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Year 2 Term 3 Control > Sensory and motor innervation of muscle > Flashcards

Sensory and motor innervation of muscle Flashcards

1
Q

The descending motor system is not a linear system, from what 2 places can motor signals originate, what is each involved in?

A

1) Motor cortex - planning, initiating and directing voluntary movements
2) Brainstem centers - basic movements and postural control

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2
Q

Which 2 parts of the brain can give inputs to the motor cortex and brainstem centers where motor signals originate, what is the role of each?

A

1) Basal ganglia - has a role in dampening down the level of movements we carry out (called proper initiation of movement)
2) Cerebellum - has a role in sensory motor coordination, making sure the correct movement has been carried out and coordinating patterns of movement

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3
Q

By which 2 circuits can UMNs make contact with LMNs?

A

1) Directly - straight from UMNs to LMN pools

2) Indirectly - via local circuit neurones - interneurones - involved in reflex coordination

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4
Q

What are the 3 basic roles of the brain in terms of the motor system?

A

1) Initiation
2) Integration
3) Coordination

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5
Q

What are the 2 basic roles of the spinal cord in terms of the motor system?

A

1) Simple reflexes

2) Pattern generation

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6
Q

What is the clinical consequence of the route taken by UMNs to reach LMNs?

A

Those which go via local circuits - ie interneurones - have a greater capacity for neuronal plasticity and thus recovery of function if damage occurs

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7
Q

What are the 4 brain stem centers involved in the motor system?

A

1) Red nucleus
2) Reticular formation
3) Vestibular-nuclear complex
4) Also houses the LMNs responsible for movements of head and neck

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8
Q

The motor cortex is mainly confined to which 2 Brodmann areas?

A

4 & 6

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9
Q

The primary motor cortex is found within which lobe?

A

Frontal

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10
Q

What is the role of the motor cortex?

A

Responsible for planning and initiating voluntary movement

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11
Q

Upper motor neurons are what kind of cells?

A

Pyramidal cells

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12
Q

The pyramidal cells which are the UMNs are located within which of the 6 layers of the cortex?

A

Located within the 5th layer

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13
Q

Via which 2 tracts do UMN axons descend?

A

1) Corticobulbar

2) Corticospinal

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14
Q

Motor signals to below the neck are carried within which tract?

A

Corticospinal

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15
Q

Motor signals to the head, neck and face are carried within which tract?

A

Corticobulbar

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16
Q

What is the other name for the corticospinal tract?

A

Pyramidal tract

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17
Q

Where in the spinal cord do UMNs terminate either directly or indirectly via interneurones on the LMNs?

A

In the ventral horn

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18
Q

In the medulla 95% of corticospinal fibres cross the midline, why do 5% continue to descend ipsilaterally?

A

It is thought to be because postural muscles require input to both sides in order for left and right postural muscles to act in a coordinated fashion thus you need corticospinal fibres running down both sides

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19
Q

The 95% of UMNs which cross the midline to travel contralaterally do so within which tract?

A

Lateral/ posterior CST

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20
Q

The 5% of UMNs which continue to descend ipsilaterally do so within which tract?

A

Anterior/ ventral CST

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21
Q

LMNs are localised within which lamina of the ventral grey horn?

A

Lamina IX

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22
Q

What are the 2 types of LMNs?

A

1) Alpha motor neurons

2) Gamma motor neurons

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23
Q

How many axons is each muscle fibre innervated by?

A

A single axon

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24
Q

Does each LMN give off a single axon?

A

No - give off many collaterals - each motor neuron can give off many different axons

