Motor units and muscle spindles

Question 1

Where can upper motor neurones be found?

Answer 1

Brain

Question 2

Where can lower motor neurones be found?

Answer 2

Soma of brain stem and ventral horn of spinal cord

Question 3

What is the relationship between upper and lower motor neurones?

Answer 3

UMN supply input to LMN to modulate their activity

Question 4

From which sources do LMNs receive input?

Answer 4

UMN
Proprioception
Interneurons

Question 5

What are the different types of LMNs?

Answer 5

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

Question 6

Where are the spinal cord enlargements?

Answer 6

Cervical (C3-T1)

Lumbar (L1-S3)

Question 7

What is a motor unit?

Answer 7

An alpha motor unit and all of the skeletal muscle it innervates

Question 8

What is a motor neurone pool?

Answer 8

The collection of alpha motor neurones that innervate a single named muscle

Question 9

What determines the force of muscle contraction?

Answer 9

Frequency of action potential discharge of the alpha motor neurone
Recruitment of additional, synergistic motor units

Question 10

Where can the cell bodies of LMNs be found?

Answer 10

Ventral horn

Question 11

Where can LMNs that innervate axial muscles be found?

Answer 11

Medial to those innervating distal muscles

Question 12

Where can MNs supplying flexors be found?

Answer 12

Dorsal to those supplying extensors

Question 13

What are the sources of input to an alpha motor neurone?

Answer 13

Central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
UMNs in motor cortex and brain stem
Spinal interneurones

Question 14

What does muscle strength depend on?

Answer 14

Activation of muscle fibres

Force production by innervated muscle fibres

Question 15

What does activation of muscle fibres depend on?

Answer 15

Firing rate of LMNs (force of motor unit increases to a max as a function of LMN firing frequency)
Number of LMNs that are simultaneously active
Co-ordination of movement

Question 16

What does the force production of innervated fibres depend on?

Answer 16

Fibre size - hypertrophy

Fibre phenotype - fast or slow contracting muscle

Question 17

What does a single action potential in an alpha motor neurone result in?

Answer 17

Twitch in muscle fibre

Question 18

What does a sustained twitching of muscles result in?

Answer 18

Tetany

Question 19

Describe the variable sizes of motor units

Answer 19

Small - extraocular eye muscles

Large - postural antigravity muscles

Question 20

Describe the relationship between the size of a cell body of an alpha motor neurone and excitability

Answer 20

Smaller cell bodies are more excitable

Question 21

What is each muscle fibre innervated by?

Answer 21

Single motor axon at endplate (neuromuscular junction) which is usually at the centre of the fibre

Question 22

What is the difference between fast and slow muscle fibres?

Answer 22

Alpha motor neurones innervating fast type tend to be larger and have faster conductive axons than those of slow units

Question 23

What determines a slow and fast twitch muscle?

Answer 23

Differs in how quickly myosin ATPase splits ATP to provide energy for the cross bridge formation
Expression of different myosin heavy chains

Question 24

Describe a slow oxidative type 1 muscle fibre

Answer 24

ATP derived from oxidative phosphorylation
Slow contraction and relaxation
Fatigue resistant
Red fibres due to high myoglobin content

Question 25

Describe a fast type 2a muscle fibre

Answer 25

ATP derived from oxidative phosphorylation Fast contraction and relaxation Fatigue resistant Red and well vascularised

Question 26

Describe a fast type 2b muscle fibre

Answer 26

ATP derived from glycolysis Fast contraction but not fatigue resistant Pale in colour and poorly vascularised White

Question 27

What are the 3 types of motor units?

Answer 27

Fast fatiguing Fatigue resistant Slow

Question 28

Describe a fast fatiguing motor unit

Answer 28

``` High tension Large alpha motor neurone High threshold Type 2b fibres Utilised in burst power ```

Question 29

Describe a fatigue resistant motor unit

Answer 29

``` High tension Slow fatiguing Intermediate alpha motor neurone Intermediate threshold Type 2a fibres Utilised in sustained locomotion ```

