The Muscular System: Structure, Function, and Physiology

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The Muscular System

Overview of Muscle Types

The muscular system is responsible for all types of body movement and consists of three basic types of muscle tissue: skeletal, cardiac, and smooth. Each type has unique structural and functional characteristics.

Skeletal Muscle: Largest, voluntary, striated, multinucleate, attached to bones by tendons.
Cardiac Muscle: Involuntary, striated, found only in the heart, branching cells joined by intercalated discs.
Smooth Muscle: Smallest, involuntary, non-striated, spindle-shaped, found in walls of hollow organs.

Key Terminology: 'Myo-' and 'mys-' refer to muscle; 'sacro-' refers to flesh.

Skeletal Muscle Structure

Skeletal muscle fibers are large, elongated cells surrounded by connective tissue sheaths. The organization of these sheaths is crucial for muscle function.

Endomysium: Encloses a single muscle fiber.
Perimysium: Wraps around a fascicle (bundle) of muscle fibers.
Epimysium: Covers the entire skeletal muscle.
Fascial: Dense connective tissue outside the epimysium.
Tendons: Cordlike structures, mostly collagen, attach muscle to bone.
Aponeuroses: Sheetlike structures, attach muscles indirectly to bones or connective tissue.

Cardiac and Smooth Muscle Structure

Cardiac Muscle: Striated, involuntary, uninucleate, contracts at a steady rate due to pacemaker cells.
Smooth Muscle: Non-striated, involuntary, uninucleate, slow contractions, found in visceral organs.

Functions of Muscles

Muscles perform several essential functions beyond movement.

Maintain posture
Stabilize joints
Generate heat

Microscopic Anatomy of Skeletal Muscle

Muscle Cell Structure

Skeletal muscle cells contain specialized structures for contraction.

Sarcolemma: Specialized plasma membrane.
Myofibrils: Long organelles inside muscle cell, responsible for striations.

Banding Pattern of Myofibrils

I band: Light band, contains only thin filaments (actin), interrupted by Z disc.
A band: Dark band, contains thick filaments (myosin), includes H zone (no actin) and M line (myosin center).

Sarcomere Organization

The sarcomere is the contractile unit of a muscle fiber, composed of myofilaments.

Thick filaments: Myosin, with ATPase enzymes and myosin heads (cross bridges).
Thin filaments: Actin, anchored to Z disc.

At rest, the H zone lacks actin filaments. During contraction, actin and myosin overlap, causing the H zone to disappear.

Sarcoplasmic Reticulum (SR)

Specialized smooth endoplasmic reticulum surrounding myofibrils.
Stores and releases calcium, which triggers contraction.

Muscle Physiology: Stimulation and Contraction

Functional Properties of Skeletal Muscle

Irritability (Responsiveness): Ability to receive and respond to a stimulus.
Contractility: Ability to forcibly shorten when stimulated.
Extensibility: Ability to be stretched.
Elasticity: Ability to recoil after stretching.

The Nerve Stimulus and Action Potential

Skeletal muscles must be stimulated by motor neurons to contract. The neuromuscular junction is the association site between the axon terminal and the sarcolemma.

Neurotransmitter: Acetylcholine (ACh) stimulates muscle contraction.
Synaptic cleft: Gap between nerve and muscle, filled with interstitial fluid.

Events at the Neuromuscular Junction:

Nerve impulse reaches axon terminal.
Calcium channels open; Ca2+ enters terminal.
Ca2+ causes release of ACh from synaptic vesicles.
ACh diffuses across synaptic cleft, binds to sarcolemma receptors.
Sarcolemma becomes permeable to Na+; depolarization occurs.
Depolarization opens more Na+ channels; action potential is generated.
Acetylcholinesterase (AChE) breaks down ACh, ending contraction.

Cell returns to resting state via diffusion of K+ and the sodium-potassium pump.

Mechanism of Muscle Contraction: Sliding Filament Model

Muscle contraction occurs as myosin heads bind to actin, forming cross bridges, and slide the filaments past each other.

Ca2+ binds regulatory proteins, exposing myosin-binding sites.
Cross bridges attach, pivot, and detach repeatedly.
ATP provides energy for the sliding process.

Contraction of Skeletal Muscle as a Whole

Graded Responses

Muscle fibers contract in an all-or-none fashion, but whole muscles can produce graded responses by varying stimulation frequency and the number of fibers activated.

Muscle twitch: Single, brief contraction.
Unfused (incomplete) tetanus: Rapid, summed contractions.
Fused (complete) tetanus: Sustained, smooth contraction with no relaxation.

