Myotatic reflexes Domina Petric, MD

The muscle spindle 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 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 spindles are concerned with proprioception.

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

Muscle tone 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.

Golgi tendon organs 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 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.

Muscle spindle and Golgi tendon organ 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.

Flexion withdrawal reflex 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.

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

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