Flashcards in Muscle Spindles Deck (56):
How many spindles are in a muscle?
40 - 60.
How long are the muscle spindles?
4 - 10 mm.
What are intrafusal fibers?
- Skeletal muscle fibers innervated by gamma motor neurons within a sheet of collagen (collagen is fusal shaped)
What are the 2 general types of intrafusal fibers?
- Nuclear bag
- Nuclear chain
Which general type of intrafusal muscle fiber is larger?
- Nuclear bag is twice the length and diameter of chains
How long is each general type of intrafusal muscle fiber>
- Bag 8mm
- Chain 4mm
What are extrafusal muscle fibers?
Normal skeletal muscle
To which type of muscle fiber does more than half the innervation go to?
How are the muscle spindle fibers oriented?
In parallel to extrafusal
Which portion of the intrafusal fibers is the receptor portion of the spindle? Which is the muscular portion?
- Middle is receptor
- Outer portions are muscle
Why are nuclear bag fibers termed as such?
The collection of nuclei in the middle make the fiber bulge outwards.
What is the term for the afferent nerve fiber that wraps around the nuclear portion of the intrafusal muscle fibers?
- Anulospiral ending
What is the name of afferent nerve fiber that innervates the outer/ muscular portion of the muscle spindle?
- Flower spray fiber
What type of sensory information do the anulospiral endings collect?
- Muscle length
- Rate of muscle length change
What type of sensory information do the flower spray fibers collect?
- Tonic/ muscle length information
What are the 2 types of nuclear bag fibers?
- Dynamic bag
- Static bag
What is the difference between the dynamic and static nuclear bag fibers?
- Dynamic bag responds to rapid changes in muscle length
- Static bag and chains respond to slow stead changes in muscle length
Are chain fibers tonic or phasic?
What efferent innervation is there to the dynamic nuclear bag fiber?
- Dynamic beta and gamma fibers
What efferent innervation is there to the static nuclear bag fiber?
- Static gamma
What is the function of the efferent fibers to the muscle spindle?
To change the length of the muscle spindle and take up, or let loose slack in the spindle.
Do type II sensory afferents have tonic or phasic components?
Do type Ia sensory afferents have tonic or phasic components?
Do Ia or II respond more readily to changes in muscle length?
Do Ia or II better register muscle length alone?
Which type of afferent fiber is more active during a static stretch of a muscle?
What type of fibers fire off when a muscle spindle/ muscle is in a shortened state for an extended period of time?
What is the adjustment of the length of the muscle spindle in response to muscle length change called?
"Re-biasing" the spindle
How do afferent responses from the muscle spindle change during dynamic fusimotor stimulation?
Afferent responses to phasic length changes are increased.
Which nuclear bag is more affected by dynamic gamma motor neurons?
- The dynamic bag/ Nuclear Bag 1
What are muscle spindles in the 2nd nuclear bag more sensitive to when static fusimotor stimulation occurs ?
- Increased response to static extension
Which spindle fibers are innervated by static gamma/ fusimotor neurons?
- Both bag and static
How is afferent input, muscle force, and gain of the stretch reflex affected by static fusiform stimulation of the 2nd nuclear bag?
- Afferent increased
- Muscle force increased
- No or little change in stretch reflex
What sensitivity is NOT increased by static fusiform stimulation in the 2nd nuclear bag?
- Sensitivity to amplitude or velocity of length change
How is the nuclear chain fiber affected static fusimotor stimulation?
- Sensitivity of stretch reflex increased when a muscle is at a constant length
What is the function of static fusiform stimulation of the nuclear chain fiber?
- Muscle can support heavier loads
Which endings are more affected by intrafusal stimulation?
Which type of intrafusal stimulation may be a means of suppressing the stretch reflex during voluntary movement? Why?
- Static intrafusal stimulation because it reduces the primary ending's sensitivity to length chang.
What are the 3 aspects of servo assistance of motor command?
- Adjustment of spindle bias during movement
- Abolition of spindle length sensitivity during movement
- Modulation of spindle length sensitvity during movement
What is meant by an adjustment of spindle bias during movement? What is its function?
- Spindle discharge is maintained following muscle shortening
- Keeps spindle responsive to unexpected length changes resulting from fatigue, change in or load, or obstruction of movement
What is meant by an abolition of spindle length sensitivity during movement? When does it occur?
- Decreased spindle discharge during muscle shortening without the need for modulation based on alpha drive
- Occurs during low velocity, normal, unobstructed movements
What efferent stimulation and which muscle spindle fiber is involved in abolishing spindle length sensitivity during movement?
- Static gamma/ intrafusal stimulation to the chain fibers
What is meant by modulation of spindle length sensitivity during movement? What is its function
- High length sensitivity
- Rapidly corrects movements that deviate from a desired trajectory
Which intrafusal efferent system is responsible for modulating spindle length sensitivity during movement?
- The dynamic system
What 2 static intrafusal systems contribute to muscle tone?
- Low frequency spontaneous activity in static bag-2 fibers increasing Ia discharge causing a reflexive mild contraction
- Gamma stimulation to chain fibers increases tone via Ia and II afferents.
What can result from overactivity of the 2 muscle tone static intrafusal systems?
What is muscle tone?
- The sensation of resistance felt as one manipulates a joint through space
What are 3 reasons for muscle tone as just defined?
- Limb inertia
- Mechanical/ elastic properties of muscle and connective tissue
Which 2 afferents are the primary proprioceptive fibers?
What 2 messages are required to model planned movements?
- Message to spindle signaling desired trajectory of movement
- A signal to extrafusal muscle that takes into account any load and fatigue
Where are golgi tendon organs found? How are they oriented?
- Found at fibers of tendons at muscle insertions, and along the fascial covering of muscles
- GTOs are arranged in series
What type of force stimulates a GTO?
- Tension/ muscle contraction
What is the sensory afferent of the GTO?
What is the efferent of the GTO?
There is no efferent component
What does the primary afferent of the GTO synapse on?
An interneuron in the spinal cord that inhibits the intrasegmental alpha motor neuron