muscle physiology

What are the three classifications of muscle tissue shown in the sources?

Skeletal muscle, cardiac muscle, and smooth muscle.

What are the classification and voluntary control status of skeletal muscle?

Striated muscle, voluntary.

What is the typical location and primary function of skeletal muscle?

Attached to bones; function is movement of body in relation to external environment.

What are the classification and voluntary control status of cardiac muscle?

Striated muscle, involuntary.

What is the location and primary function of cardiac muscle?

Wall of heart; function is pumping of blood out of heart.

What are the classification and voluntary control status of smooth muscle?

Unstriated muscle, involuntary.

What is the typical location and primary function of smooth muscle?

Walls of hollow organs and tubes, such as stomach and blood vessels; function is movement of contents within hollow organs.

Approximately how many skeletal muscles does the human body contain, and what percentage of total body weight do they make up?

650 skeletal muscles, making up around 40-50% total body weight.

List the four main functions of skeletal muscles mentioned in the sources

Force production for locomotion, breathing, postural support, heat production during cold stress, and constricting openings (sphincters).

What are the three connective tissue layers surrounding skeletal muscle structures?

Epimysium, perimysium, and endomysium.

Which connective tissue layer surrounds the entire skeletal muscle (organ)?

Epimysium.

Which connective tissue layer surrounds a muscle fascicle (bundle of cells)?

Perimysium.

Which connective tissue layer surrounds a single muscle fiber (cell)?

Endomysium.

What is the muscle cell membrane called?

Sarcolemma.

What is the cytoplasm of a muscle cell called?

Sarcoplasm.

What is the endoplasmic reticulum of a muscle cell called?

Sarcoplasmic reticulum.

What are the characteristics of skeletal muscle fibers regarding nuclei?

Multinucleated.

What two structures give skeletal and cardiac muscle their striated appearance?

Myofibrils and sarcomeres.

What are the two primary contractile filaments within myofibrils?

Actin (thin) and myosin (thick).

Describe the structure of the myosin tail

It consists of 2 intertwined heavy chains.

What extends out from the myosin molecule to form cross bridges?

Two globular heads.

What three components are contained within the globular myosin heads?

Heavy chains, light chains, binding sites for actin, and binding sites for ATP.

What forms the backbone of the thin filament?

Two actin filaments twisted together.

What is the function of tropomyosin in a relaxed muscle?

It overlaps binding sites on actin for myosin and inhibits interaction in relaxed states.

What is the function of the contractile protein actin?

Each G actin has a binding site for myosin.

What three polypeptide units make up troponin?

One binds to tropomyosin, one binds to $\text{Ca}^{+}$, and one binds to actin.

How does troponin initiate contraction when calcium is present?

It binds to $\text{Ca}^{+}$ reversibly, changes conformation, and pulls tropomyosin away from the myosin interaction site, allowing myosin to interact with actin.

What is the functional unit of the skeletal muscle?

Sarcomere.

Which sarcomere band consists only of thin filaments?

I band.

Which sarcomere band contains thick filaments and the zone of overlap?

A band.

What is the overall effect on the sarcomere during muscle contraction?

The sarcomere shortens.

What is Excitation-Contraction Coupling?

The sequence of events in which the nerve impulse (action potential) depolarizes the muscle fiber, leading to muscle shortening by cross-bridge cycling.

What is the trigger to initiate skeletal muscle contraction?

The depolarization-induced release of calcium from the sarcoplasmic reticulum.

During contraction coupling, what happens after the action potential spreads down the T tubules?

Depolarisation of T tubules activates DHPR receptors in the membrane.

How is calcium released from the sarcoplasmic reticulum (SR) during skeletal muscle contraction?

DHPR receptors mechanically couple and pull open ryanodine receptors (RyRs) on the SR, releasing $\text{Ca}^{+}$ into the sarcoplasm.

What is the role of $\text{Ca}^{2+}$ in initiating cross-bridge formation?

$\text{Ca}^{+}$ binds to troponin, which moves tropomyosin, exposing myosin binding sites on the actin filament.

In the molecular steps of cross bridge cycling, what causes the cross bridge to release from actin?

A new ATP molecule binds to the myosin head.

What step immediately follows the binding of ATP in cross bridge cycling?

ATP is hydrolyzed, causing the cross bridge to cock and become energized.

What event immediately follows the binding of the cross bridge to actin and the release of $\text{P}_{\text{i}}$?

The power stroke causes filaments to slide, and ADP is released.

What is the function of ATP hydrolysis by $\text{Na}^{+}/\text{K}^{+}$-ATPase in the plasma membrane?

Maintains the ion gradients, allowing the membrane to produce and propagate action potentials.

What is the function of ATP hydrolysis by $\text{Ca}^{2+}$-ATPase (SERCA) in the sarcoplasmic reticulum?

Provides energy for the active transport of calcium ions into the reticulum, lowering cytosolic $\text{Ca}^{2+}$ to pre-release concentrations, thus ending muscle contractions.

What is the function of ATP hydrolysis by myosin-ATPase?

Energizes the cross bridges, providing energy for force generation.

What is the junction between a neuron and a muscle fiber called?

Neuromuscular junction (NMJ).

What is a motor unit?

A motor neuron and all the fibers it innervates.

What is the motor end plate?

The pocket formed around the motor neuron by the sarcolemma.

What neurotransmitter is released from the motor neuron at the NMJ?

Acetylcholine (ACh).

What is the effect of Acetylcholine release at the NMJ?

It causes end plate potential, which is the depolarization of the muscle fiber.

What is Myasthenia Gravis?

An autoimmune disease of the NMJ caused by the destruction of Acetylcholine receptors ($\text{AChR}$).

What is the treatment for Myasthenia Gravis?

Acetylcholinesterase inhibitors, such as neostigmine.

How does Botulism toxin (BOTOX) affect the NMJ?

It inhibits Acetylcholine release from the nerve terminal.

What is twitch summation in skeletal muscle?

It occurs if a muscle fiber is restimulated before it has completely relaxed, resulting in the second twitch being added on to the first twitch.

What is tetanus?

A maximal sustained contraction that occurs if a muscle fiber is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli.

What initiates cardiac muscle contraction?

Calcium induced calcium release.

What is a key time-related characteristic of cardiac muscle contraction not seen in skeletal muscle twitches?

Cardiac muscle has a refractory period.

What allows smooth muscle to contract even when stretched?

The stress relaxation response, which reflects the plasticity of the filaments.

How is smooth muscle contraction initiated?

By an increase in intracellular $\text{Ca}^{2+}$ concentration.

How does the mechanism of cross-bridge cycling in smooth muscle differ from skeletal muscle regarding regulatory proteins?

Troponin is not present; calcium combines with calmodulin and activates myosin light chain kinase, allowing cross bridge activation.

What clinical treatment uses $\text{Ca}^{2+}$ channel blockers to cause smooth muscle relaxation and dilation of blood vessels?

Treatment for hypertension and angina.