Chapter 54

Question 1

What are the main differences between Alpha and Gamma motor neurons?

Answer 1

Alpha MN:

• larger
• innervate large skeletal muscle fibers

excites motor units

Gamma MN:

• small
• innervate intrafusal fibers

Question 2

What is the role of the spinal interneuron?

Answer 2

• allow for processing of signals from the descending tracts and sensory neurons before sending a signal to the alpha motor neuron
• Can be diverging, converging, repetitive-discharge

Question 3

What is the function of Renshaw cells?

Answer 3

-function to laterally inhibit surrounding LMNs to allow for better localization of the efferent signal

Question 4

What innervates the intrafusal muscle fibers? Extrafusal?

Answer 4

-Intrafusal: gamma nerve fibers

Extrafusal: alpha neurons

Question 5

What is the static stretch response? Is the transmission constant or intermittant?

Answer 5

• occurs when the receptor of the muscle spindle is stretched slowly
• occurs when both **primary and secondary endings increase in direct proportion to the stretch **
• causes the degree of muscle contraction to remain relitively constant

Question 6

What is the dynamic stretch response?

Answer 6

• sudden or rapid increases in muscle length cause **excitation of only the primary endings **
• as soon as the length steps increasing, the impulse returns to that of a static stretch

Question 7

What is the path of the muscle stretch reflex? What is the result of the loss?

Answer 7

• from the muscle spindle to a type Ia propioceptive nerve fiber which terminates directly on an alpha motor neuron
• smoothes out jerkiness of the contractile signal and if it is lost, muscle jerkiness results.

Question 8

Are gamma motor neurons typically stimulated at the same time as Alpha MN? Why?

Answer 8

Alpha MN allow for the contraction of muscle, while the gmma MN shortens the intrafusal muscle fibers to maintain the length of the muscle spindle.

-this allows for mainainence of tension and monitoring of the muscle contraction

Question 9

How can the knee jerk and muscle stretch reflexes help to diagnose UMN lesions?

Answer 9

-Someone with an UMN lesion will have hypersensitivity to both tests due to a laxity in the muscle spindle

Question 10

What is clonus? What is it indicative of?

Answer 10

Clonus is an oscillation of msucle jerks upon muscle stretch excitation

-indicative of an UMN lesion (highly facilitated muscle stretch reflex)

Question 11

What is the purpose of the golgi tendon organ?

Answer 11

-Montior muscle tension and inhibit muscle contraction in the case of extreme force generation.

Question 12

What is the flexor-crossed extensor reflex?

Answer 12

• A painful stimulus in one limb cuses a reflexive contraction of the flexors in the limb to withdrawl the limb.
• 200-500ms later, the extensors in the opposite limb contract to push the entire body away from the painful stimulus.

Question 13

What type of neuron in the spinal cord is resposible for the flexor-crossed extensor reflex?

Answer 13

-interneurons

Question 14

How many pages in the module did Allen Shepherd waste on the extensor-crossed reflexor pathway?

Answer 14

3.5 pages

Question 15

What causes a muscle spasm after a bone break? How can it be relieved?

Answer 15

• iritation of pain receptors at the end of the broken bone ilicit a contraction reflex
• local or general anasthetic

Question 16

What causes abdominal muscle spasm in pertonitis? How is it releved of circumvented?

Answer 16

• iritation of the peritoneum leading to a reflexive contraction.
• deep anasthesia in intr-abdominal surgery

Question 17

What causes muscle cramps? Why do they tend to just get worse?

Answer 17

• typically caused by abnormal metabolic activity, severe cold, lack of blood flow, or over exercise
• the cramp itself actually stimulates the same sensory fibers even more, causing a positive feedback

Question 18

What is the most common general cause of muscle spasm?

Answer 18

Localized pain

Question 19

How many pages did Allen spend on muscle spasm?

Answer 19

1

Question 20

What is the cause of the mass reflex?

Answer 20

-strong pain stimulus to the skin or overfilling of a viscus like the bladder or gut

Question 21

What are the effects of the mass reflex?

Answer 21

• strong flexor spasm of skeletal muscle
• evactuation of colon and bladder
• high arterial pressure (>200mmHg)
• Profuse sweating

Question 22

What is the first stage of physiologic effect after spinal cord transection?

Answer 22

-spinal shock

–>silencing of almost all cord functions due to the loss of their normal tonic input from higher levels

Question 23

What happens a few weeks after a spinal cord transeciton in humans?

Answer 23

-excitability of the cord functions return, and sometimes become hyperexcitable (due to increased sensitivity).

Question 24

How will this hyperexcitability after transection present?

Answer 24

-Tonic muscle spasm and hyper-reflexiveness

Question 25

What happens to arterial blood pressure at the onset of spinal shock? Why?

Answer 25

• falls drastically, sometimes as low as 40mmHg
• loss of sympathetic excitation
• eventually returns to normal

Question 26

What is the initial state of muscle reflexes in spinal shock? Do they retrun to normal?

Answer 26

• Lack of all reflexes
• some return in full, some never return, some become hyperexcitable

Question 27

What happens to control of bladder function in humans with a spinal cord transection?

Answer 27

-sacral reflex control of bladder function is initially lost but does return in most cases