Muscle Tissue and Physiology: Structured Study Notes for Anatomy & Physiology

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Muscle Tissue and Physiology

Overview of Muscle Tissue

Muscle tissue is essential for movement, posture, joint stabilization, heat generation, and regulation of material flow through hollow organs. It consists of muscle cells (myocytes) and the surrounding endomysium, which transmits tension and holds cells together. There are three main types of muscle tissue: skeletal, cardiac, and smooth.

Muscle Tension: The force generated by muscle tissue.
Endomysium: Extracellular matrix surrounding muscle cells.
Myocytes: Muscle cells specialized for contraction.

Types of Muscle Tissue

Each muscle tissue type has unique structural and functional characteristics.

Skeletal Muscle: Long, thin, striated, multinucleated cells arranged parallel; voluntary; attached to skeleton; must be stimulated by the nervous system.
Cardiac Muscle: Shorter, branched, striated cells with a single nucleus; involuntary; found only in the heart; connected by intercalated discs.
Smooth Muscle: Long, flattened cells with pointed ends and a single nucleus; involuntary; lines hollow organs, eyes, skin, and ducts; linked by gap junctions.

Type	Structure	Location	Voluntary/Involuntary	Function
Striated (Skeletal)	Long, cylindrical, multinucleated, striated	Attached to skeleton	Voluntary	Movement of body
Cardiac	Short, branched, single nucleus, striated, intercalated discs	Heart	Involuntary	Pumping blood
Smooth	Long, flattened, single nucleus, no striations	Hollow organs, eyes, skin	Involuntary	Changes diameter, moves materials

Comparison of muscle tissue types

Properties of Muscle Cells

Muscle cells possess five key properties that enable their function:

Contractility: Ability to contract and draw proteins together.
Excitability: Ability to respond to stimuli.
Conductivity: Ability to conduct electrical charges across the membrane.
Distensibility: Ability to stretch without rupture.
Elasticity: Ability to return to original length after stretching.

Structure of Muscle Cells

Muscle cells share organelles with other cells but have specialized structures for contraction.

Sarcoplasm: Muscle cell cytoplasm.
Sarcolemma: Muscle cell plasma membrane.
Myofibrils: Bundles of proteins for contraction; make up 50–80% of cell volume.
Sarcoplasmic Reticulum (SR): Modified smooth ER surrounding myofibrils.

Generalized cell vs muscle cell

Structure of the Skeletal Muscle Fiber

Skeletal Muscle Fiber Differences

Skeletal muscle fibers are long, cylindrical, multinucleated, and striated. They arise from fused embryonic myoblasts and are the most abundant organelle in the sarcoplasm.

Cell Shape: Long, cylindrical, up to 30 cm.
Striations: Striped appearance under microscope.
Multiple Nuclei: Result of myoblast fusion.

Size and shape of skeletal muscle fiber

Internal Structure

The sarcolemma forms inward extensions called Transverse Tubules (T-tubules) that surround each myofibril. Terminal cisternae are enlarged portions of the SR flanking T-tubules, forming a Triad.

Structure of a skeletal muscle fiber

Structure of the Myofibril

Myofilaments

Myofibrils are composed of three types of myofilaments:

Thick Filaments: Made of myosin; two heads and a tail.
Thin Filaments: Made of actin (with active sites), tropomyosin (covers active sites), troponin (holds tropomyosin), and nebulin (determines length).
Elastic Filaments: Made of titin; provides elasticity and resists overstretching.

Structure of myofilaments

Myofilament Arrangement and the Sarcomere

Sarcomere Structure

The sarcomere is the functional unit of muscle contraction, defined by the arrangement of myofilaments:

I Band: Light region; only thin filaments.
A Band: Dark region; thick and thin filaments.
H Zone: Middle of A band; only thick filaments.
M Line: Middle of A band; structural proteins hold thick filaments.
Z Disk: Anchors thin filaments; attachment for elastic filaments.

Structure and bands of the sarcomere Three-dimensional structure of the sarcomere

Structure	Description	Mnemonic
A band	Dark band; thick and thin filaments	dArk band
I band	Light band; only thin filaments	lIght band
H zone	Middle of A band; only thick filaments	"H" in A band
M line	Middle of A band; structural proteins	"M" for middle/myosin
Z disc	Bisects I band; anchors filaments	"Z" shape

Memory cues for sarcomere bands

The Sliding-Filament Mechanism of Contraction

Mechanism

Muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere and generating tension. The I bands and H zone narrow, Z-discs move closer, but the A band remains unchanged.

