Muscular Tissue: Structure, Function, and Physiology (Chapter 10 Study Notes)

Introduction to Muscular Tissue

Overview and Chapter Objectives

This chapter explores the structure and function of the three types of muscular tissue, the events at the neuromuscular junction, energy use in muscle cells, and the mechanisms controlling muscle tension. Understanding these concepts is essential for comprehending how muscles contribute to movement, stability, and homeostasis in the human body.

• Structure and function of muscular tissue

• Neuromuscular junction events

• Energy use in muscle cells

• Control of muscle tension

Types of Muscular Tissue

Classification and Characteristics

Muscular tissue is classified into three main types, each with distinct structural and functional properties. These types are essential for various physiological processes, including movement, circulation, and digestion.

• Skeletal muscle: Attached to bones, responsible for voluntary movements.

• Cardiac muscle: Found in the heart, responsible for pumping blood.

• Smooth muscle: Located in walls of hollow organs, controls involuntary movements such as peristalsis.

Comparison of Muscular Tissue Types

The following table summarizes the key differences among skeletal, cardiac, and smooth muscle tissue.

Type	Location	Function	Appearance	Control
Skeletal	skeleton	move bones	multi-nucleated & striated	voluntary
Cardiac	heart	pump blood	one nucleus, striated, & intercalated discs	involuntary
Smooth	various organs (e.g., GI tract)	various functions (e.g., peristalsis)	one nucleus & no striations	involuntary

Skeletal Muscle Tissue

Microscopic Anatomy and Organization

Skeletal muscle is composed of bundles of muscle fibers, each surrounded by connective tissue layers. This organization allows for efficient force transmission and coordinated contraction.

• Muscle fiber: The basic cellular unit, multinucleated and striated.

• Connective tissue layers: Include endomysium (surrounds individual fibers), perimysium (surrounds fascicles), and epimysium (surrounds the entire muscle).

• Tendon: Connects muscle to bone.

• Fascicle: Bundle of muscle fibers.

Additional info: The sarcolemma is the plasma membrane of a muscle fiber, and the sarcoplasm is its cytoplasm. Myofibrils within the muscle fiber contain the contractile proteins.

Muscle Development

Skeletal muscle fibers are formed by the fusion of myoblasts during embryonic development. Satellite cells remain in adult muscle and can aid in repair.

• Myoblasts: Embryonic cells that fuse to form muscle fibers.

• Satellite cells: Involved in muscle growth and repair.

• Immature muscle fiber: Result of myoblast fusion.

Microscopic Structure of Muscle

Sarcomere Arrangement

The sarcomere is the functional unit of striated muscle, defined by Z discs. It contains thick and thin filaments whose interaction produces contraction.

• Thick filaments: Composed of myosin.

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

• Z discs: Define the boundaries of a sarcomere.

• Banding pattern: A bands (dark), I bands (light), H zone (center of A band).

Muscle Proteins

Muscle contraction and structure depend on three classes of proteins: contractile, regulatory, and structural.

• Contractile proteins: Actin and myosin generate force during contraction.

• Regulatory proteins: Troponin and tropomyosin regulate contraction by controlling access to binding sites.

• Structural proteins: Titin, nebulin, alpha-actinin, dystrophin maintain alignment and elasticity.

Mechanism of Muscle Contraction

Sliding Filament Theory

Muscle contraction occurs when myosin heads pull on actin filaments, causing them to slide inward and shorten the sarcomere.

• Myosin pulls actin: Thin filaments slide toward the center of the sarcomere.

• Z discs move closer: Sarcomere shortens, resulting in muscle contraction.

• Structural proteins: Transmit force throughout the muscle.

Additional info: During contraction, the I band and H zone decrease in width, while the A band remains constant.

Contraction Cycle

The contraction cycle consists of repeated steps that allow myosin to bind to actin, perform a power stroke, and release.

1. ATP hydrolysis

2. Attachment of myosin to actin (cross-bridge formation)

3. Power stroke

4. Detachment of myosin from actin

Excitation-Contraction Coupling

This process links the action potential in the muscle fiber to the sliding filament mechanism.

• Action potential: Triggers release of Ca2+ from the sarcoplasmic reticulum (SR).

• Ca2+ binds troponin: Moves tropomyosin, exposing binding sites on actin.

• Contraction cycle begins: Myosin binds actin and contraction occurs.

Neuromuscular Junction (NMJ)

Events at the NMJ

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber. It is essential for initiating muscle contraction.

• Action potential arrives: At the axon terminal of the motor neuron.

• Voltage-gated Ca2+ channels open: Causing influx of Ca2+.

• Exocytosis of neurotransmitter (ACh): Into the synaptic cleft.

