Skeletal Muscle Structure and Function: Study Notes

Skeletal Muscle Structure and Function

Types of Muscle Cells

Muscle tissue in the human body is classified into three main types, each with distinct structure and function:

• Skeletal Muscle: Composed of long, striated fibers attached to the skeleton. These muscles are voluntary (under conscious control) and contract rapidly. They are responsible for body movement and posture.

• Cardiac Muscle: Forms the heart's structure. It is striated like skeletal muscle but is involuntary (not under conscious control). Cardiac muscle contracts rhythmically and continuously to pump blood.

• Smooth Muscle: Found in the walls of hollow organs (such as the stomach, intestines, blood vessels). It is non-striated and involuntary. Smooth muscle contracts slowly and is responsible for movements like peristalsis.

Functions of Muscle Cells

Muscle cells perform several essential functions in the body:

• Movement: Locomotion, manipulation, blood flow, and pressure regulation.

• Posture: Maintaining body position against gravity.

• Stabilization: Stabilizing joints (e.g., shoulder muscles during arm movement).

• Heat Generation: Especially skeletal muscle, which produces heat as a byproduct of contraction (accounts for at least 40% of body mass).

Functional Characteristics of Muscle

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Response: Generation of an action potential along the sarcolemma, leading to contraction.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to return to original shape after stretching.

Anatomy of a Skeletal Muscle Cell

Structure of a Muscle Fiber

Skeletal muscle fibers are long, cylindrical cells with multiple nuclei located at the periphery. The plasma membrane is called the sarcolemma. Each muscle cell contains many myofibrils, which are composed of repeating units called sarcomeres.

• Sarcolemma: The cell membrane of a muscle fiber.

• Myofibrils: Cylindrical organelles within the muscle fiber, making up about 80% of the cell's volume. Myofibrils are composed of myofilaments (actin and myosin) arranged into sarcomeres.

• Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and myoglobin (an oxygen-binding protein).

• Myoglobin: Stores oxygen for use during muscle contraction.

Connective Tissue Layers

• Endomysium: Surrounds each individual muscle fiber.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Epimysium: Surrounds the entire muscle.

Sarcomeres

The sarcomere is the basic contractile unit of muscle, extending from one Z disk to the next. Sarcomeres are responsible for the striated appearance of skeletal muscle.

• Z disk: Defines the boundaries of a sarcomere; composed of alpha-actinin and anchors thin filaments via titin (an elastic protein).

• H zone: Area with only thick filaments (myosin).

• M line: Connects adjacent thick filaments.

Myofilaments

• Actin (Thin Filament): Crosses the I band and partly into the A band. Made of F-actin (polymerized G-actin subunits).

• Myosin (Thick Filament): Entire width of the A band. Each myosin molecule has a tail and two heads, which form cross-bridges with actin during contraction. Myosin heads contain ATPase activity.

Molecular Composition of Myofilaments

• Titin: Large, elastic protein that helps anchor thick filaments and provides recoil after stretching.

• Troponin-Tropomyosin Complex: Regulates muscle contraction by blocking or exposing the myosin-binding sites on actin.

• Troponin: A 3-polypeptide complex:

  • TnI: Binds to actin

  • TnT: Binds to tropomyosin

  • TnC: Binds to calcium ions

Clinical Note: Dystrophin

Dystrophin is a protein that acts as a shock absorber or anchor for muscle force. Mutations in the dystrophin gene cause muscular dystrophy, a group of disorders characterized by progressive muscle weakness.

Sliding Filament Theory of Muscle Contraction

Mechanism of Contraction

Muscle contraction occurs when myosin heads bind to actin, forming cross-bridges, and pull the thin filaments toward the center of the sarcomere. This process shortens the sarcomere and, consequently, the muscle fiber.

1. Stimulus to contract: Action potential arrives at the muscle fiber.

2. Myosin cross-bridges attach to actin.

3. Myosin heads pivot, pulling actin filaments toward the M line.

4. ATP binds to myosin, causing it to detach from actin and reset for another cycle.

5. The cycle repeats as long as calcium ions and ATP are present.

Detailed Cycle

1. At rest, myosin head has ATP and troponin-tropomyosin covers the active site on actin.

2. Excitation: Nerve impulse releases Ca2+ from the sarcoplasmic reticulum, which binds to troponin, moving tropomyosin and exposing the active site.

3. Myosin binds to actin, hydrolyzes ATP, and performs a power stroke.

4. ADP and phosphate are released, and myosin pulls actin closer to the center of the sarcomere.

5. New ATP binds to myosin, causing it to detach from actin and reset.

Note: If ATP is not produced, myosin cannot detach from actin, leading to rigor mortis.

Muscle Shortening

• On average, muscle fibers shorten by about 30% during contraction.

• Relaxation occurs when Ca2+ is reabsorbed by the sarcoplasmic reticulum.

Sarcoplasmic Reticulum and T-Tubules

The sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum that stores and releases Ca2+ ions, essential for muscle contraction. The T-tubules are invaginations of the sarcolemma that transmit action potentials into the muscle fiber, ensuring coordinated contraction.

• Triad: Consists of two terminal cisternae of the SR and one T-tubule. The triad is crucial for coupling excitation to contraction by releasing Ca2+ in response to an action potential.

Summary Table: Types of Muscle Tissue

Type	Striations	Control	Location	Function
Skeletal	Yes	Voluntary	Attached to bones	Movement, posture, heat
Cardiac	Yes	Involuntary	Heart	Pumping blood
Smooth	No	Involuntary	Walls of hollow organs	Movement of substances

Key Equations

• ATP Hydrolysis by Myosin ATPase:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Muscle Shortening Percentage:

$\text{Muscle Shortening} = \frac{\text{Change in Length}}{\text{Original Length}} \times 100\%$

Additional info:

• Some details about the molecular structure and function of titin, dystrophin, and the triad were expanded for clarity.

• Clinical context for dystrophin and muscular dystrophy was added for relevance.