Module 2: Lower Motor Neurons Flashcards
(135 cards)
1
Q
what does ‘motor systems’ mean
A
behavior and the biology that creates it; the neural connections that hook up to our brain with our muscles that allow us to move
2
Q
where do simple behaviors usually have circuits in
A
brainstem and spinal cord
3
Q
where do complex behaviors usually have circuits in
A
higher levels of the brain
4
Q
descending systems composition
A
motor cortex, brainstem centers
5
Q
motor cortex function
A
planning, initiating, and directing voluntary movements
6
Q
brainstem centers function
A
basic movements and postural control
7
Q
basal ganglia connectivity
A
to the motor cortex
8
Q
basal ganglia function
A
gating proper initiation of movement
9
Q
cerebellum connectivity
A
brainstem centers
10
Q
cerebellum function
A
sensory motor coordination of ongoing movement
11
Q
spinal cord and brainstem circuits composition
A
lower circuit neurons and motor neuron pools
12
Q
where do motor neuron pools project to
A
skeletal muscles
13
Q
where do local circuit neurons project to
A
motor neuron pools
14
Q
where do sensory inputs first project to
A
local circuit neurons
15
Q
where do descending systems mainly project to
A
local circuit neurons
16
Q
where do descending systems mildly project to
A
motor neuron poolss
17
Q
general classes of movement
A
- reflexes
- rhythmic
- voluntary
18
Q
where does the circuit for reflexes lie
A
simple neural circuit that resides in spinal cord or brainstem
19
Q
main component of rhythmic movement
A
central component (central pattern generator) that regulates brainstem or spinal cord oscillatory circuit
20
Q
what parts of the brain does voluntary movement require
A
‘higher’ brain levels such as cortex, basal ganglia, cerebellum
21
Q
In general, a reflex uses neurons located in _____ which voluntary movement requires neurons located in _______
A
the spinal cord; the cortex
22
Q
what is the relationship between sensation and movement
A
there is blurring; you can’t separate sensory from motor systems
- behavior is continuously motivated and guided by sensory input
23
Q
feed-forward control
A
watching a ball fall and moving the arm into a position to catch it
24
Q
feedback control
A
after the ball hits the hand, the movement of the arm to return to its original position
25
kinds of tracer injections
anterograde, retrograde
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anterograde dye path
travels from cell body --> axons
- yields complete reconstruction of axons
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retrograde dye path
travels from nerve terminals back toward soma
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pseudorabies function
jump synapses giving you multiple steps in the circuit
- often toxic
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in vitro physiology
'in a dish'
- dye is injected into two cells that appear to be synaptically coupled to determine connectivity
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in vivo calcium imaging
- inject a virus encoding a protein that fluoresces in response to ca
- miniscope lets you record ca signals as a proxy for neural activity
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optogenetics
neurons are triggered to become light sensitive and fire when activated by light
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techniques to record patterns of energy use by the brain during behavior
- positron emission tomography (PET)
- functional or structural magnetic resonance imaging (MRI)
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techniques to record patterns of immediate early gene activation
immediate early gene (IEG) mRNA detection
34
what is IEG
a gene whose mRNA is rapidly transcribed following neuronal activity within the active neuron
- requires a gene probe to detect them
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what can IEG reveal
neurons that were active prior to death
- poor man's fMRI
36
which of the following techniques would be most useful for discovering whether two neurons are synaptically connected to one another?
d. in vitro electrophysiology
37
types of muscle
smooth vs striated
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types of striated muscle
cardiac vs skeletal
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what is skeletal muscle composed of
extrafusal and intrafusal muscle fibers
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extrafusal muscle fiber function
generate the mechanical forces required for movement
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intrafusal muscle fiber function
embedded in the extrafusal fibers; sensory ending wrap around and report how much tension is in the muscle
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what are the extrafusal fibers innervated by
alpha motor neurons
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where do the cell bodies of alpha motor neurons lie
ventral horn of the spinal cord
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what are the intrafusal fibers innervated by
gamma motor neurons
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where do gamma motor neuron cell bodies lie
adjacent to alphas
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synergist
muscle that produces a similar motor action when contracted
- eg soleus and gastrocnemius muscles
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antagonist
muscle that produces the opposite motor action
- eg flexors vs extensor, biceps vs triceps
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contralateral
muscle of the opposite limb
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how can alpha motor neurons be labeled
by injecting a retrograde dye into the muscle
50
what kind of neurons. do lower motor neuron circuits include
alpha neurons and local interneurons
51
how are alpha motor neurons organized relative to the muscles that they control
somatotopically
52
topography in neuroscience
mapping of an external surface or stimulus onto a biological substrate
53
which direction does the somatotopic map run?