Sliding-filament mechanism using hands Sliding-filament mechanism diagram

Membrane Potential in Muscle Cells

Electrophysiology

Muscle cells maintain a resting membrane potential due to the separation of charges across the plasma membrane. The sodium-potassium pump (Na+/K+ ATPase) maintains ion gradients, moving three sodium ions out and two potassium ions in, requiring ATP.

Voltage measurement across plasma membrane Ion gradients maintained by Na+/K+ pump Electrochemical gradient for potassium ions Generation of resting membrane potential No membrane potential Membrane potential created by K+ diffusion

Action Potentials

An action potential is a rapid change in membrane potential, involving depolarization (Na+ enters) and repolarization (K+ exits). This process is propagated along the sarcolemma and T-tubules.

Stages of an action potential

The Neuromuscular Junction

Structure and Function

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber, consisting of the axon terminal, synaptic cleft, and motor end plate. Acetylcholine (ACh) is the neurotransmitter released to stimulate muscle contraction.

Structures of the neuromuscular junction

Skeletal Muscle Contraction

Phases of Contraction

Muscle contraction occurs in three phases:

Excitation: ACh stimulates the sarcolemma, generating an action potential.
Excitation-Contraction Coupling: Action potential triggers calcium release from SR.
Contraction: Sliding-filament mechanism shortens the sarcomere.

Excitation phase at NMJ Excitation-contraction coupling Preparation for contraction: regulatory events Crossbridge cycle of sliding-filament mechanism Crossbridge cycle continued Analogy of sailors pulling rope

Muscle Relaxation

Process

Relaxation occurs when ACh release stops, ACh is degraded, calcium is pumped back into the SR, and troponin/tropomyosin block actin active sites.

Process of muscle relaxation

Rigor Mortis

Rigor mortis is the stiffening of muscles after death due to lack of ATP, preventing detachment of myosin from actin.

Rigor mortis

Energy Sources for Skeletal Muscle

ATP Regeneration

Muscle fibers regenerate ATP through three processes:

Creatine Phosphate Reaction: Creatine phosphate donates a phosphate to ADP.
Glycolytic Catabolism: Anaerobic breakdown of glucose to pyruvate/lactic acid.
Oxidative Catabolism: Aerobic breakdown of fuels in mitochondria.

Immediate energy sources for muscle Glycolytic and oxidative energy sources

Muscle Tension at the Fiber Level

Muscle Twitch and Myogram

A muscle twitch is the response to a single action potential, recorded as a myogram. It consists of latent, contraction, and relaxation periods.

Myogram of a twitch contraction

Wave Summation and Tetanus

Repeated stimulation increases tension (wave summation). Unfused tetanus allows partial relaxation; fused tetanus produces sustained contraction.

Wave summation: unfused and fused tetanus

Length-Tension Relationship

The number of crossbridges depends on sarcomere length. Optimal length allows maximum tension; too short or too stretched reduces tension.

Length-tension relationship

Classes of Skeletal Muscle Fibers

Fiber Types

Fast-Twitch Fibers: Rapid contraction, fatigue quickly, rely on glycolysis.
Slow-Twitch Fibers: Slow contraction, sustained activity, rely on oxidative catabolism.

Comparison of muscle fiber types

Motor Units

Structure and Function

A motor unit consists of a motor neuron and the muscle fibers it innervates. Recruitment increases force by activating more units. Muscle tone is maintained by involuntary activation.

Motor unit diagram

Types of Muscle Contractions

Isotonic Concentric: Muscle shortens; force > load.
Isotonic Eccentric: Muscle lengthens; force < load.
Isometric: Muscle length unchanged.

Types of muscle contractions

Changes Caused by Physical Training

Training Effects

Endurance Training: Increases oxidative capacity, resistance to fatigue.
Resistance Training: Increases fiber diameter (hypertrophy), strength.
Disuse: Decreases fiber diameter (atrophy), strength, and endurance.

Adaptive changes of muscle fibers

Muscle Fatigue and Recovery

Causes of Fatigue

Depletion of metabolites (creatine phosphate, glycogen, glucose).
Decreased oxygen availability.
Accumulation of chemicals (Ca2+, phosphate, ADP).
Environmental conditions (heat, electrolyte imbalance).

Recovery period involves excess postexercise oxygen consumption (EPOC) to restore homeostasis.

Smooth Muscle

Functions and Structure

Smooth muscle is found in hollow organs, blood vessels, and the eye. It lacks striations and sarcomeres, and contracts via a different mechanism involving calmodulin and myosin light-chain kinase.

Structure of smooth muscle tissue Contraction of smooth muscle cells

Cardiac Muscle

Cardiac muscle is striated, has sarcomeres, T-tubules, and extensive SR. Cells are branched, have one or two nuclei, and are connected by intercalated discs. Pacemaker cells allow autorhythmic contraction.

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