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Stretch Reflex
Physiological Description
&
Referees-Players Analogy
Proprioception
Awareness of
1. muscle length
2. musculo-skeletal interface
(joint angle change)
3. speed of movement
“Body Awareness in Space”
Muscle Spindles
These sensory fibers lie between
skeletal muscle (extrafusal) fibers
Small fibers (intrafusal fibers) wind
through muscles
Intrafusal fibers stretch and stimulate
gamma motor neurons
Thus, muscle spindles provide the CNS
with information about the length of
the muscles and accommodate changes
in length
Muscle Spindle
When the spindles are stretched
rapidly, the sensory feedback causes
the CNS to stimulate alpha motor
nerves. This process causes the
stretched muscle to contract!!
Spindle Schematics
Alpha MN
Extrafusal
Fiber
Muscle Spindle
Afferent
Capsule
Gamma MN
Extrafusal
Fiber
Functions of theMyotatic Reflex
Muscle tone
Maintain posture
Voluntary movement
Stretch (myotatic) reflex
The myotatic reflex is activated
from stimulation of the muscle
spindle
Information is sent to the CNS
about the degree of stretch of
the involved muscle and the
needed motor units to contract
in order to overcome stretch
Stretch (myotatic) reflex
Simply, it is a 2-neuron loop:
• afferent and efferent neurons
• The afferent neuron is connected to a
muscle spindle, which detects stretch
in the muscle
• The efferent neuron is the motor
neuron, which causes the muscle to
twitch
Muscle Spindle Action
Muscle spindles note the change in
length (and speed) and send signals to
the spinal cord
This action stimulates the stretch
reflex (myotatic reflex) which
attempts to resist the change in muscle
length by causing the stretched muscle
to contract
The more sudden the change in muscle
length, the stronger the muscle
contractions will be (plyometric training
is based on this fact)
Monitored Stretch Components
The stretch reflex has both a dynamic
component and a static component
• The static component persists as long as
the muscle is being stretched
• The dynamic component lasts for only a
moment and is in response to the initial
sudden increase in muscle length
Monitored Stretch Components
The stretch reflex has two
components because there are two
kinds of intrafusal muscle fibers:
• nuclear chain fibers, which are
responsible for the static component;
• nuclear bag fibers, which are
responsible for the dynamic component.
Intrafusal Type
Type determined by:
1. length/diameter: bag>chain
2. central nuclei:
bag> chain
3. innervation :
bag< chain
(1:1)
(1:5)
Bag: monitor rate and degree of
stretch
Chain: monitors degree of stretch
AFFERENTS
1. Annulospiral endings: on central
portion of the nuclei bags and chain
(primary afferents)
2. Flower spray endings: on extended
portion of nuclei chains (secondary
afferents)
Stretch Receptors
Two stretch receptors are sensitive to
different time scales
The first is a Ia fiber; the classification
is based on diameter and conduction
velocity
The Ia fiber fires when the muscle is
stretching, but it is rapidly adapting.
As soon as the muscle stops changing
length, the Ia adapts to the new length
and stops firing
Stretch Receptors
The second type of stretch receptor is
a II fiber
It is slowly adapting
It also responds when the muscle is
stretching, but it maintains a firing
rate after the muscle has stopped
moving (essentially, it is non-adapting)
Muscle Spindle Sensitivity
How can the muscle spindle remain
sensitive to stretch at short lengths?
Like any muscle fiber, it can contract
When it contracts, the entire spindle
shortens, and the sensitivity is intact
Muscle Spindle Sensitivity
Small motor neurons in the ventral
horn innervate the intrafusal muscle
fibers and cause them to contract via
the gamma motor neurons
The neurons are excited every time
the alpha motor neurons fire, so
that as the extrafusal fibers contract,
the intrafusals contract also
Stretching
Reasons for holding a stretch for a
prolonged period of time:
• The muscle spindle habituates (becomes
accustomed to the new length) and
reduces its signaling
• Stretch receptors are trained to allow
greater lengthening of the muscles
Stretching
When a muscle is ballistically
stretched, it responds with an amount
and rate of contraction which vary
directly with the stretch
When a muscle is statically stretched,
this reflex is inhibited
GAMMA SYSTEM
Intrafusal fibers are sometimes
referred to as the gamma system
Afferent nerves – 2 types
Gamma efferent nerves -1 type
GAMMA EFFERENT
• Innervates ends of intrafusal fibers
(contractile apparatus)
• Slower conduction velocity than the
alpha motor neurons
 sense movement before their
contractile ends are stimulated to
contract (to compensate for change in
muscle length)
CO ACTIVATION
Both alpha and gamma motoneurons
are stimulated
Gamma loop is initiated
( helps to inhibit antagonistic muscle action)
GAMMA BIAS
If high  sensitivity of the spindle
afferent to the external stretch
Muscle contraction is maintained at
varying levels of force  precision
control
At prefered extrafusal length
• minimal spindle firing
• unconscious control  less jerky
Inverse Stretch Reflex
The inverse stretch reflex is also
known as the inverse myotatic reflex,
the Golgi tendon reflex and the
clasp-knife reflex
The reflex responds to tension in a
muscle tendon which is sensed by
Golgi tendon organs
Inverse Stretch Reflex
The inverse stretch reflex is
bisynaptic
Therefore slower than the stretch
reflex
The inverse stretch reflex can override the stretch reflex:
• Protective
• Automatic maintenance of posture
Referees and Players
The intrafusal fibers are the referees
The extrafusal fibers are the players
Referees’ Roles
The referees:
• Monitor player safety and the players
playing by the rules of the game
• If the players exceed safety
components, the referee whistles play
to stop
• An adjustment is made for safety and
proper game play
Players’ Roles
Produce the action (force)
Determine movement within the
game
Do not monitor the rules of the
game, but determine game play
Keep up the movement until fatigued
or ordered to stop by authorities
(CNS: spinal cord)
Questions to ponder......
Why does the knee “jerk” when the
patellar tendon is whacked?
How could a depth jump (pre stretch)
cause one to jump higher?
How can someone kick at another
person’s head, but come just an inch
short?