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Neuro Physiology > Motor units, muscle spindles, Golgi tendon organs and joint receptors > Flashcards

Motor units, muscle spindles, Golgi tendon organs and joint receptors Flashcards

1
Q

Where do lower motor neurons receive input from?

A

Upper motor neurons, proprioceptors and interneurons

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2
Q

What do LMNs control?

A

They command muscle contraction and form the final common pathway

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3
Q

What comprises LMNs?

A

Alpha motor neurons that innervate the bulk of fibres within a muscle that generate force. Gamma motor neurons innervate a sensory organ within the muscle known as a spindle

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4
Q

Name two examples of synergistic muscles

A

Biceps brachii and brachialis, triceps brachii and anconeus

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5
Q

What are biceps brachii & brachialis and triceps brachii & anconeus to each other?

A

Antagonists

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6
Q

What muscles control movements of the trunk?

A

Axial muscles

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7
Q

Where are proximal/girdle muscles found?

A

Shoulder, elbow, pelvis and knee

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8
Q

Where are distal muscles found?

A

Hands, feet and digits

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9
Q

Where do LMNs exit the spinal cord?

A

Ventral roots

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10
Q

What are the basic components of a mixed spinal nerve?

A

Ventral root joining a dorsal root

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11
Q

Motor neurons belong to a spinal segment e.g. cervical and are not distributed within the spinal cord. Where are greater numbers of motor neurons found?

A

Cervical enlargement (C3-T1) supplying the arm and the lumbar enlargement (L1-S3) supplying the leg

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12
Q

What is the smallest functional component of the motor system?

A

The motor unit - composed of an alpha-MN and all of the skeletal muscles fibres that it innervates

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13
Q

What is a motor neuron pool?

A

The collection of alpha-MNs that innervate a single muscle

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14
Q

Force of muscle contraction is graded by alpha-MNs by 2 principle mechanisms, what are these?

A
  1. Frequency of action potential discharge of the alpha-MN

2. The recruitment of additional, synergistic, motor units

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15
Q

The cell bodies of LMNs show a distinct distribution in the ventral horn of the spinal cord. Describe the relationship between LMNs innervating axial muscles to those innervating distal muscles

A

LMNs innervating axial muscles are medial to those innervating distal muscles

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16
Q

The cell bodies of LMNs show a distinct distribution in the ventral horn of the spinal cord. Describe the relationship between LMNs innervating flexors muscles to those innervating extensors

A

LMNs innervating flexors are dorsal (posterior) to those supplying extensors

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17
Q

There are 3 sources of input to an alpha-MN that regulate its activity. Name thresde

A

Central terminals of dorsal root ganglion cells, UMNs and spinal interneurons

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18
Q

What does muscle strength depend on?

A

Neuromuscular activation and force production by innervated muscles fibres

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19
Q

What affects neuromuscular activation?

A

The firing rates of the LMNs involved, the number of LMNs that innervate a muscle and the coordination of the movement (innervation of agonist vs antagonist)

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20
Q

What affects force production by innervated muscles fibres?

A

Fibre size (hypertrophy) and fibre phenotype (fast or slow contracting muscle)

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21
Q

Describe frequency summation of muscle fibre contraction i.e. what happens when the number of APs increases?

A

A single AP in an alpha-MN causes a muscle fibre to twitch. Summation of twitches causes a sustained contraction as the number of incoming APs increases

22
Q

Motor unit size varies from small (a few fibres) to large (hundreds to thousands of muscle fibres). Name an example of both a small motor unit and a large motor unit

A

Small - extraocular eye muscles

Large - leg muscles

23
Q

How do fast and slow skeletal muscle fibres differ?

A

They differ in how fast myosin ATPase splits ATP to provide energy for cross bridge cycling (reflected in time to develop peak tension)

24
Q

Slow-oxidative (type I) fibres use ATP derived from oxidative phosphorylation. Describe their function

A

Slow contraction and relaxation. Fatigue resistant

25
Q

Type I fibres are red. Why is this?

