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632: Neuroanatomy-Neuroscience > Motor Neurons > Flashcards

Motor Neurons Flashcards

1
Q

The anticipatory use of sensory information to prepare for movement is referred to as what?

A

feedforward

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2
Q

Give an example of feedforward

A

In patients who have ACL ruptures they increase hamstring activity before joint loafing in order to prevent anterior tibial translation during acceptance of loading

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3
Q

The use of sensory information during or after movement to make corrections to the ongoing movement or to future movements is referred to as what?

A

feedback

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4
Q

What types of things continually give feedback?

A

proprioceptors, skin, vision, hearing, and vestibular receptors

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5
Q

Describe symptoms of patients with complete loss of somatosensation below the neck

A

Joint movements are not well synchronized and fast movements are decomposed such that only one joint is moved at a time, in an attempt to simplify control by eliminating interaction torques

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6
Q

What does the act of reaching depend on in the absence of vision?

A

Somatosensation and proprioception to locate objects which requires more time and is less accurate

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7
Q

What does loss of somatosensation in individuals with complete deafferentation disrupt?

A

positioning of limbs

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8
Q

Smooth, accurate movement requires information from what 3 systems?

A
  • visual
  • somatosensory
  • gravitational
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9
Q

Describe the pathway through the motor system from the time a decision is made to the point in which the muscle contracts

A

1) a decision is made in the anterior part of the frontal lobe
2) motor planning areas are activated
3) control circuits are activated to regulate the activity in upper motor neuron tracts
4) UMN tracts deliver signals to spinal interneurons and LMNs
5) LMNs transmit signals directly to skeletal muscles which elicit the contraction of muscle fibers that move the upper limbs and fingers

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10
Q

Where are the control circuits of the motor system located?

A

In the cerebellum and the basal ganglia

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11
Q

What is the basal ganglia involved in?

A

gating proper initiation of movement

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12
Q

What is the cerebellum involved in?

A

sensory motor coordination of ongoing movement

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13
Q

What are the 3 classifications of UMN tracts?

A
  • postural/gross movement tracts
  • fine movement tracts
  • nonspecific tracts
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14
Q

What do postural/gross movement tracts control?

A

autonomic skeletal muscle activity

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15
Q

What do fine movement tracts control?

A

fractioned movements (independent contractions of single muscles) of the limbs and face

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16
Q

What do nonspecific tracts control?

A

all motor neurons

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17
Q

Control circuits adjust the activity of the ___ motor neurons

A

upper

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18
Q

In summary _____ motor neurons have their cell bodies in the spinal cord or brainstem and synapse with skeletal muscle fibers. Whereas _____ motor neurons arise in the cerebral cortex or brainstem and synapse with LMNs or interneurons in the brainstem or spinal cord.

A

lower

upper

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19
Q

What are the 4 properties of skeletal muscle?

A
  • excitable
  • contractile
  • extensible
  • elastic
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20
Q

Describe the structure of an individual muscle fiber

A

Myofibrils are arranged in parallel to the long axis of the muscle fiber. These myofibrils consist of proteins arranged in sarcomeres.

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21
Q

What is the functional unit of muscle?

A

sarcomere

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22
Q

What are the proteins that provide structure to the sarcomere?

A
  • Z line
  • M line
  • titin
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23
Q

What is the Z line?

A

The fibrous structure at each end of the sarcomere

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24
Q

What is the M line?

A

It is the structure that anchors the fibers in the center of the sarcomere

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25
Q

What are the 3 functions of titin?

A
  • connects the Z line with the M line
  • maintains the position of myosin relative to actin
  • prevents the sarcomere from being pulled apart
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26
Q

When is muscle contraction produced?

A

when actin slides relative to myosin

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27
Q

What initiates muscle contraction?

A

When calcium binds to troponin, which causes a conformational change in the troponin which induces movement of the tropomyosin to uncover active sites on actin. This allows myosin cross-bridges to bind on the actin binding sites

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28
Q

What does the resistance to muscle stretch depend on?

A

Muscle length, for example a stretched spring generates more resistance to stretch than the same spring when it is shortened

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29
Q

What 3 things determine the total resistance to muscle stretch?

A
  • active contraction from descending motor commands and proprioceptive information
  • titin
  • weak actin-myosin bonds
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30
Q

What is muscle tone?

A

the resistance to stretch in a resting muscle

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31
Q

Clinically how can you assess muscle tone?

