Review of reflex arc.

Muscle Stretch Reflex

These first two slides have provided a brief outline of the muscle stretch reflex. You can

now go back to the lab exercise.

The remainder of this presentation will be used to illustrate Lecture

Objective 66.

Response characteristics of the stretch receptor—another example of frequency coding.

The intrafusal muscle fibers are tiny muscle fibers attached to either end of the stretch receptor. They are innervated by gamma efferent

neurons and are not part of the stretch reflex arc.

• This has been a brief description of the muscle stretch reflex arc and how it operates.

• The next series of slides will demonstrate how the muscle stretch reflex arc, the intrafusal muscle fibers, and the gamma efferent neurons are used in local control of muscle fibers.

D—extrafusal muscle fiberE—stretch receptorF—intrafusal muscle fiber

A—stretch receptor afferent neuron

B—alpha efferent neuronThis completes the stretch reflex arc.

Review: what happens when the stretch receptor is stretched? Notice the change

in frequency of action potentials.

What events will stretch the stretch receptor?

• Stretch of the entire muscle (tap the patellar tendon, watch the foot jerk!)

What events will stretch the stretch receptor?

• Stretch of the entire muscle (tap the patellar tendon, watch the foot jerk!)

OR

What events will stretch the stretch receptor?

• Stretch of the entire muscle (tap the patellar tendon, watch the foot jerk!)

OR

• Contraction of the intrafusal muscle fiber!

What events will stretch the stretch receptor?

• Stretch of the entire muscle (tap the patellar tendon, watch the foot jerk!)

OR

• Contraction of the intrafusal muscle fiber!– What would make this happen?

Reminder:The intrafusal muscle fibers are innervated by gamma efferent

neurons.

What events will stretch the stretch receptor?

• Stretch of the entire muscle (tap the patellar tendon, watch the foot jerk!)

OR

• Contraction of the intrafusal muscle fiber!– This will occur any time the gamma efferent

fiber stimulates the intrafusal muscle fiber.

C—gamma efferent neuron to intrafusal muscle fiber

What if we could stimulate C? Work through the sequence: C →

What if we could stimulate C? Work through the sequence: C → F

What if we could stimulate C? Work through the sequence: C → F →E

What if we could stimulate C? Work through the sequence: C → F →E →A

What if we could stimulate C? Work through the sequence: C → F →E →A →B

What if we could stimulate C? Work through the sequence: C → F →E →A →B →D

• This seems like a stupid thing to do because it would be easier just to stimulate the extrafusal muscle fiber with the alpha efferent neuron.

• But look what actually happens:

Descending neurons (pyramidal tracts!) stimulate both alpha and gamma neurons.

Both the intrafusal fibers AND the extrafusal fibers contract to the same extent. The stretch receptor doesn’t feel a thing!

What if the load is too big for the muscle to lift? What happens to the stretch receptor?

It becomes stretched (the muscle doesn’t shorten, remember?) and increases the frequency of action potentials along the stretch receptor afferent…..

…which stimulates only the alpha efferent, causing the extrafusal muscle fiber to generate greater amounts of tension.

• This combined stimulation of the alpha and gamma efferent neurons sets up a situation in which there will automatically be an increase in tension of the muscle if the load is too heavy. This combined stimulation of the two neuron types is called alpha-gamma co-activation.