Chapter 1 & 3: Muscle Structure, Excitation-Contraction Coupling & Neural Control

What are the three connective tissue sheaths that organize skeletal muscle, and what do they surround?

Epimysium surrounds the entire outer muscle belly; Perimysium surrounds bundles of muscle fibers called fascicles; Endomysium surrounds individual muscle fibers (cells).

What is the primary role of satellite cells located between the plasmalemma and basement membrane?

Satellite cells play a critical role in muscle growth, development, adaptation to exercise training, and cellular repair following muscle injury.

What are Transverse Tubules (T-tubules) and what is their function?

T-tubules are deep invaginations of the plasmalemma that allow electrical action potentials to travel rapidly from the cell surface into the interior of the muscle fiber.

What is the sarcoplasmic reticulum (SR) and what essential ion does it store?

The SR is a network of fluid-filled membranous channels wrapped around myofibrils that stores and releases calcium ions (Ca2+) for muscle contraction.

Define the functional boundary of a sarcomere.

A sarcomere is the basic functional contractile unit of a myofibril, running from one Z-disk to the next Z-disk.

Which sarcomere bands/zones shorten during a concentric contraction, and which stays constant?

The I-band and H-zone shorten (and the H-zone can disappear), while the A-band length remains completely unchanged because thick myosin filaments do not change length.

What are the roles of troponin and tropomyosin on the thin actin filament at rest?

Tropomyosin physically blocks the myosin-binding sites on actin at rest. Troponin is attached to both actin and tropomyosin and acts as the calcium sensor.

What happens when calcium (Ca2+) binds to troponin?

Calcium binding causes troponin to change shape, pulling tropomyosin off the active binding sites on actin so myosin heads can attach.

What is the function of the giant protein titin?

Titin acts like a molecular spring extending from the Z-disk to the M-line; it centers myosin filaments within the sarcomere, provides passive elastic stiffness, and prevents overstretching.

What neurotransmitter is released at the neuromuscular junction (NMJ) to initiate muscle depolarization?

Acetylcholine (ACh).

What specific action triggers the myosin "power stroke"?

The release of inorganic phosphate (Pi) from the myosin head after binding to actin causes the head to pivot, pulling actin toward the M-line.

Why is ATP required for both muscle contraction AND muscle relaxation?

ATP is required for the cross-bridge cycle, allowing myosin to detach from actin to contract muscle. ATP is also needed to power calcium pumps that move Ca²⁺ back into the SR, allowing the muscle to relax.

Compare the contractile speed and myosin ATPase activity of Type I vs. Type II muscle fibers.

Type I fibers have slow myosin ATPase activity and reach peak tension slowly (~110 ms); Type IIa and IIx fibers have fast myosin ATPase activity and reach peak tension rapidly (~50 ms).

What structural feature of Type II muscle fibers allows for faster calcium release compared to Type I fibers?

Type II muscle fibers have a significantly more highly developed sarcoplasmic reticulum (SR).

Explain Henneman's Size Principle of motor unit recruitment.

Motor units are always recruited in an orderly hierarchy based on motor neuron size: small, slow-twitch Type I motor units are recruited first, followed by Type IIa, and finally large Type IIx units for maximum force demands.

What is rate coding, and how does it progress from a single twitch to tetanus?

Rate coding is the frequency of motor unit stimulation. Rapid, repeated stimuli summate force until reaching tetanus, a smooth, maximal continuous muscle contraction.

According to the Length-Tension relationship, when does a muscle generate maximal isometric force?

At an optimal sarcomere length where there is maximal cross-bridge interaction between actin and myosin filaments.

Explain the Force-Velocity relationship for concentric vs. eccentric muscle actions.

During concentric actions, maximal force decreases as shortening velocity increases. During eccentric actions, maximal force increases as lengthening velocity increases.

What maintains the resting membrane potential of a neuron at -70 mV?

High Na+ concentration outside, high K+ inside, high membrane permeability to K+ leak, and the Na+/K+ pump (pumping 3Na+ OUT for every 2K+ IN).

What ion movement causes depolarization during an action potential once threshold (-55 mV) is reached?

Voltage-gated Na+ channels open, allowing Na+ to rush INTO the cell, flipping membrane potential to +30 mV.

What ion movement causes repolarization of the neuron?

Na+ channels close and voltage-gated K+ channels open, allowing K+ to rush OUT of the cell, restoring negative voltage.

Differentiate between the Absolute and Relative Refractory Periods.

During the absolute refractory period (depolarization/early repolarization), Na+ channels are open or inactivated, making another action potential impossible. During the relative refractory period (hyperpolarization), a secondary action potential can occur only with an abnormally strong stimulus.

What is saltatory conduction?

The rapid jumping of an action potential from one Node of Ranvier to the next along a myelinated axon.

Compare the functions of Muscle Spindles and Golgi Tendon Organs (GTOs).

Muscle spindles sense muscle stretch/length and induce reflex contraction to prevent overstretching. GTOs sense tendon tension and induce reflex inhibition/relaxation to prevent tendon tears.

Compare neurotransmitters used by the Sympathetic vs. Parasympathetic nervous systems.

The Sympathetic division ("fight-or-flight") primarily uses Norepinephrine; the Parasympathetic division ("rest-and-digest") uses Acetylcholine