Module 2: Lower Motor Neurons Flashcards
what does ‘motor systems’ mean
behavior and the biology that creates it; the neural connections that hook up to our brain with our muscles that allow us to move
where do simple behaviors usually have circuits in
brainstem and spinal cord
where do complex behaviors usually have circuits in
higher levels of the brain
descending systems composition
motor cortex, brainstem centers
motor cortex function
planning, initiating, and directing voluntary movements
brainstem centers function
basic movements and postural control
basal ganglia connectivity
to the motor cortex
basal ganglia function
gating proper initiation of movement
cerebellum connectivity
brainstem centers
cerebellum function
sensory motor coordination of ongoing movement
spinal cord and brainstem circuits composition
lower circuit neurons and motor neuron pools
where do motor neuron pools project to
skeletal muscles
where do local circuit neurons project to
motor neuron pools
where do sensory inputs first project to
local circuit neurons
where do descending systems mainly project to
local circuit neurons
where do descending systems mildly project to
motor neuron poolss
general classes of movement
- reflexes
- rhythmic
- voluntary
where does the circuit for reflexes lie
simple neural circuit that resides in spinal cord or brainstem
main component of rhythmic movement
central component (central pattern generator) that regulates brainstem or spinal cord oscillatory circuit
what parts of the brain does voluntary movement require
‘higher’ brain levels such as cortex, basal ganglia, cerebellum
In general, a reflex uses neurons located in _____ which voluntary movement requires neurons located in _______
the spinal cord; the cortex
what is the relationship between sensation and movement
there is blurring; you can’t separate sensory from motor systems
- behavior is continuously motivated and guided by sensory input
feed-forward control
watching a ball fall and moving the arm into a position to catch it
feedback control
after the ball hits the hand, the movement of the arm to return to its original position
kinds of tracer injections
anterograde, retrograde
anterograde dye path
travels from cell body –> axons
- yields complete reconstruction of axons
retrograde dye path
travels from nerve terminals back toward soma
pseudorabies function
jump synapses giving you multiple steps in the circuit
- often toxic
in vitro physiology
‘in a dish’
- dye is injected into two cells that appear to be synaptically coupled to determine connectivity
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
optogenetics
neurons are triggered to become light sensitive and fire when activated by light
techniques to record patterns of energy use by the brain during behavior
- positron emission tomography (PET)
- functional or structural magnetic resonance imaging (MRI)
techniques to record patterns of immediate early gene activation
immediate early gene (IEG) mRNA detection
what is IEG
a gene whose mRNA is rapidly transcribed following neuronal activity within the active neuron
- requires a gene probe to detect them
what can IEG reveal
neurons that were active prior to death
- poor man’s fMRI
which of the following techniques would be most useful for discovering whether two neurons are synaptically connected to one another?
d. in vitro electrophysiology
types of muscle
smooth vs striated
types of striated muscle
cardiac vs skeletal
what is skeletal muscle composed of
extrafusal and intrafusal muscle fibers
extrafusal muscle fiber function
generate the mechanical forces required for movement
intrafusal muscle fiber function
embedded in the extrafusal fibers; sensory ending wrap around and report how much tension is in the muscle
what are the extrafusal fibers innervated by
alpha motor neurons
where do the cell bodies of alpha motor neurons lie
ventral horn of the spinal cord
what are the intrafusal fibers innervated by
gamma motor neurons
where do gamma motor neuron cell bodies lie
adjacent to alphas
synergist
muscle that produces a similar motor action when contracted
- eg soleus and gastrocnemius muscles
antagonist
muscle that produces the opposite motor action
- eg flexors vs extensor, biceps vs triceps
contralateral
muscle of the opposite limb
how can alpha motor neurons be labeled
by injecting a retrograde dye into the muscle
what kind of neurons. do lower motor neuron circuits include
alpha neurons and local interneurons
how are alpha motor neurons organized relative to the muscles that they control
somatotopically
topography in neuroscience
mapping of an external surface or stimulus onto a biological substrate
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
local circuit neurons
cell bodies and axons are all located within the spinal cord
projection neuron
cell body and nerve terminal are in separate brain areas
interneuron placement
cell body and terminal are within the same general brain area
motor unit
single alpha motor neuron and all the muscle fibers it innervates
what do alpha motor neurons release at neuromuscular junction
acetylcholine
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
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
example of slow fatiguable muscles
soleus
where is the threshold for slow fatiguable muscles
low
comparative force produced by slow fatiguable
weaker
slow fatiguable soma size
small
slow fatiguable capillary bed color
red
slow fatiguable energy source
oxidative
- requires blood flow
slow fatiguable ration neuron/fiber
1:180
slow fatiguable contraction speed
slow
thanksgiving example for slow fatiguable muscles
dark meat
fast fatiguable muscle example
gastrocnemius
fast fatiguable muscle threshold
high
fast fatiguable muscle force
stronger
fast fatiguable muscle soma size
large
fast fatiguable muscle capillary bed color
pale
fast fatiguable energy source
glycolytic
- uses enzymes that run out
fast fatiguable ratio neuron/fiber
1:2000
fast fatiguable contraction speed
fast
fast fatiguable thanksgiving example
white meat
recruitment order of motor neurons
slow fatiguable –> fast fatigue resistant –> fast fatiguable
what is in charge of the muscle fiber type?
