Muscle Reflexes

Question 1

Muscle Spindles

Answer 1

Are intrafusal fibers within the muscle with the nerve wrapped around them to sense change in tension
Encapsulated by CT
Located inside the muscle body
Measure length and protect muscle from being torn
Are in parallel with extrafusal fibers on the outside

Question 2

Intrafusal vs. Extrafusal Fibers

Answer 2

Intrafusal fibers: sense length change

Extrafusal fibers affect the contraction of the muscle

Question 3

Golgi Tendon Organ

Answer 3

Golgi tendon organs/sensors: collagen fibers and neuron that comes in to act as sensor
Measures muscle force to protect tendon from being torn
Are in series with muscle fibers

Question 4

Stretch Reflex

Answer 4

Although the tendon is tapped, it is the muscle spindle which responds to the lengthening caused by the tap

Cause transient stretch of fibers and activates sensory neuron around fibers and sends AP through DRG into dorsal horn of spinal cord where is splits:

1. Excite the muscle of origin by stimulating an alpha motor neuron in spinal cord = monosynaptic
2. Inhibit the antagonist muscle by stimulating an inhibitory interneuron in spinal cord, which inhibits the alpha motor neuron of antagonistic muscle = disynaptic

Question 5

Types of Intrafusal Fibers

Answer 5

1. Large nuclear bag 1 (dynamic)
2. Large nuclear bag 2 (static)
3. Smaller nuclear chain

Each fiber receives motor and sensory innervation

Question 6

Dynamic Nuclear Bag Fibers

Answer 6

Nuclei are collected in a bundle in the middle of the fiber

Signal primarily information about the rate of change (velocity) of muscle length.

Question 7

Static Nuclear Bag Fibers

Answer 7

Nuclei are collected in a bundle in the middle of the fiber

Signal information about the static length of a muscle.

Question 8

Nuclear Chain Fibers

Answer 8

Nuclei are aligned in a single row (chain) in the center of the fiber.
Signal information about the static length of the muscle.

Question 9

Afferents and Efferents: Innervation of Intrafusal Fiber Types

Answer 9

Afferents from spindles to sense rate or length change

Efferents from gamma motor neuron (separate from ventral horn); modulate muscle spindle function

Dynamic, static, and nuclear chain = fiber types and all innervated by 1 alpha afferent (1a), which wrap around cell body of bag portion of the spindle to act as a sensor

Afferents II only innervate nuclear bag fiber and chain fibers

Efferent gamma static = static and nuclear chain fibers
Efferent gamma dynamic= dynamic fibers

Question 10

Afferents : Group Ia and Group II

Answer 10

Group Ia afferents are the largest and most rapidly conducting afferents
The endings of group I fibers occur on all three types of intrafusal fibers.
Only sense lengthening of muscles, and turn off during shortening

Group II afferents:
Group II afferents are smaller in diameter and conduct less rapidly.
The endings of group II fibers occur on nuclear bag 2 (static) and nuclear chain intrafusal fibers.
Always send APs to tell about length

Question 11

Gamma Efferents

Answer 11

Gamma efferents are the smallest somatic motor fibers, smaller than the alpha motor neurons, which innervate extrafusal fibers

Gamma dynamic: innervate mainly dynamic and static nuclear bag fibers and increase the dynamic sensitivity of the type Ia endings but have no influence on the type II endings

Gamma static: innervate mainly nuclear chain fibers and increase the tonic level of activity in both the type Ia and II endings, decrease the dynamic sensitivity of type Ia endings

Question 12

Gamma Efferent System Modulates the Strength of the Reflex

Answer 12

Trains of action potentials in the gamma efferent signals cause graded contractions of the intrafusal myofibrils.
These contractions increase the tension/stiffness of the contractile parts of the intrafusal fibers.
This increased stiffness causes any stretch imposed on the spindle to be more focused in the center of the spindles where there are no myofibrils, and where the primary nerve endings are located.
This increases the sensitivity of the spindle to stretch

Question 13

The Hoffman Reflex

Answer 13

Stimulating the peripheral nerve directly and bypassing the intrafusal and extrafusal fibers; low stimulus only activates afferent, or Hoffman reflex, and then once stimulus becomes larger it can directly activate efferent (bypassing the spinal cord) thus getting the M wave before the H reflex because the efferent being stimulated is right next to the muscle and the afferent takes times to go through the sensory nerve, DRG, dorsal horn, interneuron, ventral horn, then efferent/motor nerve (AKA the pathway from efferent to muscle directly is shorter than the afferent to muscle)
When low stimulus, only activate H reflex and NO M wave

Question 14

Alpha and Gamma Motor Neuron Coactivation

Answer 14

A: Stimulate alpha motor neuron (tendon tap) causes reflex contraction via intrafusal fibers, but if gamma not stimulated then the muscle goes slack because not being maintained

B: Gamma and alpha (extrafusal and intrafusal, respectively) innervated = maintain signaling going on and contraction of muscle continues

Question 15

Golgi Tendon Organs: Functions

Answer 15

Proprioceptive sensory receptors inside tendon (Ib afferents)
Not very sensitive to passive stretch
Very sensitive to muscle contraction
Inhibitory to agonist alpha motor neurons and excitatory to antagonist alpha motor neurons
Aid in maintenance of constant force
GTO’s, in series with the extrafusal fibers, register force generated by the muscle and prevent excess force

Question 16

GTO Feedback Loop: Stretch Reflex

Answer 16

Signals from GTO’s (disynaptic reflex):

1. Inhibit the muscle of origin by stimulating an inhibitory interneuron in spinal cord
2. Excite antagonist muscle by stimulating an excitatory interneuron in spinal cord

Together, spindles and GTO’s provide feedback important to fine control of the motor system.

*This is different than the stretch reflex for muscle spindles

Question 17

Stretch Reflex: Clinical Correlations

Answer 17

The stretch reflex is modulated by descending neural influences which alter the gain of the reflexes

Descending influences are largely inhibitory. Loss of these influences by brain or spinal cord dysfunction (i.e. cerebral palsy, stroke, spinal cord injury) gives rise to spasticity.

Question 18

Muscle Spindle vs. GTOs: Stretch Reflex

Answer 18

Muscle spindles: give rise to the stretch reflex and provide negative feedback regulation of muscle length
Induce:
Agonist muscle contraction
Antagonist muscle relaxation

Golgi tendon organs: provide negative feedback regulation of muscle force
Induce:
Agonist muscle relaxation
Antagonist muscle contraction

Question 19

Sensory Afferent Fiber Classifications: Ia, Ib, and II

Answer 19

Ia: primary spindle ending; muscle length and rate of change of length
Ib: Golgi tendon organs; muscle tension
II: secondary spindle ending; muscle length