• ACh binds to ligand-gated Na+ channels: On the motor endplate, causing Na+ influx and depolarization.

• Release of Ca2+ from SR: Initiates contraction.

• ACh breakdown: By acetylcholinesterase terminates the signal.

Additional info: Without these events, muscle contraction would not occur.

Muscle Metabolism

ATP Production in Muscle

Muscle cells require ATP for contraction, which is generated by several metabolic pathways.

• Creatine phosphate: Provides rapid ATP regeneration.

• Anaerobic glycolysis: Produces ATP without oxygen, generating lactic acid.

• Cellular respiration: Aerobic process producing large amounts of ATP.

Equation for ATP hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Muscle Fatigue

Muscle fatigue is the inability to maintain force of contraction after prolonged activity.

• Inadequate release of Ca2+ from SR

• Depletion of creatine phosphate, oxygen, and nutrients

• Build-up of lactic acid and ADP

• Insufficient release of ACh at NMJ

Oxygen Consumption After Exercise

After exercise, increased oxygen consumption (oxygen debt) helps restore muscle cells to resting conditions.

• Replenish creatine phosphate stores

• Convert lactate into pyruvate

• Reload oxygen onto myoglobin

Control of Muscle Tension

Motor Units and Recruitment

A motor unit consists of a somatic motor neuron and all the muscle fibers it innervates. The strength of contraction depends on the number of active motor units.

• Motor unit recruitment: Weakest units are activated first, followed by stronger units.

• Alternating contraction: Sustains muscle activity for longer periods.

Twitch Contraction and Frequency of Stimulation

A twitch is a brief contraction of all muscle fibers in a motor unit in response to a single action potential. The frequency of stimulation affects the strength and duration of contraction.

• Latent period: Delay between stimulus and contraction.

• Contraction period: Muscle fibers shorten.

• Relaxation period: Muscle fibers return to resting length.

• Refractory period: Muscle cannot respond to another stimulus.

• Wave summation: Increased frequency leads to stronger contractions.

• Fused tetanus: Sustained contraction without relaxation.

• Unfused tetanus: Partial relaxation between stimuli.

Factors Influencing Muscle Tension

• Number of active motor units

• Frequency of stimulation

• Length-tension relationship

Length-tension relationship equation:

$\text{Force} \propto \text{Initial length of sarcomere}$

Muscle Tone and Types of Contractions

Even at rest, skeletal muscle exhibits a small amount of tension called muscle tone, due to weak, involuntary contractions.

• Tonic contractions: Tension is constant while muscle length changes.

• Isometric contractions: Muscle contracts but does not change length.

• Concentric: Muscle shortens during contraction.

• Eccentric: Muscle lengthens during contraction.

Skeletal Muscle Fiber Types

Classification of Muscle Fibers

Skeletal muscle fibers are classified based on their contraction speed and metabolic properties.

• Slow oxidative (SO): Fatigue-resistant, used for endurance.

• Fast oxidative-glycolytic (FOG): Intermediate properties.

• Fast glycolytic (FG): Rapid, powerful contractions, fatigue quickly.

Exercise and Muscle Tissue

Effects of Exercise

Exercise influences muscle tissue through stretching and strength training, affecting muscle size and endurance.

• Stretching: Increases flexibility.

• Strength training: Increases muscle mass and strength.

Cardiac and Smooth Muscle

Cardiac Muscle

Cardiac muscle shares structural similarities with skeletal muscle but is distinguished by intercalated discs and involuntary control.

• Intercalated discs: Specialized connections for synchronized contraction.

• Involuntary control: Regulated by the autonomic nervous system.

Smooth Muscle

Smooth muscle contractions are slower and longer-lasting than those of skeletal and cardiac muscle. Smooth muscle can stretch and contract to a greater extent.

• Location: Walls of hollow organs (e.g., blood vessels, GI tract).

• Contraction: Initiated by different mechanisms, including hormones and local factors.

• Greater extensibility: Can stretch and still contract efficiently.

Regeneration and Aging of Muscle Tissue

Muscle Regeneration

Mature skeletal muscle fibers cannot undergo mitosis. Muscle growth occurs by hypertrophy (increase in fiber size), while smooth muscle can also undergo hyperplasia (increase in fiber number).

• Hypertrophy: Increase in muscle fiber size.

• Hyperplasia: Increase in muscle fiber number (mainly in smooth muscle).

• Satellite cells: Aid in repair and regeneration.

Aging and Muscle Tissue

With aging, muscle tissue is gradually replaced by fibrous connective tissue and adipose tissue, leading to decreased strength and flexibility.

• Muscle strength and flexibility decrease

• Reflexes slow

• Slow oxidative fiber numbers increase