Muscle Tissue and Physiology: Study Guide (Chapters 9–10)

Muscle Tissue Types

Overview of Muscle Types

Muscle tissue is specialized for contraction and is classified into three main types, each with distinct structural and functional characteristics.

• Skeletal Muscle: Voluntary, striated, multinucleated, and attached to bones. Responsible for body movement.

• Cardiac Muscle: Involuntary, striated, contains intercalated discs (unique to cardiac muscle), found only in the heart, with 1–2 central nuclei per cell.

• Smooth Muscle: Involuntary, non-striated (lacks sarcomeres), found in walls of hollow organs, under autonomic control, and capable of slow, sustained contractions.

Muscle Function Basics

Properties of Muscle Tissue

All muscle types share four fundamental properties that enable their function:

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched without damage.

• Elasticity: Ability to return to original length after stretching.

Neuromuscular Junction (NMJ)

Sequence of Events at the NMJ

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber, where nerve impulses initiate muscle contraction.

1. Nerve impulse arrives at the axon terminal.

2. Acetylcholine (ACh) is released into the synaptic cleft.

3. ACh binds to receptors on the motor end plate of the muscle fiber.

4. Sodium ions (Na+) enter the muscle cell, causing depolarization.

5. An action potential spreads along the sarcolemma.

• Neurotransmitter: Acetylcholine (ACh)

• Synaptic cleft: The gap between neuron and muscle fiber

• Motor end plate: Region of sarcolemma with ACh receptors

Muscle Action Potential Phases

Phases of Muscle Contraction

Muscle contraction involves a series of electrical and chemical events:

1. Excitation: ACh release and sarcolemma depolarization

2. Excitation–Contraction Coupling: Calcium ions (Ca2+) released from the sarcoplasmic reticulum (SR) bind to troponin, causing tropomyosin to shift and expose binding sites on actin.

3. Contraction Phase: Cross-bridge cycling occurs, generating force.

4. Relaxation: Ca2+ is pumped back into the SR, and ACh is broken down.

Cross-Bridge Cycle

Steps of the Cross-Bridge Cycle

The cross-bridge cycle describes the interaction between actin and myosin during muscle contraction:

1. ATP hydrolysis "cocks" the myosin head.

2. Ca2+ exposes actin binding sites.

3. Myosin binds to actin, forming a cross-bridge.

4. Power stroke: Myosin pulls actin filament.

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

• Key Rule: ATP is required for detachment, resetting the myosin head, and pumping Ca2+ back into the SR—not for cross-bridge formation.

Roles of ATP in Muscle Contraction

Functions of ATP

• Detaches myosin from actin (ending the power stroke)

• Re-energizes ("cocks") the myosin head for the next cycle

• Pumps Ca2+ back into the sarcoplasmic reticulum

• Not used for: Cross-bridge formation

Types of Muscle Contractions

Classification of Contractions

• Isometric: Muscle tension increases, but length does not change (e.g., holding a position).

• Isotonic Concentric: Muscle shortens while generating force (e.g., lifting a weight).

• Isotonic Eccentric: Muscle lengthens under tension (e.g., lowering a weight); produces the greatest tension.

Muscle Tension States

Patterns of Muscle Response

• Twitch: Single, brief contraction from one stimulus.

• Wave Summation: Increased force with repeated stimuli.

• Unfused Tetanus: Partial relaxation between stimuli.

• Fused Tetanus: No relaxation; maximal, sustained contraction.

• Muscle Tone: Low-level tension maintained at rest.

Motor Units

Definition and Recruitment

• Motor Unit: A single motor neuron and all the muscle fibers it innervates.

• Recruitment: Increasing the number of active motor units increases muscle force.

Muscle Energy Systems

Sources of ATP for Muscle Contraction

1. Creatine Phosphate System: Provides immediate ATP for 0–10 seconds of activity.

2. Glycolysis (Anaerobic): Short-term ATP production; produces lactic acid as a byproduct.

3. Oxidative (Aerobic) Metabolism: Long-term, most efficient ATP production; requires oxygen.

Muscle Fiber Types

Classification of Muscle Fibers

Type	Speed	Mitochondria	Myoglobin	Function
Type I	Slow	High	High	Endurance
Type IIx	Fastest	Low	Low	Speed/Power (fatigues quickly)

Smooth Muscle

Unique Features and Comparison

• No sarcomeres; lacks striations

• Uses calmodulin (not troponin) for Ca2+ binding

• Controlled by the autonomic nervous system

• Responsible for peristalsis and sphincter function

• Single-unit type is most common in organs

Calcium Control in Muscle Types

Role of Calcium in Contraction

• Skeletal Muscle: Ca2+ binds to troponin

• Smooth Muscle: Ca2+ binds to calmodulin

Muscle Fatigue and Recovery

Causes and Recovery Mechanisms

• Causes of Fatigue: ATP depletion, ion imbalance, waste buildup, heat

• Not a cause: Increased oxygen

• Recovery Period: Oxygen is restored, ATP is rebuilt, and waste products are removed

High-Yield Concepts and Common Traps

Key Points for Exams

• Greatest tension is produced during eccentric contraction

• Latent period: Time when action potential spreads, but contraction has not started

• Electrical gradient: Separation of charge across the membrane

• ATP is required for detachment, not for cross-bridge formation

• Smooth muscle uses calmodulin and lacks sarcomeres

Quick Reference Sheet

Essential Facts for Rapid Review

• NMJ Sequence: ACh → motor end plate → depolarization

• Calcium Binding: Skeletal (troponin), Smooth (calmodulin)

• ATP Functions: Detach myosin, reset myosin, pump Ca2+ back

• Contraction Types: Isometric (hold), Concentric (lift), Eccentric (lower, max tension)

• Fiber Types: Type I (endurance), Type IIx (speed/power)

• Smooth Muscle: No striations, peristalsis, sphincters, autonomic control