(9) Skeletal Muscle Physiology: Excitation Contraction Coupling

Step 1 of Excitation-Contraction Coupling

Motor neurons secrete ACh

Step 2 of Excitation-Contraction Coupling

The ACh receptors are non specific cation channels. The effect of Na influx is greater than K

End Plate Potential

Depolarization resulting from ACh receptors opening (excitatory), resulting in AP

What does depolarization trigger?

Depolarization triggers a muscle action potential that travels across cell membrane

Step 3 of Excitation-Contraction Coupling

The AP travels down T-tubules and alters the shape of DHP receptors

Step 4 of Excitation-Contraction Coupling

The shape change of DHP receptors physically opens the gate of ryanodine receptors (RyR)

What are ryanodine receptors

Gated ion channels on the SR and open when Ca2+ moves from SR into ICF

Step 5 of Excitation-Contraction Coupling

Ca2+ binds to troponin in the sarcomeres, moving tropomyosin and allowing crossbridges to form

Step 6 of Excitation-Contraction Coupling

Contractile cycling occurs

Step 7 of Excitation-Contraction Coupling

Actin slides toward the M-line, shortening the sarcomere

Step 8 - Relaxation

Ca2+ - ATPase pumps move Ca2+ back into the SR; they are always active

Step 9 - Relaxation

As free Ca2+ in the ICF decreases, Ca2+ unbinds from troponin

Step 10 - Relaxation

Contraction cycling stops as tropomyosin covers myosin binding sites of actin

Twitch

Muscle contraction and relaxation resulting from one AP

Latent Period

Delay between AP and start of contraction (because Ca2+ movement)

What are the three pathways that supply additional ATP to muscles

Phosphocreatine, Oxidative phosphorylation, and glycolysis

Phosphocreatine

Molecule that stores energy through a Pi

What is the first source of ATP at the onset of contractile activity

Phosphocreatine

There is ______ times more ATP stored in _______ than in free ICF

Five, phosphocreatine

Creatine Kinase

Enzyme that catalyzes reversible reaction

What provides ATP for prolonged periods? (Light-medium exercise)

Oxidative phosphorylation

Myoglobin

Stores small amounts of O2 and increases O2 transfer rate from blood to muscle fibers

Oxidative Phosphorylation produces ______ amounts of ATP, but is _______ and requires O2

Large, slow

What occurs during high intensity exercise? (Low O2 delivery or ox. phos. cannot keep up with ATP need)

Glycolysis

Glycolysis produces ATP _______ but _________

Quickly, inefficiently

Glycogen

Limited quantities of glucose stored in cells as glycogen ; quickly depleted

Lactate

Produced when pyruvate cannot enter oxidative phosphorylation

Muscle Fatigue

Reversible state where exercising muscle can no longer respond to stimulation with the same contractile activity

Central Fatigue

The feeling of fatigue, protective mechanism to prevent damage from overexertion

Peripheral Fatigue

A failure of the processes in the nerve or muscle tissue of contraction

Hypothesized source for peripheral fatigue (Nervous)

Cannot process ACh enough to keep up with stimulation. Usually a diseased state and not a cause of fatigue

What are 3 muscular hypothesized sources for peripheral fatigue?

- Too much Pi in the ICF prevents Pi release from myosin

- Ca2+ decrease because it binds to phosphates

- K+ imbalance in ECF because of T-tubules

What type of myosin isoform do Slow Twitch Fibers (Type I) have?

Type 1 myosin isoform, relatively slow

How do Slow Twitch Fibers generate ATP?

They rely on oxidative phosphorylation to generate ATP

What amount of capillaries, mitochondria, and myoglobin do slow twitch fibers have?

High amounts of capillaries, mitochondria, and myoglobin

What is the diameter of Slow Twitch Fibers compared to other fiber types?

Fibers have a smaller diameter

What is the fatigue resistance of Slow Twitch Fibers?

They are resistant to fatigue

In which type of muscles are Slow Twitch Fibers primarily found?

They are found in muscles used for posture

How quickly do Slow Twitch Fibers develop force?

Force develops slowly and the twitch is longer

What type of myosin isoforms do Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers have?

Type II myosin isoforms

What is the Ca2+ - ATPase activity level in Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers?

Higher than in other fiber types

What metabolic pathway do Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers primarily use?

Glycolysis

Can Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers be trained to rely more on oxidative phosphorylation?

Yes

What amount of capillaries, mitochondria, and myoglobin do Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers have?

Moderate

How does the diameter of Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers compare to other types?

Medium diameter

What is the fatigue resistance level of Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers?

Resistant to fatigue

In what type of activities are Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers primarily found?

Walking and other moderate exercise

How quickly do Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers develop force?

Quickly

What is the twitch duration of Fast Twitch oxidative glycolytic (FOG, Type IIA) fibers?

Short

What type of myosin isoform is found in Fast Twitch glycolytic (FG, Type IIX) fibers?

Type II myosin isoform

What metabolic process do Fast Twitch glycolytic (FG, Type IIX) fibers rely on to generate ATP?

Glycolysis

What amount of capillaries, mitochondria, and myoglobin do Fast Twitch glycolytic (FG, Type IIX) fibers have?

Very few

How does the diameter of Fast Twitch glycolytic (FG, Type IIX) fibers compare to other cell types?

They have a larger diameter.

How quickly do Fast Twitch glycolytic (FG, Type IIX) fibers develop force?

Force develops quickly.

Are Fast Twitch glycolytic (FG, Type IIX) fibers easily fatigued?

Yes, they are easily fatigued.

What types of movements are Fast Twitch glycolytic (FG, Type IIX) fibers used for?

Short powerful movements (like jumping) or fine movements.