- medial to lateral; motor neurons for distal muscles are located laterally a d proximal muscles are located more medially
- segmentally; motor neurons are located at the same segmental level as the muscles they innervate
54
local circuit neurons
cell bodies and axons are all located within the spinal cord
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projection neuron
cell body and nerve terminal are in separate brain areas
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interneuron placement
cell body and terminal are within the same general brain area
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motor unit
single alpha motor neuron and all the muscle fibers it innervates
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what do alpha motor neurons release at neuromuscular junction
acetylcholine
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what does acetylcholine release cause
an 'endplate' potential (epp), ie an epsp in the muscle fiber
- if large enough, there is an action potential --> the muscle twitches
60
three ways to increase muscle force
- stimulate a given motor unit more frequently
- recruit more motor units
- recruit fast-fatiguable motor units in addition to slow ones
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example of slow fatiguable muscles
soleus
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where is the threshold for slow fatiguable muscles
low
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comparative force produced by slow fatiguable
weaker
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slow fatiguable soma size
small
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slow fatiguable capillary bed color
red
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slow fatiguable energy source
oxidative
- requires blood flow
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slow fatiguable ration neuron/fiber
1:180
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slow fatiguable contraction speed
slow
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thanksgiving example for slow fatiguable muscles
dark meat
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fast fatiguable muscle example
gastrocnemius
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fast fatiguable muscle threshold
high
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fast fatiguable muscle force
stronger
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fast fatiguable muscle soma size
large
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fast fatiguable muscle capillary bed color
pale
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fast fatiguable energy source
glycolytic
- uses enzymes that run out
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fast fatiguable ratio neuron/fiber
1:2000
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fast fatiguable contraction speed
fast
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fast fatiguable thanksgiving example
white meat
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recruitment order of motor neurons
slow fatiguable --> fast fatigue resistant --> fast fatiguable
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what is in charge of the muscle fiber type?
alpha motor neuron
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what muscle type is a soleus muscle with a slow alpha motor neuron connection
slow
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what muscle type is a gastrocnemius muscle with a fast alpha motor neuron connection
fast
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what muscle type is a soleus muscle with a fast alpha motor neuron connectino
fast-like
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what muscle type is a gastrochemius muscle with a slow alpha motor neuron connection
slow-like
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what does 10 Hz stimulation favor
the development of slow units
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what does 100 Hz stimulation favor
the development of fast units
87
what is meant by a circuit
a series of neurons synaptically linked that work together to perform some function
- usually have an 'input' and an 'output'
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afferents
nerves that carry info into the CNS
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efferents
nerves that send signals out of the CNS
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where are most motor neurons located
in the ventral horn
91
1a afferents
sensory neurons
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where are 1a afferents cell bodies located
dorsal root ganglia
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proprioceptors
provide info about self-movement, force, & body position
- spindles & their afferents are examples
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what neurons are essential for the myotatic reflex
1a afferents
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when are mechanosensitive ion channels on the 1a afferents gated
when the intrafusal fibers are stretched
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what happens when intrafusal fibers on 1a afferents are stretched
stretch --> opens channels --> sodium flows in --> action potential travels into the spinal cord
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metaphor for myotatic reflex
like putting your fingers into a fishing net and someone stretching it
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how does the knee tap reflex occur
tap on patellar tendon stretches quadriceps --> activates the muscle spindle --> burst of APs in 1a spindle afferents --> release of glutamate onto the alpha motor neuron innervating the extensor --> motor neuron depolarizes --> fires APs --> releases ACh onto the muscle --> muscle contracts
99
what does the burst of action potentials in the 1a spindle afferents cause in the knee tap reflex
a release of glutamate onto the alpha motor neuron innervating the extensor
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what does a release of glutamate on the alpha motor neuron innervating the extensor cause in the knee tap reflex
the motor neuron depolarizes and fires action potentials
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what happens when the motor neuron depolarizes and fires an action potential in the knee tap reflex
it releases ACh onto the muscle and it contract
102
what is the function of the interneuron in terms of spindle afferents
the spindle afferents also synapse on an inhibitory interneuron which releases GABA onto the alpha motor neuron innervating the flexor; motor neuron hyperpolarizes and the muscle relaxes
103
What does stimulation of nocireceptive sensory fibers do
excite the ipsilateral flexor and inhibit the extensor
- on the contralateral side, stimulus inhibits the flexor and stimulates the extensor
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what happens when gamma motor neurons fire?
