Spinal cord and motor system

Question 1

What type of neurons are found in the center of the spinal cord? (Hint: not motor neurons)

Answer 1

Interneurons. These connect between other neurons in the spinal cord. They allow motor neurons of antagonist muscles to be inhibited when other muscles are active

Question 2

Where are sensory and motor tracts at their smallest?

Answer 2

Caudal end of the spinal cord

Question 3

What type of neurons are in the ventral horn of the spinal cord? What about in the intermediate gray matter?

Answer 3

Mostly alpha motor neurons in the ventral horn.

Interneurons are in the intermediate gray matter

Question 4

What is a motor pool?

Answer 4

All of the motor neurons innervating a single muscle

Question 5

How are motor pools oriented along the spinal cord?

Answer 5

Vertically, spanning a number of spinal segments.

Question 6

How are motor neurons organized within the ventral horn?

Answer 6

Distal muscles are more lateral. Proximal muscles are more ventral

Question 7

What is a motor unit?

Answer 7

All of the muscle fibers innervated by a single motor neuron

Question 8

Which is fast switch/slow twitch: type I muscle fibers vs type II muscle fibers?

Answer 8

Type I=slow twitch. Anaerobic with small fiber and neuron size.
Type II=fast twitch. Low aerobic metabolism, fatigues easily, with large fiber and neuron sizes

Question 9

What determines the type of muscle fiber that it becomes?

Answer 9

The neuron which controls that muscle fiber

Question 10

What are fast fatigue-resistant muscle fibers?

Answer 10

fast twitch muscle that is much more fatigue resistant

Question 11

Describe the 3 classic spinal cord reflexes and what they are.

Answer 11

Myotatic muscle stretch: Stretching a muscle results in contraction. 
Inverse myotatic (golgi tendon organ reflex): Stretch to the point of muscle damage results in Relaxation of muscle
Flexion reflex (withdrawal): Painful stimulus of a limb results in withdrawal

Question 12

What are muscle spindles?

Answer 12

Modified muscle fibers, also called intrafusal fibers.

Question 13

What are intrafusal muscle fibers innervated by?

Answer 13

gamma motor neuron.

Question 14

Describe the structure of an intrafusal muscle fiber

Answer 14

Centrally located nucleus. Contractile elements on either side of the nucleus

Question 15

What are annulospiral endings?

Answer 15

Sensory nerve process that wraps around the central portion of the muscle fiber. Activated when the muscle is stretched or gamma motor neuron is activated

Question 16

How does the muscle restore its ability to detect stretch from a new position?

Answer 16

Gamma motor neuron activation resets a new length for the muscle

Question 17

What do gamma loops do?

Answer 17

Stimulate gamma motor neurons which simultaneously activate agonist muscles while inhibiting corresponding antagonistic motor neurons

Question 18

What is spasticity?

Answer 18

Overactivity of gamma motor neurons resulting in overactivity of the stretch reflex. Caused by damage to descending inhibitory projections that inhibit gamma motor neurons

Question 19

What neuron is activated in golgi tendon organs?

Answer 19

Ib afferent neuron

Question 20

During normal stretching, is the GTO activated? When is it activated?

Answer 20

No, because most of the muscle stretches (highly elastic) without building up any tension in the tendon (inelastic). ONly at the limits of muscle stretch are GTOs activated to prevent tearing of the muscle. However, can be highly active in patients with spasticity

Question 21

Aside from extreme muscle stretching, when else is the GTO activated?

Answer 21

When muscle contracts–pulls on the tendons.

Question 22

The babinski response is an example of:

Answer 22

Withdrawal reflex or physiological flexion.

Question 23

What effect does physiologic flexion produce?

Answer 23

Flexion in the limb and extension in the contralateral limb

Question 24

What are interneuron pattern reflexes?

Answer 24

Generates patterns of repetitive movements. Gait when running