Lecture 6 Flashcards
Explain the function of the DHP and ryanodine channels in muscle contraction and describe their location in relation to the sacromere.
Dihydropyridine
- voltage-sensitive L-type calcium channels arranged in quadruplets
- Located in the sacrolemma T-tubules
- Cause a conformational change in the ryanodine receptors
- A minute amount of calcium flows into the cytoskeleton via thee channels.
Ryanodine receptors (RyRs or Ca2+ release channels)
- Located on the cisternae of the sacroplasmic reticulum
- Open in response to conformational change in DHP receptors
- Allow calcium into the cytoskeleton from the SR
- SERCA* uses ATP to pump calcium back into the SR
- (Sacroplasmic Reticulum Calcium ATPase)
- Calseuestrin in the SR maintains an optimum calcium concentration gradient to facilitate return of calcium to SR.
Define preload and describe the result
Definition: load on a muscle in the relaxed state (before it contract)
Results. . .
- stretches he muscle which stretches the sacromere
- generates passive tension in the muscle
- the muscle resists the tension applied to it
- force of resistance is measured as passive tension
- the greater the preload, the greater the passive tension in the muscle.
Define afterload and describe the result
Definition: load the muscle works against
Results. . .
- If the muscle generates more force than the afterload, an isotonic contraction occurs
- If the muscle generates less force than the afterload, an isometric contraction occurs
Differentiate between active and passive tension
Passive: produced by the preload
Active: produced by cross-bridge cycling
Total: sum of active and passive tension
Explain what is meant by cross-bridge cycling and describe the role of ATP.
- Cross-bridge cycling states when free calcium is available and attaches to troponin
- Contraction is the continuous cycling of cross-bridges.
- ATP is no required to form the cross-bridge linking to actin but is required to break the link with actin.
- Cross-bridge cycling continues until:
- withdrawal of calcium ion
- ATP is depleted.
Describe where ATP is required during muscle contraction
- Most is used for the sliding filament mechanism
- Pumping calcium ions from sacroplasm back into sacroplasmmic reticulum
- Pumping sodium and potassium ions through he sacrolemma to reestablish resting potential .
List the sources for rephosphorylation during muscle contraction and the significance of each of these sources.
Concentration of ATP in muscle fiber:
- About 4 mmol
- Enough to maintain contraction for 1-2 seconds
Phosphocreatine:
- Releases energy rapidly
- Reconstitute ATP
- ATP + phosphocreatine provides enough energy for 5-8 seconds of contraction
Glycolysis:
- Lactic acid build-up
- Can sustain contraction for 1 minute
Oxidative metabolism:
- Provides more than 95% of all energy needed for long-term contraction.
Compare isotonic and isometric contractions
Isometric:
- occurs when there is an increase in tension but not in length
Isotonic:
- muscle length changes
- Eccentric: occurs when muscle lengthens
- Concentric: occurs when muscle shortens.
Characteristics of fast fibers
Light, fast fibers (white fibers):
Contract rapidly but have less endurance
- fewer mitochondria
- primarily use anaerobic respiration resulting in a buildup of pyruvic and lactic acids
- little myoglobin
- larger concentrations of ATPase
Characteristics of slow fibers
Dark, slow fibers (red fibers) Slow twitch fibers contract more slowly but have more endurance - more mitochondria - primarily use aerobic respiration - more myoglobin - smaller concentration of ATPase
Muscles predominantly composed of dark fibers. . .
Soleus
Muscles predominantly composed of light fibers. . .
Gastrocnemius
Define motor unit
A single nerve cell (neuron) may innervate from a few to several hundred myofibers.
- A neuron and the myofibers it innervates constitute a motor unit
- When a neuron fires, all the myofibers in the motor unit contract
- All-or-none really refers to a motor unit.
Define summation and explain the mechanism behind it
- Electrical events occur faster than mechanical event:
- An additional spike can occur before the previous calcium ions have been returned to the SR.
- This increases the total amount of calcium ion in the cytoskeleton and increases the rate of cycling between the myosin and actin cross-bridges.
- This increases muscle tension
- Each additional spike adds to the effects of the previous spike.
Define Tetany and explain the mechanism behind it.
- If the frequency of spikes is fast enough, there is no time for relaxation between spikes.
- The muscle remains at maximal contraction.
