Exam2Lec5SpinalCordReflexesandLocomotion Flashcards
What is a motor unit
single motor neuron and the muscle fibers that it innervates (can inn. multiple mucles)
What is a motor neuron pool
all motor neurons inn. a given muscle (ex. biceps)
What are the three different types of motor units with:
* motor unit size
* force
* contraction velocity
* rate of fatigue
* muscle type (ATP synthesis)
* Myoglobin content (oxygen binding protein)
* Common recruitment order
What is the size principle
Order of motor unit recruitment – First: smaller motor neurons and slow muscle fibers; last: larger motor neurons with fast fatigable muscle fibers.
What is a muscle spindle, what does it contain and how are they arranged?
- A spindle is a connective tissue sheath that contains intrafusal muscle fibers.
- Spindles are arranged in parallel with extrafusal fibers
What inn. intrafusal muscle fibers?
Sensory nerve endings innervate intrafusal muscle fibers: their cell bodies are in the dorsal root ganglion and their axons enter the cord via dorsal roots.
Besides stretching the skeletal muscle, what also stretches? what happens?
Stretch of skeletal muscles also stretches primary Ia (faster conduction velocity) and group II (slower) endings and increases their firing rate.
What does Ia afferents measure? II?
- Ia afferents “measure” muscle length and rate of change of length (velocity)
- Group II measure length
to send signal to SC
intrafusal muscle + capsule =
muscle spindle
Compare group Ia and II afferent firings and their importence
Ia:
* when stretch happens, it fires immediately but then goes away immediately
* initial mvt (thinking about mvt)
II:
* when stretch happens, it fires slowly but remains longer
* used to maintain movement
What do you hit in the stretch reflex? and what does it cause (first thing)
hitting tendon attached to muscle then casues the muscle to stretch
What happens in the stretch reflex when the muscle is stretched?
- When a muscle is stretched, muscle spindle senses it and sends signal through spindle afferents (Ia and II) to excite alpha motor neurons that innervate the stretched muscle (directly). This is the monosynaptic stretch reflex
- Ia afferents also inhibit alpha motor neurons that innervate “antagonistic” muscles through inhibitory interneuron (indirectly). This is a di-synaptic reflex for “reciprocal inhibition”: an inhibitory interneuron is interposed between the Ia afferent and the motor neurons (that innervate the un-stretched muscle).
How is the stretch reflex regulates muscle length through neg. feedback
- acts to reduce the length of the muscle by exciting the alpha motoneurons that innervate the muscle with which it is associated
- This is an expample of direct activation of alpha motor neurons
help us remove hand from hand stove
Presynaptic inhibition normally regulates the ____ or “ ____ ” of the stretch reflex.
Presynaptic inhibition normally regulates the strength or “gain” of the stretch reflex.
What is proprioception?
Proprioception is the sense of the relative position of neighboring parts of the body and strength of effort being employed in movement
What does the Ia afferents also provide?
proprioceptive information about limb position & joint angle.
* vibration of a muscle (~100 Hz, 500 micron amplitude) can selectively activate spindle afferents and change the perception of joint angle!!
What does gamma motor neurons selectively inn?
intrafusal muscle fibers
What does the activation of gamma motoneurons by descending signals cause?
- Causes contraction of the two ends of the intrafusal muscle fibers
- This action stretches the middle and promotes activity in the Ia afferents
What are the two classes of decending routes for alpha motoneuron activation?
- First there is “direct” activation of alpha motoneurons by descending signals (this can be monosynaptic or through interneurons*).
- The second “route” is “indirect” activation via the gamma loop: descending signals excite gamma -> intrafusal fibers contract -> stretches Ia -> activates alpha motor neuron
What do gamma neurons do?
regulate length of spindle at same time as alpha motor neuron that reg. muscle
What is alpha gamma co-activation
- In many instances, movements are produced by alpha - gamma co-activation. This process keeps the intrafusal fibers contracted while extrafusal fibers contract.
- Thus, the Ia spindle afferents remain active - ready for increase in load!
- FACILIATES LOAD COMPENSATION
_ afferent nerve endings are found on bundle of small tendon fascicles adjacent to _ junction.
Ib afferent nerve endings are found on bundle of small tendon fascicles adjacent to musculotendinous junction.
What is Ib afferent activated by?
tension produced by muscle contraction
What can the tendon organ reflex, also be called?
inverse myotatic reflex
tendon organs are in _ and meauses what?
- in series
- muscle force
the tendon organ reflex is what type of synaptic reflex
a di- (two) synaptic reflex arc
* The alpha motor neurons to same and synergistic muscles are inhibited. Motor neurons to antagonistic muscles are excited.