intrafusal fibers contract
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what do small gamma motor neurons do
adjust the tension, or 'gain' in the spindle
106
what does increasing the tension of muscle fibers do
increases the sensitivity of the system
107
what are Golgi tendon organs
second sensory system associated with muscles
108
what is the difference between Golgi tendon organs and muscle spindles
tendons are in series with the muscle while spindles are in parallel
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what evokes the biggest response in the Golgi tendon system
contraction
110
happens when a tendon is stretched
mechanosensitive ion channels on the 1b afferents gate
- happens when the muscle contracts
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characteristics of muscle spindles (signal, position, and senstivity)
- signals mostly about muscle length
- it is in parallel with extrafusals
- sensitivity is set by gamma motor neurons in spinal cord
112
characteristics about Golgi tendon organ (signals, position, and sensitivity)
- signals mostly about muscle contraction
- is in series with extrafusals
- no known setting of sensitivity
113
does rhythmic walking require sensory info?
no
114
first step of rhythmic activity in a spinal interneuron
without glutamate the receptors are closed and there is no activity
115
what happens when glutamate is applied in rhythmmic activity in a spinal interneuron
it binds to both the AMPA and NMDA receptors
116
what happens when glutamate binds to the AMPA and NMDA receptors in rhythmic activity in a spinal interneuron
the membrane begins to depolarize due to sodium ions entering through the AMPA receptor
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what initially happens to the NMDA receptor in rhythmic activity in a spinal interneuron
initially no ions flow through the NMDA receptor because the channel is blocked by magnesium
118
what happens as the membrane starts to depolarize in rhythmic activity in a spinal interneuron
the magnesium is electrostatically repulsed
119
what happens when the AMPA and NMDA receptors open in rhythmic activity in a spinal interneuron
they let in sodium leading to lots of action potentials
- NMDA receptor is also permeable to calcium
120
what happens over time in rhythmic activity in a spinal interneuron
the AMPA receptor desensitizes and closes, then resensitizes and re-opens because glutamate is still around
121
what happens when calcium that comes in through the NMDA receptor binds to the calcium-activated potassium channel
the receptor opens and lets potassium out of the cell, which repolarizes the membrane
122
what happens during the hyperpolarization in rhythmic activity in a spinal interneuron
magnesium re-blocks the NMDA receptor but once the AMPA receptors resensitize, the cycle repeats
123
true or false: in vitro application of glutamate ensures that those neurons expressing glutamate receptors will be tonically (consistently) active
false
124
what is one way to translate tonic excitation into alternating rhythmic
reciprocol inhibition
125
first step of reciprocal inhibition
action potentials causing glutamate release onto both excitatory interneurons
126
what happens after glutamate is released onto both excitatory interneurons in reciprocal inhibition
stochastically, the extensor excitatory interneuron fires first
127
what happens after the extensor excitatory interneuron fires first in reciprocal inhibition
both excitatory interneurons have axon collaterals that release glutamate onto inhibitory interneurons
128
what happens after both excitatory interneurons release glutamate onto inhibitory internuerons in reciprocal inhibition
one of the inhibitory interneurons releases GABA onto the top excitatory interneuron, hyperpolarizing it so it doesn't fire
129
what happens after an excitatory interneuron is hyperpolarized in reciprocal inhibition
an inhibitory interneuron also releases GABA onto itself via an autapse
- so it no longer inhibits the flexor interneuron
130
what happens after the first interneuron autapse?
the flexor interneuron is free to respond to the glutamate and begins firing
- also releases glutamate onto the other inhibitory interneuron
131
what happens after the flexor interneuron responds to the glutamate
the inhibitory interneuron releases GABA onto the extensor excitatory interneuron, hyperpolarizing it so it no longer fires
132
what happens after the excitatory interneuron is hyperpolarized in reciprocal inhibition DELETEEEE
an inhibitory interneuron also releases GABA onto itself via an autapse
- it inhibits itself and stops inhibiting the extensor excitatory interneuron
133
what happens after an inhibitory interneuron inhibits itself and stops inhibiting the extensor motor neuron
the extensor excitatory interneuron is now able to respond to the glutamate and starts firing, thereby regenerating the cycle
134
what mechanisms can lead to escape from inhibition
inhibitory autapse, fatigue of presynaptic release, or accommodation of the postsynaptic response
135
