Muscle Spindles

Question 1

How many spindles are in a muscle?

Answer 1

40 - 60.

Question 2

How long are the muscle spindles?

Answer 2

4 - 10 mm.

Question 3

What are intrafusal fibers?

Answer 3

• Skeletal muscle fibers innervated by gamma motor neurons within a sheet of collagen (collagen is fusal shaped)

Question 4

What are the 2 general types of intrafusal fibers?

Answer 4

• Nuclear bag

- Nuclear chain

Question 5

Which general type of intrafusal muscle fiber is larger?

Answer 5

• Nuclear bag is twice the length and diameter of chains

Question 6

How long is each general type of intrafusal muscle fiber>

Answer 6

• Bag 8mm

- Chain 4mm

Question 7

What are extrafusal muscle fibers?

Answer 7

Normal skeletal muscle

Question 8

To which type of muscle fiber does more than half the innervation go to?

Answer 8

Intrafusal.

Question 9

How are the muscle spindle fibers oriented?

Answer 9

In parallel to extrafusal

Question 10

Which portion of the intrafusal fibers is the receptor portion of the spindle? Which is the muscular portion?

Answer 10

• Middle is receptor

- Outer portions are muscle

Question 11

Why are nuclear bag fibers termed as such?

Answer 11

The collection of nuclei in the middle make the fiber bulge outwards.

Question 12

What is the term for the afferent nerve fiber that wraps around the nuclear portion of the intrafusal muscle fibers?

Answer 12

• Anulospiral ending

Question 13

What is the name of afferent nerve fiber that innervates the outer/ muscular portion of the muscle spindle?

Answer 13

• Flower spray fiber

Question 14

What type of sensory information do the anulospiral endings collect?

Answer 14

• Muscle length

- Rate of muscle length change

Question 15

What type of sensory information do the flower spray fibers collect?

Answer 15

• Tonic/ muscle length information

Question 16

What are the 2 types of nuclear bag fibers?

Answer 16

• Dynamic bag

- Static bag

Question 17

What is the difference between the dynamic and static nuclear bag fibers?

Answer 17

• Dynamic bag responds to rapid changes in muscle length

- Static bag and chains respond to slow stead changes in muscle length

Question 18

Are chain fibers tonic or phasic?

Answer 18

Tonic.

Question 19

What efferent innervation is there to the dynamic nuclear bag fiber?

Answer 19

• Dynamic beta and gamma fibers

Question 20

What efferent innervation is there to the static nuclear bag fiber?

Answer 20

• Static gamma

Question 21

What is the function of the efferent fibers to the muscle spindle?

Answer 21

To change the length of the muscle spindle and take up, or let loose slack in the spindle.

Question 22

Do type II sensory afferents have tonic or phasic components?

Answer 22

Only tonic

Question 23

Do type Ia sensory afferents have tonic or phasic components?

Answer 23

Both.

Question 24

Do Ia or II respond more readily to changes in muscle length?

Answer 24

Ia

Question 25

Do Ia or II better register muscle length alone?

Answer 25

II

Question 26

Which type of afferent fiber is more active during a static stretch of a muscle?

Answer 26

II

Question 27

What type of fibers fire off when a muscle spindle/ muscle is in a shortened state for an extended period of time?

Answer 27

Gamma fibers

Question 28

What is the adjustment of the length of the muscle spindle in response to muscle length change called?

Answer 28

"Re-biasing" the spindle

Question 29

How do afferent responses from the muscle spindle change during dynamic fusimotor stimulation?

Answer 29

Afferent responses to phasic length changes are increased.

Question 30

Which nuclear bag is more affected by dynamic gamma motor neurons?

Answer 30

- The dynamic bag/ Nuclear Bag 1

Question 31

What are muscle spindles in the 2nd nuclear bag more sensitive to when static fusimotor stimulation occurs ?

Answer 31

- Increased response to static extension

Question 32

Which spindle fibers are innervated by static gamma/ fusimotor neurons?

Answer 32

- Both bag and static

Question 33

How is afferent input, muscle force, and gain of the stretch reflex affected by static fusiform stimulation of the 2nd nuclear bag?

Answer 33

- Afferent increased - Muscle force increased - No or little change in stretch reflex

Question 34

What sensitivity is NOT increased by static fusiform stimulation in the 2nd nuclear bag?

Answer 34

- Sensitivity to amplitude or velocity of length change

Question 35

How is the nuclear chain fiber affected static fusimotor stimulation?

Answer 35

- Sensitivity of stretch reflex increased when a muscle is at a constant length

Question 36

What is the function of static fusiform stimulation of the nuclear chain fiber?

Answer 36

- Muscle can support heavier loads

Question 37

Which endings are more affected by intrafusal stimulation?

Answer 37

Primary endings

Question 38

Which type of intrafusal stimulation may be a means of suppressing the stretch reflex during voluntary movement? Why?

Answer 38

- Static intrafusal stimulation because it reduces the primary ending's sensitivity to length chang.

Question 39

What are the 3 aspects of servo assistance of motor command?

Answer 39

- Adjustment of spindle bias during movement - Abolition of spindle length sensitivity during movement - Modulation of spindle length sensitvity during movement

Question 40

What is meant by an adjustment of spindle bias during movement? What is its function?

Answer 40

- 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

Question 41

What is meant by an abolition of spindle length sensitivity during movement? When does it occur?

Answer 41

- Decreased spindle discharge during muscle shortening without the need for modulation based on alpha drive - Occurs during low velocity, normal, unobstructed movements

Question 42

What efferent stimulation and which muscle spindle fiber is involved in abolishing spindle length sensitivity during movement?

Answer 42

- Static gamma/ intrafusal stimulation to the chain fibers

Question 43

What is meant by modulation of spindle length sensitivity during movement? What is its function

Answer 43

- High length sensitivity | - Rapidly corrects movements that deviate from a desired trajectory

Question 44

Which intrafusal efferent system is responsible for modulating spindle length sensitivity during movement?

Answer 44

- The dynamic system

Question 45

What 2 static intrafusal systems contribute to muscle tone?

Answer 45

- 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.

Question 46

What can result from overactivity of the 2 muscle tone static intrafusal systems?

Answer 46

- Hypertonicity

Question 47

What is muscle tone?

Answer 47

- The sensation of resistance felt as one manipulates a joint through space

Question 48

What are 3 reasons for muscle tone as just defined?

Answer 48

- Limb inertia - Mechanical/ elastic properties of muscle and connective tissue - Reflexed

Question 49

Which 2 afferents are the primary proprioceptive fibers?

Answer 49

- Ia | - II

Question 50

What 2 messages are required to model planned movements?

Answer 50

- Message to spindle signaling desired trajectory of movement - A signal to extrafusal muscle that takes into account any load and fatigue

Question 51

Where are golgi tendon organs found? How are they oriented?

Answer 51

- Found at fibers of tendons at muscle insertions, and along the fascial covering of muscles - GTOs are arranged in series

Question 52

What type of force stimulates a GTO?

Answer 52

- Tension/ muscle contraction

Question 53

What is the sensory afferent of the GTO?

Answer 53

- Ib

Question 54

What is the efferent of the GTO?

Answer 54

There is no efferent component

Question 55

What does the primary afferent of the GTO synapse on?

Answer 55

An interneuron in the spinal cord that inhibits the intrasegmental alpha motor neuron

Question 56

What are the 7 functions of the GTO?

Answer 56

- Regulates muscle force by bringing about changes in muscle tension - Proprioception - Clasp knife reflex - Reciprocal excitation to antagonist - Protection - Muscle tone - Contract - relax