Muscular System: Histology and Physiology – Study Notes

Muscular System: Histology and Physiology

Functions of the Muscular System

The muscular system is essential for various physiological processes and body functions.

• Movement of the body: Muscles contract to produce movement of body parts.

• Maintenance of posture: Muscles stabilize joints and maintain body position.

• Respiration: Muscles such as the diaphragm are vital 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 regulates diameter of blood vessels and 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.

• 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.

Types and Comparison of Muscle Tissue

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth. Each type has distinct structural and functional characteristics.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to bones	Heart	Walls of hollow organs
Cell Shape	Long, cylindrical, multinucleated	Branched, usually one nucleus	Spindle-shaped, one nucleus
Striations	Present	Present	Absent
Control	Voluntary	Involuntary	Involuntary
Special Features	Fast contraction, fatigue easily	Intercalated discs, autorhythmic	Can contract slowly, autorhythmic

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 (SR): 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 and protected by connective tissue layers.

• Endomysium: Loose connective tissue surrounding individual muscle fibers.

• Perimysium: Dense connective tissue surrounding muscle fascicles (bundles of muscle fibers).

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Muscular fascia: Connective tissue sheet external to epimysium, separates muscles or groups of muscles.

Muscle Cell Anatomy

Muscle cells (myofibers) contain specialized structures for contraction.

• Myofibrils: Thread-like structures packed in myofibers, composed of contractile proteins.

• Myofilaments: 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, responsible for contraction.

• Extends from one Z disk to the next Z disk.

• Z disk: Filamentous network of protein serving as attachment for actin myofilaments.

• Calcium ions are required for contraction, enabling myosin heads to bind to actin filaments.

• Regulatory proteins: Troponin and Tropomyosin control access to actin binding sites.

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 move tropomyosin and expose binding sites.

• 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 filaments slide over myosin filaments, shortening the sarcomere.

• Actin and myosin do not change length; sarcomeres shorten as filaments slide past each other.

• 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 actin binding sites. Myosin heads attach and perform power strokes, shortening the sarcomere.

• Calcium is pumped back into the SR during relaxation, and actin slides back to its resting position.

The Action Potential

Muscle contraction is controlled by action potentials generated by the nervous system.

• Resting membrane potential (RMP): The voltage difference across the cell membrane at rest, typically -70 mV to -90 mV.

• Cell membranes are polar, with a separation of charge maintained by the Na+/K+ pump.

Physiology of Skeletal Muscle Fibers

At rest, the inside of muscle cells is negative relative to the outside. This electrical difference is necessary for action potential generation.

• Potassium ions (K+) are higher inside the cell; sodium (Na+), chloride (Cl-), and calcium (Ca2+) are higher outside.

Establishing Resting Membrane Potential (RMP)

• Na+/K+ pump maintains ion gradients.

• K+ tends to leak out, making the inside more negative.

• Much less tendency for K+ to re-enter due to charge attraction.

Ion Channels and Action Potential Phases

Ion channels regulate the movement of ions across the membrane, leading to action potentials.

• Ligand-gated channels: Open in response to binding of specific molecules (e.g., neurotransmitters).

• Voltage-gated channels: Open in response to changes in membrane potential.

Action potential phases:

• Depolarization: Na+ channels open, Na+ enters, membrane becomes less negative.

• Repolarization: K+ channels open, K+ exits, membrane returns to resting potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Action Potential Propagation

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

• All-or-none principle: If threshold is reached, action potential occurs.

• Propagation: Action potential spreads from one location to another along the membrane.

• Frequency: Number of action potentials produced per unit time.

Neuromuscular Junction

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

• Neurotransmitter acetylcholine (ACh) is released from synaptic vesicles.

• ACh binds to receptors on the muscle cell membrane, triggering an action potential.

• Acetylcholinesterase breaks down ACh to prevent continuous stimulation.

