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Neuroscience > L41 Muscle Innervation and Motor Unit > Flashcards

L41 Muscle Innervation and Motor Unit Flashcards

1
Q

where are the cell bodies of all muscle afferents?

A

DRG

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2
Q

what are the two major sensory organs?

A

muscle spindle
golgi tendon organ
(also mechanoreceptors)

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3
Q

what does the muscle spindle measure and what type of fiber does it use?

A

measures length and rate of change of length

Ia fiber

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4
Q

what does the golgi tendon organ measure and what type of fiber does it use?

A

measure tension

Ib fiber

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5
Q

what make up the additional mechanoreceptors?

A

thermo and nociceptors (temperature and pain)

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6
Q

what is a muscle spindle

A

encapsulated structures found in most skeletal muscles used to monitor its length

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7
Q

the number of spindles in the muscle is directly related to what?

A

the muscles function

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8
Q

muscles involved in fine movement (hand) have ___ spindles than coarse movement (back)

A

more

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9
Q

what are the components of the muscle spindle?

A
  • small group of 2-12 intrafusal gamma fibers
  • central regions are non contractile
  • large diameter myelinated sensory axon (Ia axon) with sensory ending in the intrafusal belly
  • small diameter myelinated motor axon that innervate the distal contractile regions (gamma motor ending)
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10
Q

where do the intrafusal fibers lie?

A

within the spindle which is between/parallel to the extrafusal fibers and are attached to the muscles tendons

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11
Q

when the muscle contracts, it shortens and ____ I a firing rate

A

decreases

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12
Q

when the muscle stretches, Ia firing rate _____

A

increases

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13
Q

what are the 3 types of intrafusal fibers?

A

dynamic nuclear bag fibers
static nuclear bag fibers
nuclear chain fibers

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14
Q

what are dynamic nuclear bag fibers sensitive to?

A

length change and rate of change

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15
Q

what are static nuclear bag fibers sensitive to?

A

length change

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16
Q

what are nuclear chain fibers sensitive to?

A

length change

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17
Q

what are the primary sensory ending and what does it wrap around

A

group Ia
each intrafusal nuclear bag and chain fiber

static and dynamic changes

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18
Q

what does Group II sensory ending wrap around?

A

all nuclear chain and static nuclear bag fiber

only static changes

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19
Q

describe A-Gamma fibers responsible for

A

small myelinated and low velocity axons carry motor innervation to the intrafusal fibers

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20
Q

what are the A-Alpha fibers responsible for?

A

motor innervation of the extrafusal fibers

21
Q

during a sustained stretch reflex, as the spindle is continuously stretched, there is a _____ AP firing in the muscle spindle afferent

A

sustained

22
Q

what happens to the muscle spindle during alpha motor stimulation?

A

the muscle shortens and briefly alleviates the stretch on the spindle, at this time, there is no stimulation of the Ia axon = NO afferent discharge!

23
Q

what happens to the muscle spindle during simultaneous alpha and gamma stimulation?

A

the intrafusal and extrafusal fibers will contract together

because the intrafusal spindle consensually contracts, it continues to relay afferent information thus able to continually measure muscle length

24
Q

where are golgi tendon organs located?

A

in series between tendon and extrafusal fibers which are sensitive to tension

25
Q

what are each golgi tendon organ innervated by?

A

single Ib axon which is large myelinated and has high velocity

26
Q

once the Ib axon enters the tendon organ and intertwine in the collagen chains what happens?

A

the Ib axon will lose its myelin

27
Q

what happens to the Ib axon firing rate when the contraction of the muscle compresses the free, non-myelinated endings?

A

increase firing rate

28
Q

what happens to the alpha motor neuron when you increase Ib axon firing rates?

A

increase inhibition of alpha motor neurons - ensures even distribution of tension and contraction as well as avoiding excessive contraction to the point of damage

29
Q

what is the reverse myotactic reflex?

A

increasing Ib axon firing increases inhibition of alpha motor neurons - ensures even distribution of tension and contraction as well as avoiding excessive contraction to the point of damage

30
Q

what are the 2 types of motor neurons?

A

A-alpha

A-gamma

31
Q

describe A-alpha motor neurons?

A

large, myelinated, high velocity passing via ventral horn to limb muscle extrafusal fibers

32
Q

describe A-gamma motor neurons

A

small, myelinated, lower velocity passing via ventral horns to limb muscle intrafusal fibers

33
Q

where are the alpha and gamma motor neurons located in the spinal cord?

A

ventral horn

34
Q

what are renshaw cells?

A

inhibitory interneurons of the anterior horn that receive collaterals from the alpha motor neuron and work to avoid damage in the event of excessive contraction (like golgi)

35
Q

what makes up a motor unit?

A

motor neuron and the muscle it activates

36
Q

what is a motor nuclei?

A

100 motor neurons (average) to control a typical muscle

they congreate to create a nuclei in the anterior horn of the spinal cord as well as in the brainstem for motor CNs

37
Q

a motor axon branches at its muscle and each fiber receives ____ ending of the branching

A

one

38
Q

where is the NMJ located

A

in the center of the muscle fiber where APs are generated

39
Q

the number of muscle fibers in a motor unit ___ with increasing fine motor control

A

decreases

40
Q

what are the 3 types of motor units?

A

Type I
Type IIA
Type IIB

41
Q

describe Type IA motor units

A

slow twitch muscle fibers, low tension, fatigue resistant, aerobic, innervated by small motor neuron and axon

42
Q

describe Type IIA motor units

A

fast fatigue resistant, large tension, some aerobic capacity, innervated by large motor neuron and axon

43
Q

describe Type IIB motor units

A

fast fatigable, large tension, anaerobic, innverated by large motor neuron and axon

44
Q

the force developed by the muscle fibers increase with _____ firing rate of the motor neuron

A

increasing

45
Q

force will also increase as ____ motor units are recruited to the excited state

A

more

46
Q

describe hypotonia?

A

reduced muscle tone (atrophy) due to damage to either Ia afferent or the alpha motor neuron

47
Q

describe LMN syndrome

A

causes by destruction of the motor neuron in the anterior horn, the axons in the ventral roots or the peripheral nerves

can cause

  • atrophy
  • loss of voluntary and reflex responses
  • hyporeflexia
  • fasciculations and fibrillations
48
Q

what does conduction velocity depend on

A
  1. myelination

2. axon diameter - the larger the axon diameter, the faster the conduction velocity

49
Q

what factors control contraction

A
  1. firing rates of motor neurons

2. number of motor units recruited

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