PHYSIOLOGY OF MOVEMENT AND COORDINATION OF MUSCLES

1. Stretch reflex
Department of Physiology
School of Medical Sciences and Research
Sharda University
SANJOG BAM
HUMAN BIOLOGIST
Msc. Medical Physiology

2. Learning objectives
• Introduction
• Muscles fibers and motor innervation
• Muscles spindles and its innervation
• Control of γ efferent discharge
• Phasic stretch reflex
• Tonic stretch reflex and role in posture maintenance

3. INTRODUCTION
• Reflex is a relatively predictable,
involuntary and stereotyped
response to an eliciting stimulus
• Skeletal muscles with intact nerve
supply gets stretched ,it contracts
• It is the fundamental reflex which
plays an important role in control
of body posture.
• It is highly developed in
antigravity muscles.
• Changes in the stretch reflex are
involved in actions commanded
by the brain.

4. Knee jerk reflex
Reaction time:19-20ms
Central delay:0.6 – 0.9ms

5. Types of Muscle fibers
Types of muscle fibers
Extrafusal fibers
• Forms bulk of muscle
• are innervated by alpha-motor
neurons
• Provide the force for muscle
contraction
Intrafusal fibers
• Smaller than extrafusal muscle
fibers
• innervated by gamma-motor
neurons
• Encapsulated in sheaths
• Runs in parallel with extrafusal
fibers
Stretch reflex refers to contraction of muscle when it is stretched

6. Motor innervation…
Two types of motor neurons:
I. α motor neurons
II. γ motor neurons
α Motoneurons:
• innervate extrafusal skeletal muscle fibers
• Large multipolar ,up to 70 µm diameter.
• Activation of α- motorneurons leads to contraction of extrafusal
muscle fibers
γ Motoneurons
• innervates intrafusal muscle fibers
• Constitute 30% of fibers(3-6)µm in ventral root
• function is to adjust the sensitivity of the intrafusal muscle spindles

7. Muscle spindle
• Muscles stretch receptors
• Found in almost all skeletal muscles,
particularly concentrated in muscles that
exert fine motor control.
 e.g: the small muscles of the hand
and eyes
• They are also found in good number in
muscles involved in control of posture,
especially the muscles rich in slow twitch
fibers type.
• Muscles spindles lie in parallel with the
regular muscle fibers

8. • Innervated part of the muscle spindle
encased in a connective tissue capsule
• Respond to change in the muscles
length and velocity of lengthening .
• consists intrafusal fibers
• 2-12 fibers in a muscle spindle
• Spindle or fusiform shaped organ
• 100µm in diameter and up to 10mm
long.

9. Intrafusal
fibers
• Individual intrafusal fibers are much
narrower and do not run the length of
the muscle.
• Specialized muscles fibers innervated
both by sensory and motor axons.
• Central non-contractile part contains
receptor whereas the peripheral parts
contain contractile element.
• Morphologically , two types:
1. nuclear bag fibers
2. nuclear chain fibers

10. Nuclear bag fibers
• Larger
• Nuclei are bunched together centrally
• Functionally these are divided into
two types:
1. Bag fiber 1: low myosin ATPase
activity and responds best in the
dynamic phase of muscle stretch.
2. Bag fiber 2: high myosin ATPase
activity and responds best in the
static phase of muscles stretch
Nuclear chain fibers
• Thinner and shorter fibers
• Present by the side of the nuclear bag fiber
and donot have a definite bag
• Nuclei arranged in a row in the form of a
chain
• Usually, four or more nuclear chain fibers
are present in a spindle.

11. sensory innervation of muscle spindles
Group Ia fibers
• Forms single primary ending (annulo
spiral) in each spindle.
• 12- 20µm
• 72-120 m/sec
• Wraps around the center of the
nuclear bag(dynamic and static) and
nuclear chain fibers
• Sensitive to change in length and
velocity of lengthening of the muscle.
Group II fibers
• Forms up to eight secondary
endings(flower spray).
• 6 - 12µm
• 36-72 m/sec
• Located adjacent to the center of the
static nuclear bag and nuclear chain
fibers
• Donot innevate the dynamic bag
fibers
• Sensitive to absolute length

12. Motor innervation to muscle spindles
γ -Motoneurons :do not receive input from primary sensory afferents.
: two types
Dynamic γ1 neurons
• Supply the dynamic nuclear bag
fibers.
• Increases the dynamic sensitivity
of the group Ia ending.
• Plate ending
Static γ2 neurons
• Supply the static nuclear bag and
nuclear chain fibers.
• Increases tonic level of activity in
both group Ia and group II endings
• Also decreases the dynamic
sensitivity of group Ia afferents
• Can prevent silencing of Ia
afferents during muscle stretch
• Trail ending

13. Control of γ efferent discharge
• No direct contact with primary sensory
afferents .
• descending pathways adjust the rate of γ
neuron discharge.
• γ neuron increases sensitivity of muscle
spindles.
• Sensory pathways also indirectly
influences γ neuron.
• α-γ Colinkage

14. • muscles spindle continues to
discharge even when the muscle is in
contracted state.

15. Phasic stretch reflex
• Elicited by stimulating the primary endings of the muscle spindle
• Stimulus : sudden stretch of the muscle
• In the spinal cord ,afferent fibers divides into two branches:
 One branch of Ia fiber directly terminate mono synaptically on the homonymous
neurons
 Other branch terminate di synaptically via an Golgibottle neuron on the
heteronymous motor neurons.
• Reciprocal innervation and reciprocal inhibition.
• Phasic reflex contraction results in rapid limb movement.
• The phasic stretch reflex mediate tendon jerk and control rapid correction
in motor output.

17. Tonic stretch reflex
• Elicited by a sustained stretch of the muscles
• Receptors are both the primary and secondary endings.
• Group II afferents direct contact with α motor neurons, thus
monosynaptic reflex
• Contribute to the muscle tone

18. Posture
• Tonic stretch reflex is important for regulation of posture
• Stretch reflex helps to restore and maintain the posture for a very
long period.
Action of
gravity on
medial
extensor of
thigh
These antigravity
muscles are
stretched
Sustained
contractions of
antigravity muscles
Maintains
extension at knee
joint
Standing position is
maintained

19. Thank you