Proprioceptors & Spinal Reflexes

What is the function of muscle spindles?

To maintain muscle length

What is the function of Golgi tendon organs?

To maintain muscle tension

What fibers (axons) are involved in reflexes?

• Ia and II sensory afferents from muscle spindles

• Ib sensory afferents from GTO

• alpha motor neurons to extrafusal fibers in skeletal muscle

• gamma motor neurons to intramural fibers in muscle spindles

What are the characteristics of Ia afferents?

• Rapidly-adapting

• Firing rate changes during the dynamic phase

• Firing rate during static phases is proportional to the amount of stretch

• Senses velocity of stretch & amount of muscle stretch

What are the characteristics of II afferents?

• Slowly-adapting

• Firing rate during static phases is proportional to amount of stretch

• Senses amount of muscle stretch during static phases

How do GTO signal muscle tension?

The tension on the GTO compresses the branches of the Ib afferents, causing them to depolarize. Muscle tension increases, but muscle length stays the same (isometric contraction).

How does the stretch reflex work?

1) Monosynaptic activation of the agonist muscle (quadricep); Ia excites alpha motor neuron for agonist muscle

2) Disynaptic inhibition of the antagonist muscle (hamstring); Ia excites an inhibitory interneuron that inhibits the alpha motor neuron for the antagonist muscle

How does feedback correction for increased load work?

1) Ia afferent from muscle spindle is activated by stretch from added weight

2) alpha motor neuron for the agonist muscle is activated

3) agonist muscle contracts

How does feedback correction for decreased load work?

1) Ia afferent from muscle spindle becomes less active due to reduced weight

2) alpha motor neuron for the agonist becomes less active

3) agonist muscle relaxes

How does alpha-gamma co-activation adjust muscle spindle sensitivity (what happens to muscle spindles when a muscle shortens during contraction)?

1) A muscle contracts

2) Gamma motor neuron contracts intrafusals fiber in muscle spindle

3) Muscle spindle contracts & remains taut

How is the stretch reflex involved in postural control?

How does the GTO tendon reflex work? What is its function?

1) Activation of Ib fibers in the GTO excites Ib inhibitory interneurons in the spinal cord

2) The interneurons inhibit the alpha motor neurons in the muscle, causing it to relax

To resist increased muscle tension that may cause injury

How does the withdrawal reflex work? What is its function?

A-delta nociceptive sensory afferents innervate interneurons in the spinal cord that excite ipsilateral flexor motor neurons & inhibit ipsilateral extensor motor neurons; can be suppressed by descending control of the interneurons

How does the crossed-extensor reflex work? What is its function?

A-delta nociceptive sensory afferents:

• Excite ipsilateral flexor motor neurons & inhibit ipsilateral extensor motor neurons

• Excite contralateral extensor motor neurons & inhibit contralateral flexor motor neurons

Moves a limb away from a painful stimulus while keeping the body upright