Exam 5: lower motor neurons Flashcards
1
Q
what components make up the complexity of movement? (6)
A
- uncertainty
- Variability between sensory and motor action
- multiple degrees of freedom (joints and muscles)
- complex dynamics and postural instability
- time delays for sensory and motor
- time varying properties like fatigue
2
Q
how does the knee jerk reflex work if a hammer hits the knee?
A
- Hammer is pressed onto the knee
- There is stretch of tendon and extensor muscle
- Depolarization and transmitter release
- Sensory afferents enter the dorsal root and the ventral root is responsible for the alpha motor neuron output => exciting the motor neuron in the spinal cord and interneuron
- interneuron synapses and inhibits the flexor motor neuron while motor neuron conducts action potentials to synapse on extensor muscle fibers for contraction (no interneuron on extensor)
- flexor muscles relax
3
Q
where do sensory afferents enter in the spinal cord?
A
the dorsal root
4
Q
where does the alpha motor neuron output come from?
A
ventral root
5
Q
what two things does the sensory afferent axon synapse on in the knee jerk reaction?
A
- the motor efferent for the extensor muscle
- the interneuron for the flexor muscle
6
Q
T/F the muscles have the same kind of mechanoreceptors as touch but located in muscle rather than skin?
A
True
7
Q
what is the entire motor circuit top down of brain and spinal cord regions?
A
- upper motor neurons from the motor cortex and brainstem centers
- mainly synapse on local circuit neurons but also motor neuron pools (lower motor neurons)
- the local circuit neurons also synapse on the pools
- motor neuron pools synapse on skeletal muscles
8
Q
what is the entire motor circuit bottom up of brain and spinal cord regions?
A
- sensory inputs go to the local circuit neurons (go to MNP and skeletal) and the cerebellum
- the cerebellum synapses on the brainstem and motor cortex => the basal ganglia also synapse separately from sensory input
9
Q
what does the motor cortex do?
A
planning, initiating, directing voluntary movements
10
Q
what does the brainstem center do for upper motor neurons?
A
rhythmic, stereotyped movements and postural control
11
Q
what does the basal ganglia do in motor?
A
initiates intended movement and suppresses unwanted movement
12
Q
what does the cerebellum do in motor units?
A
coordinates ongoing movement
13
Q
what do local circuit neurons do?
A
provide sensorimotor integration and central pattern generation
14
Q
Upper motor neurons
A
in the brain and motor cortex/brainstem
15
Q
Lower motor neurons
A
the alpha motor neurons that make direct synaptic contact on the muscle to drive the action
16
Q
what happens when you inject dye into muscle fibers?
A
it gets taken up by innervating motor neurons and retrogradely transported to neuron cell bodies in the spinal cord from alpha neurons
17
Q
motor neuron pool
A
many neurons innervate the same muscle
18
Q
what does the lateral spinal cord control? ventral?
A
distal; medial
19
Q
Muscle
A
made up of many muscle fibers
20
Q
Myofibrils
A
many of them make up each muscle fiber (myocite)
21
Q
what is the muscle structure progression from smallest to largest?
A
myofibril => muscle fiber/myocyte => bundle of fiber/muscle
22
Q
do motor neurons innervate the whole muscle or muscle fibers?
A
muscle fibers
23
Q
The motor unit
A
all of the fibers innervated by the same neuron
24
Q
1 muscle fiber is innervated by ___?
A
1 neuron
25
1 neuron innervates ________
multiple muscle fibers (motor unit)
26
a muscle has multiple ______?
motor units
27
what is released from muscle fiber nerve endings onto other muscles via the cholinergic N-receptors?
acetylcholine vesicles
- 2 ACh sit in small holes per each receptor where they bind
- Ca2+ comes in and K+ goes out through the same receptor
28
what are larger motor units usually (not always) associated with? (4)
- Larger motor neuron ⇒ larger cell body and axon diameter
- Increased dendritic branching ⇒ more inputs to the motor neuron
- More axonal branches ⇒ 1 branch for each muscle fiber vs for each muscle
- Decreased excitability of the motor neuron ⇒ higher threshold
29
T/F larger motor units have lower threshold for evoking spikes?
