Muscle Physiology

Chapter 11: Muscle Physiology

Muscular Tissue

• Review of Muscle Types:

  • Skeletal Muscle

  • Striated, Multinucleate, thin nonbranching cells (fibers)

  • Voluntary control

  • Found in bones, skin

  • Cardiac Muscle

  • Striated, branched cells

  • Typically single nucleus per cell

  • Intercalated discs between cells for communication

  • Involuntary control

  • Smooth Muscle

  • Nonstriated, fusiform cells

  • Typically single nucleus

  • Involuntary control

  • Found in visceral and vessel walls

Cellular Components of Skeletal Muscle Fibers

• Key Structures for Muscle Contraction:

  • T-Tubules: Invaginations of the sarcolemma that facilitate the transmission of action potentials into the muscle fiber.

  • Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum in muscle cells that stores calcium ions.

  • Terminal Cisternae: Enlarged areas of the sarcoplasmic reticulum adjacent to T-tubules, important in calcium release during muscle contraction.

  • Myofibrils: Long, cylindrical structures that run parallel to the length of the muscle fiber, composed of contractile myofilaments.

Sarcomere Structure

• Definition: The basic functional unit of muscle contraction, defined as the segment between two Z discs.

• Contractile Proteins:

  • Myosin: Thick filament protein with a head that binds to actin.

  • Actin: Thin filament protein that contains binding sites for myosin heads.

• Structural Proteins:

  • Z Disc: Attachment site for thin filaments, marking the boundary of sarcomeres.

  • Titin: Elastic protein that extends from the Z disc to the M line, providing stability and elasticity.

• Regulatory Proteins:

  • Troponin: Binds calcium ions and facilitates muscle contraction by moving tropomyosin.

  • Tropomyosin: Winds around actin filaments, covering myosin binding sites during relaxation.

Neuromuscular Junction Structure

• A specialized synapse where the motor neuron communicates with muscle fibers, involving the release of acetylcholine (ACh) into the synaptic cleft.

• Components:

  • Axon Terminal: Contains synaptic vesicles filled with ACh.

  • Synaptic Cleft: Gap between the neuron and muscle cell.

  • Motor End Plate: Specialized region of the sarcolemma with receptors for ACh.

Physiology of Muscle Contraction

• Ligand-Gated Channels: Open in response to the binding of a ligand (e.g., ACh).

• Voltage-Gated Channels: Open in response to changes in membrane potential.

Excitation Phase

1. Action Potential Arrival: An action potential (AP) travels down the motor neuron.

2. ACh Release: ACh is released into the synaptic cleft and binds to receptors on the motor end plate.

3. End Plate Potential (EPP): Local depolarization of the muscle fiber membrane occurs due to Na+ influx.

Excitation-Contraction Coupling

1. Propagation of Action Potential: The EPP stimulates voltage-gated Na+ channels, propagating the AP along the sarcolemma and into T-tubules.

2. Calcium Release: AP triggers the release of Ca++ from the sarcoplasmic reticulum through voltage-gated channels.

3. Binding of Ca++ to Troponin: Ca++ binds to troponin, inducing a conformational change that moves tropomyosin away from myosin binding sites on actin.

Contraction Phase

• Sliding-Filament Theory: Describes how myosin heads pull on actin filaments, resulting in contraction.

  • Cross Bridge Cycle:

  • Myosin heads bind to actin (crossbridge formation).

  • The power stroke occurs as myosin pulls actin toward the center of the sarcomere.

  • ATP binds to myosin, causing it to release actin.

  • Repeat for continued contraction.

Relaxation Phase

1. ACh Breakdown: ACh is hydrolyzed by acetylcholinesterase, terminating the signal for contraction.

2. Calcium Re-uptake: Ca++ is pumped back into the sarcoplasmic reticulum via ATP-dependent pumps.

3. Restoration of Tropomyosin blocking: Tropomyosin returns to cover myosin binding sites on actin, leading to muscle relaxation.

Muscle Twitch Dynamics

• Definition: A muscle twitch is a single contraction-relaxation cycle in response to an action potential.

• Phases of Muscle Twitch:

  1. Latent Period: Time between stimulus and beginning of contraction; no tension is detectable.

  2. Contraction Phase: Muscle fibers actively pull together, tension increases.

  3. Relaxation Phase: Tension decreases as Ca++ returns to the sarcoplasmic reticulum.

Factors Affecting Muscle Contraction Strength

• Length-Tension Relationship:

  • Too short: actin and myosin bump each other, leading to minimal tension.

  • Too long: myosin heads cannot engage properly, resulting in minimal tension.

  • Optimal overlap of actin and myosin produces maximum tension.

• Stimulation Frequency: Greater frequency causes increased tension due to incomplete relaxation.

• Motor Unit Recruitment: Activating more motor units increases muscle strength.

Types of Motor Units

• Small Motor Units: Provide fine control, found in muscles like fingers or eyes.

• Large Motor Units: Associated with gross movement, found in muscles like quadriceps.

Types of Muscle Contractions

• Isometric Contractions: Muscle maintains tension but does not change length (e.g., holding a weight).

