1. THE SKELETAL MUSCLE SERVO
MECHANISM
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2. CONTENTS
• DEFINITION
• ANATOMY
• MECHANISM
• ALPHA LOOP CONTROL
• GAMMA LOOP CONTROL
• SCHEMATIC DIAGRAM
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3. SERVO MECHANISM
• servomechanism, automatic device used to
correct the performance of a mechanism by
means of an error-sensing feedback. The term
servomechanism properly applies only to
systems in which the feedback and error-
correction signals control mechanical position
or one of its derivatives such as velocity or
acceleration.
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4. ANATOMY
• EXTRAFUSAL CONTRACTING FIBERS
1. Extra fusal muscle fibers is the term
given to standard Muscle fibers
2. Extrafusal muscle fibers are innervated
by alpha motor neurons and generate
tension by contracting, thereby
allowing for skeletal movement.
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5. INTRAFUSAL MUSCLE FIBER
• Intrafusal muscle fibers are
skeletal muscle fibers that
comprise the muscle spindle
and are innervated by gamma
motor neurons.
• These fibers are proprio-
-ceptors that detect the
amount and rate of change in
length of a muscle.
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6. Intrafusal fibers are wrapped by two types of
afferent endings that send sensory
inputs to the CNS
• Primary afferent endings (annulo-spiral endings)
– group 1 fibers
– Innervate the center of the spindle
• Secondary afferent endings(flower spray endings)
– Type II fibers
– Associated with the
ends of the spindle
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7. MECHANISM
• The extrafusal fibres are attached to bone
through tendons exhibit a finite spring
stiffnesss,and which are embedded in golgi
tendon organs
• These organs are mechano receptors sensitive
to the total muscle tension.
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8. Alpha loop control
• The tendon organs provides inhibitory
feedback to the alpha motor neuron but do
not constitute the basic positive feedback
element for this servomechanism
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9.  Loop control
• Activating the  motor neurons that stimulate
the distal ends of the intrafusal fibers to contact
thus stretching the mid-portion of the spindle
(internal stretch)
• Whatever the stimulus, when the spindles are
activated their associated sensory neurons
transmit impulses at a higher frequency to the
spinal cord
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11. • The spindle cells primary afferents have an
excitatory effect on alpha motor neurons
travel via spinal cord in a fast mono synaptic
reflex arc
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12. • The spindle cells secondary afferent conduct
more slowly through a multi synaptic
connection
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13. • A local feedback loop exist in the spinal
cord called renshaw cell to the local motor
neuronal pool
• This feedback is inhibitory.
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14. 
a
1a
Descending influence (UMN)
MUSCLE
Muscle spindle
GAMMA LOOP CONTROL
Activation of the -loop
results in increased
muscle tone
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15. • Branches of the afferent fibers also synapse
with inter- neurons that inhibit motor neurons
controlling the antagonistic muscles
• Inhibition of the antagonistic muscles is called
reciprocal inhibition
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16. SCHEMATIC OF SKELETAL MUSCLE
CONTROL SYSTEM
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17. CONCLUSION
• Servo mechanics can apparently be driven by
either of two reference command inputs from
higher nervous centers,so called as alpha and
gamma efferent pathways.
• Firing frequency of spindle’s primary afferent
in the steady state is approxiamately
propotional to muscle extension over the
physiological range.
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19. References
• en.wikipedia.org/wiki/spindle mechanism
• www.slideshare.net
• John H.Milsum text book
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