Fig. 13-15. Neuronal

Pools. Obj 265-266. See

text pp. 446-448

• Go to your notes for a special case of a reverberating circuit, called reciprocal inhibition.

Fig. 13-16. Review of reflex arc. Obj. 267.

Fig. 13-19. Obj. 267. Muscle Stretch Reflex

Fig. 13-21. Obj. 268. Details of stretch receptor and the intrafusal muscle fiber

• Now let’s see how these elements of the stretch reflex 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? Here is Fig. 13-21:

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!)

• 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!)

• 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 → 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 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.