Chapter 9: Muscles and Muscle Tissue – Comprehensive Study Notes

Muscles and Muscle Tissue

Main Function of Muscle

Muscle tissue is specialized for contraction, enabling movement of the body and its parts. The main function of muscle is to convert chemical energy (ATP) into mechanical energy, producing force and movement.

• Movement: Muscles generate movement by contracting and pulling on bones or tissues.

• Posture: Muscles maintain body posture and position.

• Joint Stability: Muscles stabilize joints.

• Heat Production: Muscle contraction produces heat, helping maintain body temperature.

Muscle Terminology

Understanding muscle-related prefixes and terms is essential for anatomy and physiology.

• Myo- and Mys-: Refer to muscle (e.g., myocyte).

• Sarco-: Refers to flesh or muscle (e.g., sarcoplasm).

• Muscle Fiber vs. Muscle Cell: In skeletal muscle, these terms are used interchangeably. A muscle fiber is a single muscle cell, typically long and cylindrical.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle is attached to bones and is responsible for voluntary movements.

• Function: Moves the skeleton, maintains posture, stabilizes joints, and produces heat.

• Description: Long, cylindrical, multinucleated cells with striations.

• Control: Voluntary (conscious control).

Cardiac Muscle

Cardiac muscle is found only in the heart and is specialized for continuous rhythmic contraction.

• Function: Pumps blood throughout the body.

• Description: Branched, striated cells with one or two nuclei; contains intercalated discs.

• Control: Involuntary (regulated by the autonomic nervous system).

Smooth Muscle

Smooth muscle is found in walls of hollow organs (e.g., intestines, blood vessels).

• Function: Moves substances through internal passageways (e.g., peristalsis).

• Description: Spindle-shaped, non-striated cells with a single nucleus.

• Control: Involuntary.

Special Characteristics of Muscle Tissue

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

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

Skeletal Muscle Gross Anatomy

Nerve and Blood Supply

Each muscle receives nerves and blood vessels to supply nutrients and remove waste.

• Nerves: Stimulate muscle contraction.

• Blood Vessels: Deliver oxygen and nutrients; remove metabolic waste.

Connective Tissue Sheaths

• Endomysium: Surrounds each muscle fiber.

• Perimysium: Surrounds groups of muscle fibers, forming fascicles.

• Epimysium: Surrounds the entire muscle.

• Fascicle: Bundle of muscle fibers within a muscle.

Muscle Attachments

• Origin: Attachment to immovable bone.

• Insertion: Attachment to movable bone.

• Direct Attachment: Epimysium fused to bone or cartilage.

• Indirect Attachment: Connective tissue extends as a tendon or aponeurosis.

• Tendon: Cord-like connective tissue.

• Aponeurosis: Sheet-like connective tissue.

Muscle Fiber Structure

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber, contains glycogen and myoglobin.

• Myofibril: Rod-like structures within muscle fiber, composed of sarcomeres.

• Sarcomere: Functional unit of muscle contraction; contains actin and myosin filaments.

Parts of a Sarcomere

• Actin (thin) filaments

• Myosin (thick) filaments

• Z disc: Boundary of sarcomere

• M line: Center of sarcomere

• H zone: Area with only thick filaments

• I band: Area with only thin filaments

• A band: Area with both thick and thin filaments

Myosin Myofilament

• Structure: Composed of myosin molecules with heads and tails.

• Heads: Bind to actin and ATP, form cross-bridges during contraction.

Actin Myofilament

• Structure: Composed of actin molecules, tropomyosin, and troponin.

• Binding Sites: Myosin heads attach to actin during contraction.

Sarcoplasmic Reticulum (SR)

• Description: Specialized smooth endoplasmic reticulum.

• Purpose: Stores and releases calcium ions (Ca2+), essential for muscle contraction.

T-tubules

• Description: Invaginations of the sarcolemma.

• Purpose: Conduct action potentials deep into muscle fiber.

Triad

• Description: Structure formed by a T-tubule and two terminal cisterns of the SR.

• Purpose: Coordinates release of Ca2+ for contraction.

Sliding Filament Model

The sliding filament model explains how muscles contract. Myosin heads attach to actin, pulling the filaments past each other, shortening the sarcomere.

• Key Process: Thin filaments slide past thick filaments, causing muscle shortening.