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25
Which type of LMNs are directly responsible for the generation of force by muscle?
Alpha motor neurones
26
What is a muscle fibre composed of?
Each muscle fibre is composed of fibrils which are composed of sarcomeres
27
Why is damage to LMNs often catastrophic?
Because the only route by which impulses can reach muscles is via LMNs
28
How does the size of lamina IX within the ventral grey horn vary?
Varies rostro-caudally, the size variation reflects demands of peripheral musculature and its innervation Lamina IX is largest within the cervical and upper lumbar segments which control upper and lower limb musculature
29
How many alpha motor neurons are there estimated to be on each side of the cord?
150,000
30
What is meant by the motor neuron pool?
The motor neuron pool of a muscle is all the motor neurons which innervate that single muscle
31
What is the distribution of a motor neuron pool within the spinal cord?
Each motor neuron pool extends rostro-caudally along the spinal cord axis
32
How would the motor neuron pools for a powerful muscle such as gastrocnemius and a more postural muscle such as soleus vary?
There motor neuron pools would lie close to each other as they come from similar spinal cord levels (Gastrocnemius is L5-S2 and soleus is L4-L7) but gastrocnemius' pool is likely to be larger as it is a more powerful muscle
33
How does the segmental distribution of LMNs in the ventral grey horn vary depending on position and function of peripheral muscle?
1) Motor neurons that control flexors lie dorsal to those which control extensors 2) Motor neurons that control axial muscles lie medial (closer to the spinal canal) to those which control more distal muscles
34
What is a motor unit?
It is comprised of the alpha motor neuron and all of the muscle fibres that it innervates As each motor neuron gives off more than one axon (and each muscle fibre is innervated by one axon) then each motor neuron innervates more than one muscle fibre so motor units contain more than one muscle fibre
35
What are the 3 sizes of motor unit, give an example of each?
1) Small motor unit - ratio of 3 muscle fibres per single alpha motor neurone eg. extraocular muscles of the eye (need fine movement) 2) Medium motor unit - ratio of 180 muscle fibres per single alpha motor neurone eg. soleus muscle 3) Large motor unit - ratio of 1000-2000 muscle fibres per alpha motor neurone eg. gastrocneumius (need less fine but more powerful movement)
36
How does the size of motor neurons vary between different sized motor units?
Small motor units = small motor neurons | Large motor units = large motor neurons
37
In muscles with motor units of more than one size, in what order are motor units recruited and why?
Small motor units are recruited before large motor units to get an increase in force
38
Motor units vary in the type of muscle fibres they innervate, what are the 3 types of motor units in terms of force and fatigability?
1) Slow motor units 2) Fast fatigable motor units 3) Fast fatigue-resistant motor units
39
Slow motor units, innervate what kinds of muscle fibres with what kinds of adaptations?
Slow motor units are composed of a small motor units and small red muscle fibres. These red muscle fibres generate low force but are resistant to fatigue. They are red because: 1) They have high levels of myoglobin 2) Good vascular supply to bring O2 and take away wast products ie CO2 to avoid acidosis 3) High levels of mitochondira All the above lead to resistance to fatigue
40
Give an example of a type of muscle which would contain lots of slow motor units?
Postural muscles - dont need to produce high force but need to be resistant to fatigue
41
Fast fatiguable motor units are composed of what types of muscle fibre with what adaptations?
Fast fatigable motor units are composed of large motor units and larger paler muscle fibres that generate large forces, pale muscle fibres are fatiguable as: 1) They contain lower amounts of myoglobin 2) Contain less mitochondria Thus they resort to anaerobic respiration more readily so are fatigued by lactic acidosis
42
Give an example of a muscle containing many fast fatigable motor units?
Biceps muscle - generates large amounts of force over short periods of time
43
What is the role of fast fatigue resistant motor units?
They are an intermediate type and can produce moderate force and some fatigue
44
How does the percentage of maximum force generated differ between 3 types of motor unit?
Slow motor units - produce 100% of maximum force over a sustained period of time but their maximum force is not that high Fast fatigable motor units - maximum force is v high but only produce 100% maximum force for a short period of time before it declines Fast fatigue-resistant motor units - maximum force is moderate, can produce 100% maximum force for a moderate length of time before it reduces
45
How can motor unit activity be recorded?
Using electromyography (EMG) - place a fine gauge needle into the selected muscle to pick up the electrical activity of the motor unit
46
How does the appearance of the electromyograph vary with mild voluntary effort to maximal effort?
1) Mild voluntary effort - fewer peaks 2) Moderate effort - more peaks and motor units recruited following the size principle - ie. smaller followed by larger 3) Maximal effort - Many more peaks
47
How is a clinical diagnosis made using an electromyograph?
Degradation in the EMG or spontaneous or asynchronous activity forms the basis for clinical diagnosis
48
Can complete fused tetanus be achieved in one motor unit in vivo?
No - cannot reach the level of Hz needed. Only unfused tetanus can be achieved in each individual motor unit
49
