Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors Flashcards
(102 cards)
1
Q
What does the somatic motor system consist of?
A
The skeletal muscles and the elements of the nervous system that control them
2
Q
What do the neural elements of the somatic motor system comprise of?
A
The UPPER MOTOR NEURONES (UMN’S) within the brain.
+
The LOWER MOTOR NEURONES (LMN’s) with soma within brainstem and ventral horn of the spinal cord.
3
Q
What do UMN’s do?
A
Supply input to LMN’s to modulate their activity
4
Q
Aside from UMN’s, what else do LMN’s receive input fro,?
A
Proprioceptors
AND
Interneurones
5
Q
What do LMN’s do?
A
Command muscle contraction and form the ‘final common pathway’
6
Q
Name 2 components of LMN’s and state what they do.
A
Alpha Motor Neurones – innervate the bulk of fibres within a muscle that generate force.
Gamma Motor Neurones – innervate a sensory organ within the muscle known as a MUSCLE SPINDLE.
7
Q
What innervates the muscle spindle?
A
Gamma Motor Neurones
8
Q
Biceps brachia and brachiallis work together as ……….
A
SYNERGISTS
9
Q
What do axial muscles control?
A
Movements of the trunk (maintain posture)
10
Q
Where are proximal muscles found?
A
In the shoulder, elbow, pelvis and knee
11
Q
What do distal muscles do?
A
Move the hands, feet and digits
12
Q
What do axons on LMN’s exit the spinal cord in?
A
Ventral roots of via cranial nerves
13
Q
What do each ventral (anterior) and dorsal (posterior) root join together to form? What types of fibre do these contain?
A
A MIXED spinal nerve
14
Q
What does a mixed spinal nerve contain?
A
Sensory AND motor fibres
15
Q
Name the 2 enlargements of the spinal cord.
A
Cervical
Lumbar
16
Q
What is the level of the cervical enlargement?
A
C3-T1
17
Q
What is the level of the lumbar enlargement?
A
L1-S3
18
Q
What is contained within each enlargement?
A
Large numbers of motor neurones
19
Q
What does the cervical enlargement supply?
A
ARM
20
Q
What does the lumbar enlargement supply?
A
LEG
21
Q
Where are the motor neurones that innervate distal and proximal musculature mainly found?
A
In the cervical and lumbar-sacral segments of the spinal cord
22
Q
Where do the motor neurones innervating the axial musculature occur?
A
AT ALL LEVELS
23
Q
What is a ‘motor unit’?
A
An α-MN and all of the skeletal muscle fibres that it innervates
24
Q
A motor unit is the SMALLEST functional component of the motor system
A
TRUE
25
In terms of motor units, what does muscle contraction result from?
The individual and combined actions of the motor units, which must be co-ordinated
26
What is the motor neurone pool?
The collection of alpha motor neurones that innervate a single muscle
27
The force of muscle contraction is graded by alpha motor neurones by two principle mechanisms. What are these?
1. Frequency of action potential discharge of the α-MN ie. the faster the AP freq, the greater the force of contraction.(note: each action potential causes a muscle ‘twitch’ (rapid sequence of contraction followed by relaxation)
2. The recruitment of additional, synergistic, motor units.
28
The cell bodies of LMN’s show a distinct (somatotopic) distribution in the ventral (anterior) horn. Describe this.
* LMN’s innervating axial muscles MEDIAL to those innervating distal muscles.
* LMN’s innervating flexors DORSAL to those supplying extensors.
29
LMN's innervating axial muscles are LATERAL to those innervating distal muscles
FALSE - medial
30
There are only ___ sources of input to alpha motor neurones to regulate its activity
3
31
Name the 3 sources of input to alpha motor neurones.
* Central terminals of DORSAL ROOT GANGLION CELLS whose axons innervate the muscle spindles.
* UMN’s in the motor cortex and brain stem.
* SPINAL INTERNEURONES.
32
What 2 factors does muscle strength depend on?
1. Neuromuscular Activation.
| 2. Force production by innervated muscle fibres.
33
What does neuromuscular activation depend on?
* Firing rates of LMN's involved
* The number of LMN's that innervate a muscle
* The co-ordination of the movement
34
What does force production by innervated muscle spindles depend on?
* Fibre size (hypertrophy)
| * Fibre phenotype (fast or slow contracting)
35
What does a single action potential in an alpha motor neurone cause?
A muscle fibre to twitch
36
What does the summation of muscle fibre twitches cause?
A sustained contraction as the number of incoming action potentials increases
37
Motor units are of variable size
TRUE
38
What are small motor units of few fibres used for?
Fine movements ie. extraocular eye muscles
39
What are large motor units of hundreds of fibres used for?
Large postural (antigravity) muscles e.g leg muscles
40
Small motor units are innervated by small alpha-motor neurones (soma diameter), whereas the converse is true for large motor units
U got that right gurlllll
41
What is the force of contraction that each motor unit produces dependant on?
Size
42
Compare the alpha-motor neurones innervating the fast type to those innervating the slow units.
They tend to be larger and have faster conducting axons.
43
Fast and slow type fibres contain both types of fibres
FALSE - only contain one type
44
What is each muscle fibre innervated by? Where?
A single motor axon at the endplate (NMJ) which is usually at the centre of the fibre
45
What are the 2 major skeletal muscle types?
Fast and Slow
46
How do fast and slow muscle types differ?
Differ in how fast myosin ATPase splits ATP (different myosin ATPase isoforms and activities) to provide energy for cross-bridge cycling.
47
Describe slow oxidative (type I fibres)
* ATP derived from oxidative phosphorylation
* Contraction and relaxation is SLOW
* RESISTANT TO FATIGUE
* DARK red - due to high myoglobin content
48
Describe fast type IIa fibres.
