Motor units, muscle spindles, golgi tendon organs and joint receptors Flashcards
1
Q
What does the somatic motor system consist of?
A
- Skeletal muscles + elements of the nervous system controlling them.
2
Q
What is the neural element of the motor system comprised of?
A
- Upper motor neurones within the brain.
- Lower motor neurones with soma within the brain stem and ventral horn of the spinal cord.
3
Q
UMNs supply input to LMNs to modulate what?
A
LMN activity.
4
Q
LMNs receive input from UMNs and what else?
A
- Proprioceptors.
- Interneurons.
5
Q
LMNs command what?
A
Muscle contraction.
6
Q
LMNs form what pathway?
A
The “final common pathway”.
7
Q
LMNs comprise what?
A
- Alpha motor neurones.
- Gamma motor neurones.
8
Q
What is the function of alpha motor neurones of LMNs?
A
Innervate most fibres that generate force within a muscle.
9
Q
What is the function of gamma motor neurones of LMNs?
A
Innervate the muscle spindle - a sensory organ within the muscle.
10
Q
Biceps brachii and brachialis may be described as?
A
Synergists.
11
Q
Triceps brachii and anconeous may be described as?
A
Synergistic muscles.
12
Q
Biceps brachii and brachialis work as what?
A
Flexors of the elbow.
13
Q
Triceps brachii and anconeus work as what?
A
Extensors of the elbow.
14
Q
Axial muscles control movement of the what?
A
Trunk.
15
Q
What is the function of axial muscles?
A
To maintain posture.
16
Q
Where are proximal muscles found?
A
- Shoulder.
- Elbow.
- Pelvis.
- knee.
17
Q
What is the function of proximal muscles?
A
To mediate locomotion.
18
Q
Distal muscles move what?
A
- Hands.
- Feet.
- Digits (allow fine manipulation of objects).
19
Q
Axons of LMNs exit the spinal cord via?
A
Ventral roots or cranial nerves.
20
Q
Each ventral root joins with a dorsal root to form what?
A
A mixed spinal nerve.
21
Q
A mixed spinal nerve composed of both a dorsal and ventral root contains what fibres?
A
- Sensory fibres.
- Motor fibres.
22
Q
Motor neurones/motoneurones belong to a what?
A
A spinal segment.
23
Q
How are motor neurones distributed in the spinal cord?
A
Unequally.
24
Q
Where are the greatest numbers of motor neurones found in the spinal cord?
A
- Cervical enlargement (C3-T1).
- Lumbar enlargement (L1-S3).
25
At what spinal levels is the cervical enlargement found?
C3-T1.
26
At what spinal levels is the lumbar enlargement found?
L1-S3.
27
The cervical enlargement mostly supplies what?
The arm.
28
The lumbar enlargement mostly supplies what?
The leg.
29
Motor neurones innervating distal and proximal musculature are mainly found in which segments of the spinal cord?
- Cervical.
| - Lumbar-sacral.
30
Motor neurones innervating the axial musculature mainly occur at which segments of the spinal cord?
All levels.
31
What is a motor unit?
An alpha-motor neuron and all the skeletal muscle fibres it innervated.
32
What is the smallest functional component of the motor system?
Motor unit.
33
Collection of alpha-motor neurones innervating a single muscle is termed what?
A motor neurone pool.
34
Force of muscle contraction is graded by alpha-motor neurones by what two principle mechanisms?
- Frequency of AP discharge of a-motor neurone.
| - Recruitment of additional, synergistic motor units.
35
Describe the "twitch" caused by AP discharge of an alpha-motor neuron.
A rapid sequence of contraction follow by relaxation.
36
Cell bodies of LMNs show a distinct somatotropic distribution where?
In the ventral horn.
37
Where are LMNs innervating axial muscles found in relation to those innervating distal muscles?
Medially.
38
Where are LMNs innervating flexors found in relation to those supplying extensors?
Dorsally.
39
What three sources of input to an alpha-motor neuron regulate its activity?
- Central terminal of dorsal root ganglion cells whose axons innervate muscle spindles.
- UMNs in the motor cortex and brain stem.
- Spinal interneurones.
40
Muscle strength is dependent on what?
Neuromuscular activation.
41
What features of neuromuscular activation impact muscle strength?
- (involved) LMN firing rate.
- Number of LMNs (motor units) innervating a muscle.
- Co-ordinated movement i.e. agonist vs antagonist, integration/control of reflex responses.
