Muscular System: Histology and Physiology – Study Notes

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Muscular System: Histology and Physiology

Functions of the Muscular System

The muscular system is essential for various physiological processes and body functions. Muscles are responsible for movement, posture, and several other vital roles.

Movement of the body: Skeletal muscles contract to produce voluntary movements.
Maintenance of posture: Muscles stabilize joints and maintain body position.
Respiration: Muscles such as the diaphragm are crucial for breathing.
Production of body heat: Muscle contractions generate heat, helping regulate body temperature.
Communication: Muscles enable speech, writing, and facial expressions.
Constriction of organs and vessels: Smooth muscle controls diameter of blood vessels and movement within hollow organs.
Contraction of the heart: Cardiac muscle pumps blood throughout the body.

Properties of Muscle Tissue

Muscle tissue possesses unique properties that enable its function in the body.

Contractility: Ability of a muscle to shorten with force.
Excitability: Capacity of muscle to respond to stimuli, usually from nerves.
Extensibility: Muscle can be stretched beyond its normal resting length and still contract.
Elasticity: Ability of muscle to recoil to its original resting length after being stretched.

Muscular System Terms

Myofiber: Long, rod-shaped skeletal muscle cell.
Sarcolemma: Cell membrane of a muscle cell, capable of carrying action potentials.
Sarcoplasm: Cytoplasm of a muscle cell.
Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores calcium ions needed for contraction; releases Ca2+ in response to action potentials.

Connective Tissue Coverings of Muscle

Muscle fibers are organized into bundles and surrounded by connective tissue layers.

Fascicles: Bundles of muscle fibers.
Endomysium: Loose connective tissue surrounding individual muscle fibers.
Perimysium: Dense connective tissue surrounding fascicles.
Epimysium: Dense connective tissue surrounding the entire muscle.
Muscular fascia: Connective tissue sheet external to epimysium, separates muscles or groups of muscles.

Whole Muscle Structure

Muscles are highly vascularized and innervated, with specialized structures for communication and coordination.

Three connective tissue layers: endomysium, perimysium, epimysium.
Muscular fascia surrounds muscles.
Neuromuscular junction: synapse between motor neuron and muscle fiber; acetylcholine (ACh) is the neurotransmitter.

Muscle Cell Anatomy

Muscle cells (myofibers) contain myofibrils, which are composed of myofilaments.

Myofibrils: Thread-like structures packed in myofibers.
Myofilaments: Contractile proteins; two types: Actin (thin) and Myosin (thick).
Sarcomeres: Highly ordered repeating units of myofilaments; functional contractile unit of muscle.

Sarcomere Structure and Function

The sarcomere is the basic functional unit of muscle fiber, extending from one Z disk to the next.

Z disk: Filamentous network of protein; serves as attachment for actin myofilaments.
Calcium is required for contraction; it enables myosin heads to bind to actin filaments.
Regulatory proteins: Troponin and Tropomyosin control myosin-actin interactions.

Organization of Sarcomeres

Sarcomeres have a striated appearance due to the arrangement of actin and myosin filaments.

Bands: I band (actin only), A band (actin and myosin overlap), H zone (myosin only), M line (center of sarcomere).
Z disk marks the boundary of each sarcomere.

Actin and Myosin Myofilaments

Actin and myosin are the primary contractile proteins in muscle fibers.

Actin (Thin) Filaments: Composed of fibrous (F) actin, tropomyosin, and troponin. Tropomyosin blocks binding sites on actin; troponin binds calcium to initiate contraction.
Myosin (Thick) Filaments: Made of elongated myosin molecules with heads that form cross-bridges with actin during contraction.

Sliding Filament Model

Muscle contraction occurs as actin myofilaments slide over myosin, shortening sarcomeres.

Actin and myosin do not change length; only their relative position changes.
During relaxation, sarcomeres lengthen due to external forces or contraction of antagonistic muscles.

Sarcomere Shortening

Calcium ions bind to troponin, causing tropomyosin to move and expose binding sites on actin for myosin heads, resulting in contraction.

During relaxation, Ca2+ is pumped back into the sarcoplasmic reticulum, and actin slides back to its resting position.

The Action Potential

Nervous system controls muscle contractions through action potentials.

Resting membrane potential (RMP): Voltage difference across cell membrane; inside is negative, outside is positive.
Cell membranes are polar due to ion distribution.
Na+/K+ pump maintains this distribution.

Physiology of Skeletal Muscle Fibers

Action potentials are necessary for muscle contraction.

At rest, inside of cell is negative (-70 mV to -90 mV).
Potassium ions are higher inside; sodium, chloride, and calcium are higher outside.

Establishing Resting Membrane Potential (RMP)

Na+/K+ pump is always active.
K+ tends to leak out; less tendency to re-enter due to charge attraction.
When equilibrium is reached, RMP is established.

Measuring the Resting Membrane Potential

RMP can be measured using electrodes and an oscilloscope, showing the voltage difference across the membrane.

Ion Channels

Ion channels regulate the movement of ions across the membrane, affecting membrane potential.

Ligand-gated channels: Open in response to binding of specific molecules (ligands).
Voltage-gated channels: Open in response to changes in membrane potential.

Action Potential Phases

Depolarization: Na+ channels open, Na+ enters cell, membrane potential becomes less negative.
Repolarization: K+ channels open, K+ leaves cell, membrane potential returns to resting value.
Hyperpolarization: Membrane potential becomes more negative than resting value.

Action Potential Terms

All-or-none principle: If threshold is reached, action potential occurs; if not, it does not.
Propagation: Action potential spreads from one location to another along the membrane.
Frequency: Number of action potentials produced per unit time.

Action Potential Propagation

Action potentials travel along the muscle fiber membrane, leading to muscle contraction.