False they have a higher threshold for evoking spikes
- Correlates with dexterity of the muscle
30
T/F the more precise the movement the muscle makes, the smaller the motor units will be?
True
31
what are the types of motor units? (3)
1. slow motor units (S)
2. fast fatigue resistant (FR)
3. fast fatigable (FF)
32
slow motor units (size, energy, threshold, sustainability?)
- Small
- Muscle fibers contain lots of mitochondria and blood vessels (energy) => Slow to fatigue
- Low threshold, therefore tonic activity
- For sustained, low force muscle movements ⇒ upright posture
33
fast fatigue resistant?
- Intermediate size
- High threshold
34
fast fatigable (size, energy, threshold, sustainability?)
- Large
- Relatively few mitochondria and blood vessels => Fast to fatigue
- For brief, high force contraction ⇒ sprinting
- High threshold
35
what type of motor neuron would be used for running and walking?
fast fatigue resistant
36
what type of motor unit would be used for jumping and galloping?
fast fatigable
37
what type of motor unit would be used for standing?
slow
38
Muscle tension
the force exerted by the muscle on an object
39
Load
the force exerted by an object on the muscle
40
Isometric
when muscle tension changes without any corresponding changes in muscle length
41
what is the order of activation of the different types of motor units?
S => FR => FF
42
which motor unit type has the highest force with 1 AP?
fast fatiguable is the largest, then fast fatigue-resistant, slow
43
what is the latency and adaptation for each motor unit type?
- the latency for FF and FR are the same but FF is at a higher force while S is longer in latency
- the FF adapts first, then FR, then S
44
what is the force generated by a muscle proportional to? (3)
- Number of motor units activated
- Size of the motor units
- Frequency of motor neuron activation
45
when do muscle fibers relax?
between simulations
- this is a twitch
46
temporal summation
muscle fibers get increased magnitude of contraction with increasing frequency of stimulation
47
what is the force of fused tetanus?
maximum force
- no relaxation twitches
48
what is walking an example of for contraction types?
smooth titanic contraction
49
what is eye blinking an example of for contraction?
twitch => single action potential
50
T/F for almost everything we do there is fused tetanic stimulation to generate smooth muscle activation?
True
51
what are the 4 types of contraction?
1. single muscle twitches (5 Hz)
2. temporal summation (20 Hz)
3. unfused tetanus (80 Hz)
4. fused tetanus (100 Hz)
52
what effect does chronic low level electrode stimulation of a muscle have?
most fibers become slower
- FF motor units converted to S
- Properties of the innervating a motor neuron also shift to be similar to those neurons that normally control S motor units
53
what does a loss of lower motor neurons lead to?
severe muscle loss
- Injury, tumor, etc.
54
what happens if you train your dorsiflexor (ankle joint) for 12 weeks via foot pedal training for intervals that no fatigue occurs?
- After training the force generated by the muscle can happen quicker
- Action potentials of the motor neurons are occurring earlier after training
- peak torque (~strength) increased
- Increased and earlier surface EMG (~muscle Vm)
- Increased initial firing rate and amplitude of alpha motor neuron
55
Motor neurons
neurons that activate contraction of motor units or muscle spindles
56
examples of motor neurons
- gamma
- alpha
57
Gamma motor neurons
synapse in spindle
- innervate muscle spindles ⇒ deep inside the muscle and control sensitivity
58
Alpha motor neurons
synapse on muscle fibers
- innervate striated muscle which is the outside of the muscle fibers that are intrafusal
59
proprioceptive (sensory) neurons
innervate and sense change in the muscle
60
what are the proprioceptive (sensory) neurons?
- muscle spindle Ia and II
- Golgi tendon organ (Ib)
61
where do local proprioceptive signals go?
to the alpha motor neuron and local circuit neurons
62
where do ascending proprioceptive signals go?
to the cerebellum
63
T/F even the simplest reflexes need to integrate information from motor and proprioceptive neurons?