What is the purpose of the tendon organ reflex through neg feedback
acts to reduce force
how does the inverse myotatic reflex act on muscles
you relax the same muscle becasue neg feedback
* motor neurons to synergictic muscles are inhibited are inhibited via inhibitory interneuron
* motor neurons to antagonistic muscles are excited via excitartory interneuron
What does stretch and tendon organ reflexes contribute to?
generation of appropritate stiffness for support and shock absorber functions
What is muscle stiffness equation
change in force/change in length
How does the stretch reflex and tendon organ reflex affect muscle stiffness
- Stretch reflex tends to increase muscle stiffness
- Tendon organ reflex tends to decrease muscle stiffness
polysynaptic reflex (flexor reflex) responds to what?
noxious stimulation of the skin or muscle.
What happens in polysnaptic reflex?
Cutaneous nociceptor evoke a sensory signal to the spinal cord to cause poly synaptic spinal reflex for withdrawal and support
* inhibitory interneuron stops extendor on the stimulated leg and interneuron activates the extensor on the other leg.
* interneuron activates flexor on effect leg and inhibitory interneron stops the flexor on the other leg
What is an upper motor neuron
is a clinical term for neurons in the brain that project to
spinal motor circuits & motor neurons that directly innervate muscles
When you have damage to an upper motor neuron, what changes?
the resting level of tension in muscle (motor tone)
With upper motor neuron damage, what happens first and what is followed?
First hypotonia (spinal shock) followed by hypertonia (“spasticity”)
What happens with damage with LMN? UMN?
LMN-> paralysis
UMN-> spasmtic (contractions)
What are inhibitory NT for the UMN and LMN
gaba and glycine-> neurons must have low levels of cl- inside
what does gaba a and glycine hibition of motor neurons depends on what? What is it maintain by?
depends upon low intracellular chloride concentration maintained by the potassium-chloride cotransporter KCC2
What happens with the cotransporter, KCC2, after spinal cord injury
cotransporter is downregulated in motoneurons after spinal cord injury in rats -> depolarizing the chloride equilibrium potential and reducing the strength of postsynaptic inhibition-> contributing to hyper-excitability (SPASTICITY)
What is something that upregulates the transport after spinal cord injury
neurotrohic factor BDNF
what normally does the activation of GABAa and glycine receptors that inhibit motor neurons result in?
esult of low intracellular chloride concentration maintained by the potassium-chloride cotransporter KCC2.
Explain what is going on with central pattern generator for locomotion
Stepping cycle of a leg has two phases: extensor and flexor muscles alternate Central mechanisms for generating stepping rhythm in spinal cord:
* “Spinal” animals with deafferented hind limbs can walk on tread mill
* Temporal sequence of muscles activated not altered
* Central pattern generator sets basic rhythm & order of activation of different muscles
Reflex feedback is not necessary for rhythm generation … but reflex feedback does influence phase durations & pattern.
Phase dependent reflex reversal:
* In the swing phase, stimulating the dorsal surface of the foot can produce additional flexion.
* During the stance phase, on the other hand, the most effective way for the animal to get over a moving object touching its paw would be to perform an increased rapid extension followed by flexion.”
Rhythm persists in spinal animal after dorsal roots cut
Reflex feedback is not necessary for rhythm generation …
but reflex feedback does influence phase durations & pattern. Phase dependent reflex reversal
What are the two classes of mechanism for CPG
- pacemaker cells have intrinsic membrane properties that cause periodic “bursts” of action potentials. (For example, alternating inward (Ca++) and outward (K+) currents)
- “Emergent” network property: no cell has bursting properties. Reciprocal inhibition & adaptation in a network account for the alternating activity in flexors & extensors.
What is evidence for multople rhythmogenic modules for locomotion
Optogenetic evidence: Light-gated ion channels or transporters -> expressed in neurons using cell specific promoters.
What is activated by blue light and what does it do?
ChR2 nonspecific cation channel gated by blue light conducts H+, Na+, K+, and Ca2+ ions
DEPOLARIZATION
What is activated by green/yellow light and what does it do?
Halorhodopsin gated by green/yellow light: conducts Cl- ions
HYPERPOLARIZATION
Selective _ of glutamatergic neurons turn on CPG
excitation
Are multiple spinal cord levels involved in rhythmogenic modules underlying locomotion
yes
coordination of left and right sides are provided by
commissural interneurons
Brainstem control of _ _ and _ and its modulation during REM Sleep
Brainstem control of muscle tone and locomotion and its modulation during REM Sleep
why do we have decreased brain activity so we dont sleep walk normally? how are our muscle (toned, relaxed, etc)
pons sends signals to locomotion path to stop it from firing
muscle has tone while sleeping because pons also sends signal to the medulla to regulate muscule tone
Which cells prevent unloading of muscle spindle afferents during muscle contraction and facilitate load compensation?
a. Alpha motor neurons
b. Gamma motor neurons
c. Golgi tendon organ afferents d. Cutaneous nociceptors
B. gamma
Which cells prevent unloading of muscle spindle afferents during muscle contraction and facilitate load compensation?
a. Alpha motor neurons
b. Gamma motor neurons
c. Golgi tendon organ afferents d. Cutaneous nociceptors
B. gamma