42
Force production by innervated muscle fibres is dependent on what?
- Fibre size (hypertrophy).
| - Fibre phenotype ("fast" or "slow" contracting muscle).
43
A single AP in an alpha-motor neuron causes what?
A muscle fibre to twitch.
44
Summation of muscle fibre twitches causes what?
- Sustained contraction.
45
Why does summation of muscle fibre twitches cause sustained contraction?
Increasing number of incoming action potentials.
46
What is the function of small motor units?
Fine movements e.g. extra-ocular eye muscles.
47
Give a function of large motor units.
E.g. large postural (antigravity) muscles - leg muscles.
48
Small motor units are innervated by what size of alpha-motor neurones?
Small alpha-motor neurons.
49
Large motor units are innervated by what size of alpha-motor neurones?
Large alpha-motor neurons.
50
Force of contraction that each motor unit produces is dependent on what?
Its size.
51
Alpha-motor neurons innervating fast-type muscle fibres tend to be what and have what?
- Larger.
- Faster conducting units.
(Than those of slow units).
52
Each muscle fibre innervation occurs via what?
A single motor axon at endplate (NMJ), usually at the centre of the fibre.
53
How do fast and slow muscle fibres differ (generally)?
In the speed of myosin ATPase splitting ATP to provide energy for cross bridge cycling.
54
In slow-oxidative Type I muscle fibres, ATP is derived from what?
Oxidative phosphorylation.
55
Describe the action of slow-oxidative (type-I) muscle fibres.
Fatigue resistant fibres performing slow contraction and relaxation.
56
Why are slow-oxidative fibres red?
High myoglobin content.
57
In fast type-IIa muscle fibres, ATP is derived from what?
Oxidative phosphorylation.
58
Describe the action of fast type-IIa fibres.
Fatigue resistant fibres performing fast contraction and relaxation.
59
How does vascularisation differ between fast type-IIa and IIb/x muscle fibres?
Type-IIa: (red) reasonably well vascularised.
Type-llb: (pale) poorly vascularised.
60
Describe the action of fast type-IIb/x fibres.
Non-fatigue resistant muscle fibres performing fast contraction.
61
In fast type-IIb/x muscle fibres, ATP is derived from what?
Glycolysis.
62
What are the three types of motor unit?
- Fast fatiguing.
- Fatigue resistant.
- Slow.
63
Describe fast fatiguing motor units.
- Very high tension.
- Fast fatiguing.
- Large alpha-motor neurons of high threshold.
- Type IIb/x fibres.
64
Describe fatigue resistant motor units.
- High tension.
- Slow fatiguing.
- Intermediate alpha-motor neurons and threshold.
- Type IIa fibres.
65
Describe slow motor units.
- Low tension.
- Fatigue resistant.
- Small alpha-motor neurons of low threshold.
- Type I fibres.
66
Which motor units are more easily activated and "trained" by training that activates the muscle?
Slow motor units.
67
Which motor units are associated with muscles of fine gradation of force?
Small motor units.
68
Which motor units are associated with muscles of coarse gradation of force?
Large motor units.
69
Recruitment of alpha-motor neurons by size allows for what?
Fine and graded development of muscle force.
70
Which reflex is being described? When a skeletal muscle is pulled, it pulls back.
Myotatic reflex.
71
What sensory organ registers change in length within a muscle?
Muscle spindle.
72
What does a muscle spindle consist of?
- Fibrous capsule.
- Intrafusal muscle fibres.
- Sensory afferents.
- Gamma-motor neuron efferents.
73
What is the function of sensory afferents in a muscle spindle?
To innervate intrafusal muscle fibres.
74
What is the function of gamma motor neurone efferents in a muscle spindle?
To innervate intrafusal fibres.
75
Striking of the paterllar tendon with a tendon hammer elicits what?
A knee jerk reaction underpinned by the myotatic reflex.
76
Describe the myotatic reflex (basic).
It is a monosynaptic reflex arc. Rapid and most prominent in extensor muscles.
77
Describe the myotatic reflex in detail.
1. Muscle spindle is stretched.
2. Ia afferent activated.
3. Excitatory synaptic transmission in spinal cord (monosynaptic).
4. Alpha-motor neuron activated.
5. Contraction of homonymous muscle.
78
The only monosynaptic reflex humans.
Myotatic reflex.
79
Intrafusal fusal fibres consist of?