A

Passively rotate the limb through it’s range of motion

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32
Q

When muscle tone is normal, resistance to passive stretch is _____.

A

minimal

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33
Q

Normal resting muscle tone is provided by what 2 things?

A
  • titin

- weak actin-myosin bonds

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34
Q

When are weak actin-myosin bonds formed?

A

When myosin attaches to actin but the myosin heads do not swivel, so there is no power stroke. Thus no muscle contraction occurs.

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35
Q

Are fast or slow stretches more resistant to stretch? Explain why…

A

Fast stretches because cross-bridges do not have the opportunity to detach, making the muscle more resistant to stretch.

This is apparent in a muscle that has been immobilized for a period of time in which many weak actin-myosin bonds have formed.

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36
Q

What happens to sarcomeres when a healthy innervated muscle is continuously immobilized in a shortened position for a prolonged period?

A

They begin to disappear from the ends of myofibrils (contracture)

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37
Q

Why do sarcomeres disappear when a healthy muscle is immbolized in a shortened position?

A

It is a structural adaptation that allows the muscle to generate optimal force at the new (shorter) resting length

38
Q

What happens to sarcomeres when a muscle is immobilized in a lengthened position for a prolonged period?

A

The muscle will add new sarcomeres

39
Q

How can a joint’s resistance to movement be increased?

A

cocontraction

40
Q

What does concontraction refer to?

A

The simultaneous contraction of antagonist muscles

41
Q

essentially what does cocontraction do?

A

stabilizes joints

42
Q

What does cocontraction in the UEs do?

A

enables precise movements

43
Q

What does cocontraction in the LEs do?

A

allows an individual to stand on unstable surfaces

44
Q

___ motor neurons are the only neurons that convey signals to extrafusal and intrafusal skeletal muscle fibers.

A

lower

45
Q

What are the 2 types of LMNs?

A
  • alpha

- gamma

46
Q

Where are the cell bodies of all LMNs found?

A

in the ventral horn of the spinal cord

47
Q

Describe the path LMN axons take once they leave the spinal cord

A

They leave the spinal cord via the ventral root, travel through the spinal nerve, and then travel through the peripheral nerve to reach skeletal muscle

48
Q

Alpha motor neurons have ____-sized myelinated axons

A

large

49
Q

Alpha motor neurons project to ___fusal skeletal mucle

A

extrafusal

50
Q

What type of NT do LMNs release?

A

ACh

51
Q

Gamma motor neurons have ____-sized myelinated axons

A

medium

52
Q

Gamma motor neurons project to ___fusal skeletal mucle

A

intrafusal

53
Q

What is the function of a gamma motor neuron?

A

To control and adjust the sensitivity of spindle afferents

In other words to “gain” control

54
Q

Define “gain”

A

the amount of extrafusal contraction elicited by a given stretch (output/input)

55
Q

When is gain adjustment necessary?

A

When muscle is shortened to prevent the muscle spindle from being unloaded

56
Q

During most movements, the alpha and gamma motor neuron systems function simultaneously. This pattern is called what?

A

alpha-gamma coactivation

57
Q

What does alpha-gamma coactivation maintain?

A

The stretch on the central region of the muscle spindle intrafusal fibers when the muscle actively contracts

58
Q

Alpha-gamma coactivation occurs because of what 2 things?

A
  • most sources of input to alpha motor neurons have collaterals that project to gamma motor neurons
  • gamma motor neurons, with their smaller cell bodies, require less excitation to reach threshold than alpha motor neurons do
59
Q

What does a motor unit consist of?

A

An alpha motor neuron and all of the muscle fibers it innervates

60
Q

What are the 2 classifications of motor units?

A
  • Slow-twitch fibers

- Fast-twitch fibers

61
Q

What type of motor neurons innervate fast-twitch muscle fibers?

A

larger-diameter, faster-conducting motor neuron

62
Q

Fast-twitch muscle fibers are recruited in what types of movements?

A

walking, running, etc.

explosive movements

63
Q

What type of motor neurons innervate slow-twitch muscle fibers?

A

Smaller-diameter, slower-conducting alpha motor neuron

64
Q

Slow-twitch muscle fibers are found in what type of muscle?

A

Postural and slowly contracting muscles

65
Q

Which muscle fibers are activated first and why?

A

Slow twitch fibers are activated first due to smaller cell bodies

66
Q

The order of recruitment from small to larger motor neurons is called what?

