Skeletal Muscle Structure and Function: Study Guide

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 contract to pull on bones, enabling actions such as walking, running, and lifting.

• Posture and Body Position: Muscles stabilize the body, allowing us to sit, stand, and maintain an upright posture.

• 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 orifices 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, each with a specific role:

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

• Perimysium: Encloses a bundle of muscle fibers, known as a fascicle.

• Epimysium: Envelops 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:

• Sarcolemma: The muscle cell membrane.

• Sarcoplasm: The cytoplasm of the muscle cell.

• 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: Invaginations of the sarcolemma that transmit electrical signals into the cell.

Sarcomere Organization

The sarcomere is the basic contractile unit of muscle fibers, defined by specific regions:

• Z line: Boundary of the sarcomere.

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

• A band: Length of thick filaments (myosin), includes overlap with thin filaments.

• H zone: Central region with only thick filaments.

• M line: Center of the sarcomere, where thick filaments are anchored.

Changes 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 muscle fiber.

• Myofibrils: Bundles of myofilaments responsible for contraction.

Contraction of Skeletal Muscles

Muscle contraction is a multi-step process:

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 ions are released from the sarcoplasmic reticulum.

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

6. Myosin heads bind to actin, forming cross-bridges.

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

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

9. 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 relaxation.

The Triad

The triad is a structural feature of muscle fibers:

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

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

Components of Actin (Thin Filament)

• Actin: Main protein forming the filament.

• Troponin: Binds calcium and regulates tropomyosin position.

• Tropomyosin: Blocks active sites on actin 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, releases ACh.

• Acetylcholine (ACh): Neurotransmitter that initiates muscle contraction.

• Motor End Plate: Specialized region of the sarcolemma with ACh receptors.

Types of Muscle Contractions

• Twitch: Single, brief contraction.

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

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

• Incomplete Tetanus: Partial relaxation between contractions.

• Complete Tetanus: Sustained contraction with no relaxation.

Isometric vs. Isotonic Contractions

• Isometric: Muscle tension increases, but 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

Fatigue is a temporary loss of ability to contract, caused by:

• ATP depletion

• Ion imbalances

• Lactic acid accumulation

• Nervous system factors

Types of Muscle Fibers

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 Muscle
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 responsible for 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 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: Key Features to Know

For each muscle, be able to identify:

• Origin

• Insertion

• Innervation (nerve supply)

• Major action

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