Neuromuscular Junction

The neuromuscular junction is a specialized synapse where a motor neuron communicates with a muscle fiber.

Synaptic vesicles: Contain neurotransmitter acetylcholine (ACh).
Acetylcholinesterase: Enzyme that breaks down ACh in the synaptic cleft.

Excitation-Contraction Coupling

Describes how an action potential leads to muscle contraction.

Involves sarcolemma, T tubules, sarcoplasmic reticulum, and Ca2+ release.
Action potential triggers Ca2+ release, which enables cross-bridge formation between actin and myosin.

Action Potentials and Muscle Contraction

Action potential is propagated along the sarcolemma and T tubules.
Depolarization of T tubules causes Ca2+ release from sarcoplasmic reticulum.
Calcium binds to troponin, moving tropomyosin and exposing actin binding sites.
Myosin heads bind to actin, forming cross-bridges and initiating contraction.

Cross-Bridge Movement

Describes the cyclical interaction between actin and myosin during contraction.

Myosin heads bind to actin, perform a power stroke, release ADP, and detach after ATP binds.

Muscle Relaxation

Ca2+ is actively transported back into sarcoplasmic reticulum.
Troponin-tropomyosin complex re-establishes its position, blocking myosin binding sites.
Actin filaments slide back to original resting length.

Physiology of Skeletal Muscle

Muscle twitch is a contraction in response to a stimulus that causes an action potential in one or more muscle fibers.

Phases: Lag, Contraction, Relaxation.

Motor Units

Motor unit: A single motor neuron and all muscle fibers it innervates.
Large muscles have motor units with many muscle fibers; small muscles have motor units with fewer fibers for precise control.

Stimulus Strength and Motor Unit Response

All-or-none law: Muscle fibers contract fully in response to action potential.
Threshold stimulus: Minimum stimulus required for contraction.
Graded strength: Strength of contraction ranges from weak to strong depending on stimulus strength.
Multiple motor unit summation: Strength of contraction depends on number of motor units activated.

Multiple Wave Summation and Tetanus

As frequency of action potentials increases, contraction force increases.
Incomplete tetanus: Muscle fibers partially relax between contractions.
Complete tetanus: No relaxation between contractions.

Muscle Contractions

Isometric: Tension increases, but muscle length does not change.
Isotonic: Tension is constant, but muscle length changes (e.g., lifting an object).
Muscle tone: Constant tension by muscles for long periods.

Muscle Length versus Tension

The greatest amount of force is generated when muscle fibers are at optimal length for cross-bridging.

Stretched muscle: too long, not enough cross-bridging.
Crumpled muscle: too short, myofilaments overlap excessively, preventing contraction.

Muscle Fatigue

Decreased capacity to work and reduced efficiency of performance.
Types:
- Psychological: Depends on emotional state; highly variable.
- Muscular: Results from ATP depletion.
- Synaptic: Occurs in neuromuscular junction due to lack of acetylcholine; least common.

Physiological Contracture and Rigor Mortis

Physiological contracture: State of fatigue due to lack of ATP; neither contraction nor relaxation can occur.
Rigor mortis: Rigid muscles several hours after death due to Ca2+ leakage and ATP depletion; myosin heads cannot release actin filaments.

Energy Sources for Muscle Contraction

ATP provides immediate energy for muscle contractions, produced from:

Creatine Phosphate: Rapid ATP regeneration.
Anaerobic Respiration: Occurs in cytoplasm, produces ATP without oxygen.
Aerobic Respiration: Occurs in mitochondria, requires oxygen, produces more ATP.

Equation:

Aerobic Respiration:

Slow and Fast Twitch Fibers

Slow-twitch: Contract more slowly, smaller diameter, better blood supply, more mitochondria, more fatigue-resistant, use lipids and carbohydrates for energy, used for endurance.
Fast-twitch: Contract rapidly, larger diameter, densely packed, use anaerobic respiration and creatine phosphate, fatigue quickly, used for quick, powerful bursts.

Slow vs. Fast Twitch Fibers (Comparison Table)

Feature	Slow-Twitch	Fast-Twitch
Contraction Speed	Slow	Fast
Diameter	Smaller	Larger
Blood Supply	Better	Less
Mitochondria	More	Less
Fatigue Resistance	High	Low
Energy Source	Lipids, Carbohydrates	CP, Glycogen
Use	Endurance	Powerful bursts

Effects of Exercise on Muscle Fibers

Hypertrophy: Increase in muscle size due to increased synthesis of actin and myosin.
Atrophy: Decrease in muscle size due to inactivity or disease.

Smooth Muscle

Smooth muscle cells are not striated, have a central nucleus, and are found in walls of hollow organs.

No sarcomeres; actin/myosin complexes attach to dense bodies.
Ca2+ required for contraction, can come from outside the cell.
Enzymes (e.g., myosin kinase) replace troponin/tropomyosin for regulation.

Types of Smooth Muscle

Visceral (unitary): Cells in sheets, function as a unit, numerous gap junctions, often autorhythmic.
Multiunit: Cells or groups of cells act as independent units (e.g., arrector pili, iris).

Electrophysiology and Key Features of Smooth Muscle

Does not follow all-or-none principle.
Stimuli can summate to create action potential.
Slow to contract, slow to relax.
Can be autorhythmic (pacemaker cells).
Controlled by nervous system, hormones, and autorhythmicity.

Cardiac Muscle

Cardiac muscle is found only in the heart and is responsible for pumping blood.

Striated, usually one nucleus per cell.
Has intercalated disks and gap junctions for coordinated contraction.
Autorhythmic cells (SA node) set pace.
Action potentials have longer duration and refractory period.
Ca2+ regulates contraction.