NB8-5 - Muscle Innvervation and Spinal Reflexes and DLA

Question 1

Where is muscle spindle density the highest?

Answer 1

In muscle groups that need fine motor control, such as in the hand.

Question 2

List the fibers found within the muscle spindle, what they are sensitive to, and their innervations.

Answer 2

• Nuclear Chain Fiber - sensitive to static changes and innervated by gamma motor neurons, Type Ia, and Type II fibers.
• Static Nuclear Bag Fiber - sensitive to static changes and innervated by gamma motor neurons, Type Ia, and Type II fibers
• Dynamic Nuclear Bag Fiber - sensitive to dynamic changes and innervated by gamma motor neurons and Type Ia fibers.

Question 3

Answer 3

4

Question 4

Answer 4

3

Question 5

Use the patellar reflex to describe how myotatic reflexes work in general.

Answer 5

• Tapping the patellar tendon stretches the quadriceps
• Ia fibers in the muscle spindle detect this and synapse on the quadriceps alpha motor neurons, activating them.
• The Ia fibers also send out a collateral axon to an inhibitory interneuron which will synapse on the hamstrings alpha motor neurons causing the hamstring muscles to relax

Question 6

Describe how an inverse myotatic reflex occurs.

Answer 6

When a muscle is placed under a dangerous amount of tension, Ib fibers in the golgi tendon organ will synapse on an inhibitory interneuron which will synapse on the alpha motor neuron for that muscle, causing it to relax.

Question 7

Answer 7

D

Question 8

Describe how the flexion reflex occurs

Answer 8

When a noxious stimulus is encoutered on a limb, the type III pain fiber, in addition to sending the signal to the brain, will also give out three colateral axons:

• 1 to a stimulatory interneuron and 1 to an inhibitiory neuron that will synapse on the ipsilateral alpha motor neurons of flexors and extensors fo the that limb, respectively (withdraws limb in danger).
• 1 to a bifid interneuron that will decussate and synapse on a stimulatory and a inhibitory interneuron that will synapse on the extensor and flexor alpha motor neurons for the contralateral limb, respectively (provides balance)

Question 9

Answer 9

E

Question 10

What are the technical names for diminished or absent reflexes. What are the most common causes for this?

Answer 10

Diminished reflexes - hyporeflexia

Areflexia - absent reflexes

These can be caused by:

• Peripheral Nerve Lesion - afferent or efferent limb
• Lesion of a spinal cord segment
• Disease of NMJ
• Lesion of muscle

Question 11

What are the common causes of hypotonia?

Answer 11

• Lesion to alpha motor neurons
• Lesion to Ia afferents
• Cerebellar lesions (you’d typically see ataxia also)

Question 12

Answer 12

D

Question 13

List and describe the types of motor units.

Answer 13

Type I - slow twitch fibers that generate low tension and are fatigue resistant. They are aerobic and are small units.

Type IIa - fast twitch fibers that generate large tension and are moderately fatigue resistant. They are somewhat aerobic and large units

Type IIb - fast twitch fibers that generate large tension and are fatigue susceptible. They are anaerobic and are large units.

Question 14

What are the factors that control muscle contraction speed and strength.

Answer 14

• The firing rates of the motor neurons in the muscle belly
• The number of motor units recruited during the action

Question 15

Describe the hierarchy of motor unit recruitment.

Answer 15

The Type I fibers are recruited first, which usually isn’t enough to perform the desired action.

The Type IIa fibers are then recruited

The Type IIb fibers will then be recruited if needed to perfrom action

Question 16

Answer 16

D