- Non-contractile equatorial region.
| - Contractile polar ends.
80
The non-contractile equatorial region of intrafusal fibres is innervated by?
- Ia sensory neurones.
81
Contractile polar ends of intrafusal fibres receive efferent input from what?
- Gamma-motor neurons with cell bodies in the ventral horn of the spinal cord.
82
Stimulation of what causes the spindle to contract?
- Gamma motor neurons.
83
Why are alpha- and gamma-motor neurons normally co-activated during voluntary movement?
To allow parallel contraction of intrafusal and extrafusal fibres thus maintaining the sensitivity of the spindle.
84
Intrafusal fibres can be divided broadly into which two categories?
- Nuclear bag fibres.
| - Chain fibres.
85
Name the 2 types of nuclear bag fibres (intrafusal fibre subtype).
- Bag 1/ dynamic.
| - Bag 2/ static.
86
Describe Nuclear bag fibres - Bag 1/ dynamic.
- Very sensitive to rate of change in muscle length.
| - Innervated by dynamic, but not static Gamma-motor neurons.
87
Describe Nuclear bag fibres- Bag 2/ static.
- More sensitive to the absolute length of the muscle.
| - Innervated by static gamma-motor neurons.
88
Describes chain fibres of intrafusal fibres.
Sensitive to the absolute length of the muscle and innervated by static gamma-MNs.
89
What two types of afferent fibre innervate the intrafusal fibres?
- Ia afferents.
| - II fibres.
90
Ia afferents form a primary annulospiral nerve ending, winding around the centre of all what?
Intrafusal fibres.
91
II fibres, form flowerspray endings on all intrafusal fibres except what?
Bag 1/ dynamic type.
92
Ia sensory fibres respond to what?
- Rate of change of muscle length.
| - Absolute length of muscle.
93
Stimulation of static gamma fibres causes what?
- Steady state/static response to increase.
94
Stimulation of dynamic gamma fibre enhances what?
Dynamic response to stretch.
95
In activities in which muscle length changes slowly and predictably, what type of gamma-MNs are active?
Only static gamma-MNs.
96
Which type of gamma-MNs are active during activities in which muscle length changes rapidly and unpredictably?
Dynamic gamma-MNs.
97
Where are golgi tendon organs located?
At the junction of muscle and tendon.
98
What is the function of golgi tendon organs?
Monitor changes in muscle tension to regulate it.
99
Describe the anatomical relationship of golgi tendons and extrafusal fibres.
They exist in series, rather than parallel to each other.
100
Golgi tendon organs are innervated by what?
Group Ib sensory afferents.
101
Why do golgi tendon organs regulate muscle tension?
- To protect the muscle from overload in extreme circusmtances e.g. weight lifting.
- Generally to regulate muscle tension to an optimal range.
102
Group Ib afferents enter spinal cord and synapse upon inhibitory interneurones, which then synapse onto what?
Alpha-motor neurons of the homonymous muscle.
103
Group Ib afferents enter spinal cord and synapse upon inhibitory interneurones, which then synapse onto Alpha-motor neurons of the homonymous muscle to form what?
The basis of the reverse myotatic reflex.
104
An inhibitory interneuron interposed between the Ib afferent and alpha-MN is an example of what kind of pathway?
Polysynaptic pathway.
105
How do golgi tendon organs gauge the force generated by a muscle?
By measuring the tension in its tendon.
106
Coding of force by the Golgi Tendon Organ and the Reverse Myotatic reflex supplies important proprioceptive feedback for what?
Proper execution of fine motor acts.
107
Proprioceptive axons are present in connective tissue of joints and therefore respond to what?
Changes in joint:
- Angle.
- Direction and velocity of movement.
108
The position of a joint is transmitted to what?
The CNS.
109
Free nerve endings are found where within a joint?
- Capsule.
| - Connective tissue.
110
Free nerve endings within a joint have which function?
Nociceptive.
111
Golgi-type endings are only found where within a joint?
Only in ligaments.
112
Function of golgi-type endings in a joint?
Protective?
113
Paciniform ending are found where within a joint?
- In periosteum near articular attachments.
| - Fibrous part of joint capsule.
114
Ruffini endings are found where within a joint?
Mainly in joint capsules.
115
Function of Ruffini endings in joints?
Static position and speed of movements.
116
Proprioceptive information arises from?
- Muscle spindles.
- Golgi tendon organs.
- Joint receptors.