Muscle force increases with more fibers stimulated; maximum contraction occurs when all motor units are active.

Energy for Muscle Contraction

ATP and Regeneration Pathways

ATP is the only direct energy source for muscle contraction. Muscle fibers store limited ATP and regenerate it via three pathways:

Direct phosphorylation by creatine phosphate (CP): Fastest, supplies ATP for ~15 seconds.
Aerobic respiration: Uses oxygen, occurs in mitochondria, produces ~32 ATP per glucose.
Anaerobic glycolysis: No oxygen, produces 2 ATP per glucose, converts pyruvic acid to lactic acid.

Relevant Equations:

Muscle Fatigue and Oxygen Deficit

Muscle fatigue results from ion imbalances, oxygen deficit, lactic acid accumulation, and decreased ATP. Recovery involves rapid, deep breathing to repay oxygen deficit.

Types of Muscle Contractions

Isotonic: Muscle shortens; movement occurs (e.g., lifting weights).
Isometric: Tension increases, muscle does not shorten (e.g., pushing palms together).

Muscle Tone

Muscle tone is a state of continuous partial contraction, keeping muscles firm and ready for action.

Effect of Exercise on Muscles

Aerobic (endurance) exercise: Increases strength, flexibility, resistance to fatigue, improves metabolism and coordination.
Resistance (isometric) exercise: Increases muscle size and strength by enlarging individual fibers.

Rules and Types of Skeletal Muscle Activity

Five Rules of Skeletal Muscle Activity

All skeletal muscles cross at least one joint (with few exceptions).
Bulk of muscle lies proximal to the joint crossed.
Each muscle has at least two attachments: origin and insertion.
Muscles pull, never push.
Insertion moves toward origin during contraction.

Types of Body Movements

Origin: Attachment to immovable bone.
Insertion: Attachment to movable bone.
Flexion: Decreases joint angle (e.g., bending knee).
Extension: Increases joint angle (e.g., straightening elbow).
Hyperextension: Extension beyond 180°.
Rotation: Bone moves around longitudinal axis (e.g., shaking head 'no').
Abduction: Limb moves away from midline.
Adduction: Limb moves toward midline.
Circumduction: Combination of flexion, extension, abduction, adduction; distal end moves in a circle.

Special Movements

Dorsiflexion: Lifting foot toward shin.
Plantar flexion: Pointing toes away from head.
Inversion: Turning sole of foot medially.
Eversion: Turning sole of foot laterally.
Supination: Forearm rotates laterally, palm faces anteriorly; radius and ulna are parallel.
Pronation: Forearm rotates medially, palm faces posteriorly; radius and ulna cross.
Opposition: Thumb touches tips of other fingers.

Interactions of Skeletal Muscles

Prime mover: Main muscle responsible for movement.
Antagonist: Opposes or reverses prime mover.
Synergist: Aids prime mover, reduces undesirable movements.
Fixator: Specialized synergist, stabilizes origin of prime mover.

Naming Skeletal Muscles

Muscles are named based on several criteria:

Direction of fibers: e.g., rectus (straight).
Relative size: e.g., maximus (largest).
Location: e.g., temporalis (temporal bone).
Number of origins: e.g., triceps (three heads).
Origin and insertion: e.g., sterno (on sternum).
Shape: e.g., deltoid (triangular).
Action: e.g., flexor, extensor.

Arrangement of Fascicles

Pattern	Description
Circular	Fascicles arranged in concentric rings
Convergent	Fascicles converge on a single insertion tendon
Parallel	Fascicles run parallel to the long axis of the muscle
Fusiform	Modified parallel arrangement, spindle-shaped muscle
Pennate	Short fascicles attach obliquely to a central tendon

Developmental Aspects of the Muscular System

Muscular control reflects nervous system maturation.
Control develops superior/inferior and proximal/distal.
Regular exercise is essential for muscle health.
Without exercise, muscles atrophy; with vigorous exercise, they hypertrophy.
Muscle mass decreases with age; exercise helps retain mass and strength.

Example: Muscle Contraction

When lifting a weight, skeletal muscle contracts via the sliding filament mechanism, using ATP regenerated by aerobic respiration if oxygen is available, or anaerobic glycolysis if not.

Additional info: The sliding filament model is fundamental to understanding muscle physiology and is supported by the presence of actin and myosin filaments within the sarcomere. Muscle fatigue is a complex phenomenon involving multiple biochemical and physiological factors.