Excitation-Contraction Coupling

This process links the action potential to muscle contraction.

• Action potential travels along sarcolemma and T-tubules.

• Triggers release of Ca2+ from the sarcoplasmic reticulum.

• Ca2+ binds to troponin, moving tropomyosin and exposing actin binding sites.

Cross-Bridge Movement and Muscle Relaxation

• Myosin heads bind to actin, perform power strokes, and detach using ATP.

• Relaxation occurs when Ca2+ is pumped back into the SR and actin binding sites are covered.

Physiology of Skeletal Muscle: Twitch and Motor Units

• Muscle Twitch: Contraction in response to a stimulus causing an action potential in one or more muscle fibers.

• Phases: Lag, contraction, relaxation.

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

Stimulus Strength and Motor Unit Response

• All-or-none law: Muscle fibers contract fully or not at all.

• Strength of contraction is graded depending on stimulus strength.

• Multiple wave summation: Increased frequency of action potentials increases contraction force.

• Tetanus: Sustained contraction without relaxation.

Muscle Contractions: Types

• Isometric: Tension increases, but muscle length does not change.

• Isotonic: Muscle length changes, tension remains constant.

• Muscle tone: Constant tension by muscles for long periods.

Muscle Length versus Tension

The amount of tension a muscle can produce depends on its length at the time of stimulation.

• Optimal length allows maximum cross-bridge formation.

• Overly stretched or crumpled muscles produce less tension.

Muscle Fatigue and Rigor Mortis

• Muscle Fatigue: Decreased capacity to work and reduced efficiency. Can be psychological, muscular (ATP depletion), or synaptic (rare).

• Physiological Contracture: State of fatigue due to lack of ATP; neither contraction nor relaxation can occur.

• Rigor Mortis: Rigid muscles after death due to lack of ATP; myosin heads cannot release actin filaments.

Energy Sources for Muscle Contraction

ATP is the immediate energy source for muscle contraction, produced by:

• Creatine Phosphate: Rapidly regenerates ATP from ADP.

• Anaerobic Respiration: Produces ATP without oxygen, yields lactic acid.

• Aerobic Respiration: Uses oxygen, produces more ATP and CO2.

Equation:

$\text{ADP} + \text{CP} \rightarrow \text{Creatine} + \text{ATP}$

Aerobic respiration (in mitochondria):

$\text{Glucose} + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\text{ATP}$

Slow and Fast Twitch Fibers

Muscle fibers are classified based on contraction speed and fatigue resistance.

• Slow-twitch: Contract slowly, resist fatigue, have more mitochondria and myoglobin, use aerobic respiration, specialized for endurance.

• Fast-twitch: Contract rapidly, fatigue quickly, have less myoglobin, use anaerobic respiration and creatine phosphate, specialized for quick, powerful bursts.

Feature	Slow-Twitch	Fast-Twitch
Contraction Speed	Slow	Fast
Fatigue Resistance	High	Low
Myoglobin	High	Low
Mitochondria	Many	Few
Primary Energy Source	Aerobic respiration	Anaerobic respiration, CP

Smooth Muscle

Smooth muscle is found in the walls of hollow organs and blood vessels. It is not striated and has unique contractile mechanisms.

• Cells are spindle-shaped with a central nucleus.

• No sarcomeres; actin/myosin complexes attach to dense bodies.

• Contraction can be initiated by Ca2+ from outside the cell.

• Enzymes (e.g., myosin kinase) replace troponin/tropomyosin for regulation.

• Types: Visceral (unitary) – cells function as a unit; Multiunit – cells function independently.

• Can be autorhythmic and controlled by nervous system, hormones, or local factors.

Cardiac Muscle

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

• Striated, usually one nucleus per cell.

• Cells connected by intercalated discs and gap junctions.

• Autorhythmic cells (SA node) set the pace.

• Action potentials have longer duration and refractory period.

• Ca2+ regulates contraction.