BIO66 - Chapter 12: Muscle

What are the three types of muscle cells, and how do they differ in structure and function?

Skeletal muscle: Striated, voluntary, attached to bones, responsible for movement.

Cardiac muscle: Striated, involuntary, found in the heart, contracts rhythmically.

Smooth muscle: Non-striated, involuntary, found in organs, blood vessels, and skin, controls movements like digestion and blood flow.

Why do skeletal and cardiac muscles have striations, while smooth muscle does not?

Striations are caused by the organized arrangement of myofilaments (actin and myosin) in skeletal and cardiac muscle.

Smooth muscle lacks this organized structure, so it does not appear striated.

What is the difference between voluntary and involuntary muscle control?

Voluntary muscles (skeletal) are controlled consciously by the somatic nervous system.

Involuntary muscles (cardiac and smooth) are controlled automatically by the autonomic nervous system.

Why are skeletal muscle cells called muscle fibers?

They are elongated in shape and can be very long (up to 20 cm), resembling fibers.

What is the role of tendons in skeletal muscle movement?

Tendons connect muscles to bones and transmit the force generated by muscle contraction to produce movement.

How do myofibrils and myofilaments contribute to the striated appearance of skeletal muscle?

Myofibrils are made of alternating dark A bands and light I bands, which give skeletal muscle its striated appearance. These bands result from the orderly arrangement of actin (thin) and myosin (thick) filaments.

What happens at the neuromuscular junction when a motor neuron activates a skeletal muscle fiber?

The motor neuron releases acetylcholine (ACh), which binds to nicotinic ACh receptors on the muscle fiber, triggering an action potential that leads to muscle contraction.

How does acetylcholine (ACh) trigger muscle contraction?

ACh binds to receptors, opening ion channels, causing depolarization of the muscle fiber, which leads to calcium release from the sarcoplasmic reticulum, initiating contraction.

What is a motor unit, and why are smaller motor units important for fine muscle control?

A motor unit consists of a motor neuron and all the muscle fibers it controls. Smaller motor units allow for precise and fine muscle movements, like those in the fingers or eyes.

How does recruitment of motor units affect muscle tension?

The more motor units that are activated, the greater the muscle tension. This allows for controlled and graded force production.

What are the two main types of myofilaments in skeletal muscle, and what are their roles in contraction?

Thick filaments (myosin): Bind to actin and generate force.

Thin filaments (actin, troponin, tropomyosin): Provide a structure for myosin to pull on during contraction.

What is the function of troponin and tropomyosin in muscle contraction?

Tropomyosin blocks myosin-binding sites on actin.

Troponin binds to calcium (Ca²⁺), shifting tropomyosin and exposing binding sites for myosin.

Describe the sliding filament mechanism of muscle contraction.

Myosin heads bind to actin, pulling it toward the center of the sarcomere, shortening the muscle fiber.

What is the sarcomere, and why is it considered the basic contractile unit of striated muscle?

The sarcomere is the functional unit of contraction, consisting of thick and thin filaments arranged in a repeating pattern.

What is the role of calcium (Ca²⁺) in muscle contraction?

Calcium binds to troponin C, which moves tropomyosin and allows myosin to bind to actin, initiating contraction.

How does ATP contribute to the cross-bridge cycle?

ATP is required to detach myosin from actin, reset the myosin head, and provide energy for the next contraction cycle.

What is excitation-contraction coupling, and how does it link an action potential to muscle contraction?

The action potential travels through T-tubules, causing the release of Ca²⁺ from the sarcoplasmic reticulum, which initiates contraction.

What is the latent period in muscle contraction?

The short delay between an action potential and the start of muscle contraction, due to calcium release and cross-bridge formation.

How does summation differ from a single muscle twitch?

Summation occurs when multiple stimuli lead to increased muscle tension, while a twitch is a single contraction in response to one stimulus.

What is tetanus (tetany), and how is it achieved in muscle fibers?

A sustained muscle contraction due to rapid, repetitive stimulation before the muscle can relax.

How do isotonic and isometric contractions differ? Give an example of each.

Isotonic: Muscle changes length (e.g., lifting a weight). Isometric: Muscle generates tension but doesn't change length (e.g., holding a plank).

What happens when a sarcomere is too short or too long in terms of tension generation?

Too short: Actin overlaps too much, reducing tension.

Too long: Not enough actin-myosin overlap, reducing force production.

Compare and contrast slow-twitch (Type I) and fast-twitch (Type IIx) muscle fibers.

Slow-twitch: Fatigue-resistant, rich in mitochondria, used for endurance.

Fast-twitch: Fatigue quickly, generate powerful contractions, used for sprinting.

How does muscle fiber type influence endurance and strength?

More slow-twitch fibers = greater endurance. More fast-twitch fibers = greater strength and power.

What are some potential causes of muscle fatigue?

Ion imbalances, lactic acid buildup, glycogen depletion, impaired Ca²⁺ release.

How does cardiac muscle differ from skeletal and smooth muscle?

Involuntary, striated, connected by intercalated discs, contracts rhythmically.

What is the function of intercalated disks in cardiac muscle?

Allow for electrical and mechanical coupling between cardiac cells.

How does calcium (Ca²⁺) play a role in cardiac muscle contraction?

Ca²⁺ enters from extracellular fluid and SR, triggering contraction.

Why does smooth muscle not have striations like skeletal and cardiac muscle?

Filaments are arranged irregularly, not in sarcomeres.

How is cross-bridge activation in smooth muscle different from that in skeletal muscle?

Uses calmodulin instead of troponin to activate myosin.

What is the role of calmodulin in smooth muscle contraction?

Binds to Ca²⁺ and activates myosin light-chain kinase (MLCK).

How does the source of Ca²⁺ differ between skeletal and smooth muscle?

Skeletal: Mostly from SR. Smooth: From SR and extracellular fluid