Skeletal Muscle Structure and Function: Study Notes

Skeletal Muscle Structure and Function

Functions of Skeletal Muscles

Skeletal muscles are essential for movement and maintaining the body's structure. They perform several key functions:

• Movement: Muscles pull on bones to create voluntary movements such as walking, running, or lifting objects.

• Posture and Body Position: Muscles help maintain posture, allowing you to sit, stand, and hold your body upright.

• Support of Soft Tissues: Muscles protect and support internal organs, especially in the abdominal and pelvic regions.

• Guarding Body Openings: Sphincter muscles control the opening and closing of passages such as the mouth, urethra, and anus.

• Heat Generation (Thermogenesis): Muscle contractions produce heat, which helps maintain body temperature (e.g., shivering).

Connective Tissue Organization in Muscle

Skeletal muscles are organized by layers of connective tissue that provide structure and support:

• Endomysium: Surrounds each individual muscle fiber (cell).

• Perimysium: Surrounds a fascicle, which is a bundle of muscle fibers.

• Epimysium: Surrounds the entire muscle.

Mnemonic: ENDO > PERI > EPI (smallest to largest)

Anatomy of a Skeletal Muscle Fiber

Each muscle fiber is a specialized cell with unique structures for contraction:

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber.

• Nuclei: Multiple nuclei are located peripherally in each fiber.

• Myofibrils: Contractile organelles composed of myofilaments.

• Sarcoplasmic Reticulum (SR): Stores calcium ions necessary for contraction.

• T-tubules (Transverse Tubules): Carry electrical signals (action potentials) into the muscle fiber.

Sarcomere Organization

The sarcomere is the functional unit of muscle contraction, composed of repeating segments along the myofibril:

• Z line: Boundary of each sarcomere.

• I band: Contains only thin filaments (actin).

• A band: Entire length of thick filaments (myosin).

• H zone: Region with only thick filaments.

• M line: Center of the sarcomere.

During contraction:

• I band and H zone decrease in width.

• A band remains the same length.

Key Terms in Muscle Structure

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Muscle cell cytoplasm.

• Sarcoplasmic Reticulum: Stores and releases calcium ions.

• T-tubules: Carry action potentials into the cell.

• Myofibrils: Bundles of myofilaments responsible for contraction.

Contraction of Skeletal Muscles

Muscle contraction is a complex process involving electrical and chemical signals:

1. Nerve impulse arrives at the neuromuscular junction.

2. Acetylcholine (ACh) is released into the synaptic cleft.

3. Action potential spreads along the sarcolemma.

4. Calcium is released from the sarcoplasmic reticulum.

5. Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.

6. Myosin binds to actin, forming cross-bridges.

7. Power stroke occurs, sliding filaments past each other.

8. ATP binds to myosin, causing it to detach from actin.

9. Muscle relaxes when calcium is reabsorbed into the SR.

Importance of the Sarcoplasmic Reticulum

• Stores calcium ions (Ca2+).

• Releases calcium to initiate contraction.

• Reabsorbs calcium to allow relaxation.

The Triad

The triad is a structural feature important for excitation-contraction coupling:

• Consists of one T-tubule and two terminal cisternae of the sarcoplasmic reticulum.

Components of Actin (Thin Filament)

• Actin: Main protein forming the thin filament.

• Troponin: Binds calcium and regulates tropomyosin position.

• Tropomyosin: Blocks active sites on actin when muscle is at rest.

Parts of the Neuromuscular Junction

• Motor Neuron: Nerve cell that stimulates the muscle fiber.

• Synaptic Cleft: Gap between neuron and muscle fiber.

• Axon Terminal: End of the motor neuron.

• Acetylcholine (ACh): Neurotransmitter released to stimulate muscle contraction.

• Motor End Plate: Specialized region of the muscle fiber's sarcolemma.

Types of Muscle Contractions

• Twitch: Single, brief contraction.

• Treppe: Gradual increase in contraction strength after repeated stimulation following rest.

• Wave Summation: Increased force due to repeated stimuli before the muscle fully relaxes.

• Incomplete Tetanus: Partial relaxation between contractions.

• Complete Tetanus: Sustained contraction with no relaxation.

Isometric vs. Isotonic Contractions

• Isometric: Muscle tension increases, but muscle length does not change (e.g., pushing against a wall).

• Isotonic: Muscle changes length, producing movement.

  • Concentric: Muscle shortens.

  • Eccentric: Muscle lengthens.

Energy Sources for Muscle Contraction

• At Rest: Aerobic respiration using fatty acids and glucose.

• During Contraction:

  1. Stored ATP

  2. Creatine phosphate

  3. Glycolysis

  4. Aerobic respiration

Muscle Fatigue

Muscle fatigue is a temporary loss of ability to contract due to:

• ATP depletion

• Ion imbalances

• Lactic acid accumulation

• Nervous system factors

Muscle Fiber Types

Type	Color	Mitochondria	Fatigue Resistance	Example
Slow Oxidative	Red	Many	High	Marathon runner
Fast Glycolytic	White	Few	Low	Sprinter
Fast Oxidative Glycolytic	Intermediate	Moderate	Moderate	Middle-distance runner

Skeletal Muscle Arrangements

Arrangement	Example
Parallel	Biceps brachii
Parallel with tendinous bands	Rectus abdominis
Convergent	Pectoralis major
Circular	Orbicularis oris
Unipennate	Extensor digitorum
Bipennate	Rectus femoris
Multipennate	Deltoid
Wrapping (spiral)	Supinator

Muscle Attachments: Origin and Insertion

• Origin: Attachment point that remains relatively fixed during contraction.

• Insertion: Attachment point that moves during contraction.

Example: Biceps brachii Origin: Scapula Insertion: Radius When the biceps contract, the radius moves toward the scapula.

Muscle Roles in Movement

• Agonist (Prime Mover): Main muscle producing a movement (e.g., biceps brachii during elbow flexion).

• Antagonist: Opposes the agonist (e.g., triceps brachii during elbow flexion).

• Synergist: Assists the agonist (e.g., brachialis assists the biceps).

• Fixator: Stabilizes the origin of the agonist (e.g., muscles stabilizing the scapula during arm movement).

Axial and Appendicular Musculature

• Axial Muscles: Attach to the skull, vertebral column, ribs, or sternum. Functions include posture, breathing, and movement of the head and trunk.

  • Examples: Sternocleidomastoid, rectus abdominis, external oblique, intercostals

• Appendicular Muscles: Attach to the limbs or girdles (shoulder/pelvic). Responsible for movement of arms and legs.

  • Examples: Deltoid, biceps brachii, triceps brachii, quadriceps femoris

Muscle Identification Essentials

For each muscle, know the following:

1. Origin

2. Insertion

3. Innervation (nerve supply)

4. Major action

Additional info: For exam preparation, be able to identify these features for all major muscles discussed in class or listed in your course materials.