Muscle Mechanics Flashcards
What is an isometric contraction?
When muscle is fixed at both ends to that it is unable to shorten.
What is the latent period in muscle contraction?
The time delay between the stimulus and the first recorded tension increase (length will vary based on fiber type).
What is mechanical summation?
If a second stimulus is applied to a muscle before it has relaxed completely, the resulting contraction will be greater than the first.
What is a tetanic contraction (“fused tetanus”)?
With a high frequency of stimuli, a sufficiently high fusion frequency can be reached resulting in a smooth and sustained contraction with a peak tension of 3 or 4x an individual twitch. This is the MAXIMUM force a muscle can develop.
Do muscle fibers have elasticity even when 100% extended/under tetanic tetany?
Yes
What does titin do?
Links thick filaments to the Z-band. Major source of passive elasticity.
What happens in terms of tension when relaxed muscles are extended?
Rise in passive tension
What is stress relaxation?
When a relaxed muscle is elongated there is a quick rise in passive tension, but given time the initial tension decreases exponentially with time to a lower stable value. Thus, extensible elements in muscle are visco-elastic.
What muscle type shows the greatest stress relaxation? Which the least?
Most = smooth Least = cardiac
What is the maximum active isometric force a sum of?
Passive tension in muscle prior to stimulation and active tension developed when crossbridges start cycling.
What is the “rest length” of a muscle?
Paradoxically, it is the length at which maximum active tetanic tension can be generated.
At rest, the tension is not due to SEC (which is not active), but rather the _____.
PEC from the recoil force of titin (aka preload)
When does maximum tension occur?
When there is optimal/complete overlap of actin and myosin filaments.
Resting tension is greater in _____ muscle.
Cardiac
Smooth muscle is able to function over _____ range of tension than skeletal muscle. What is the significance of this?
Wider range. Smooth muscle surrounds hollow organs and thus may experience large changes in volume.
How does the latent period change when lifting a larger load?
Latent period will increase with load increase due to the greater time it takes to build up isometric tension equal to the weight of the load.
What is an isotonic contraction?
When the tension in the muscle is constant and equals the load and it shortens at a constant velocity while bearing the load.
What transphosphorylating enzyme keeps creatine phosphate and ATP in equilibrium?
Creatine phosphokinase
What is the Lohmann reaction?
PCr + ADP -> ATP + Cr
For sustained contractions, what substrates do muscles use to generate ATP if sufficient O2? If insufficient O2
FFAs
Glucose
AAs
via glycolysis.
Insufficient O2 will lead to anaerobic glycolysis.
What does adenylate kinase (myokinase) do?
Converts 2 ADP to ATP + AMP if ADP levels rise
How is muscle fatigue/ATP conservation achieved?
Decreased release of Ca2+ from the SR inhibits crossbridge cycling
What “tells” the cell that significant ATP hydrolysis is occurring?
As ATP levels fall, Mg2+ will be an unbound free cation (MG2+ has a high affinity for ATP) and end up in the cytoplasm. This along with Pi levels contribute to the fall in maximum tension capability.
Rising free Mg2+ levels lead to what?
Decreased Ca2+ release and RyR is most likely to be closed
In addition to Mg, what are 2 other protective measure/indicators?
Excessive extracellular K+ brings the resting potential closer to zero (paradoxically) but leads to less activation of Na+ channels thus decreasing the amplitude of the AP.
Cl- conductance inside the cella re greater than normal. Together with K+ it counteracts the Na+ current.
What does preload refer to and what “causes it”?
Passive tension in the muscle, largely due to titin
In a fast fiber, calcium is reqsequestered ______.
Rapidly
Does calcium explain the difference between twitch and tetanic contraction?
No! They both release the same amount of calcium. No the series elastic component (SEC) just doesn’t have time to reach maximum in the twitch.
Is there any tension in SEC at rest?
No! Only activated upon active tension
What 3 changes result from increasing load?
- Increase latent period (time between first muscle activation and when the load lifts off)
- Increase in extent of shortening
- Decrease in velocity of shortening
