Midterm 6 Flashcards
Muscle function Reflexes
1
Q
A neuron that synapses with multiple muscle fibers and provides excitation for muscle fibers to generate tension
A
Alpha motor neuron
2
Q
A motor unit is comprised of…
A
The alpha motor neuron and the skeletal fibers it innervates
3
Q
A motor unit follows an “all or none” principle, meaning…
A
All of the motor units in a muscle are excited if one is excited
4
Q
What is a motor pool?
A
All the motor neurons that innervate a single muscle
5
Q
Motor neurons are _________________ on the longitudinal axis of the spinal cord
A
grouped together
6
Q
What are the three parts of the neuromuscular junction (relevant to this course)?
A
1) Presynaptic terminal
2) Synaptic cleft
3) Postsynaptic muscle fiber (motor endplate)
7
Q
What is the presynaptic terminal in the neuromuscular junction?
A
The presynaptic terminal is the motor neuron
8
Q
What is the synaptic cleft in the neuromuscular junction?
A
The synaptic cleft is the area/space between the presynaptic neuron and the postsynaptic muscle fiber
9
Q
Where is neurotransmitter released in the neuromuscular junction?
A
Neurotransmitter is released into the synaptic cleft
10
Q
What is the postsynaptic muscle fiber in the neuromuscular junction?
A
The motor endplate
11
Q
What part of the neuromuscular junction contains acetylcholine receptors?
A
The motor endplate
12
Q
List the steps of synaptic transmission at the neuromuscular junction:
A
- Depolarization of the presynaptic membrane causes an influx of calcium (Ca2+)
- Acetylcholine is released into the synaptic cleft via vesicles
- Acetylcholine binds to receptors on muscle membrane
- Causes depolarization of motor endplate (end plate potential, specific to muscle fibers, similar to graded potential)
- End plate potential triggers an action potential if it reaches threshold
13
Q
What happens after synaptic transmission
A
Acetylcholine is broken down by the enzyme acetylcholinesterase and the vesicles are recycled
14
Q
What is the principle function of the safety factor relating to the neuromuscular junction?
A
The safety factor increases the chance that the muscle will contract even if fatigued.
15
Q
Explain the safety factor of the neuromuscular junction:
A
Motor neurons release extra acetylcholine to depolarize the postsynaptic membrane, this increases the chance that the muscle will contract even if fatigued
16
Q
Myasthenia gravis results in a loss of acetylcholine receptors in the muscle fibers, what does this cause?
A
Reduced membrane depolarization and smaller safety factor which in turn results in weakness of the muscles (a normal level of depolarization does not occur; therefore, the safety factor is also decreased)
17
Q
Each muscle fiber is surrounded by the ___________________.
A
Sarcolemma
18
Q
What is the function of sarcolemma?
A
Allows depolarizing current to reach the sarcomeres
19
Q
What are T-tubules?
A
Openings in the sarcolemma that allow current to travel
20
Q
What is the sarcoplasmic reticulum?
A
A net-like structure that stores, releases, and retrieves calcium
21
Q
_______________ are the smallest functional unit of the muscle fiber
A
Sarcomeres
22
Q
Sarcomeres consist of _____ and ______ filaments.
A
Actin and myosin filaments
23
Q
Actin and myosin are arranged in series along the entire length of the muscle fiber, true or false?
A
True
24
Q
Thick filament =
Think filament =
A
Thick filament = myosin
Thin filament = actin
25
Name the (general) steps of excitation-contraction coupling:
1: Endplate potential triggers action potential
2: Action potential travels into muscles via T-tubules
3: Release of calcium (Ca2+) from the sarcoplasmic reticulum
4: Interaction of actin and myosin
26
What aspect of the muscle fiber is responsible for calcium primarily?
The sarcoplasmic reticulum
27
If calcium is not released, actin and myosin are still able to interact to cause contraction of the muscle, true or false?
False, if calcium is not released, actin and myosin will not interact
28
Calcium is a ________________ from action potential to muscle contraction.
Secondary messenger
29
Muscle contraction is dependent of release of _________________ from the sarcoplasmic reticulum.