• Isotonic Contractions: Muscle changes length while maintaining tension.

  • Concentric: Muscle shortens (e.g., bicep curl).

  • Eccentric: Muscle lengthens while under tension (e.g., lowering weight).

Energy Demand in Skeletal Muscle

• Energy Sources During Exercise:

  • Immediate Energy: Uses ATP and creatine phosphate for initial bursts of activity.

  • Short-Term Energy: Anaerobic glycolysis provides ATP for 30-40 seconds leading to lactic acid production.

  • Long-Term Energy: Aerobic respiration requires oxygen and provides a large amount of ATP, lasting for prolonged activities.

Excess Post-Exercise Oxygen Consumption (EPOC)

• The increased rate of oxygen intake following strenuous activity aimed at replenishing ATP and creatine phosphate stores, and processing lactic acid.

Muscle Fatigue

• Factors contributing to muscle fatigue include:

  • Build-up of Potassium Ions: Affecting membrane potential and ion channels.

  • Accumulation of ADP and inorganic phosphate (Pi): Compromising muscle contraction capability.

  • Depletion of Energy Sources: Reduced ATP and glycogen availability leads to feelings of fatigue.

Properties of Cardiac Muscle

• Characteristics:

  • Rhythmic contraction: Consistent contraction and relaxation cycles.

  • Fatigue resistance: Can sustain prolonged contractions without tiring.

  • Intercalated Discs: Structures that allow electrical signals to pass rapidly between cells, ensuring synchronized contractions.

Cardiac Contraction Mechanism

• Depolarization occurs autonomously via pacemaker cells that can adjust contraction rate based on autonomic input.

• Calcium Influx: Calcium arrives both from the sarcoplasmic reticulum and extracellular fluid to facilitate contraction.

Properties of Smooth Muscle

• Characteristics:

  • Non-striated appearance, slow contraction, and incredible fatigue resistance.

  • Has functional differences based on location (e.g., large versus fine control).

Types of Smooth Muscle Control

• Multiunit Smooth Muscle: Composed of independent fibers, each regulated by its own nerve supply (e.g., in the iris).

• Unitary Smooth Muscle: Sheets of cells that respond to stretch and communicate with gap junctions (e.g., in the digestive tract).

• Caveolae: Pocket-like structures on the cell membrane that facilitate calcium influx during contraction.

Calcium Mechanism in Smooth Muscle

1. Calcium binds to calmodulin, activating myosin light chain kinase.

2. This enzyme facilitates the ATPase activity needed for contraction, resulting in a latch-bridge mechanism that maintains contraction with minimal energy usage.

3. Plasticity: The ability of smooth muscle to adjust its tension in response to stretch, allowing organs to function properly.

Smooth Muscle and Contraction Patterns

• Smooth muscle can respond to multiple stimuli (e.g., hormones, neural signals) with varied contraction patterns, essential for proper physiological functions such as digestion and vascular regulation.

Chapter 11: Muscle Physiology

Muscular Tissue

• Review of Muscle Types:

  • Skeletal Muscle

  • Striated, Multinucleate (syncytium forming large, cylindrical cells, also called fibers)

  • Voluntary control (receives signals from the somatic nervous system)

  • Found attached to bones and some skin (e.g., facial muscles)

  • Responsible for body movement, posture, and heat generation

  • Cardiac Muscle

  • Striated, branched cells (cardiomyocytes)

  • Typically single nucleus per cell, sometimes binucleate

  • Intercalated discs between cells for structural and electrical communication (contain desmosomes for strength and gap junctions for ion passage)

  • Involuntary control (regulated by the autonomic nervous system and intrinsic pacemaker activity)

  • Found exclusively in the heart wall

  • Smooth Muscle

  • Nonstriated, fusiform cells (spindle-shaped)

  • Typically single nucleus

  • Involuntary control (regulated by the autonomic nervous system, hormones, and local factors)

  • Found in the walls of visceral organs (e.g., digestive tract, bladder), blood vessels, and airways

  • Responsible for peristalsis, vasoconstriction/dilation, and other internal movements

Cellular Components of Skeletal Muscle Fibers

• Key Structures for Muscle Contraction:

  • Sarcolemma: The plasma membrane of a muscle fiber, capable of conducting action potentials.

  • Sarcoplasm: The cytoplasm of a muscle fiber, containing glycogen, myoglobin, and mitochondria.

  • T-Tubules (Transverse Tubules): Deep invaginations of the sarcolemma that penetrate into the cell's interior, running perpendicular to the myofibrils. They facilitate the rapid transmission of action potentials from the sarcolemma to the sarcoplasmic reticulum.

  • Sarcoplasmic Reticulum (SR): A specialized smooth endoplasmic reticulum in muscle cells that forms a network around each myofibril. Its primary function is to store, release, and reabsorb calcium ions ($Ca^{++}$), which are crucial for muscle contraction.

  • Terminal Cisternae: Enlarged, flattened sacs of the sarcoplasmic reticulum located on either side of a T-tubule. A T-tubule and its two flanking terminal cisternae form a