Why can a smooth muscle contraction still be achieved despite the fact that fused tetanus cannot be achieved in individual motor units in vivo?
Because different motor units are being recruited at slightly different time, so the fluctuation in unfused tetanus are at slightly different points and the overall effect of all the motor units working together is smooth muscle contraction
50
What is meant by tetanus in muscles?
Each AP produces a muscle twitch, in tetanus muscle fibres are innervated by the next AP before they have chance to relax leading to a maintenance of tension
51
Name the 3 main sources of inputs to the alpha motor neurons?
1) Sensory inputs from peripheral proprioceptors 2) Local inputs from spinal interneurons 3) Descending inputs from UMNs
52
Which 3 kinds of proprioceptor is found within the joint capsule?
Ruffini-like proprioceptor Pacinian-like proprioceptor Bare nerve endings
53
What is the role of the proprioceptors found within the joint capsule?
Protective function against hyperextension
54
Which type or proprioceptor is found in tendons?
Golgi tendon organ
55
What do golgi tendons detect?
Muscle tension
56
The muscle spindle is made up of what type of muscle fibres?
Intra-fusal muscle fibres - ie. within a capsule
57
What do muscle spindles detect?
Change in muscle length
58
What is the function of muscle spindles?
Enable detection of body position in space and enables regulation of muscle contraction and precisely matches force generated to the task
59
What is the role of muscle spindle in maintaining muscle tone?
Interaction between muscle spindles and alpha motor fibres ensures the muscle is always under some degree of stretch - this is muscle tone
60
What are the 2 main classes of muscle spindle?
1) Chain spindles | 2) Bag spindles
61
What is the difference between chain and bad muscle spindles?
- Chain spindles innervated by Ia and II afferent fibres and they encode the static response of the fibre - ie they continue firing for as long as the muscle is stretched - Bag spindles are innervated by Group Ia afferents and only encode mainly the dynamic response of the fibre ie. the rate of change during dynamic stretch
62
Extrafusal muscle fibres receive innervation from what type of motor neurones?
Alpha motor neurones
63
Intrafusal fibres have a sensory function and do not generate tension but they have to receive some motor innervation, which type of motor neurons provide this motor innervation?
Gamma motor neurones
64
Describe the mechanism of muscle contraction and the role of gamma motor neurons in ensuring sensitivity of muscle spindles?
1) Alpha MN cause extrafusal fibres to contract 2) Intrafusal fibres not innervated by alpha MN so dont contract, the intrafusal fibres thus lose tension and sensitivity to changes in muscle length as muscle spindle collapses 3) This is corrected by the gamma motor neuron which matches the length of the intrafusal fibres to the extrafusal fibres thus restoring the sensitivity of the muscle spindle to the muscle length
65
What kind of receptor is the golgi tendon organ?
Mechanoreceptor
66
What is the rough structure of golgi tendon organ?
Axons of the Ib afferent neurons are intertwined with the collagen fibrils of the tendon
67
What do golgi tendon organs detect?
change in muscle tension as when the muscle contracts the force acts directly on the tendon, acts like a strain gauge ie monitors muscle tension and the force of contraction
68
What kind of fibres are golgi tendon organs innervated by?
Ib afferents
69
What is the difference in the firing of golgi tendon organs and muscle spindles during isometric and isotonic contraction?
During isometric contraction (tension without a change in muscle length) - Ib (golgi tendon organ) is firing but Ia (muscle spindles are not) During isotonic contraction (change in muscle length with no change in tension) - the Ia axon (muscle spindle) is active and Ib axon (golgi tendon organ) is not
70
What are the relative conduction velocities of Ia, Ib and II afferent nerve fibres, what is this related to?
Ia>Ib>II | Large diameter and greater myelination
71
Group Ia afferents innervate what receptors involved in proprioception?
Both bag and chain spindles
72
Group II afferent nerve fibres innervate which receptors involved in proprioception?
Chain spindles
73
Group Ib afferent nerve fibres innervate which receptors involved in proprioception?
Golgi tendon organ
74
What is the babinski sign?
To do with plantar response When sharp object scraped along the plantar surface of the foot the toes should plantar flex Babinski's sign is when the toes instead dorsiflex and span out This is a symptoms of upper motor neurone syndrome NB. it also occurs in babies for the first few days of life
75
Damage to descending motor pathways gives rise to what syndrome?
Upper motor neurone syndrome
76
After several days what are the symptoms of upper motor neurone syndrome? 4
1) Babinski sign 2) Spasticity 3) Hyporeflexia 4) Loss of fine movements
77
Why is it thought that fine movements are lost in damage to descending motor pathways?
Fine movement signals travel via UMN which make direct contact with LMNs - thus no involvement of interneurones so less capacity for neuronal plasticity and recovery of function than the less fine movements whcih travel via interneurones
78
What disease is caused by damage to lower motor neurones?
Amyotrophic lateral sclerosis/ Motor neurone disease
79
What are the symptoms of motor neurone disease?
1) In early stages weakness of muscles and atrophy 2) Inexorable decline with loss of all voluntary movement (walking, speaking, swallowing, breathing) 3) No loss of cognitive function
80
What kind of neurons degenerate in motor neuron disease?
Progressive alpha neuron degeneration

Year 2 Term 3 Control (11 decks)

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