property: no cell has bursting properties. Reciprocal inhibition & adaptation in a network account for the alternating activity in flexors & extensors.
53
Q

What is evidence for multople rhythmogenic modules for locomotion

A

Optogenetic evidence: Light-gated ion channels or transporters -> expressed in neurons using cell specific promoters.

54
Q

What is activated by blue light and what does it do?

A

ChR2 nonspecific cation channel gated by blue light conducts H+, Na+, K+, and Ca2+ ions

DEPOLARIZATION

55
Q

What is activated by green/yellow light and what does it do?

A

Halorhodopsin gated by green/yellow light: conducts Cl- ions

HYPERPOLARIZATION

56
Q

Selective _ of glutamatergic neurons turn on CPG

A

excitation

57
Q

Are multiple spinal cord levels involved in rhythmogenic modules underlying locomotion

A

yes

58
Q

coordination of left and right sides are provided by

A

commissural interneurons

59
Q

Brainstem control of _ _ and _ and its modulation during REM Sleep

A

Brainstem control of muscle tone and locomotion and its modulation during REM Sleep

60
Q

why do we have decreased brain activity so we dont sleep walk normally? how are our muscle (toned, relaxed, etc)

A
  • Pons sends signals to locomotion path to stop it from firing
  • Muscle has tone while sleeping because pons also sends signal to the medulla to regulate muscle tone
61
Q

Which cells prevent unloading of muscle spindle afferents during muscle contraction and facilitate load compensation?
a. Alpha motor neurons
b. Gamma motor neurons
c. Golgi tendon organ afferents
d. Cutaneous nociceptors

A

B. gamma

62
Q
  1. Cross-country athletes require significant levels of endurance to maintain a steady pace during long-lasting races. Given the nature of the sport, which motor unit is likely functioning for extended periods of time with minimal fatigue?
    A. Large
    B. Intermediate
    C. Small
    D. None of the above
A

C. Small

63
Q
  1. Which of the following is considered a sensory afferent fiber that innervates muscle spindles?
    A. Group Ia neurons
    B. α motor neurons
    C. Group Ib neurons
    D. γ motor neurons
A

A. Group Ia neurons

64
Q
  1. An MSP3 student is getting a workout in after a long day of studying. After increasing the weight on the barbell, he notices that his muscles very easily “give out.” Based on his recent studies, he believes this is likely negative feedback regulation of muscle force at the level of the spinal cord. What reflex is the student referring to?
    A. Alpha-Gamma
    B. Inverse Myotatic
    C. Spinal Flexor
    D. Myotatic
A

B. Inverse Myotatic

65
Q
  1. Upper motor neuron (UMN) syndrome is caused by lesions of descending motor pathways and often presents has hypotonia followed by spasticity. Which of the following is responsible for the secondary spasticity?
    A. Downregulation of KCC2
    B. Increased hyperpolarization of UMN
    C. Upregulation of BDNF
    D. Increased GABA release
A

A. Downregulation of KCC2

66
Q
  1. Channelrhodopsin 2 (ChR2) is a _________ light activated ion channel used to stimulate cellular _________ in optogenetic studies.
    A. Blue; Hyperpolarization
    B. Green; Depolarization
    C. Blue; Depolarization
    D. Green; Hyperpolarization
A

C. Blue; Depolarization

Neuroscience Exam 2 (9 decks)

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