M9: Skeletal and Smooth Muscle

What are the primary functions of skeletal muscle?

Movement, support, posture, and temperature regulation.

What percentage of body mass does skeletal muscle constitute?

Approximately 40%.

How do skeletal muscles facilitate movement?

By contracting and pulling on tendons connected to bones.

What role does skeletal muscle play in supporting visceral organs?

It provides abdominal wall support and shields tissue from injury.

How do skeletal muscles contribute to posture?

They continuously contract to maintain an upright sitting or standing position.

What is the effect of skeletal muscle on body temperature?

Skeletal muscle has a disproportionate effect on body temperature regulation.

What are some modes of interpersonal communication involving skeletal muscle?

Speaking, typing, writing, facial expressions, and gestures.

What is the macrostructure of skeletal muscle?

It consists of muscle tissue (red) and tendons (white).

What is the function of the fibrous connective tissue sheath in skeletal muscle?

It surrounds the entire muscle, providing anchorage and support to local nerves and blood vessels.

What specialized structures are found in skeletal muscle fibers?

Sarcoplasm and sarcoplasmic reticulum.

What is myogenesis?

The process when myoblasts fuse to form myocytes.

What is the typical diameter range of skeletal muscle fibers?

10-100 μm.

How long can skeletal muscle fibers grow?

Up to 23 inches.

What determines the orientation of skeletal muscle fibers?

The type of muscle.

What are the two types of muscle fibers?

Thick (myosin) and thin (actin) filaments.

What is the role of satellite cells in muscle tissue?

They are regenerative myoblasts that did not fuse during development.

What is the smallest contractile unit of skeletal muscle?

The sarcomere.

What are the boundaries of a sarcomere formed by?

Z-lines.

What initiates muscle contraction?

Excitation of the muscle fiber leading to action potential.

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

They regulate the binding of myosin to actin.

What is excitation-contraction coupling?

The process linking muscle fiber excitation to contraction.

What happens during the relaxation phase of muscle contraction?

Calcium levels decrease, and binding sites on actin become covered.

What is the neuromuscular junction?

A chemical synapse between an alpha motor neuron and a muscle fiber.

What is the sliding filament model?

A model explaining muscle contraction through the sliding of actin over myosin.

What is the latent phase of a muscle twitch?

The phase where action potential depolarizes the sarcolemma, and no tension is produced yet.

What is the contraction phase of a muscle twitch?

The phase where muscle fiber tension peaks due to cross-bridge interactions.

What is the effect of stimulation frequency on muscle contraction?

Higher stimulation frequency can lead to unfused or fused tetanus.

What is muscle tone?

The continuous and passive partial contraction of the muscles.

What factors affect muscle tone?

Structure of the muscle, including connective tissue and elastin component size.

What is the role of T-tubules in muscle fibers?

They carry depolarization waves deep into muscle fibers.

What is the role of calcium ions in muscle contraction?

They bind to troponin, causing tropomyosin to change conformation and expose binding sites on actin.

What happens during the power stroke in muscle contraction?

Myosin head moves into low-energy conformation, pulling actin towards the M line.

What is the role of acetylcholinesterase at the neuromuscular junction?

It degrades acetylcholine in the synaptic cleft, ending stimulation.

What is a motor unit?

A motor neuron and the muscle fibers it innervates.

What is the significance of motor unit recruitment?

Greater tension requires more motor units to be recruited.

What is the function of myofibrils in muscle cells?

They are the contractile elements of muscle fibers.

What is the role of mitochondria in muscle cells?

They support the high ATP demand of muscle contraction.

What is active muscle tone?

The number of motor units stimulated even when the muscle is at rest.

What is the length-tension relationship in muscles?

Muscle tension is ideal at a specific muscle length due to optimal placement of actin and myosin for cross-bridge formation.

What happens in resting muscle regarding ATP production?

More ATP is produced than needed, and ATP is generated from creatine phosphate.

What is the reaction for ATP generation from creatine phosphate?

Creatine phosphate + ADP → ATP + creatine.

How is ATP continuously generated in muscle?

ATP is continuously generated at the same rate it is used through the reaction ADP + ADP → ATP + AMP (myokinase).

What are the energy sources during short/intense versus prolonged exercise?

Prolonged exercise initially uses the same metabolism as short/intense exercise but shifts to the aerobic pathway for most of the duration.

What causes muscle fatigue?

Lactic acid build-up after high-intensity exercise and glycogen depletion after medium-intensity exercise over long periods.

What is the Cori cycle?

The process where lactic acid is shuttled through blood to the liver and back to muscle for recycling.

What is delayed-onset muscle soreness (DOMS)?

Muscle soreness that usually lasts 3-4 days, most intense after eccentric contractions, possibly caused by small muscle tears.

What is sarcopenia?

Age-related loss of both muscle fiber size and number of muscle fibers.

What is fibrosis in muscle tissue?

Age-related development of fibrous tissue that restricts movement and circulation.

What are the main functions of smooth muscle?

Line walls of hollow organs, responsible for involuntary movements, and regulate contraction/relaxation of passageways.

What are the structural characteristics of smooth muscle cells?

Smaller than skeletal muscle cells, spindle-shaped with a centrally-located nucleus, and no visible striations.

What is excitation-contraction coupling in smooth muscle?

Ca2+ enters the sarcoplasm, interacts with calmodulin, MLCK phosphorylates myosin, forming cross-bridges that develop muscle tension.

What is plasticity in smooth muscle tissue?

The ability to maintain contractibility over a wide range of lengths.

How does smooth muscle differ from skeletal muscle in contraction?

Smooth muscle contraction is involuntary and regulated at the level of myosin phosphorylation rather than troponin binding.

What happens to muscle fibers with age?

Muscle fibers get thinner, contain less ATP, CP, and glycogen, leading to decreased strength and endurance.

What is the impact of aging on muscle recovery?

Decreased number of satellite cells and poorer circulation hinder optimal healing of injured tissue.

What is the principle of overload in muscle training?

The body adapts after rest and becomes stronger in response to increased demands.