Skeletal Muscle Structure and Function: Study Notes

Skeletal Muscle Structure and Function

Functions of Skeletal Muscles

Skeletal muscles are essential for movement, posture, and various physiological processes. Their main functions can be summarized as follows:

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

• Posture and Body Position: Muscles maintain posture, allowing us to sit, stand, and hold the 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, helping to 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 (bundle of muscle fibers).

• Epimysium: Surrounds the entire muscle.

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

Anatomy of a Skeletal Muscle Fiber

Skeletal muscle fibers are specialized cells with unique structures for contraction:

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber.

• Nuclei: Multiple, located at the periphery of the cell.

• 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 cell.

Sarcomere Organization

The sarcomere is the functional unit of muscle contraction, composed of organized protein filaments:

• Z line: Defines the boundary of each sarcomere.

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

• A band: Contains the entire length of thick filaments (myosin).

• H zone: Contains only thick filaments.

• M line: Center of the sarcomere.

Changes During Contraction:

• I band: Decreases in width.

• H zone: Decreases in width.

• A band: Remains the same.

Key Terms in Muscle Structure

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Muscle cell cytoplasm.

• Sarcoplasmic Reticulum: Stores calcium ions (Ca2+).

• T-tubules: Carry action potentials into the muscle fiber.

• Myofibrils: Bundles of myofilaments responsible for contraction.

Contraction of Skeletal Muscle: Steps

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.

6. Tropomyosin moves, exposing active sites on actin.

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

8. Power stroke occurs (myosin pulls actin).

9. ATP binds to myosin, causing detachment from actin.

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

Importance of the Sarcoplasmic Reticulum (SR)

• Stores calcium ions (Ca2+).

• Releases calcium to initiate contraction.

• Reabsorbs calcium to allow muscle relaxation.

The Triad Structure

The triad is a specialized structure in muscle fibers:

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

• Facilitates rapid transmission of action potentials and release of calcium.

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.

Neuromuscular Junction (NMJ) Components

• Motor Neuron: Nerve cell that stimulates 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 membrane.

Types of Muscle Contractions

• Twitch: Single, brief contraction.

• Treppe: Gradual increase in contraction strength after repeated stimulation ("staircase effect").

• Wave Summation: Increased force due to repeated stimuli before relaxation is complete.

• 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 (e.g., lifting a weight).

  • Eccentric: Muscle lengthens (e.g., lowering a weight).

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: Muscle that 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 vs. Appendicular Musculature

Type	Attachment	Function	Examples
Axial Muscles	Skull, vertebral column, ribs, sternum	Posture, breathing, movement of head/trunk	Sternocleidomastoid, rectus abdominis, external oblique, intercostals
Appendicular Muscles	Limbs or girdles (shoulder/pelvic)	Movement of arms and legs	Deltoid, biceps brachii, triceps brachii, quadriceps femoris

Muscle Identification: Key Features to Know

For each muscle, be able to identify:

1. Origin

2. Insertion

3. Innervation (nerve supply)

4. Major action

Example: Biceps brachii Origin: Scapula Insertion: Radius Innervation: Musculocutaneous nerve Major action: Flexes elbow and supinates forearm

Additional info: For exam preparation, students should be able to apply these concepts to specific muscles, understand the sequence of contraction, and relate muscle structure to function.