SM 224a - Muscle Physiology and Mechanics Flashcards
What are the 4 things that determine the force of muscle contraction?
- Ca2+ available
- Rate of action potential firing (force-stimulus frequency relationship)
- Length of the muscle (length-tension relationship)
- How fast the muscle moves (force-velocity relationship)
What determines whether a given motor unit is fast or slow?
The type of myosin ATPase it has
Fast motor units will fatige, either quickly or slowly depending on the type
Slow motor units do not fatigue
Describe the force-velocity relationship of muscle mechanics
Faster muscle shortening = less force generated
- Fast-twitch muscle fibers can contract 20-30x per second
- Slow-twitch muscle fibers are still contracting in fast movements, but not contributing significantly
- They will help more with slower movements
Which section is labeled by #3?
H band
Which cellular energy source dominates in an effort that lasts 5-10 seconds?
Creatine phosphate (CP)
Does your biceps brachi contain fast or slow motor units?
Both!
Every muscle is heterogeneous - contains both fast and slow motor units
- Different muscles have different percentages of each fiber
- Your muscles are a bit trainable - you can work to increase endurance (by increasing slow motor units) or contraction velocity (by increasing fast motor units)
- Different people natrually have more of one type or another
- Think about Usain Bolt vs. Shalane Flanagan (accomplished marathoner)
Describe the length-tension relationship in muscle mechanics
More cross-bridges = stronger contractrion force
More overlap between actin and myosin = more cross-bridges
- When the sarcomere length is very short, actin fibers overlap each other and one blocks the other from forming cross briges
- As the sarcomere lengthens, more actin is available to form cross bridges and contractile force increases
- When the sarcomere gets too long, overlap between actin and myosin decreases, which reduces the number of corss briges that can be formed. Contractile force decreases
Which structure is labeled by #7?
Which molecules make up this structure?
Thin filament
Actin + Tropomyosin + Troponin T, C, and I
Describe the following for stored fat:
- Mobilization time:
- Amount stored:
- Mobilization time: 20-60 minutes
- Amount stored: SOOO much
Note: this pathway is oxygen dependent
What are the mechanical characteristics of muscle?
- Spring-like characteristics
- Cushion the landing of the jump
- Viscous-like characteristics
- Prevent too much oscillation at the landing of a jump
Which cellular energy source dominates within the first second of effort?
ATP
Which molecule is labeled by #6?
What is its function?
Titin
A length-sensing giant molecule that acts as a stiff spring
It prevents over-stretching of the sarcomere, and recoils after stretch to pull the sarcomere back to its normal length
Which structure is labeled by #9?
Which molecules make up this structure?
Thick filament
Made from myosin light and heavy chains
Describe the troponin complex
The troponin complex is attached to tropomyosin
It is made up of…
- Troponin T (TnT), which binds to tropomyosin
- Troponin C (TnC), which binds to Ca2+
- Troponin I (TnI), which inhibits contraction when it is bound to actin
- It covers up the myosin binding site on actin
Which structure is labeled by #8?
M Line
During a 10 s maximum speed running race, which energy substrate is the main source of ATP to support the force generated in the muscles being used?
- glycogen via aerobic pathways
- glycogen via anaerobic glycolysis
- creatine phosphate (CP)
- fat via anaerobic pathways
- fat via aerobic pathways
C. creatine phosphate (CP)
What elements contrubute to the overall stiffness of a muscle?
- Contractile element (sarcomere)
- Stiffness comes from the force-length relationship
- Connective tissue surrounding muscle fibers
- Passive elasticity
- Tendons
- Series elasticity
- Tendons are in series with the contractile element and connective tissue that surrounds the contractile element
Describe the configuration of actin and myosin in the released state of cross-bridge cycling
ATP binding causes the myosin head to be released from actin
List and describe the steps of cross-bridge cycling
- Attached state - The myosin head is attached to the thin filament after power stroke completion
- Released state - ATP binding causes the myosin head to be released from actin
- Cocked state - ATP is hydrolyzed to ADP + Pi. The products remain, and the myosin head unhinges 11 nm. It lines up with the acin monomer 2 positions over from the previous binding site
- Cross-bridge state - The cocked myosin head binds actin
- Power stroke state - Pi dissociates from myosin, leaving the muscle fiber in a rigid state
What molecule makes up #5?
(The pink cord encircling the green)
What is its function?
Nebulin
Regulates thin filament length
Describe the steps requred to generate a muscle twitch, starting with a motor command from the CNS
- A motor command from the causes action potential in a motor neuron
- The action potential travels down motor neuron’s axon
- The action potential reaches the neuromuscular junction (Each neuron terminates in several muscle fibers)
- The muscle fiber depolarizes, causing an action potential in the membrane of the muscle fiber
- Depolarization travels down T-tubule system, causing Ca2+ release from sarcoplasmic reticulum
- Calcium release allows actin and myosin to bind, which causes a muscle twitch
- Force is not sustained because as soon as Ca2+ is released, SERCA works to remove Ca2+ from the cytoplasm
Describe the following for stored glycogen (aerobic):
- Mobilization time:
- Amount stored:
- Mobilization time: 30 - 120 seconds
- Amount stored: 30-120 minutes
How long after starting a physical activity does it take for fat to cont10-ribute significantly as a power source?
20-60 minutes
Describe the interaction of actin and myosin in the power stroke state of cross-bridge cycling
Power stroke state - Pi dissociates from myosin, leaving the muscle fiber in a rigid state