True
64
what does compelx movement require from UMNs and LMNs?
bilateral integration
65
what are properties of alpha motor neurons? (3)
- Not encapsulated ⇒ extrafusal
- Force generators
- Determine muscle contraction
66
what are properties of gamma motor neurons? (3)
- Encapsulated in the spindle ⇒ intrafusal
- Regulates sensitivity (gain) of the spindle ⇒ if you need to make a change in mid flight of your muscle you cannot do it without engaging the gamma motor neurons
- To retain sensitivity of the spindle the gamma motor neuron synapses on the contractile poles of the spindle, causing the poles to contract, increasing tension on the spindle and increasing sensitivity of the group Ia and II sensory neurons
67
where are proprioceptive mechanoreceptors?
they are internal stimuli
68
do sensory afferents leave the extrafusal fibers?
no
69
Muscle spindle (Ia, II)
activated when the muscle is stretched
70
Golgi tendon organ (Ib)
activated with muscle contraction
- Located In the tendon at the golgi tendon organ ⇒ different afferent to muscle spindle
71
do the muscle spindle fibers and the Golgi tendon organ work together?
No, they work in opposition to one another
72
what are the sensory neuron receptors important for? (3)
- 3D perception of self
- Coordination of movement
- Balance
73
what type of feedback does the Golgi tendon organ give?
negative feedback
- if you contact the muscle you stretch the GTO ⇒ need stretch to activate and cause firing
74
properties of spindle sensory neurons (Ia and II)? (3)
- Encapsulated in the spindle (intrafusal)
- when the muscle stretches, increased spindle length is detected
- Most sensitive to muscle stretch
75
what are Ia afferents most sensitive to?
velocity of stretch ⇒ dynamic (big) change
76
what are II afferents most sensitive to?
sustained stretch
77
what is the conduction of Ia and II fibers?
80-120 m/s
78
which is faster I or II? a or b?
I; a
79
where is the GTO located?
anchored between the highly contractile muscle and the relatively inert tendon
80
what does contraction of the muscle do to the GTO?
it stretches the GTO and activates the Ib sensory afferent
- GTO is the most sensitive to alpha motor unit contraction and relatively insensitive to stretch
- Functions in a negative feedback loop to maintain steady level of force
81
what does passive stretch vs active contraction look like for muscle spindles (1a)?
- when the muscle is stretched during passive stretch, the intrafusal muscle fibers fire and send out 1a afferents
- when there is active contraction the spindle afferent does not fire aside from spontaneous APs
82
what does passive stretch vs active contraction look like for the GTO (1b)?
- when the muscle has a passive stretch the Golgi tendon organ is compressed and only fires spontaneous APs
- when the muscle is actively contracted the alpha motor neuron is stimulated and the GTO fires
83
what happens to the stretch fibers when the alpha motor neuron is stimulated without the gamma neuron activation? with the game neuron?
the intrafusal muscle fibers start firing after the contraction is done taking place; the 1a fibers fill in the contraction time with activity as the stimulated gamma motor neuron synapses on the muscle spindle
84
what do gamma motor neurons do to other sensory afferents?
they modulate sensitivity of the Ia and II afferents
- the mechanism to maintain tension in the spindle afferent despite the muscle contraction is what the gamma motor region does
85
what happens when you electrically stimulate the gamma motor neuron on the muscle spindle?
you maintain tension so the spindle is stretched rather than flaccid and can fire action potentials
- The issue is whether you can make a change in the muscle contraction ⇒ you need gamma motor neuron activation
86
what are the descending inputs to the gamma motor neuron? (3)
- Motor cortex
- Cerebellum ⇒ making these adjustments
- Co-activate the alpha and gamma motor neuron
87
when is the gamma motor neuron more active during movement?
with more complex movements
- work on modulating the spindle
88
what happens with damage to the cerebellum?
big effect on modulation of the gamma motor neurons in modulating the spindle
89
what is ALS?
neurodegenerative disease with loss of myelin, etc.
- Beyond skeletal muscles so diaphragm too affecting breathing
- ALS is both an upper and lower motor disease
90
what factors affect ALS? (9)
- release of inflammatory mediators
- dysfunction of axonal transport systems
- 3 Na+/2 K+ pump dysfunction
- neurofilament dysfunction in the axon
- mitochondrial dysfunction
- impaired glutamate intake in astrocytes and glutamate excitotoxicity in the neurons
- secretion of toxic factors from astrocytes
- oxidative stress and RNA dysregulation
- SOD1 aggregation and mutants
91
end card
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