alpha motor neuron
what muscle type is a soleus muscle with a slow alpha motor neuron connection
slow
what muscle type is a gastrocnemius muscle with a fast alpha motor neuron connection
fast
what muscle type is a soleus muscle with a fast alpha motor neuron connectino
fast-like
what muscle type is a gastrochemius muscle with a slow alpha motor neuron connection
slow-like
what does 10 Hz stimulation favor
the development of slow units
what does 100 Hz stimulation favor
the development of fast units
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’
afferents
nerves that carry info into the CNS
efferents
nerves that send signals out of the CNS
where are most motor neurons located
in the ventral horn
1a afferents
sensory neurons
where are 1a afferents cell bodies located
dorsal root ganglia
proprioceptors
provide info about self-movement, force, & body position
- spindles & their afferents are examples
what neurons are essential for the myotatic reflex
1a afferents
when are mechanosensitive ion channels on the 1a afferents gated
when the intrafusal fibers are stretched
what happens when intrafusal fibers on 1a afferents are stretched
stretch –> opens channels –> sodium flows in –> action potential travels into the spinal cord
metaphor for myotatic reflex
like putting your fingers into a fishing net and someone stretching it
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
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
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
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
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
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
what happens when gamma motor neurons fire?
intrafusal fibers contract
what do small gamma motor neurons do
adjust the tension, or ‘gain’ in the spindle
what does increasing the tension of muscle fibers do
increases the sensitivity of the system
what are Golgi tendon organs
second sensory system associated with muscles
what is the difference between Golgi tendon organs and muscle spindles
tendons are in series with the muscle while spindles are in parallel
what evokes the biggest response in the Golgi tendon system
contraction
happens when a tendon is stretched
mechanosensitive ion channels on the 1b afferents gate
- happens when the muscle contracts
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
characteristics about Golgi tendon organ (signals, position, and sensitivity)
- signals mostly about muscle contraction
- is in series with extrafusals
- no known setting of sensitivity
does rhythmic walking require sensory info?
no
first step of rhythmic activity in a spinal interneuron
without glutamate the receptors are closed and there is no activity
what happens when glutamate is applied in rhythmmic activity in a spinal interneuron
it binds to both the AMPA and NMDA receptors
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
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
what happens as the membrane starts to depolarize in rhythmic activity in a spinal interneuron
the magnesium is electrostatically repulsed
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
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
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
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
true or false: in vitro application of glutamate ensures that those neurons expressing glutamate receptors will be tonically (consistently) active
false
what is one way to translate tonic excitation into alternating rhythmic
reciprocol inhibition
first step of reciprocal inhibition
action potentials causing glutamate release onto both excitatory interneurons
what happens after glutamate is released onto both excitatory interneurons in reciprocal inhibition
stochastically, the extensor excitatory interneuron fires first
what happens after the extensor excitatory interneuron fires first in reciprocal inhibition
both excitatory interneurons have axon collaterals that release glutamate onto inhibitory interneurons
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
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
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
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
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
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
what mechanisms can lead to escape from inhibition
inhibitory autapse, fatigue of presynaptic release, or accommodation of the postsynaptic response