Calcium
30
Explain the sliding filament theory:
1: Myosin head is in a 'cocked' position with ATP present
2: Calcium reveals binding site on actin (pushes troponin out of the way)
3: Myosin head binds with actin (cross-bridge)
4: 'Powerstroke' occurs - myosin pulls actin towards the center of the sarcomere
5: When ATP is still present: myosin dissociates from actin, myosin head is 're-cocked', myosin binds to next active site
6: Cross-bridge cycling (repeats)
31
When does muscle relaxation occur?
Relaxation occurs when simulation of the motor neuron stops
31
How does muscle relaxation occur? (in simple terms)
Calcium is pumped back into the sarcoplasmic reticulum, breaking the link between actin and myosin, causing the muscle to relax
Relaxation will also occur when ATP is no longer available
32
The amount of force (tension) a muscle can generate is dependent on: (6 factors)
- Muscle length
- Muscle velocity and direction
- Innervation ratio
- Motor unit types
- Size principle
- Rate coding
32
How does muscle length affect the amount of force a muscle is able to produce?
The force produced by a muscle varies with muscle length.
33
_______________ and _____________________ contribute to total tension in the muscle.
Active contraction and passive structures
34
Active contraction and passive structures contribute to total tension in the muscle, what are these passive structures and how do they contribute?
Weak actin-myosin bonds, connective tissue, and titin act like an elastic.
Increased tension with increased length.
35
Passive structures contribute to total tension in the muscle how?
Increased length = increased tension (elastic like)
36
How does the active contribution relate to muscle length?
Smaller force generated when moving away from the optimal length during active contraction (as there is less overlap between actin and myosin).
37
The total tension of a muscle gets higher as a muscle is _____________ (lengthened or shortened?)
Lengthened
38
During concentric contractions of the muscle there is __________ tension with increasing velocity. Why?
During concentric contractions (shortening) there is less tension with increasing velocity.
Cross-bridges cannot cycle quickly enough.
39
During eccentric contractions of the muscle there is __________ tension with increasing velocity. Why?
During eccentric (lengthening) contractions of the muscle there is more tension with increasing velocity.
There is a greater percentage of cross-bridges that remain attached (greater passive contribution)
40
What is innervation ratio?
Innervation ratio is the number of muscle fibers innervated by a single motor neuron.
41
A muscle fiber is synapsed by _________(#) motor neuron(s).
A muscle fiber is synapsed by one motor neuron.
42
What type of muscles have a higher innervation ratio?
Larger, more powerful muscles
43
Innervation ratio differs by muscle type:
Power/force muscles =
Precision muscles =
Power/force muscles = higher innervation ratio
Precision muscles = lower innervation ratio
44
The number of muscle fibers innervated by a single alpha motor neuron
Innervation ratio
45
Explain the characteristics of Type I motor units:
- Slow
- Small muscle fibers
- Rich in myoglobin, mitochondria, and capillaries
- Sustained muscular contraction
- Fatigue-resistant
- Low-threshold for activation (activated first according to size principle)
46
Explain the characteristics of Type IIa motor units:
- Fast fatigue-resistant
- Medium sized muscle fibers
- Contract less quickly than type IIx
- Generate larger forces
47
Explain the characteristics of Type IIx motor units:
- Fast fatigable
- Large muscle fibers that contract quickly
- Generate larger forces
- Sparse mitochondria
- Easily fatigued
- High activation threshold
48
What muscle type (motor unit type) has the highest activation threshold?
Type IIx (fast-fatigable)
49
Explain the size principle of motor recruitment
With increasing levels of motor activation, motor units are recruited from smallest to largest
50
Explain rate coding:
Rate coding is the motor unit firing rate.
Smaller muscles typically rely on rate coding while larger muscles rely more on the size principle for recruitment.
51
Age has an impact on neuromuscular function, action potential propagation is ______________ along alpha motor neuron axons.
slowed
52
Age has an impact on neuromuscular function, there is often reduced ____________ of active areas in the presynaptic terminal.
