LG 1.3 - Skeletal Muscle Phys Flashcards
Contraction of Sarcomere
- Myosin heads move against actin filaments 2. sarcomere gets shorter as myosin and actin move against each other 3. Z disks get closer together as sarcomere contracts
Outline the excitation-contraction coupling
- Motor neuron in anterior (ventral) horn of spinal cord send action potential through axon to motor end plate 2. Depolarization at motor end plate generates action potential in muscle 3. Sarcolemma depolarizes 4. Depolarization transfers down transverse tubules 5. DHP receptor on T-tubule undergoes conformational change in response to depolarization 6. Causes ryanodine receptor in SR to open and Ca2+ flows out
Locate and describe the Dihydropyridine receptor (2)
- located in T-tubules 2. L-type Ca2+ channel 3. Mechanically coupled to ryanodine receptor in SR –> when DHP recetpor is depolarized, the ryanodine receptor channel is activated and the Ca2+ flows out of the sarcoplasmic reticulum
Locate and describe the Ryanodine receptor channels (2)
- located in SR 2. opening of channel allows for Ca2+ to be released from SR 3. Ca2+ release allows for muscle contraction
Troponin I
strong affinity for actin
Troponin T
strong affinity for Tropomyosin
Troponin C
strong affinity for Ca2+
Explain Ca2+ role in excitation-contraction coupling
Ca2+ binds to troponin, which leads to a conformational change in the tropomyosin and exposes myosin binding sites on the actin
Cross-bridge cycling steps (4)
- ATP binds myosin head, which dissociates the actin-myosin complex 2. Myosin head is in the released state, ATP is hydrolyzed (to ADP + Pi) causing myosin to return to resting conformation (cocked state) 3. Myosin head binds to new position on actin and cross- bridge forms 4. Phosphate is released, which causes the myosin head to change confirmation, resulting in power stroke as the filaments slide past one another 5. ADP is released and myosin head is in attached state with actin
SERCA (3)
- Sarcoplasmic and endoplasmic reticulum Ca2+ ATPase 2. Pumps Ca2+ back into SR
Calsequestrin, Calreticulin (3)
Ca2+ binding proteins in the SR
Describe the Electrical component of Excitation-Contraction Coupling (5)
propagation of an action potential through the muscle
Describe the Chemical component of Excitation-Contraction Coupling (5)
release of Ca2+ from the sarcoplasmic reticulum
Describe the Mechanical component of Excitation-Contraction Coupling (5)
development of tension in the muscle fiber
Isometric Contraction
- Muscle length does not change as muscle is active 2. Maximum isometric tension is at optimum muscle
Isotonic contraction
same load, changing muscle lengths
Twitch
A single action potential through the muscle will yield a given amount of Ca2+ release. That Ca2+ release will cause a contraction in the muscle = a twitch
Tetanus
If the action potentials are at a high enough rate, the twitches will start to fuse. Fused tetanus is the state where the muscle does not have a chance to relax.
Fast twitch muscle fibers
- white, type II 2. extensive sarcoplasmic reticulum, more active enzymes that promote rapid release of energy
Slow twitch muscle fibers
- red, type I 2. more oxidative metabolism, myoglobin (iron containing substance similar to hemoglobin, combines with oxygen and stores it, this speeds oxygen delivery to mitochondria)
Locate and define the Sarcomere (1)
basic contractile unit, that is delineated by the Z disks
Locate and describe what composes the A band (1)
located in the center of the sarcomere and contain the thick (myosin) filatment
Locate and describe what composes the I band (1)
located on either side of A band, contain thin (actin) filaments, intermediate filamentous proteins, and Z disks NO THICK FILAMENTS
Locate and describe what composes the Z disks (1)
run down the middle of each I band, delineate the ends of each sarcomere
Locate and describe M line (1)
bisects the bare zone and contains proteins that link the central portions of the thick filaments together
Locate and describe Bare Zone (1)
located in center of each sarcomere. No thin filaments here, thus there can be NO overlap of thick and thin filaments or cross-bridge formation here
Locate and describe Thin filaments (1)
Actin - globular protein (G-actin), which is polymerized into 2 strands that are twisted into alpha-helical structure to form filamentous actin (F-actin).
Locate and describe Thick filaments (1)
Myosin
Tension vs. Muscle length (or preload) graph (6)
Looking at tension created for different muscle lengths, as you stretch a muscle (“load it up with weights”) you effect the length of the muscle and how much overlap of actin-myosin cross-bridge occurs
What does the dotted orange line represent (6)
Active tension - represents the active force developed during cross bridge cycling
What does the dotted green line represent (6)
Passive tension - tension developed by simply stretching a muscle to different lengths
What does the purple line represent (6)
Total tension - tension developed when a muscle is stimulated to contract at different preloads. Sum of active and passive tension
What is occurring at 1 (6)
the muscle length is too short, and thin filaments collide with each other at the center of the sarcomere –> reduces number of cross-bridges and therefore reduces tension
What is occurring at 2 (6)
This peak is created by maximal cross-bridge overlap –> optimal muscle length to generate force
What is occurring at 3 (6)
the muscle length is too long and the number of cross-bridges is decreased. Therefore, active tension is reduced
What is occurring at 4 (6)
This is the total tension, and the drop is due to the minimal overlap of cross-bridge in the active tension (muscle length is too long)
What is occurring at 5 (6)
The tension begins to increase again because of the passive tension created by the simple stretch of the muscle
Velocity vs. Afterload (7)
Describes the velocity of shortening when the force against a muscle contracts. Velocity of shortening reflects the speed of cross-bridge cycling
Vmax (7)
Velocity of shortening is maximal when the afterload on the muscle is zero. All muscle lengths have the same Vmax b/c it is dependent on how fast you can cycle cross-bridges (not dependent on the overlap). As the afterload on the muscle increases, the velocity will be decreased b/c the cross-bridges can cycle less rapidly against the higher resistance
Describe the yellow dashed line labelled 2 (7)
Muscle length is shortened so there is minimal overlap. Less overlap means lower velocities. As muscle length is shortened, the afterload at V0 decreases
Describe the green dashed line labelled 3 (7)
Longest muscle length = most overlap (maximal overlap in green curve). Therefore, largest initial velocities and it can lift the largest load b/c of the maximal overlap
