The Muscular System

Introduction to Muscle Tissue

The muscular system is responsible for all types of body movement and is essential for maintaining posture, stabilizing joints, and generating heat. There are three basic types of muscle tissue found in the human body: skeletal, cardiac, and smooth muscle.

• Skeletal muscle: Voluntary, striated muscle attached to bones.

• Cardiac muscle: Involuntary, striated muscle found only in the heart.

• Smooth muscle: Involuntary, non-striated muscle found in the walls of hollow organs.

Key Terminology:

• myo- and mys-: Prefixes referring to muscle.

• sarco-: Prefix referring to flesh.

Comparison of Muscle Types

Structural and Functional Differences

The three muscle types differ in structure, control, and location. The following table summarizes their main characteristics:

Skeletal Muscle Structure

Connective Tissue Organization

Skeletal muscle fibers are organized and protected by several layers of connective tissue:

• Endomysium: Encloses a single muscle fiber.

• Perimysium: Wraps around a fascicle (bundle) of muscle fibers.

• Epimysium: Covers the entire skeletal muscle.

• Fascia: Outermost layer, surrounds the epimysium.

Muscle attachments include:

• Tendons: Cordlike structures, mostly collagen, attach muscle to bone; often cross joints.

• Aponeuroses: Sheetlike structures, attach muscles indirectly to bones, cartilages, or connective tissue coverings.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

• Sarcolemma: Specialized plasma membrane of muscle cell.

• Myofibrils: Long organelles inside muscle cell, responsible for striations.

• Light (I) bands and dark (A) bands: Give muscle its striped appearance.

Sarcomere Organization

The sarcomere is the contractile unit of a muscle fiber, defined as the segment between two Z discs. It contains:

• Thick filaments (myosin): Contain myosin protein and ATPase enzymes; have myosin heads that form cross bridges during contraction.

• Thin filaments (actin): Composed of actin protein, anchored to the Z disc; during contraction, actin and myosin filaments overlap.

• H zone: Lighter central area within the A band, lacks actin filaments; disappears during contraction.

• M line: Center of the H zone.

Sarcoplasmic Reticulum (SR)

• Specialized smooth endoplasmic reticulum surrounding myofibrils.

• Stores and releases calcium ions, which are essential for muscle contraction.

Physiology of Skeletal Muscle Contraction

Functional Properties

• Irritability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length after stretching.

Nerve Stimulus and Action Potential

• Skeletal muscles must be stimulated by a motor neuron to contract.

• Motor unit: One motor neuron and all the muscle fibers it stimulates.

• Neuromuscular junction: Site where the axon terminal of a motor neuron meets the sarcolemma of a muscle fiber.

• Neurotransmitter: Chemical messenger (acetylcholine, ACh) released by nerve to stimulate muscle.

• Synaptic cleft: Gap between nerve and muscle, filled with interstitial fluid.

Steps of Muscle Stimulation

1. Nerve impulse reaches axon terminal of motor neuron.

2. Calcium channels open; Ca2+ enters axon terminal.

3. Ca2+ entry causes synaptic vesicles to release ACh.

4. ACh diffuses across synaptic cleft, binds to receptors on sarcolemma.

5. Sarcolemma becomes more permeable to Na+; depolarization occurs.

6. Depolarization triggers action potential along muscle fiber.

7. Acetylcholinesterase (AChE) breaks down ACh, ending contraction.

Restoration: Potassium ions (K+) diffuse out; sodium-potassium pump restores original ion positions.

Sliding Filament Theory of Contraction

Muscle contraction occurs as actin and myosin filaments slide past each other, shortening the sarcomere. The process is powered by ATP and regulated by calcium ions.

• Ca2+ binds to regulatory proteins on actin, exposing myosin-binding sites.

• Myosin heads attach to actin, forming cross bridges.

• Cross bridges pivot, pulling actin filaments toward sarcomere center.

• ATP is required for myosin head detachment and reattachment.

Contraction of a 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:

• The frequency of stimulation.

• The number of muscle fibers stimulated.

Types of Muscle Responses

• Muscle twitch: Single, brief contraction; not typical in normal function.

• Summing of contractions: Rapid, repeated stimulation leads to stronger, smoother contractions.

• Unfused (incomplete) tetanus: Partial relaxation between contractions.

• Fused (complete) tetanus: No relaxation; contractions are smooth and sustained.

Muscle force increases with the number of fibers stimulated; maximal contraction occurs when all motor units are active.

Energy for Muscle Contraction

ATP and Its Regeneration

ATP is the direct energy source for muscle contraction, but is stored in limited amounts. Muscles regenerate ATP by three main pathways:

1. Direct phosphorylation of ADP by creatine phosphate (CP): Fastest; CP donates phosphate to ADP to form ATP. Supplies last ~15 seconds.

2. Aerobic respiration: Occurs in mitochondria, uses oxygen to break down glucose into CO2 and H2O, yielding about 32 ATP per glucose. Slower but efficient; used during rest and moderate activity.

3. Anaerobic glycolysis and lactic acid formation: Glucose broken down without oxygen, producing 2 ATP and lactic acid. Used during intense, short bursts of activity.

Muscle Fatigue and Oxygen Deficit

Muscle fatigue occurs after prolonged, strenuous activity due to:

• Ion imbalances (Ca2+, K+).

• Oxygen deficit and lactic acid accumulation.

• Decreased ATP supply.

After exercise, rapid, deep breathing repays the oxygen deficit.

Types of Muscle Contractions

• Isotonic contractions: Muscle shortens and movement occurs (e.g., bending the knee, lifting weights).

• Isometric contractions: Muscle does not shorten; tension increases (e.g., pushing palms together).

Muscle Tone and Effects of Exercise

Muscle Tone

• State of continuous partial contraction.

• Maintains muscle firmness and readiness for action.

Effects of Exercise

• Aerobic (endurance) exercise: Increases muscle strength, flexibility, and resistance to fatigue; improves metabolism and coordination.

• Resistance (isometric) exercise: Increases muscle size and strength by enlarging individual muscle fibers (hypertrophy).