A

Due to high myoglobin content

26
Q

Fast type IIa fibres also use ATP derived from oxidative phosphorylation. Describe their function

A

Fast contraction and relaxation. Fatiguen resistant. Red and reasonably well vascularised

27
Q

Fast type IIb fibres use ATP derived from glycolysis. Describe their function

A

Fast contraction but NOT fatigue resistant, pale in colour and poorly vascularised (‘white meat’)

28
Q

There are 3 types of motor unit. Name these

A

Fast fatiguing, fatigue resistant and slow

29
Q

Describe fast fatiguing motor units

A

Very high tension, fast fatiguing, large alpha-MN, high threshold, type IIb fibres

30
Q

Describe fatigue resistant motor units

A

High tension, slow fatiguing, intermediate alpha-MN and threshold, type IIa fibres

31
Q

Describe slow motor units

A

Low tension, fatigue resistant, small alpha-MN, low threshold, type I fibres

32
Q

What is Henneman size principle?

A

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

33
Q

What component of a muscle registers the change in length of the muscle?

A

Muscle spindle

34
Q

What do muscle spindles consist of?

A

A fibrous capsule, intrafusal muscle fibres, sensory afferents (innervate the intrafusal muscle fibres) and gamma motor neuron (efferent that innervate the intrafusal fibres)

35
Q

What do intrafusal muscle fibres consist of?

A

A non-contractile equatorial region innervated by the Ia sensory neurons. Contractile polar ends that receive efferent input from gamma-MNs with cell bodies in the ventral horn of the spinal cord

36
Q

Stimulation of gamma-MNs causes what?

A

The spindle to contract

37
Q

Why are the alpha and gamma-MNs normally coactivated during voluntary movement?

A

So that the intrafusal muscle fibres contract in parallel with the extrafusal fibres. This serves the function of maintaining the sensitivity of the spindle to stop it going slack when the extrafusal fibres contract

38
Q

There are two morphological/functional types of intrafusal fibres, what are these?

A

Nuclear bag fibres and chain fibres. Bag fibre 1/dynamic = very sensitive to rate of change of muscle length, innervated by dynamic, but not static, gamma-MNs. Bag fibre 2/static = more sensitive to the absolute length of the muscle, innervated by static gamma-MNs. Chain fibres are sensitive to the absolute length of the muscle and are innervated by static gamma-MNs

39
Q

Two types of afferent fibre innervate the intrafusal fibres. Name these

A

Ia afferents form a primary annulospiral nerve ending winding around the centre of all intrafusal fibres (more sensitive to rate of change of the intrafusal fibres).
II fibres, which are more slowly conducting, form flower spray endings on all intrafusal fibres except the bag 1 dynamic type (more sensitive to absolute length of the intrafusal fibres)

40
Q

In which kind of activities would only static gamma-MNs be active?

A

Activities in which muscle length changes slowly

41
Q

When would dynamic gamma-MNs be active?

A

During behaviours in which muscle length changes rapidly and unpredictably

42
Q

Where are Golgi tendon organs located

A

The junction of muscle and tendon

43
Q

What is the innervation of Golgi tendon organs?

A

Innervated by group Ib sensory afferents

44
Q

What is the function of Golgi tendon organs?

A

Act to regulate muscle tension to protect muscle from overload and more generally regulate muscle tension to an optimal range

45
Q

Where do group Ib afferent synapse?

A

They enter the spinal cord and synapse upon inhibitory interneurons which in turn synapse upon the alpha motor neurons of the homonymous muscle forming the basis of the reverse myotatic reflex

46
Q

Golgi tendon organs gauge the force generated by measuring the tension in its tendon. Why is this thought to be important?

A

This type of proprioceptive feedback is thought to be important for the proper execution of fine motor acts - fragile objects require a steady, but not too powerful, a grip

47
Q 
Free nerve endings, found in capsule and connective tissue are:
A - high threshold, rapidly adapting
B - low threshold rapidly adapting
C - high threshold, slowly adapting
D - low threshold, slowly adapting?
A

C - HT, SA - they have a nociceptive function

48
Q

Golgi type endings, found only in ligaments are low threshold, rapidly adapting. (T/F)

A

False - they are slowly adapting, high threshold and have a protective role

49
Q

Paciniform endings, found in periosteum near the articular attachments and the fibrous part of the joint capsule are low threshold, slow adapting (T/F)?

A

True :) - they are acceleration detectors

50
Q

Ruffini endings, found mainly in joint capsule are low threshold, rapidly adapting (T/F)

A

False - low threshold, slowly adapting - static position and speed movements

Neuro Physiology (9 decks)

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