* ATP derived from oxidative phosphorylation
* Contraction and relaxation is FAST
* Red - well vascularised
49
Describe slow oxidative (type I fibres)
* ATP derived from oxidative phosphorylation
* Contraction and relaxation is SLOW
* RESISTANT TO FATIGUE
* DARK red - due to high myoglobin content
* Low tension
* Slow and low aMN threshold
50
Describe fast type IIa fibres.
* ATP derived from oxidative phosphorylation
* Contraction and relaxation is FAST
* Red - well vascularised
* Fatigue resistant
* High tension
* Intermediate aMN threshold
51
Describe fast type IIb/x fibres.
* ATP derived from glycolysis
* FAST contraction
* NOT fatigue resistant
* Pale - not well vascularised
* VERY high tension
* Large aMN, high threshold
52
Slower, smaller motor units are recruited last
FALSE - first
53
Do smaller alpha-motor neurones have a higher or lower threshold than larger ones?
Lower
54
What type of motor unit is more easily activated and 'trained'?
SLOW motor units
55
What are smaller aMN's part of?
SLOW motor units
56
What are large aMN's part of?
Fatigue-resistant, or fast fatiguing, motor units
57
aMN's are recruited by size, what does this allow for?
The fine and graded development of muscle force
58
What does 'myotatic reflex' mean?
When a skeletal muscle is pulled, it pulls back
59
What is the change in length (+ rate of change) of a muscle registered by?
The muscle spindle
60
What do spindles consist of?
* A fibrous capsule
* Intrafusal muscle fibres
* Sensory afferents
* Gamma motor neurone efferents
61
What do EXTRAfusal fibres do?
Generate force
62
The striking of the patellar tendon with a tendon hammer causes a ‘knee jerk.’ What is this knee jerk underpinned by?
Myotatic reflex
63
What can the myotatic reflex be used to assess?
The functional integrity of the spinal cord at specific levels
64
Supinator/wrist?
C5-6
65
Biceps/elbow?
C5-6
66
Triceps/elbow?
C7
67
Quads/knee?
L3-4
68
Gastrocnemius/ankle?
S1
69
What do intrafusal fibres consist of?
* A non-contractile equatorial region
| * Contractile polar ends
70
What is the non-contractile equatorial region of intrafusal fibres innervated by
The Ia sensory neurones
71
What do the contractile polar ends of intrafusal fibres receive efferent input from?
y MN's
72
Where are the cell bodies of these gamma neurones located? What are these driven by?
In the ventral horn of the spinal cord
| These are driven by higher centres, NOT Ia afferents
73
What causes the spindle to contract?
Stimulation of y MN's
74
What are normally co-activated by higher centres in voluntary movement?
a MN's and y MN's
75
What is the effect of co-ordination of a MN's and y MN's?
Means that intrafusal muscle fibres contract in parallel with the extrafusal fibres
76
What are type 1 dynamic nuclear bag fibres sensitive to?
The RATE OF CHANGE of muscle length
77
What are type 1 dynamic nuclear bag fibres innervated by?
Dynamic, but not static, gamma motor neurones
78
What are type 2 static nuclear bag fibres sensitive to?
The ABSOLUTE length of the muscle
79
What are type 2 static nuclear bag fibres innervated by?
Static gamma motor neurones.
80
What are chain fibres sensitive to?
The absolute length of the muscle
81
What are chain fibres innervated by?
Static gamma motor neurones
82
What 2 types of afferent fibre innervate the intrafusal fibre?
Ia afferents
| II fibres
83
What do the Ia afferents form?
A primary annulospiral nerve ending winding around the centre of all the intrafusal fibres
84
What do II fibres, which are more slowly conducting, form?
Flowerspray endings on all intrafusal fibres, except the bag 1 dynamic type
85
What do both Ia and II respond to?
STRETCH
86
What do Ia fibres respond to?
Both the rate of change of muscle length (dynamic response) and absolute length (steady state, or static response) of the muscle
87
What does the stimulation of static gamma fibres cause?
Steady state, or static response, to increase
88
What does the stimulation of dynamic gamma fibre enhance?
The dynamic response to stretch
89
In activities in which muscle length changes slowly and predictably, only what are active?
Static gamma motor neurones
90
When are dynamic gamma motor neurones active?
During behaviours in which muscle length changes rapidly and unpredictably.
91
Where are golgi tendon organs located?
At the junction of the muscle and the tendon
92
What do golgi tendon organs monitor changes in?
Muscle tension
93
Golgi tendon organs are IN SERIES with, rather than PARALLEL to, extrafusal fibres
TRUE
94
What are golgi tendon organs innervated by?
Group 1b sensory afferents (myelinated, slightly slower conducting than Ia fibres)
95
Golgi tendon bodies....
Act to regulate muscle tension to:
protect muscle from overload (in extreme circumstances – e.g. weight lifting)
more generally regulate muscle tension to an optimal range
96
Once group Ib efferents enter the spinal cord they synapse upon what?
Inhibitory interneurones
97
What do inhibitory interneurones then synapse on?
The alpha motor neurones of the HOMONYMOUS muscle forming the basis of the REVERSE MYOTATIC REFLEX
98
What happens in a POLYSYNAPTIC PATHWAY?
An inhibitory interneurone is interposed between the Ib afferent and alpha motor
99
How do golgi tendon organs gauge the force generated by a muscle?
By measuring the tension in its tendon
100
Where are proprioceptive axons present?
In the connective tissue of joints ie. joint capsules and ligaments
101
What do proprioceptive axons do?
* Respond to changes in angle, direction and velocity of movement of a joint.
* Also prevent excessive flexion, or extension
102
What does proprioceptive information arise from?
* Muscle spindles
* Golgi tendon organs
* Joint receptors