A

The size principle

67
Q

When are movements generated?

A

When somatosensory information is integrated with descending motor commands in the spinal cord

68
Q

What are LMN pools?

A

Groups of cell bodies in the spinal cord whose axons project to a single muscle

69
Q

Where are LMN pools located?

A

In the ventral horn of the spinal cord

70
Q

The actions of LMN pools are correlated with what?

A

their anatomic position

71
Q

What do medially located LMN pools innervate?

A

axial and proximal muscles

72
Q

What do laterally located LMN pools innervate?

A

distal muscles

73
Q

What do anteriorly located LMN pools innervate?

A

extensors

74
Q

What do posteriorly located LMN pools innervate?

A

flexors

75
Q

The inhibition of antagonist muscles during agonist contraction is called what?

A

Reciprocal Inhibition

76
Q

When is reciprocal inhibition used extensively and why?

A

During voluntary motion to prevent antagonist opposition to the movement

77
Q

What are central pattern generators (CPGs)?

A

They are basic patterns of coordinated rhythmical activity that are “wired” into the spinal cord circuits

78
Q

What do CPGs produce?

A

locomotory behaviors including walking, running or swimming

79
Q

What are the 3 types of spinal reflexes?

A
  • stretch reflex
  • GTO reflex
  • flexor-withdrawal reflex
80
Q

What are the 2 types of stretch reflexes?

A

phasic and tonic

81
Q

Describe the steps involved in a phasic stretch reflex

A

1) the muscle is stretched
2) group Ia afferent fibers in the muscle spindle are activated and their firing rate increases
3) group Ia afferents enter the spinal cord and synapse directly on and activate α motoneurons which innervate the homonymous muscle
4) Activation of the α motoneurons causes contraction of the muscle that was originally stretched
5) this contraction decreases the stretch on the muscle spindle and the firing rate of the group Ia afferents returns to normal

82
Q

When is the only time in which tonic stretch reflexes are present?

A

Only following UMN lesions

83
Q

What is the difference between phasic and tonic receptors

A

The tonic stretch reflex continues as long as the stretch is maintained

84
Q

Describe the steps involved in the Golgi tendon reflex

A

1) the muscle contracts and extrafusal fibers shorten, activating the Golgi tendon organs attached to them
2) group Ib afferent fibers are activated and synapse on inhibitory interneurons in the spinal cord
3) the inhibitory interneurons synapse on α motoneurons inhibiting them, producing relaxation of the homonymous muscle
4) as the homonymous muscle relaxes, the reflex also causes synergistic muscles to relax and antagonistic muscles to contract

85
Q

What is the function of the GTO reflex?

A

To protect against potential damage and prevent muscle fatigue

86
Q

Describe the steps involved in the flexor-withdrawal reflex

A

1) painful stimulus occurs
2) groups II, III, and IV afferents are activated
3) these flexor reflex afferents synapse on multiple interneurons in the spinal cord
4) on the same side of the pain stimulus flexor muscles contract and extensor muscle relax causing a withdrawal from the noxious stimulus
5) on the opposite side of the pain stimulus extensor muscles contract and flexor muscles relax producing extension which helps to maintain balance
6) the contracted muscles remain contracted for a period of time after the reflex is activated because of a persistent neural discharge called an afterdischarge

87
Q

What is the Hoffmann’s Reflex or H-Reflex?

A

A monosynaptic reflex elicited by electrically stimulating a nerve

88
Q

What is the purpose of the H-reflex?

A

To quantify the level of alpha motor neuron facilitation or inhibition

89
Q

Describe the process of eliciting a H-reflex

A

1) electrodes are placed over the tibial nerve in the popliteal fossa to and on the inferomedial gastrocnemius
2) a weak current, adequate to stimulate only the largest axons is administered to the popliteal fossa electrode
3) APs spread travel both toward the muscle and toward the spinal cord each with different latencies that can be recorded over the skin

90
Q

What are the 2 latencies that are produced?

A

The M-wave and the H-reflex

91
Q

The shorter-latency M-wave is produced by what?

A

Impluses traveling along motor fibers, casuing an almost immediate muscle contraction

92
Q

What elicits the H-reflex?

A

Afferent fiber activity which transmits across synapses to facilitate alpha motor neurons. If activation of this alpha motor neuron is near threshold it will cause depolarization of the calf muscle membranes, and the H-reflex is recorded

632: Neuroanatomy-Neuroscience (26 decks)

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