Steps of Muscle Contraction

1. Action potential arrives at neuromuscular junction.

2. Acetylcholine released, binds to receptors on sarcolemma.

3. Depolarization triggers action potential in muscle fiber.

4. Action potential travels down T-tubules.

5. SR releases Ca2+.

6. Ca2+ binds to troponin, moves tropomyosin, exposes actin binding sites.

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

8. Power stroke occurs, filaments slide.

9. ATP binds to myosin, cross-bridge detaches.

10. Cycle repeats as long as Ca2+ and ATP are present.

Regulation of Muscle Contraction

Motor Unit

• Definition: A motor neuron and all the muscle fibers it innervates.

• Function: Controls the strength and precision of muscle contraction.

Muscle Twitch

• Latent Period: Time between stimulus and contraction.

• Period of Contraction: Muscle shortens.

• Period of Relaxation: Muscle returns to resting length.

Graded Muscle Contraction

• Wave Summation: Increased frequency of stimulation increases contraction strength.

• Unfused Tetanus: Partial relaxation between stimuli.

• Fused Tetanus: No relaxation; sustained contraction.

• Recruitment: Increasing number of motor units activated.

• Size Principle: Small motor units recruited first, then larger ones.

Types of Muscle Contractions

• Isotonic Contraction: Muscle changes length.

  • Concentric: Muscle shortens.

  • Eccentric: Muscle lengthens.

• Isometric Contraction: Muscle does not change length.

• Muscle Tone: Constant, slightly contracted state of muscle.

Muscle Metabolism

ATP Storage and Usage

• ATP: Only a small amount stored; must be regenerated quickly.

Pathways for ATP Regeneration

• Direct Phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP.

• Anaerobic Pathway: Glycolysis produces ATP and lactic acid without oxygen.

• Aerobic Pathway: Cellular respiration produces ATP with oxygen.

Aerobic Endurance and Anaerobic Threshold

• Aerobic Endurance: Length of time muscle can sustain activity using aerobic metabolism.

• Anaerobic Threshold: Point at which muscle switches to anaerobic metabolism.

Muscle Fatigue

• Definition: Inability to contract despite stimulation.

Excess Postexercise Oxygen Consumption (EPOC)

• Definition: Oxygen needed to restore muscle to pre-exercise state.

Force of Contraction

• Depends on: Number of muscle fibers stimulated, size of fibers, frequency of stimulation, and degree of muscle stretch.

Muscle Fiber Types

Muscle fibers are classified based on speed of contraction and metabolic pathway.

Type	Speed	ATP Source	Characteristics
Slow Oxidative	Slow	Aerobic	Fatigue-resistant, endurance activities
Fast Oxidative	Fast	Aerobic	Moderate resistance to fatigue
Fast Glycolytic	Fast	Anaerobic	Fatigues quickly, short bursts of power

Load and Recruitment in Muscle Contraction

• Load: Heavier loads require more force and slow contraction.

• Recruitment: More motor units activated increases force.

Effects of Exercise on Muscle

• Endurance Training: Increases capillaries, mitochondria, and myoglobin; improves fatigue resistance.

• Resistance Exercise: Increases muscle size and strength.

Smooth Muscle

Differences from Skeletal Muscle

• No striations

• Spindle-shaped cells

• Single nucleus

Peristalsis

• Definition: Wave-like contractions moving substances through organs.

• Structure: Layers of smooth muscle contract alternately.

Smooth Muscle Structural Composition

• Varicosities: Swellings in nerve fibers releasing neurotransmitters.

• Diffuse Junction: Neurotransmitter release over wide area.

• Caveolae: Membrane invaginations for Ca2+ entry.

• Intermediate Filaments: Non-contractile, support cell shape.

• Gap Junctions: Allow communication between cells.

• Dense Bodies: Anchor actin filaments.

Source of Ca2+ in Smooth Muscle

• Extracellular fluid and sarcoplasmic reticulum

Smooth Muscle Contraction

• Slower, sustained contractions

• Regulated by autonomic nervous system

Skeletal Muscle Development

Skeletal muscle develops from embryonic mesoderm cells called myoblasts, which fuse to form multinucleated muscle fibers.

• Myoblasts: Embryonic cells that become muscle fibers.

• Satellite Cells: Aid in muscle repair and growth.

Example: During running, skeletal muscles contract to move limbs, cardiac muscle pumps blood, and smooth muscle in blood vessels regulates flow.

Additional info: Muscle contraction is governed by the interaction of actin and myosin, regulated by calcium ions and ATP. The sliding filament model is central to understanding muscle physiology.