Reduced density
53
Age has an impact on neuromuscular function, there is often a loss of motor units (number decreases) but an increased...
Size of remaining motor units due to collateral sprouting
54
What is the impact of ALS on neuromuscular function?
ALS affects specific motor pools and types of motor units.
Rapid loss of fast-fatigable neurons, followed by a loss of fast-fatigue resistant neurons.
There is relative sparing of type I neurons (slow).
55
What is the impact of stroke on neuromuscular function?
Loss of motor units of paretic side.
Smaller muscle cross-sectional area.
Reduced ability to use size-principle.
56
What is the impact of spinal cord injury of neuromuscular function?
Extensive atrophy (preferentially affects type II fibers)
Transformation toward type IIx fibers in most individuals (possible increase in contractile velocity)
Loss of motor units, increased size of remaining motor units
57
What is the impact of Parkinson's disease on neuromuscular function?
Muscle atrophy
Loss of motor units (selective loss of type II motor units)
Compensatory type I muscle fiber hypertrophy
Loss of strength and power
58
Integrates excitatory and inhibitory signals from the cortex and brainstem and translates them into a signal to modulate lower motor neuron activity
Upper motor neurons
59
Receives the signal from upper motor neurons and transmits it to effector muscle to perform a movement
Lower motor neuron
60
_______________ neurons are the final common pathway.
Lower motor neurons are the final common pathway.
61
How do we know if a movement is correct?
Sensory feedback provides information about the movement.
Sensory information is also integrated to modulate the activity of lower motor neurons and upper motor neurons.
62
Sensory feedback provides information about the movement (allows modification), where is this feedback coming from?
Proprioception (receptors in muscles, joints, tendons, etc.), vision, and/or vestibular
63
Sensory information is integrated to modulate the activity of lower motor neurons and upper motor neurons, how does this occur and what does it assist with?
Sensory inputs provide feedback to upper motor neurons.
Assists with motor learning.
Involved in most simple reflexes.
64
Cell bodies of sensory neurons are located in the _____________________.
Cell bodies of sensory neurons are located in the posterior (dorsal) root ganglia.
65
Sensory neurons send their messages to the spinal cord via the ______________________.
Posterior (dorsal) roots.
66
At each level of the spinal cord, sensory neurons contribute to a mixed ___________________.
Spinal nerve
66
The somatosensory systems convey ________, ___________, _____________, and ____________ information.
The somatosensory systems convey touch, proprioceptive, pain and temperature information.
67
Peripheral nerves are typically mixed, true or false?
True, peripheral nerves are typically mixed (sensory and motor fibers)
67
Axon size and level of myelination vary, out of the following axon types, which is the fastest and which is the slowest?
A-beta (touch)
C (pain, temperature, itch, chemoreception)
I (proprioception)
A-delta (pain, temperature)
Fastest:
I (proprioception)
A-beta (touch)
A-delta (pain, temperature)
C (pain, temperature, itch, chemoreception)
68
Intrafusal muscle fibers are encapsulated in connective tissue, and detect ________ and __________ of the muscle.
Length and velocity of the muscle
69
There are two-types of sensory axons:
What are they, where are they located and what are their functions?
Group 1a axons: around nuclear bag fibers, detects changes in length and velocity
Group II axons: around nuclear chain fibers, measure length
70
If a muscle is stretched, the spindle is _______________ (active or inactive?), why?
If a muscle spindle is stretched, the spindles are active because the embedded intrafusal fibers also stretch (embedded within the extrafusal muscle fibers).
71
If a muscle is contracted, via stimulation of the alpha motor neuron, the spindle is ______________ (active/inactive), why?
If a muscle is contracted, via stimulation of the alpha motor neuron, the activity of the spindle decreases, because the muscle length is shortened.
72
Alpha motor neurons innervate ___________ muscle fibers.
Alpha motor neurons innervate extrafusal muscle fibers.
73
Gamma motor neurons directly cause muscle contraction, true or false?
False, Gamma motor neurons DO NOT directly cause muscle contraction, they innervate muscle spindles (intrafusal fibers) and dictate their sensitivity.
