Muscle Physiology Concepts Flashcards
Identify the 4 roles of muscular tissue.
- Movement
- Maintain posture
- Store and move substances within the body (through the sphincter, peristaltic contractions, dilation, and constriction)
- Generate heat (thermogenesis)
Discuss the 4 characteristics that make muscle tissue special when compared to other tissues.
Excitable, have the ability to respond to a stimulus
Contractible, can shorten in length
Extensible, can extend or stretch
Elxastic, can return to original shape
Describe the microscopic anatomy of a skeletal muscle fiber, highlighting the key ions, molecules, organelles, and cells.
Skeletal muscle fibers are long, elongated cells with many nuclei that are wrapped by a plasma membrane known as the sarcolemma. The cytoplasm of the muscle fiber, the sarcoplasm, contains many myofibrils that run parallel to the long axis of the muscle fiber. Sarcomeres are the contractile units of muscle fibers and are made up of repeating units termed myofibrils.
Sarcomeres are made up of thick filaments made of the protein myosin and thin filaments made of the protein actin. The tiny filaments are attached to a Z-disc at each end of the sarcomere, while the thick filaments are attached to the M-line in the center. When muscle contraction, myosin heads engage with actin filaments within the sarcomere, generating force with ATP.
The sarcoplasmic reticulum, which stores calcium ions generated during muscular contraction, and mitochondria, which provide ATP for muscle contraction, are two other important organelles within the muscle fiber. Satellite cells, which are crucial in muscle repair and regeneration, are also found in muscle fibers.
Identify and describe the function of contractile proteins in skeletal muscles
Actin, Myosin
Proteins that generate force during muscle contractions
Identify and describe the function of regulatory proteins in skeletal muscles
Troponin, Tropomyosin
Proteins that help switch the muscle contraction processes on and off.
Identify and describe the function of structural proteins in skeletal muscles
Titin
Proteins that keep thick and thin filaments of myofibrils in proper alignment, give myofibrils elasticity and
extensibility, and link myofibrils to the sarcolemma and extracellular matrix.
Describe how muscle action potentials arise at the neuromuscular junction, as a part of the excitation-contraction coupling. Describe the excitation-contraction sequence in detail, naming the key structures and ordering the major steps. As you order the steps, know which are pre-synaptic and which are post-synaptic.
Step 1:Excition
A somatic nerve carries a nerve impulse to a muscle fiber, and at the neuromuscular junction, the impulse is transmitted from a nerve impulse to a muscle signal. The sarcolemma (cell membrane), with the help of transverse tubules to spread the signal, depolarizes so that all the myofibrils of the muscle cell contract.
Step 2: Releasing Calcium
The depolarization of the sarcolemma in step 1 causes Ca2+ release channels to open in the membrane of the sarcoplasmic reticulum through facilitated diffusion. The calcium fills the sarcoplasm to trigger muscle contraction and unlocks the contractile mechanism by exposing myosin-binding sites on actin. Troponin binds to calcium to help move tropomyosin out of the way to expose the binding sites on actin. Myosin heads binding to actin causes muscle contraction.
Step 3: Cross-bridging
Myosin heads can bind to the actin filaments with the tropomyosin out of the way and the muscle can contract through the sliding filament theory.
Describe a sarcomere in detail, naming all the relevant structures,
The sarcomere, I band, and H zone shrinks during contraction.
Myosin heads attach to actin during contraction, pushing the thin filaments toward the center of the sarcomere in a power stroke and shortening the H-zone and I-band. The myosin heads disengage and realigns themselves for another power stroke during the recharging phase. The sarcomere returns to its resting state after the motor neuron stops firing.
Outline the 4 steps involved in the sliding filament mechanism of muscle contraction. Know which structures change length during contraction.
Step 1: ATP hydrolysis
A water molecule passes across the covalent bond between the 2nd and 3rd phosphate groups. Hydrolyzing ATP into ADP and P, create cellular energy.
Step 2: Attachment
Myosin heads bind to actin to form cross-bridges
Step 3: Power Stroke
Myosin cross-bridges rotate toward the center of the sarcoma, towards the M line, shortening the sarcomere and contract the muscle
Step 4: Detachment
Through active transport, the calcium ions are pumped back to the sarcoplasmic reticulum. When the power stroke is made, the ADP detaches from the myosin head, leaving the myosin head in a weak state. So, new ATP binds to the myosin head to reenergize it for another cross-bridge attachment and power stroke.
Understand the role of ATP dehydration synthesis & hydrolysis in powering the 4 steps of the sliding filament theory. Be able to relate the regulatory actions of troponin & tropomyosin (as part of the excitation-contraction coupling) to the 4 steps of the sliding filament theory.
ATP hydrolysis creates the energy needed to power the 4 steps of the sliding filament theory. Dehydration synthesis gives energy for the repositioning of myosin heads for the next power stroke. Troponin and tropomyosin are regulatory proteins that modulate muscle contraction by exposing and covering actin-binding sites, permitting or inhibiting myosin head binding. The regulatory proteins move out of the way in the second stage of the sliding filament theory, allowing the power stroke to occur.
Contrast the basics of limited length-tension relationships in skeletal muscle with better relationships in smooth muscle. Does skeletal muscle contract efficiently to develop tension if it starts off very understretched or overstretched?
Smooth muscle is better at generating tension from all different lengths of overlap. At optimal length, an ideal amount of overlap, skeletal muscle can create maximum tension. If the skeletal muscle is under-stretched, there’s too much overlap in myosin and actin, giving no range for actin fibers to slide, so there’s poor tension. Likewise, if the skeletal muscle is overstretched, there’s too little overlap which creates poor tension.
Explain the phases of a twitch contraction. Is the tension of contracting sarcomeres immediately converted to force? If not, where are the “delays” in the system?
There are three phases of a twitch contraction: the latent period, contraction period, ad relaxation period.
The tension created by contracting sarcomeres does not instantly translate into force. The first delay occurs during the latent phase, which allows the action potential to go down the sarcolemma and calcium ions to be released from the sarcoplasmic reticulum. The second delay occurs during the contraction period when cross-bridges between actin and myosin form and muscle tension begins to rise. The final delay occurs during the relaxation stage when calcium ions are withdrawn from the sarcoplasm and the cross-bridges are released, allowing the muscle to return to rest.
Contrast single twitch force vs. wave summation force.
The force created by a single stimulus is known as a single twitch force, but the force generated by numerous stimuli in fast succession is known as wave summation force. Because the muscle does not have time to completely relax between stimuli, wave summation force is larger than single twitch force, resulting in greater overall tension.
contrast unfused tetanus with fused tetanus in terms of the amount of force generated.
Unfused tetanus is distinguished by a series of separate twitches that are not fully fused together, resulting in a “bumpy” contraction. In contrast, fused tetanus is a smooth, persistent contraction caused by a high enough frequency of stimulation that the twitches fuse together and there is no rest between twitches. As there is no time for the muscle to relax between twitches, fused tetanus generates more force than unfused tetanus.
Relate action potentials to force contraction graphs & know whether maximum force is generated via larger action potentials or more frequent action potentials.
The amount of force produced during muscle contraction is proportional to the action potentials produced by muscle fibers. The maximum force is generated via more frequent action potentials.
