Muscle Flashcards
What is an agonist muscle?
Prime mover of any skeletal muscle movement
What is an antagonist muscle?
Flexors and extensors that act on the same joint to produce opposite actions
What is the epimysium?
Tendon connective tissue extends muscle in an irregular arrangement to form this fibrous sheath
What are the columns of muscle fibers inside the epimysium?
Fascicles
What is each fascicle surrounded by?
Perimysium
What are the striations in muscle’s appearance produced by?
Alternating A and I bands
What stimulates the muscle fiber to contract?
Motor neuron stimulates muscle fiber to contract by liberating Ach at the neuromuscular junction (motor end plate)
What is a motor unit?
Each somatic motor neuron, together with all of the muscle fibers that it innervates
What is each muscle fiber innervated by?
Each muscle fiber receives a single axon terminal from a somatic motor neuron
-Each axon can produce a number of collateral branches to innervate an equal number of muscle fibers
How is innervation ratio related to strength of muscle contraction?
Small motor units -> fine neural control (one neuron per small amount of muscle fibers), i.e. in extraocular muscles that position eyes
-Larger motor units (recruitment) -> stronger and more powerful contractions, i.e. in gastrocnemius
What is each myofibril composed of?
Myofilaments
What are the thick filaments primarily composed of?
Myosin
-Thick filaments are in A bands
What are then thin filaments primarily composed of?
Actin
- Thin filaments are in I bands
- Center of each I band is Z disc
What is a sarcomere?
Subunit from Z disc to Z disc
-M lines are produced by protein filaments in a sarcomere, they anchor myosin during contraction
What is titin?
A type of elastic protein that runs through the myosin from the M lines to the Z lines
-Contributes to elastic recoil of muscle
What is the sliding filament theory of contraction?
Muscle contraction results from shortening of the sarcomeres, which is produced not by shortening of filaments but rather by the sliding of thin filaments over and between thick filaments (action of cross bridges)
What causes a myofiber to shorten?
Shortening of the distance from Z disc to Z disc
Which bands shorten during contraction?
I bands (distance between A bands of successive sarcomeres) and H bands (thick filaments, contain only myosin) decrease in length -A bands do NOT shorten, they move closer together (toward the origin of the muscle)
What action produces the sliding of the filaments?
Action of numerous cross bridges that extend out from the myosin toward the act
-These cross bridges are part of the myosin proteins that extend from the axis of the thick filaments to form “arms” that terminate in globular “heads”
What serves as cross bridges in the myosin protein?
Its 2 globular heads
- Each myosin head has an ATP-binding site and an actin-binding site
- The heads function as myosin ATPase enzymes, splitting ATP into ADP and Pi
When are the myosin heads attached to actin?
During contraction only
What must occur before the myosin heads can bind to actin?
ATP must be hydrolyzed to ADP and Pi
-Phosphate binds to the myosin head, phosphorylating it -> “cocked” conformation -> energized myosin can bind to actin
What happens once the myosin head binds to actin?
A cross bridge is formed, the bound Pi is released (myosin head becomes dephosphorylated) -> conformational change in the myosin -> cross bridge produces a power stroke
-This is the force that pulls the thin filaments toward the center of the A band
After the power stroke is completed, what is required for the myosin head to break its bond with actin?
The bound ADP is released as a new ATP molecule binds to the myosin head -> ready to bind again
Why is the action of the cross bridges described as asynchronous?
Only 50% of the cross bridges are attached at any given time
-Like the actions of a team engaged in tug-of-war
When is the cross bridge not attached to actin?
Resting fiber
What works together to regulate the attachment of cross bridges to actin, and thus serve as a switch for muscle contraction and relaxation?
Tropomyosin (lies within groove between double row of G-actin) and troponin (attached to tropomyosin)
How do the troponin and tropomyosin attach during muscle relaxation?
When sarcoplasmic [Ca2+] is low, Ca2+ is not bound to troponin -> tropomyosin is in a position that blocks myosin binding to actin -> prevent muscle contraction
What is the role of Ca2+ in muscle contraction?
Ca2+ attaches to troponin -> movement of troponin-tropomysin complex -> exposes binding sites on actin -> myosin cross bridges can attach to actin and undergo a power stroke
What happens when a somatic motor neuron releases ACh?
Binds to nicotinic ACh receptors in the sarcolemma, causing a depolarization that stimulates voltage-gated channels -> Na+ diffuses in, producing depolarizing stimulus (+ ions are attracted to negative plasma membrane) -> if depolarization sufficient, threshold occurs -> APs
Once an AP is produced what happens to produce muscle contraction?
APs travel down sarcolemma and T tubules -> stimulates opening of voltage-gated Ca2+ channels in T tubulues -> these channels are mechanically coupled to Ca2+ release channels in the SR, causes them to open -> Ca2+ diffuses out of the SR so it can bind to troponin in the myofibrils -> muscle contraction
T or F: As long as APs continue to be produced–which is as long as neural stimulation of the muscle is maintained–the Ca2+ release channels in the SR will remain open, Ca2+ will passively diffuse out of the SR, and the SR [Ca2+] will remain high. Thus, Ca2+ will remain attached to troponin, and the cross-bridge cycle will continue to maintain contraction.
True
What is necessary for the muscle to relax?
APs must cease
1) ACh-esterase degrades ACh
2) Ca2+ release channels close -> Ca2+ pumped back into SR through Ca2+-ATPase pumps
3) Choline recycled to make more ACh
What is a twitch?
When the muscle is stimulated with a single electric shock of sufficient voltage, it quickly contracts and relaxes
What is summation?
If second electrical shock is administered before complete relaxation of muscle -> produces a second twitch that may partially “ride piggyback” on the first
What is incomplete tetanus?
If the stimulator is set to deliver an increasing frequency of electric shocks automatically, the relaxation time between successive twitches will get shorter and shorter as the strength of contraction increases in amplitude
What is complete tetanus?
At a particular “fusion frequency” of stimulation, there is no visible relaxation between twitches -> contaction is smooth and sustained
-If the stimulation is continued, the muscle will demonstrate fatigue
What is treppe (the staircase effect)?
If a series of electrical shocks are delivered at maximum voltage -> each shock produces a separate twitch, each of the twitches evoked will be successively stronger, up to a higher maximum
- Due to increase in intracellular Ca2+
- Represents “warm-up”
What must happen in order for muscle fibers to shorten when they contract?
They must generate a force that is greater than the opposing forces that act to prevent movement of the muscle’s insertion