74
Gamma motor neurons innervate ________________________________ and dictate their sensitivity.
Gamma motor neurons innervate muscle spindles (intrafusal fibers) and dictate their sensitivity.
75
Gamma-motor neurons have a higher activation threshold than Alpha-motor neurons, true or false?
False, Gamma-motor neurons have a lower activation threshold.
76
Gamma-motor neurons can be modulated by _________________________.
Gamma motor neurons can be modulated by descending pathways.
77
Normally, during active muscle contraction, Gamma-motor neurons cause intrafusal fibers to contract at ___________________ as extrafusal fibers, to maintain sensitivity of the muscle spindles during different muscle lengths.
Normally, during active muscle contraction, Gamma-motor neurons contract AT THE SAME TIME as extrafusal fibers, to maintain sensitivity of the muscle spindles during different muscle lengths.
78
Normally, during active muscle contraction, Gamma-motor neurons contract at the same time as extrafusal fibers, why?
To maintain sensitivity of the muscle spindles during different muscle lengths.
79
Alpha-Gamma co-activation only occurs in what type of movements?
Alpha-Gamma co-activation only occurs in movements initiated by the central nervous system.
This co-activation does not occur during reflexes (only Alpha-motor neurons activate during reflex).
80
Alpha-Gamma co-activation occurs in reflexes, true or false?
False, Alpha-Gamma co-activation only occurs in movements initiated by the CNS.
81
Golgi tendon organs fall into the Group _____ afferents.
Group 1b afferent
82
Where are golgi tendon organs located?
Golgi tendon organs are located near the musculotendinous junction.
83
Golgi tendon organs are designed to measure...
muscle tension/force
84
If a muscle is stretched, and the Golgi tendon organ afferent is measured, does the afferent activity increase or decrease?
The muscle stretch does not change the afferent activity of the Golgi tendon organ.
85
If a muscle is contracted, and the Golgi tendon organ afferent is measured, does the afferent increase or decrease?
The Golgi tendon organ afferent activity increases when the muscle is contracted.
The Golgi tendon organ is located in the musculotendinous junction, at this junction there is limited tension when the muscle is lengthened; however, when the muscle is actively contracted there is increased tension at the junction.
86
Reflexes are the most complex level of movement, true or false?
False, reflexes are the most simple level of movement.
87
There is a small number of synapses (neurons) involved in reflexes, true or false?
True
88
Reflexes are controlled largely by...
Local circuitry in the spinal cord
89
Reflexes are stereotyped, in what way?
A stimulus input = set motor output
A reflex is stereotyped in terms of which muscle groups are activated to a stimulus.
90
In terms of reflex modulation, amplitude is larger with increasing ____________.
Amplitude is larger with increasing stimulus input.
91
Reflexes can be excited or inhibited via descending control, true or false?
True
92
What are the five components to a reflex?
1. Receptor
2. Sensory afferent
3. Central nervous system (spinal cord) interneuron
4. Motor efferent (lower motor neuron)
5. Muscle
93
What are the 5 components of the stretch reflex?
1. Muscle spindle (of triceps brachii)
2. 1a afferent (sensory fiber)
3. Spinal cord (direct connection to lower motor neuron)
4. Alpha motor neuron
5. Contraction of muscle (triceps brachii in this case)
94
The stretch reflex is homonymous, what does this mean?
The same muscle that is stretched is excited (40-50ms) and contracts.
95
What is the purpose of the stretch reflex (in a general sense)?
Returns joint to a desired position, maintains muscle length, provides feedback to compensate for limb perturbation during limb movement.
96
What is reciprocal inhibition?
Automatic inhibition (turn off) of the antagonist alpha motor neuron caused by contraction of the agonist muscle.
97
Automatic inhibition of the antagonist muscle alpha motor neuron caused by contraction of the agonist muscle.
Reciprocal inhibition
98
Reciprocal inhibition occurs in conjunction with the stretch reflex, true or false?
True
99
What are the 5 components of reciprocal inhibition?
