Muscle Tissue and Physiology: Structured Study Notes

Muscle Tissue and Physiology

Overview of Muscle Tissue

Muscle tissue is essential for movement, posture, and various physiological functions. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle. All muscle tissues generate force, known as muscle tension, which is fundamental to their function.

• Skeletal muscle: Long, multinucleated fibers; voluntary control; attached to skeleton for movement.

• Cardiac muscle: Short, branched cells; involuntary control; found only in the heart; connected by intercalated discs.

• Smooth muscle: Spindle-shaped cells; involuntary control; found in hollow organs, eyes, skin, and ducts.

Other functions include maintaining posture, stabilizing joints, generating heat, and regulating flow through hollow organs.

Properties of Muscle Cells

Muscle cells possess unique properties that enable their function:

• Contractility: Ability to contract and generate tension.

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to conduct electrical changes across the membrane.

• Extensibility: Ability to stretch without rupturing.

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

Structure of Muscle Cells

Muscle cells (myocytes) have specialized terminology:

• Sarcoplasm: Cytoplasm of muscle cell.

• Sarcolemma: Plasma membrane of muscle cell.

• Sarcoplasmic reticulum (SR): Modified endoplasmic reticulum; stores and releases calcium ions.

• Myofibrils: Bundles of contractile proteins; responsible for contraction.

Other organelles, such as mitochondria, are packed between myofibrils. Smooth muscle cells have a different arrangement compared to skeletal and cardiac muscle.

Structure and Function of Skeletal Muscle Fibers

Skeletal muscle fibers are long, cylindrical cells formed by fusion of embryonic myoblasts, resulting in multiple nuclei. They are surrounded by endomysium and contain abundant myofibrils.

• Transverse tubules (T-tubules): Deep extensions of sarcolemma; surround myofibrils and are filled with extracellular fluid.

• Terminal cisternae: Enlarged sections of SR; flank each T-tubule; together with T-tubule form a triad.

Structure of the Myofibril

Myofibrils are composed of myofilaments:

• Thick filaments: Bundles of myosin; heads bind actin.

• Thin filaments: Actin, tropomyosin, and troponin; actin has active sites for myosin binding.

• Elastic filaments: Titin; stabilizes myofibril and resists excessive stretching.

Tropomyosin covers actin active sites; troponin holds tropomyosin in place and regulates contraction.

Myofilament Arrangement and the Sarcomere

The sarcomere is the functional unit of muscle contraction, defined by Z-discs. Striations are due to alternating light (I band) and dark (A band) regions:

• I band: Only thin filaments; Z-disc in center.

• A band: Both thick and thin filaments; zone of overlap.

• H zone: Only thick filaments; middle of A band.

• M line: Structural proteins; anchor thick filaments.

Levels of Organization Within a Skeletal Muscle

Muscle fibers are grouped into fascicles, surrounded by perimysium. Bundles of fascicles form a muscle, surrounded by epimysium. Tendons connect muscles to bones, and fascia anchors muscles to surrounding tissues.

Duchenne Muscular Dystrophy (DMD)

DMD is a degenerative muscular disease caused by a defective gene for dystrophin, a structural protein. Without dystrophin, muscle fibers break down and are replaced by connective tissue, leading to progressive weakness and early death.

The Sliding-Filament Mechanism of Contraction

Muscle contraction occurs when myosin heads pull actin filaments toward the M line, shortening the sarcomere. The sliding-filament mechanism explains tension generation:

• I band and H zone narrow; A band remains unchanged.

• Myosin heads attach to actin and pull thin filaments.

• Sarcomeres contract simultaneously, shortening the muscle fiber.

Skeletal Muscle Fibers as Electrically Excitable Cells

Muscle fibers maintain a resting membrane potential due to unequal ion distribution. Ion channels (leak and gated) and pumps maintain gradients, essential for action potentials and contraction.

• Resting membrane potential: Electrical potential across sarcolemma; typically -85 mV.

• Ion channels: Leak, ligand-gated, voltage-gated, and mechanically-gated channels.

• Electrochemical gradient: Sum of concentration and electrical gradients; drives ion movement.

Action Potentials

Action potentials are brief changes in membrane potential, essential for muscle contraction:

• Depolarization: Voltage-gated sodium channels open; sodium enters, making membrane positive.

• Repolarization: Sodium channels close; potassium channels open; potassium exits, restoring negativity.

• Action potentials propagate across sarcolemma, triggering contraction.

The Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and muscle fiber. It transmits signals for contraction:

• Axon terminal: Contains acetylcholine (ACh) vesicles.

• Synaptic cleft: Space between neuron and muscle fiber.

• Motor end plate: Region of sarcolemma with ligand-gated sodium channels.

Skeletal Muscle Contraction and Relaxation

Muscle contraction occurs in three phases:

1. Excitation phase: Action potential arrives; ACh released; sodium enters muscle fiber, generating end-plate potential.

2. Excitation-contraction coupling: End-plate potential opens voltage-gated channels; action potential propagates; calcium released from SR.

3. Contraction phase: Calcium binds troponin; tropomyosin moves; actin active sites exposed; myosin binds actin; crossbridge cycle repeats.

Relaxation occurs when action potentials stop, ACh is degraded, calcium is pumped back into SR, and actin active sites are blocked.

Energy Sources for Skeletal Muscle

ATP is required for contraction, crossbridge cycling, and relaxation. Muscle fibers use three sources:

• Immediate: Stored ATP and creatine phosphate (lasts ~10 seconds).

• Glycolytic: Glycolysis in cytosol; produces ATP for 30–40 seconds; can be anaerobic.

• Oxidative: Aerobic catabolism in mitochondria; uses glucose, fatty acids, or amino acids; provides ATP for hours.

Creatine supplementation can improve short bursts of activity but has limited effects on endurance.

Muscle Tension at the Fiber Level

Muscle twitch is the smallest contraction, consisting of latent, contraction, and relaxation periods. Tension varies with timing, frequency, fiber length, and fiber type.

• Wave summation: Repeated stimulation increases tension.

• Unfused tetanus: Partial relaxation between stimuli.

• Fused tetanus: No relaxation; tension is maximal.

• Length-tension relationship: Optimal sarcomere length generates maximal tension.

Classes of Skeletal Muscle Fibers

Muscle fibers are classified by speed and energy source:

• Slow-twitch (Type I): Small diameter; oxidative; fatigue-resistant; postural muscles.

• Fast-twitch (Type II): Large diameter; glycolytic; fatigue quickly; powerful contractions.

• Type IIa (FOG): Intermediate; both oxidative and glycolytic.

• Type IIb (FG): Fast glycolytic; very fast, powerful, fatigue quickly.

Motor Units and Muscle Contractions

A motor unit consists of a motor neuron and all fibers it innervates. Small motor units allow fine control; large units generate more power. Recruitment increases force by activating more units. Muscle tone is the baseline activation for posture and readiness.

• Isotonic contractions: Muscle changes length; tension constant.

• Isometric contractions: Muscle length unchanged; tension equals external force.

Skeletal Muscle Performance and Adaptation

Physical training induces myoplasticity—structural and functional changes in muscle:

• Endurance training: Increases oxidative capacity, fatigue resistance, and blood supply.

• Resistance training: Increases fiber size (hypertrophy) and myofibril number.

• Disuse: Causes atrophy; decreases strength and endurance.

Muscular fatigue results from depletion of metabolites, decreased oxygen, accumulation of chemicals, and environmental factors. Recovery requires excess postexercise oxygen consumption (EPOC).

Smooth and Cardiac Muscle

Smooth Muscle

Smooth muscle propels materials, forms sphincters, and regulates flow. It lacks sarcomeres and striations; actin and myosin are arranged obliquely. Contraction is triggered by calcium binding to calmodulin, activating MLCK and myosin ATPase. Single-unit smooth muscle contracts as a wave; multi-unit contracts independently.

Cardiac Muscle

Cardiac muscle cells are branched, have one or two nuclei, and are linked by intercalated discs. They contract as a unit, coordinated by pacemaker cells, and do not require nervous stimulation.

Summary Table: Types of Muscle Tissue

Type	Structure	Control	Location	Function
Skeletal	Long, multinucleated, striated	Voluntary	Attached to skeleton	Movement, posture
Cardiac	Short, branched, striated	Involuntary	Heart	Pumping blood
Smooth	Spindle-shaped, non-striated	Involuntary	Hollow organs, vessels	Regulate flow, movement

Summary Table: Classes of Skeletal Muscle Fibers

Type	Diameter	Speed	Energy Source	Fatigue Resistance
Type I (Slow)	Small	Slow	Oxidative	High
Type IIa (FOG)	Intermediate	Fast	Oxidative/Glycolytic	Moderate
Type IIb (FG)	Large	Very Fast	Glycolytic	Low