The document summarizes key aspects of myotatic reflexes and muscle spindles. It describes: 1) Muscle spindles contain intrafusal muscle fibers that are monitored by Ia and II afferent axons. They provide proprioceptive feedback on muscle length and tension. 2) The myotatic reflex maintains muscle length through a monosynaptic connection between Ia afferents and motor neurons. It causes muscle contraction when the muscle is stretched. 3) Golgi tendon organs sense muscle tension and inhibit motor neurons through Ib interneurons, reducing muscle tension. They provide feedback different from muscle spindles.

1. Domina Petric, MD
Myotatic reflexes

2. • The myotatic reflex (strech reflex) is a system that monitors
and maintains the length of a muscle.
• The muscle spindle is a collection of muscle fibers
(intrafusal muscle fibers) that are found within a capsule.
Two types of afferent axons:
• A) Ia afferent axons supply the central portion of the
intrafusal muscle fibers where are the nuclei of the muscle
fibers
• B) II afferent axons supply the contractile portions of these
intrafusal muscle fibers (flower spray ending)
The muscle spindle

3. • Motor supply for intrafusal muscle fibers comes from axons of y
(gamma) motor neuron.
• Nuclear bag intrafusal muscle fibers tend to have a more
bulbous collection of nuclei in their central reach.
• Nuclear bag fibers activate the most when there is a change in a
muscle length.
• Nuclear chain intrafusal muscle fibers have their nuclei
arranged in a single row along the central portion along the
muscle fibers.
• Nuclear chain fibers are concerned with maintenance of
stretched length of the muscle.
The muscle spindle

4. The muscle spindles are
concerned with
proprioception.
The muscle spindle

5. • For example, reflex hammer and knee-jerk reflex (patella
tendon).
• First there is activation of group Ia axons that supply the
nuclear bag fibers and sensory information runs into the
spinal cord.
• Afferent fiber that enters the dorsal horn of the spinal cord
makes monosynaptic excitatory connection with efferent
neuron in the ventral horn.
• Efferent neuron causes contraction of the quadriceps
muscle.
Myotatic (stretch) reflex

6. The hamstring muscles tone is
reduced: inhibitory interneuron
recieves excitatory input from Ia
muscle fiber and inhibits the
motor output to the flexor muscle.
Myotatic (stretch) reflex

7. Myotatic (stretch) reflex
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Ia afferent axon
Excitatory input for quadriceps
Inhibitory input for hamstring muscles

8. Muscle tone is the resting level of tension
in the sceletal muscle.
This tension depends upon the firing rate
of the alpha motor neurons that supply
the extrafusal fibers of that muscle.
Muscle tone

9. • Golgi organ is in the tendon that attaches the muscle fiber
to the skeleton.
• Ib afferent neuron axons supply the Golgi organ.
• Golgi organ is activated by the tension.
• Ib afferent neuron axons go into the dorsal horn of the
spinal cord.
• There is Ib inhibitory interneuron that causes synaptic
inhibition: alfa motor neuron is inhibited.
• Tension of the muscle is going to be diminished.
Golgi tendon organs

10. At the same time there is disynaptic
activation of the alfa motor neuron that
supplies the antagonistic muscle.
The tension of the antagonistic muscle is
increased.
Golgi tendon organs

11. Muscle passively stretched is going to activate muscle
spindle afferent, but not the Golgi tendon afferent.
Active contraction of the muscle will unload the muscle
spindle and activate the Golgi tendon afferent.
The muscle spindle is sensitive to length, but not
tension.
The Golgi tendon organ is sensitive to tension, but not
length.
Muscle spindle and Golgi tendon organ

12. • It is withdrawal of limb from the damaging stimulus.
• It is also called crossed extensor reflex.
• For example, information is processed via tha pain afferents from
foot into the dorsal horn of the spinal cord where there is a set of
interneurons.
• There will be increase of the activity of ipsilateral flexor muscles
and decrease of the activity of ipsilateral extensor muscles:
withdrawal of the ipsilateral leg from damaging stimulus.
• There will be increase of the activity of contralateral extensor
muscles and decrease of the activity of contralateral flexor
muscles: maintaining the posture via the contralateral leg.
Flexion withdrawal reflex

13. muscle atrophy (later sign)
fibrillations and
fasciculations (early signs)
muscle paralysis or paresis
areflexia (loss of myotatic
reflexes)
profound reduction in
muscle tone
Lower motor neuron syndrome

14. https://www.coursera.org/learn/
medical-neuroscience/lecture:
Leonard E. White, PhD, Duke
University
Wikipedia.org
Literature