Question 30

Describe a slow motor unit

Answer 30

``` Low tension Fatigue resistant Small alpha motor neurone Low threshold Type 1 fibres Utilised in antigravity and sustained movement ```

Question 31

What is the henneman size principle?

Answer 31

Smaller alpha motor neurones (slow motor units) have a lower threshold than larger ones Slow motor units are more easily activated and trained by any training that activates the muscle

Question 32

Describe what occurs with the activation of an UMN

Answer 32

LMN excited

Question 33

Describe muscle fibre recruitment

Answer 33

Motor units (LMN and muscle fibres) are recruited in order of size i.e. progressively increasing - small LMNs are more easily excited than large ones

Question 34

What does this activation in terms of increasing size of LMNs allow for?

Answer 34

Fine control of muscle force across a wide range of tensions developed

Question 35

Which will be recruited first, type 1 or type 2 fibres?

Answer 35

Slow type 1 before fast fatigue resistant type 2a which comes before type 2b This results in increasing increments towards the maximal force the muscle exerts

Question 36

What detects changes in length and rate of change of a muscle?

Answer 36

Muscle spindles

Question 37

What is contained within the sensory organ the muscle spindle?

Answer 37

Fibrous capsule Intrafusal muscle fibres Sensory afferents; 1a class Gamma motor neurone efferents which innervate intrafusal fibres

Question 38

Describe a 1a sensory afferent

Answer 38

Myelinated | Very fast conduction

Question 39

What do extrafusal fibres generate?

Answer 39

Muscle force

Question 40

Which neurotransmitter mediated the myotatic reflex?

Answer 40

Glutamate

Question 41

Which muscles is the monosynaptic myotatic reflex most prominent in?

Answer 41

Extensor muscles

Question 42

Which spinal levels mediates the biceps reflex?

Answer 42

C5-6

Question 43

Which spinal level mediates the supinator reflex?

Answer 43

C5-6

Question 44

Which spinal level mediates the triceps reflex?

Answer 44

C7

Question 45

Which spinal levels mediate the quads (knee) reflex?

Answer 45

L3-4

Question 46

Which spinal levels mediate the gastrocnemius (ankle) reflex?

Answer 46

S1

Question 47

What are intrafusal fibres?

Answer 47

Non contractile equatorial regions innervated by 1a sensory afferents Contractile polar ends that receive efferents from gamma motor neurones

Question 48

Where are gamma motor neurones cell bodies?

Answer 48

Ventral horn of spinal cord

Question 49

What will cause the intrafusal fibres of the muscle spindles to contract?

Answer 49

Stimulation of gamma motor neurones

Question 50

What are the different types of nuclear bag fibres?

Answer 50

Bag 1/dynamic: very sensitive to the rate of change of muscle length Bag 2 / static: very sensitive to the absolute length of muscles

Question 51

What innervates bag 1 nuclear fibres?

Answer 51

Dynamic gamma motor neurones

Question 52

What innervates bag 2 nuclear fibres?

Answer 52

Static gamma motor neurones

Question 53

What are chain fibres?

Answer 53

Sensitive to absolute length

Question 54

What innervates chain fibres?

Answer 54

Static gamma motor neurones

Question 55

What are the 2 types of afferent fibres that innervate the intrafusal fibres?

Answer 55

1a | 2

Question 56

Which nerve ending will 1a afferents form?

Answer 56

Annulospiral nerve ending that winds around the centre of all intrafusal fibres

Question 57

Which nerve ending will 2 fibres form?

Answer 57

Flowerspray endings on all intrafusal fibres EXCEPT bag 1 dynamic type

Question 58

What will 1a fibres respond to?

Answer 58

Rate of change of muscle length (dynamic) and absolute length (steady state)

Question 59

What will stimulation of the static gamma fibre result in?

Answer 59

Steady state response

Question 60

What will stimulation of the dynamic amma fibre result in?

Answer 60

Dynamic response to stretch

Question 61

When will static gamma motor neurones be active?

Answer 61

In activities in which muscle length changes slowly and predictably

Question 62

When will dynamic gamma motor neurones be active?

Answer 62

In activities in which muscle length changes rapidly and unpredictably

Question 63

Where are golgi tendon organs found?

Answer 63

Junction of muscle and tendon

Question 64

What will golgi tendon organs monitor?

Answer 64

Changes in muscle tension

Question 65

Are golgi tendon organs in series or parallel to extrafusal fibres?

Answer 65

In series | Muscle spindles are parallel

Question 66

What innervates golgi tendon organs?

Answer 66

Group 1b sensory afferents

Question 67

What will golgi tendon organs protect the muscles from?

Answer 67

Overload e.g. weight lifting | Regulates muscle tension to an optimal range

Question 68

Where will group 1b afferents enter the spinal cord and synapse?

Answer 68

Upon inhibitory interneurons which will synapse on alpha motor neurones of homonymous muscle forming the reverse myotatic reflex

Question 69

Which reflex does the golgi tendon organ mediate?

Answer 69

Inverse myotatic reflex; tells a contracting muscle to relax

Question 70

Where can proprioceptive axons be found?

Answer 70

Connective tissue of joints (joint capsules and ligaments)

Question 71

What will proprioceptive axons respond to?

Answer 71

Changes in angle, direction and velocity of movement of a joint Prevents excessive flexion and extension

Question 72

Where will proprioceptive information arise from?

Answer 72

Muscle spindles Golgi tendon organs Joint receptors

Question 73

Describe the golgi tendon pathway

Answer 73

Activation of 1b afferents from tendon organ Excitation of inhibitory spinal interneuron Inhibition of alpha motor neurone supplying homonymous muscle Relaxation of muscle

Question 74

What does activation of gamma motor neurones along with alpha motor neurones during voluntary movement prevent?

Answer 74

Slackening as the body of the muscle contract due to alpha motor neurones activation - allowing continued signalling by the spindle