Lecture 5: Spinal Cord Reflexes and Locomotion Flashcards
What is a motor unit?
single motor neuron and the muscle fibers that it innervates
What is the motor neuron pool?
all motor neurons innervating a given muscle
What is the size recruitment/principle?
1st: small
2nd: intermediate
3rd: large
if small isnt enough, then inter., then large
What are the functions of group 1a and 2 afferent axons?
- innervate intrafusal muscle fibers
- detect muscle length and stretch
What are the characteristics of small motor units?
- doesnt generate as much force
- can last for a long time
What are the characteristics of large motor units?
- generate large amount of force
- cannot last as long
If there is a change in muscle length, what will the capsule do?
detect it and send the signal to the spinal cord
What is used to measure muscle length?
muscle spindle: capsule + nerves + intrafusal myofibers
What are the characteristics of group 1a fibers?
- measures muscle extension
- once stretched -> fire immediately but goes back
- doesnt last because it’s required for movement
What are the characteristics of group 2 fibers?
once stretched -> slowly fires but remains at high levels of firing
What are the steps of the stretch (myotatic) reflex of the extensor muscle?
- once patella is hit, the extensor muscle takes longer
- elongation activates sensory axons
- afferent sensory nerves go to spinal cord (enters through dorsal horn)
- alpha-motor neuron and sensory neuron innervate the extensor ->cause contraction
What are the steps of the stretch (myotatic) reflex of the flexor muscle?
- sensory axons activate inhibitory neurons -> inhibits other alpha neuron that innervates the flexor muscle
- flexor becomes inhibited -> becomes relaxed -> able to kick forward
What is stretch reflex?
negative feedback regulation of muscle length
What are the 2 paths of alpha-gamma co-activation?
direct and indirect activation
What is direct ativation?
- by upper motor neuron
- muscle gets extended -> aloha activation goes back to muscle -> puts arm back
- muscle spindle already extended, flappy
- no gamma activation
What is indirect activation?
- by gamma loop
- with gamma activation
- gamma afferent axon will transduce a signal to muscle spindle to make it tight again
What does the golgi tendon (inverse myotatic) reflex measure?
force
What is the inverse myotatic reflex?
negative feedback regulation of muscle force
What triggers the tendon organ in the capsule?
increase muscle force in flexor muscle
What is the tendon organ innervated by?
group 1b afferent neurons which measure the stretch of the tendon
Motor neurons to synergistic muscles are (inhibited or excited) via (inhibitory or excitatory) neurons?
inhibited via inhibitory neurons
Motor neurons to antagonistic muscles are (inhibited or excited) via (inhibitory or excitatory) neurons?
excited via excitatory neurons
During the golgi tendon reflex, the flexor is … and the extensor is …..
flexor is relax; extensor is activated
What reflex tends to increase muscle stiffness?
stretch (myotatic)
What reflex tends to decrease muscle stiffness?
tendon organ (inverse myotatic)
What is the spinal flexor reflex a response to?
noxious stimulation of skin or msucle
Is the stimulated limb withdrawn or extended?
withdrawn (flexed)
When the stimulated limb is withdrawn, what are the actions of the flexor and extensor muscles?
- flexor: contracts
- extensor: relaxed
When the non-stimulated limb is extended, what are the actions of the flexor and extensor muscles?
- flexor: relaxed
- extensor: extended
That neuron never touches muscles?
upper motor neuron
How do upper motor neurons regulate muscle strength?
through lower motor neurons
How are spinal cord neurons influenced?
by descending pathways
What is the upper motor neuron syndrome?
down regulation of KCl cotransporter contributes to spasticity
What occurs during the upper motor neuron syndrome?
- cotransporter is downregulated
- depolarization of Cl equilibrium potential and reducing the strength of postsynaptic inhibition
- hyperexcitability
What is the future therapy for upper motor neuron syndrome?
BDNF upregulates transporter after injury
What is the central pattern generator?
locomotion in spinal cord
What are the mehcanisms to generate the central pattern generator?
- pacemaker activates neuron -> activates extensor
- emergent network property: neuron fires at constant rate and activates extensor/flexor neurons
What are the 2 things related to optogenetics and central pattern generator?
channelrhodopsin 2 and halorhodopsin
What are the characteristics of channelrhodopsin?
- associated with glutamatergic neurons
- gated by blue light and conducts H+, Na+, K+, and Ca+ ions
- leads to depolarization
What are the characteristics of halorhodopsin?
- associated with glutamatergic neurons
- gated by green/yellow light and conducts inly Cl- ions
- leads to hyperpolarization
What turns on the central pattern generator?
selective excitation of glutamatergic neurons
With optogenetics and CPG, coordination of left and right sides are provided by what interneurons?
commissural
What does the brainstem modulate during REM sleep?
muscle tone and locomotion
What occurs during quiet sleep?
rapid firing
What occurs during deep sleep?
brain activity decreases
Cross-country athletes require significant levels of endurance to maintain a steady pace during long-lasting races. Given the nature of the sport, which motor unit is likely functioning for extended periods of time with minimal fatigue?
A. Large
B. Intermediate
C. Small
D. None of the above
C
Which of the following is considered a sensory afferent fiber that innervates muscle spindles?
A. Group Ia neurons
B. α motor neurons
C. Group Ib neurons
D. γ motor neurons
A
An MSP3 student is getting a workout in after a long day of studying. After increasing the weight on the barbell, he notices that his muscles very easily “give out.” Based on his recent studies, he believes this is likely negative feedback regulation of muscle force at the level of the spinal cord. What reflex is the student referring to?
A. Alpha-Gamma
B. Inverse Myotatic
C. Spinal Flexor
D. Myotatic
B
Upper motor neuron (UMN) syndrome is caused by lesions of descending motor pathways and often presents has hypotonia followed by spasticity. Which of the following is responsible for the secondary spasticity?
A. Downregulation of KCC2
B. Increased hyperpolarization of UMN
C. Upregulation of BDNF
D. Increased GABA release
A
Channelrhodopsin 2 (ChR2) is a _________ light activated ion channel used to stimulate cellular _________ in optogenetic studies.
A. Blue; Hyperpolarization
B. Green; Depolarization
C. Blue; Depolarization
D. Green; Hyperpolarization
C
