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Neurology > Motor units and muscle spindles > Flashcards

Motor units and muscle spindles Flashcards

1
Q

UMNs supply input to LMNs to ________ their activity

A

UMNs supply input to LMNs to modulate their activity

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2
Q

LMNs command muscle ___________ and form the ‘_____ ______ _______’

A

LMNs command muscle contraction and form the ‘final common pathway’

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3
Q

LMNs comprise _____ _____ _____ that innervate the bulk of fibres within a muscle that generate force, _____ ______ _____ innervate a sensory organ within the muscle known as a muscle spindle

A

LMNs comprise alpha motor neurones that innervate the bulk of fibres within a muscle that generate force, gamma motor neurones innervate a sensory organ within the muscle known as a muscle spindle

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4
Q

Biceps brachii and brachialis work together as _________. Similarly triceps brachii and anconeus are _________ ______

A

Biceps brachii and brachialis work together as synergists. Similarly triceps brachii and anconeus are synergistic muscles

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5
Q

Biceps brachii and brachialis (as _____) oppose the triceps brachii and anconeus (as _______), these groups are __________ to each other

A

Biceps brachii and brachialis (as flexors) oppose the triceps brachii and anconeus (as extensors), these groups are antagonists to each other

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6
Q

____ muscles control movements of the trunk, _______ ____ muscles are found in the shoulder, elbow and ______ _______ are found in the digits

A

axial muscles control movements of the trunk, proximal girdle muscles are found in the shoulder, elbow and distal muscles are found in the digits

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7
Q

Axons of the LMNs exit the spinal cord in the _______ roots (or via ______ nerves)

A

Axons of the LMNs exit the spinal cord in the ventral roots (or via cranial nerves)

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8
Q

Each ventral (anterior) root joins with a dorsal (posterior) root to form a _____ _____ _____ containing…..

A

Each ventral (anterior) root joins with a dorsal (posterior) root to form a mixed spinal nerve containing;

  • sensory fibres
  • motor fibres
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9
Q

What do motoneurons belong to?

A

A spinal segment;

  • cervical (C) 1-8
  • thoracic (T) 1-12
  • lumbar (L) 1-5
  • sacral (S) 1-5
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10
Q

How are motoneurones distributed within the spinal cord?

A

Unevenly, more in the cervical and lumbar enlargements

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11
Q

What forms a motor unit?

A

An a-MN and all of the skeletal muscle fibres that it innervates

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12
Q

what is a motor neurone pool?

A

The collection of an a-MNs that innervate a single muscle

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13
Q

What are the two principle mechanisms that allow a-MN to grade muscle contraction?

A
  • frequency of action potential discharge of the a-MN

- the recruitment of additional, synergistic motor units

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14
Q

Where are the cell bodies of LMN distributed in the spinal cord?

A

They show a somatotopic distribution in the ventral horm

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15
Q

LMNs innervating axial muscles are ______ to those innervating distal muscles.
LMNs innervating flexors are ______ to those supplying extensors.

A

LMNs innervating axial muscles are medial to those innervating distal muscles.
LMNs innervating flexors are dorsal to those supplying extensors.

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16
Q

What are the three sources of input to an a-MN that regulate its activity?

A
  • central terminals of dorsal root ganglion cells whose axons innervate muscle spindles
  • UMNs in the motor cortex and brainstem
  • spinal interneurones
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17
Q

Muscle strength depends on which two things?

A

Activation of muscle fibres and force production by innervated muscle fibres

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18
Q

What components determine level of activation of muscle fibres?

A
  • the firing rates of the LMNs involved
  • the number of LMNs that are simultaneously active
  • the co-ordination of the movement
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19
Q

What components determine force production of muscle fibres?

A 
Fibre size (hypertrophy)
Fibre phenotype (fast or slow contracting muscle)
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20
Q

A single action potential in an a-MN causes a muscle fibre to ______. The summation of ________ causes a _________ __________ as the number of incoming action potentials increases.

A

A single action potential in an a-MN causes a muscle fibre to twitch. The summation of twitches causes a sustained contraction as the number of incoming action potentials increases.

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21
Q

What are the discriminatory features of motor units?

A
  • variable size

- fast and slow types

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22
Q

describe the variable sizes of motor units

A

small (a few fibres)- for fine movement- innervated by small a-MNs (diameter)

large (hundreds to thousands of fibres) in large postural muscles (antigravity muscles)

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23
Q

Motor units contain muscle fibres of only a single type (_______ ________ or ____ ________) a-MNs innervating fast type tend to be _____ and have faster conducting axons than slow units

A

Motor units contain muscle fibres of only a single type (rapidly fatuguing or slow fatiguing) a-MNs innervating fast type tend to be larger and have faster conducting axons than slow units

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24
Q

Each muscle fibre is innervated by a single motor neurone at _______ (___________ junction) which is usually at the centre of the fibre

A

Each muscle fibre is innervated by a single motor neurone at endplate (neuromuscular junction) which is usually at the centre of the fibre

25
Q

What are the two major types of skeletal muscle fibres?

