Skeletal Muscle Structure, Function, and Physiology: Study Notes

Skeletal Muscle Structure

Connective Tissue Layers of Skeletal Muscle

Skeletal muscles are organized into bundles surrounded by connective tissue layers that provide support and compartmentalization.

• Epimysium: Outermost layer; surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Endomysium: Surrounds individual muscle fibers within a fascicle.

Example: In the biceps brachii, the epimysium encases the whole muscle, perimysium wraps each fascicle, and endomysium envelops each muscle cell.

Microscopic Structure of Skeletal Muscle Fiber

Each muscle fiber is a long, cylindrical cell with specialized structures for contraction.

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Myofibrils: Rod-like units within muscle fibers, composed of repeating units called sarcomeres.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions necessary for contraction.

• Transverse Tubules (T-tubules): Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

Additional info: Mitochondria are abundant in muscle fibers to meet high energy demands.

Sarcomere: The Contractile Unit

The sarcomere is the basic functional unit of muscle contraction, defined by the area between two Z lines.

• Thick filaments: Composed of myosin protein.

• Thin filaments: Composed of actin, troponin, and tropomyosin.

• A band: Dark area; length of thick filaments.

• I band: Light area; contains only thin filaments.

• H zone: Center of A band; contains only thick filaments.

• M line: Center of sarcomere; holds thick filaments together.

• Z line: Defines the boundary of each sarcomere.

Zone	Description
Zone of overlap	Region where thick and thin filaments overlap
H zone	Thick filaments only
I band	Thin filaments only

Muscle Contraction: Physiology

Sliding Filament Theory

Muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere.

• Thick filaments: Myosin heads bind to actin, forming cross-bridges.

• Thin filaments: Actin molecules interact with myosin; troponin and tropomyosin regulate binding.

• ATP is required for myosin head movement and detachment.

Key observations:

• I bands and H zones get smaller during contraction.

• Z lines move closer together.

• A bands remain the same size.

Steps in Skeletal Muscle Contraction (Overview)

1. Neural stimulation: Motor neuron releases acetylcholine (ACh) at the neuromuscular junction, triggering an action potential in the muscle fiber.

2. Excitation-contraction coupling: Action potential travels along the sarcolemma and T-tubules, causing calcium release from the SR.

3. Calcium binding: Calcium binds to troponin, shifting tropomyosin and exposing actin binding sites.

4. Cross-bridge formation: Myosin heads bind to actin, forming cross-bridges.

5. Power stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

6. Detachment: ATP binds to myosin, causing it to release actin and reset for another cycle.

Equation:

$\text{Muscle contraction:} \quad \text{Actin} + \text{Myosin} + \text{ATP} \rightarrow \text{Actomyosin} + \text{ADP} + \text{P}_i$

Types of Muscle Contraction

• Isotonic contraction: Muscle changes length (shortens or lengthens) while tension remains constant.

• Isometric contraction: Muscle tension increases, but muscle length does not change.

Muscle Tone

Muscle tone is the continuous and passive partial contraction of muscles, important for posture and readiness.

Muscle Metabolism and Energy Sources

Muscle contraction requires ATP, which is generated by several metabolic pathways.

• Creatine phosphate (CP): Provides rapid ATP regeneration.

• Glycolysis: Anaerobic breakdown of glucose to produce ATP.

• Aerobic respiration: Oxidative metabolism in mitochondria; most efficient ATP production.

Equation:

$\text{CP} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

Muscle Types and Properties

Cardiac Muscle

Cardiac muscle is found only in the heart and is responsible for pumping blood.

• Cells are branched and connected by intercalated discs (gap junctions and desmosomes).

• Contraction is involuntary and rhythmic.

• Contains thick and thin filaments similar to skeletal muscle.

Smooth Muscle

Smooth muscle is found in the walls of hollow organs and blood vessels.

• Non-striated, involuntary contraction.

• Contains calmodulin protein for calcium binding (instead of troponin).

Muscle Attachments and Actions

Muscle Attachment Sites

• Origin: The stationary bone where the muscle attaches.

• Insertion: The moving bone where the muscle attaches.

Movement and Muscle Groups

• Agonist: Main muscle responsible for movement.

• Antagonist: Muscle that opposes the agonist.

• Synergist: Muscle that assists the agonist.

Naming and Grouping of Muscles

Action	Meaning	Example
Flexor	Decreases angle of joint	Biceps brachii
Extensor	Increases angle of joint	Triceps brachii
Abductor	Moves limb away from midline	Deltoid
Adductor	Moves limb toward midline	Adductor longus

Selected Muscles and Their Functions

Muscles of Facial Expression

• Orbicularis oculi: Closes eyelids

• Orbicularis oris: Closes lips

• Buccinator: Compresses cheeks

• Zygomaticus major: Smiling

• Risorius: Draws corners of mouth to side

Muscles of Mastication

• Masseter: Elevates mandible

• Temporalis: Elevates and retracts mandible

• Medial pterygoid: Elevates mandible, closes jaw

• Lateral pterygoid: Opens jaw

Muscles of the Neck

• Sternocleidomastoid: Flexes neck, rotates head

• Stylohyoid: Elevates hyoid and larynx

Abdominal Muscles

• Rectus abdominis: Flexes vertebral column

• External oblique: Compresses abdomen, rotates trunk

• Internal oblique: Compresses abdomen, rotates trunk

• Transversus abdominis: Compresses abdomen

Muscles of the Pectoral Girdle

• Trapezius: Elevates, rotates, and depresses scapula

Muscles that Move the Arm

• Deltoid: Abducts arm

• Rotator cuff muscles: Stabilize shoulder joint

Breathing Muscles

• Diaphragm: Main muscle of respiration

• External intercostals: Elevate ribs

• Internal intercostals: Depress ribs

Muscles of the Vertebral Column

• Erector spinae group: Extends neck and back

• Transversospinalis group: Rotates and extends vertebral column

Muscle Disorders and Regeneration

• Muscular dystrophy: Genetic disorder causing muscle weakness and degeneration.

• Atrophy: Decrease in muscle size due to inactivity.

• Hypertrophy: Increase in muscle size due to increased use.

• Fibrosis: Replacement of muscle tissue with fibrous tissue after injury.

Summary Table: Muscle Types

Type	Location	Striations	Control
Skeletal	Attached to bones	Striated	Voluntary
Cardiac	Heart	Striated	Involuntary
Smooth	Walls of organs	Non-striated	Involuntary

Additional info: These notes expand on brief points with academic context and examples for clarity and completeness.