Motor System

Question 1

What is the difference between upper and lower motor neuron?

Answer 1

Upper - brain, supply input to LMN

Lower - soma in brainstem and ventral horn of spinal cord

Question 2

Where do lower motor neurons receive input from?

Answer 2

• UMN
• proprioceptors
• interneurons

Question 3

What are the two types of lower motor neuron?

Answer 3

alpha - innervate the bulk of fibres that generate force

gamma - innervate a sensory organ (muscle spindle)

Question 4

State the difference between axial, proximal and distal muscles

Answer 4

Axial - movements of the trunk
Proximal - shoulder, elbow, pelvis
Distal - hands, feet and digits

Question 5

What causes the two enlargements of the spinal cord?

Answer 5

Uneven distribution of motor neurons particularly distal and proximal ones

Question 6

Where are the two spinal cord enlargements?

Answer 6

Cervical - C3-T1

Lumbar - L1-S3

Question 7

What is a motor unit?

Answer 7

All the fibres supplied by one alpha motor neuron

Question 8

What is a motor neuron pool?

Answer 8

A collection of alpha motor neurons that innervate a single muscle

Question 9

Name two things force of contraction depends on

Answer 9

• frequency of AP discharge of alpha motor neuron

- recruitment of additional synergistic motor units

Question 10

How are the LMN arranged?

Answer 10

LMN innervating axial muscles are medial to those that innervate distal muscles
Flexors are dorsal to extensors

Question 11

What does activation of muscle fibres depend on?

Answer 11

• firing rate of LMNs
• number of LMNs active
• co-ordination of movement

Question 12

How does a muscle movement arise?

Answer 12

Summation of twitches causes a sustained contraction as the number of APs increases

Question 13

What differentiates slow and fast fibres?

Answer 13

Differ in how quickly myosin ATPase splits ATP to provide fro cross bridge cycling

Question 14

Describe slow type 1 fibres

Answer 14

ATP largely derived from oxidative phosphorylation, slow contraction and relaxation means it is fatigue resistant
High myoglobin - red fibres
Low tension, small alpha motor neurons, low threshold for antigravity sustained movement

Question 15

Describe type IIa fibres

Answer 15

ATP derived from oxidative phosphorylation, fast contraction and relaxation but fatigue resistant
Red and reasonably well vascularised
High tension, intermediate alpha motor neuron and threshold for sustained locomotion

Question 16

Describe type IIb/x fibres

Answer 16

ATP derived from glycolysis, fast contraction and susceptible to fatigue
Pale and poorly vascularised
High tension, large alpha motor neurons and threshold for burst power

Question 17

What type of fibre does not occur in mammals?

Answer 17

IIb

Question 18

In what order are alpha motor neurons recruited?

Answer 18

In order of size, allows fine control of muscle force across a range of tensions

Question 19

Name the parts of a muscle spindle

Answer 19

• fibrous capsule
• intrafusal muscle fibres
• sensory afferents (Ia fast conducting)
• gamma motor neurons

Question 20

Describe the monosynaptic reflex

Answer 20

1. Stretch of muscle spindle
2. Activation of Ia afferents
3. Excitatory synapse in spinal cord
4. Activation of alpha motor neuron
5. Contraction of homoymous muscle

Question 21

What is the excitatory synapse mediated by?

Answer 21

Release of glutamate

Question 22

Which level is the biceps reflex?

Answer 22

C5-6

Question 23

Which level is the supinator reflex?

Answer 23

C5-6

Question 24

Which level is the triceps reflex?

Answer 24

C7

Question 25

Which level is the quads reflex?

Answer 25

L3-4

Question 26

Which level is the gastrocnemius reflex?

Answer 26

S1

Question 27

Describe Jendrassik Manoevre

Answer 27

Ask the patient to interlock their fingers and pull hands apart to exaggerate the patellar reflex

Question 28

Name the two parts of intrafusal muscle fibres

Answer 28

- non-contractile equatorial region (Ia sensory neurons) | - contractile polar ends with efferent from gamma motor neurons with cell bodies in the spinal cord

Question 29

Name the two types of intrafusal muscle fibres

Answer 29

Nuclear bag fibres | Chain fibres

Question 30

Describe chain fibres

Answer 30

Sensitive to the absolute length of the muscle, innervated by static gamma motor neurons

Question 31

What are the two types of nuclear bag fibres?

Answer 31

1/dynamic | 2/static

Question 32

What is the difference between dynamic and static infrafusal fibres?

Answer 32

Dynamic - very sensitive to rate of change of muscle length innervated by dynamic gamma motor neurons Static - more sensitive to the absolute length of muscle, innervated by static gamma motor neurons

Question 33

State the two types of afferent fibres

Answer 33

Ia - form a primary annulospinal nerve ending winding around fibres IIa - more slowly conducting, form flowerspray endings on all fibres except bag 1

Question 34

In activities where muscle length changes slowly and predictably which motor neurons are active?

Answer 34

Static gamma motor neurons

Question 35

When are dynamic gamma motor neurons active?

Answer 35

Behaviours where muscle length changes rapidly and unpredictably

Question 36

Where are golgi tendon organs found?

Answer 36

Junction of muscle and tendon

Question 37

What is the function of golgi tendon organs?

Answer 37

Monitor changes in muscle tension in series to extrafusal fibres

Question 38

What innervates golgi tendon organs?

Answer 38

Ib sensory afferents

Question 39

Why do golgi tendon organs act to regulate tension?

Answer 39

- protect muscle from overload | - regulate tension to an optimal range

Question 40

How do group Ib afferents enter the spinal cord?

Answer 40

Inhibitory interneurons that synpase onto alpha motor neurons

Question 41

Describe the pathway of the reverse myotatic reflex

Answer 41

Tendon - Ib afferent to posterior horn Inhibitory interneuron to anterior horn Alpha motor neuron to muscle

Question 42

Where are proprioceptive axons found?

Answer 42

In connective tissue of joints

Question 43

What is the function of proprioceptive axons?

Answer 43

Prevent excessive flexion/extension

Question 44

What structures help with proprioception?

Answer 44

- free nerve endings in capsule connective tissue - paciniform endings in periosteum near the articular attachments & fibrous joint capsule - Ruffini endings mainly in joint capsule

Question 45

Where do spinal interneurons receive input from?

Answer 45

- primary sensory axons - descending axons from brain - LMN collateral branches - other interneurons

Question 46

What do inhibitory interneurons mediate?

Answer 46

Inverse myotatic response | Reciprocal inhibition between extensor and flexor muscles

Question 47

Describe reciprocal inhibition

Answer 47

Myotatic reflex causes the extensor muscle to contract (quads) but the antagonist flexor (hamstring) must relax simultaneously to extend the leg Ia fibres synapse with inhibitory interneuron which synapses with antagonist muscle

Question 48

How do descending pathways contribute to reciprocal inhibition?

Answer 48

Inhibit the alpha motor neuron supplying the flexor to allow unopposed extension

Question 49

What do excitatory interneurons mediate?

Answer 49

- the flexor reflex | - the crossed extensor reflex

Question 50

Describe the flexor reflex

Answer 50

Noxious stimulus causes limb to flex by contraction of flexor muscles and relaxation of extensor muscles

Question 51

Describe the crossed extensor reflex

Answer 51

Noxious stimulus causes limb to extend by contraction of extensor and relaxation of flexor