Properties of Skeletal Muscle

Question 1

What is passive force vs active force?

Answer 1

Passive - Force generated by innate pulling back of sarcomeres due to connective tissue, has nothing to do with contraction. Increases as you go above the optimum length

Active - Amount of force generated by stimulation of a muscle.

Difference between total force measured before and after stimulation is the passive force.

Question 2

What is the preload reserve?

Answer 2

Filling of ventricles with blood creates a tension on the heart wall, stretching the sarcomeres of cardiomyocytes to their optimal length ~2.4 micrometers. The preload contractile force is the part of the contraction between that optimal stretched length, and the normal length of 2.2 micrometers.

Question 3

What is the afterload? How does this relate to ejection velocity?

Answer 3

The tension of the heart wall produced during contraction. It is generally slower and caused by cardiomyocyte contraction. It has a decreased velocity because some cardiomyocytes must be used to carry the load, while others extrude the blood.

Decreased afterload = relatively more preload = greater ejection velocity

Question 4

What is isometric vs concentric vs eccentric contraction?

Answer 4

These are all contractions involving crossbridge cycling, but that does not always lead to muscle shortening.

Concentric - sarcomeres come together - contraction with muscle shortening
Isometric - Resistance = Tension, no change in sarcomere length and muscles stay in the same spot
Eccentric - contraction with muscle lengthing, load exceeds force of contraction

Question 5

What is an alpha motor neuron and what does it innervate?

Answer 5

A motor neuron with cell bodies in ventral horn. Axon progressively bifurcates and innervates many muscle fibers, all in a collective “motor unit”

Question 6

What is a motor unit?

Answer 6

A collection of muscle fibers innervated by a single motor neuron. They overlap and interdigitate, providing coordination

Question 7

What types of muscles would utilize small vs large motor units?

Answer 7

Small - about 10 fibers per unit - for precise control and faster reactions (i.e. extraocular muscles)

Large - about 10000 fibers per unit - for coarse control and slower reactions (i.e. quadriceps)

Question 8

What defines the rate of force development for a particular muscle?

Answer 8

The relative proportions of the three types of motor units, some slow, some fast, fatigue resistant, some fast, fatiguing

Question 9

What are the three types of alpha motor neurons? How does the speed of their actinomyosin ATPase (myosin heavy chain) differ?

Answer 9

1. Type I - Slow-twitch, non-fatigable - (MHC I, low Vmax)
2. Type IIa - Fast-twitch, non-fatigable (fatigue resistant) - (MCHIIa, High Vmax)
3. Type IIb - Fast-twitch, fatiguing - (MHC IIx, High Vmax)

Question 10

What is the main difference in red color, capillary supply, mitochondria, and oxidative enzymes between fatiguable and fatigue-resistant cells?

Answer 10

Fatiguable fast-twitch cells are basically anaerobic, and have almost no capillary supply, few mitochondria, and low oxidative enzymes since they do not depend on them for energy. They generate energy thru glycolysis

Question 11

What is the function of phosphocreatine?

Answer 11

Accounts for 3x as much ATP stored in muscle (8-10 sec of work). It is 4x faster than aerobic production of ATP

Question 12

What causes oxygen debt?

Answer 12

When anaerobic glycolysis exceeds oxidative phosphorylation as a source of ATP at the beginning of exercise, leading to buildup of pyruvate (taken to mitochondria) + lactate (taken to liver)

Question 13

What are the two mechanisms to increase the force of contraction of skeletal muscle?

Answer 13

1. Frequency summation

2. Fiber (motor unit) recruitment

Question 14

Why does frequency summation only work in skeletal muscle?

Answer 14

Short action potentials make it possible.

Question 15

What is the mechanism of frequency summation?

Answer 15

Twitches become of high frequency until reaching “fused tetanus”, when the alpha motorneuron is firing sufficiently often such that intracellular Ca+2 cannot be fully resolved by SERCA and NCX from the previous contraction.

Question 16

What is the mechanism of fiber (motor unit) recruitment?

Answer 16

Increases in voluntary force leads to additional motor unit recruitment.

Smaller axons have a lower threshold for activation, and will innervate smaller motor units. Increase in voluntary force will recruit motor axons of increasing size

Question 17

What are the two definitions of fatigue?

Answer 17

1. Failure to maintain required or expected force or contractile response
2. Gradual increase in effort required to maintain a constant submaximal task

Question 18

What are the two types of fatigue?

Answer 18

1. Peripheral fatigue - repeated contractions decrease the cellular contractile response, while Ca+2 transients are reduced, probably due to issues with EC coupling
   - > can be restimulated with caffeine to open RyR
2. Central fatigue - electrical stimulation of muscle can elicit a force above what can be attained voluntarily, and functionality can be returned with emotional or visual cues. May be due to inhibitory afferents depressing excitability.

Question 19

Roughly how many vesicles are needed at the end plate potential to stimulate an action potential?

Answer 19

0.4 mV depolarization per vesicle. ~40 mV depolarization is needed to trigger AP, thus 100 vesicles of AcH are needed.

Question 20

What is the Golgi tendon organ?

Answer 20

Proprioceptive organ sensing tension of tendon by stretch or muscle contraction. It is located in series between the end of extrafusal muscle fibers and tendon using 1b afferents interwoven with collagen fibers.

Question 21

What innervates intrafusal fibers?

Answer 21

Gamma-motor neurons - function to regulate gain of stretch reflex by adjusting tension in the intrafusal muscle to maintain their sensitive, nerve bodies in spinal cord.

Afferent (sensory neurons) 1a for primary, and 2 for secondary (1b is Golgi tendon organ)

Question 22

Where do intrafusal fibers (muscle spindle fibers) run and what do they do?

Answer 22

In PARALLEL with extrafusal fibers within the muscle body

They sense muscle stretch

Question 23

How does a surface EMG work?

Answer 23

Place electrodes across a distal muscle, i.e. median nerve between elbow and wrist. Stimulate a nerve to measure the evoked response. The compound muscle action potential (CMAP) is the evoked response at the surface of the muscle

Question 24

How is conduction speed determined by surface EMG (electromyogram)?

Answer 24

Time from stimulation to CMAP (compound muscle action potential)

Question 25

How would an axonal or ventral horn cell disease manifest in EMG?

Answer 25

Normal nerve conduction velocity, but reduced CMAPs due to alpha motoneuron deficiencies causing fewer muscles to contract

Question 26

How would a peripheral nerve disease manifest in EMG?

Answer 26

Both conduction velocity and CMAPs are generally affected if both motor and sensory axons are affected

Question 27

How would a demyelinating disease manifest in EMG?

Answer 27

Slower nerve conduction velocity, but normal CMAPs (action potential height is the same once it gets there)

Question 28

How would a primary muscle disease manifest in EMG?

Answer 28

Normal nerve conduction velocity, but reduced CMAP amplitudes (muscle is diseased)

Question 29

What is the most frequently used electrodiagnostic test for myasthenia gravis (an autoimmune disease)? What does it show?

Answer 29

EMG - shows decremental CMAP responses from 6-10 stimulations of neuromuscular junction at 2-3 Hz. A normal muscle would show no change in CMAP amplitude with repetitive nerve stimulation.