1. Muscle spindle (triceps brachii)
2. 1a afferent
3. Inhibitory interneuron in spinal cord
4. Alpha motor neuron
5. Relaxation of the antagonist muscle (biceps brachii in this case)
100
What is a component of reciprocal inhibition that is different than that of the stretch reflex?
The stretch reflex involves direct connection of the lower motor neuron to the spinal cord.
Reciprocal inhibition involves an inhibitory interneuron for connection to the spinal cord (additional step).
101
In reciprocal inhibition there is an additional step that is not included in the stretch reflex, what is it and what is the purpose?
1a afferent excitatory input to an inhibitory interneuron leading to inhibition of antagonist alpha motor neuron, the purpose is that it reduces antagonist muscle activity to allow agonist to contract.
102
What are the components of the Golgi-Tendon organ/inverse stretch reflex?
1. Golgi tendon organ in biceps brachii
2. 1b afferent
3. Spinal cord interneurons
4. Disynaptic reflex pattern
5. Inhibition of alpha motor neuron (to biceps in this example)
6. Excitation of alpha motor neuron (to triceps)
7. Biceps brachii relaxes
7. Triceps brachii contracts
103
Group 1b afferent fibers are known as...
Tendon organ afferents.
104
Group 1a afferent fibers are known as..
The primary afferents, they supply all three types of intrafusal fibers.
105
Explain the Golgi tendon organ/ inverse stretch reflex, starting with the input...
Input: muscle force activates the 1b afferent
Excitation of the inhibitory interneuron
(inhibit agonist homonymous muscle)
Excitation of the excitatory interneuron
(Excites antagonist muscle)
106
What reflex modulates activity of an inhibitory interneuron and an excitatory interneuron?
Golgi tendon/inverse stretch reflex
107
What is the function of the Golgi tendon organ/inverse stretch reflex?
Release the load to prevent muscle tearing and maintains muscle tone during muscle fatigue.
108
Explain the five components of the flexor withdrawal reflex:`
1. Ipsilateral nociceptor
2. Group III afferent
3. Interneuron network in the spinal cord
4. Alpha motor neuron
5. Flexor muscles excite
5. Extensor muscles inhibit
109
Name the type of afferent fiber associated with each reflex:
Stretch reflex
Golgi tendon organ/ Inverse stretch reflex
Flexor withdrawal reflex
Crossed extensor reflex
Stretch reflex: Group 1a afferent
Golgi Tendon organ reflex: Group 1b afferent
Flexor withdrawal reflex: Group III afferent
Crossed extensor reflex: Group III afferent
110
Explain the flexor withdrawal reflex, starting with the input...
Input: noxious stimulus detected and activated Group III afferent
Synapse on interneuron networks
Ipsilateral limb (painful stimulus)
- Interneuron excites flexors
- Inhibitory interneuron inhibit extensors
111
What is the function of the flexor withdrawal reflex?
Remove the limb from noxious stimulus
112
Crossed extensor reflex occurs with the flexor withdrawal reflex, true or false?
True
113
Name the components (5) of the crossed extensor reflex:
1. Ipsilateral nociceptor
2. Group III afferent
3. Contralateral interneuron network in spinal cord
4. Alpha motor neuron
5. Extensor muscles of contralateral limb (excite)
5. Flexor muscles of contralateral limb (inhibit)
114
Crossed extensor reflex is enacted for __________________ support.
Postural support
115
Explain the crossed extensor reflex, starting with the input...
Input: noxious stimulus detected
Synapse on interneuron networks
Contralateral limb (cross-extensor)
- Excite extensors (postural)
- Inhibit flexors
116
What is the function of the crossed-extensor reflex?
Ground stance limb to allow ipsilateral limb to withdraw.
117
The crossed extensor reflex crosses the spinal cord, true or false?
True
118
The crossed extensor reflex spans multiple spinal levels (involves long distance interneuron networks), true or false?
True
119
Damage to the descending tract before the anterior horn of the spinal cord is classified as a __________________ lesion.
