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 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 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.
Selective _ of glutamatergic neurons turn on CPG
excitation
Are multiple spinal cord levels involved in rhythmogenic modules underlying locomotion
yes