A

Slow-twitch and fast-twitch

26
Q

Describe slow-oxidative (type I) fibres?

A

ATP largely derived from oxidative phosphorylation. Slow contraction and relaxation. Fatigue resistant..

27
Q

What colour are slow-oxidative fibres?

A

Red fibres because of high myoglobin content

28
Q

Describe fast (type II) fibres?

A

There are two categories-

IIa, IIb, IIx

29
Q

Describe type IIa fibres?

A

ATP largely derived from oxidative phosphorylation. Fast contraction and relaxation. Fatigue resistant.

30
Q

What colour are type IIa fibres?

A

Red and reasonably well vascularised

31
Q

Describe type IIb,IIx fibres?

A

ATP mainly derived from glycolysis. Fast contraction but not fatigue resistant

32
Q

What colour are type IIb and IIx fibres?

A

white and poorly vascularised

33
Q

Which fibres do humans have?

A

I, IIa

34
Q

Which kind of motor units are responsible for burst power?

A

fast fatiguing, type IIx fibres

35
Q

Which kind of motor units are responsible for sustained locomotion?

A

Fatigue resistant type IIa fibres

36
Q

Which kind of motor units are responsible for antigravity, sustained movement?

A

Slow type I fibres

37
Q

What is the henneman size principle?

A

The susceptibility of an a-MN to discharge action potentials is a function of its size, smaller a-MNs have a lover threshold than larger ones. Slow motor units are more easily activated and trained by any training that activates the muscle.

38
Q

Activation of an upper motor neurone causes the ____ _____ ________ that it supplies to be excited

A

Activation of an upper motor neurone causes the ____ _____ ________ that it supplies to be excited

39
Q

Motor units are recruited in order of their ____, progressively increasing

A

Motor units are recruited in order of their size, progressively increasing

40
Q

Recruitment in the order ____ (___) before ____ _______ _________ (___) preceding ____ __________ (___) results in increasing increments towards the maximal force the muscle exerts

A

Recruitment in the order slow (I) before fast fatigue resistant (IIa) preceding fast fatiguable (IIx) results in increasing increments towards the maximal force the muscle exerts

41
Q

What is the myotactic reflex

A

when a skeletal muscle is pulled it pulls back

42
Q

Describe how the myotactic reflex occurs

A

Change in length (and rate of change) is registered by a sensory organ within the muscle, the muscle spindle

43
Q

What does a muscle spindle consist of ?

A
  • fibrous capsule
  • intrafusal muscle fibres
  • sensory afferents (Ia class, myelinated and very fast conducting) that innervate intrafusal fibres
  • gamma motor neurone efferents that innervate intrafusal fibres
44
Q

The myotactic reflex is a __________ reflex ___

A

The myotactic reflex is a monosynaptic reflex arc

45
Q

Stretch of muscle spindle –> activation of __ ________ –> excitatory synaptic transmission in _____ ____ (modified by release of _________) –> activation of ____ –> contraction of ___________ muscle

A

Stretch of muscle spindle –> activation of Ia afferent –> excitatory synaptic transmission in spinal cord (modified by release of glutamate) –> activation of a-MN –> contraction of homonymous muscle

46
Q

Where is monosynaptic reflex most prominent in humans?

A

In extensor muscles

47
Q

What can the myotactic reflex be used to assess?

A

The functional integrity of the spinal cord at specific levels

48
Q

Which spinal level is assessed in biceps (elbow) deep tendon reflex?

A

C5-C6

49
Q

Which spinal level is assessed in supinator (wrist) deep tendon reflex?

A

C5-C6

50
Q

Which spinal level is assessed in triceps (elbow) deep tendon reflex?

A

C7

51
Q

Which spinal level is assessed in quadriceps (knee) deep tendon reflex?

A

L3-L4

52
Q

Which spinal level is assessed in gastrocnemius (ankle) deep tendon reflex?

A

S1

53
Q

What is jendrassik manoeuvre

A

Interlocking fingers and pilling apart when instructed- modulate descending control

54
Q

What do intrafusal fibres consist of?

A
  • non-contractile equatorial region

- contractile polar ends

55
Q

What innervates the non-contractile equatorial region of intrafusal fibres?

A

innervated by Ia sensory neurones

56
Q

What input do contractile polar ends of intrafusal fibres receive?

A

Efferent input from y-MNs with cell bodies in the ventral horn of the spinal cord (driven by higher centres)

57
Q

stimulation of y-MNs causes the _______ __ ________

A

stimulation of y-MNs causes the spindle to contract

58
Q

During ________ movement _ and _ MNs are normally co-activated, so that the ________ fibres contract in parallel with the ________ fibres

A

During voluntary movement a and y MNs are normally co-activated, so that the intrafusal fibres contract in parallel with the extrafusal fibres

59
Q

What function does co-acitvated a and y MNs have?

A

Maintains the sensitivity of the spindle to keep it on the air, to stop it going slack when extrafusal fibres contract

Neurology (41 decks)

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