Upper motor neuron lesion
120
In the grading of reflexes, explain 0-5+
0 - No response (areflexia)
1+ - Present but depressed (hyporeflexia)
2+ - Normal (average)
3+ - Brisk (increased; possibly abnormal)
4+ - Very brisk, hyperactive with clonus (abnormal)
5+ - Sustained clonus
121
What is considered "normal" on the grading of reflexes scale 0-5?
2+
122
Clinically, it is likely to observe hyperreflexia in what case(s)?
Upper motor neuron lesion
123
Clinically, it is not likely to observe hyperreflexia in what case(s)?
Lower motor neuron lesions
Nerve root lesions
Spinal nerve lesions
Peripheral nerve lesions
124
Why can we observe hyperreflexia and hypertonia with upper motor neuron lesions?
Hyperreflexia and hypertonia is due to a lose of descending control, the ability to inhibit is often lost, causing excessive firing of lower motor neurons.
125
The reticulospinal tract originates in the _________________ and terminates _______________.
The reticulospinal tract originates in the reticular formation (neuron network in the brainstem) and terminated bilaterally in the spinal cord.
126
The reticulospinal tract receives inputs from the ipsilateral and contralateral cortex, true or false?
True
127
The dorsal/lateral reticulospinal tract is responsible for _________________ and the medial reticulospinal tract is responsible for _________________.
The dorsal/lateral reticulospinal tract is responsible for descending inhibition.
The medial reticulospinal tract is responsible for descending excitation.
128
Explain the relation between spinal cord injury and stretch reflex (damage to both pyramidal and extrapyramidal tracts):
Damage to both the lateral corticospinal tract and both reticulospinal tracts (medial and lateral) causes the inhibitory interneurons to lose the ability to inhibit lower motor neurons.
Hypertonia is due to a loss of descending control.
129
What is the car analogy related to spinal cord injury and stretch reflex?
You are in a car going down a hill, with no gas and no brake (the excitatory effect is not the problem necessarily in this case, the issue lies in the lack of inhibition (brakes), causing excessive firing of lower motor neurons).
130
Explain the relation between stroke and stretch reflex (damage to right lateral corticospinal tract):
Right lateral corticospinal tract damaged.
Reduced corticospinal drive to the dorsal/lateral corticospinal tract (descending inhibition).
Medial reticulospinal tract increases descending drive (excitatory) from the contralateral side.
Results in excessive reticulospinal drive from medial tract.
131
What is the car analogy related to stroke and stretch reflex?
You are in a car that is driving fast using gas, and you have no brakes.
There is increased excitatory drive from the medial reticulospinal tract and less drive from the lateral reticulospinal tract (inhibitory).
132
Hypertonia is due to a lose of _______________.
descending control
133
In upper motor neuron lesions, there is excessive neural signaling due to a loss of _________________.
descending control
134
In upper motor neuron lesions, there is excessive neural signaling due to a loss of descending control, this contribute to hyperreflexia in which ways:
1.
2.
1. Hypersensitivity of the muscle spindle (sensitivity is controlled by Gamma-motor neurons, which receives input from upper motor neurons)
2. Excitable Alpha-motor neurons (increased chance of firing)
135
Clonus is caused by...
Hypersensitivity of the muscle spindle and excitable Alpha-motor neuron
136
Upper motor neuron lesions are usually what level on the grading scale for reflexes?
3+ - 5+ (hyperreflexia)
137
Lower motor neuron lesions are usually what level on the grading scale for reflexes?
0 - 1+ (hyporeflexia)
138
A client has been experiencing muscle weakness and spasticity in her legs and arms. She has noticed difficulty with walking, frequent tripping, and increasing trouble with toileting and getting dressed independently.
Is this a UMN or LMN lesion?
UMN = Hypertonia
139
_____________ are responsible for detecting muscle length and velocity; _______________ are responsible for detecting muscle tension and force.
Muscle spindles are responsible for detecting muscle length and velocity; Golgi tendon organs are responsible for detecting tension and force.
140
___________________ maintains sensitivity of the muscle spindles during active contraction.
Alpha-Gamma co-activation
