Muscle Tissue and Physiology: Structure, Function, and Mechanisms

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Muscle Tissue and Physiology

Overview of Muscle Tissue

Muscle tissue is essential for movement, posture, joint stability, heat generation, and regulation of material flow through hollow organs. It consists of specialized cells called myocytes and a surrounding extracellular matrix known as the endomysium. There are three main types of muscle tissue: skeletal, cardiac, and smooth.

Muscle Tension: The force generated by muscle tissue, responsible for movement and other physiological functions.
Myocytes: Muscle cells that contract to produce force.
Endomysium: Connective tissue that holds muscle cells together and transmits tension.

Types of Muscle Tissue

Muscle tissue is classified based on structure, location, control, and function.

Skeletal Muscle: Long, multinucleated, striated fibers; voluntary control; attached to skeleton; responsible for body movement.
Cardiac Muscle: Short, branched, striated cells with one nucleus; involuntary; found only in the heart; contracts as a unit due to intercalated discs.
Smooth Muscle: Spindle-shaped, non-striated cells with one nucleus; involuntary; found in hollow organs, eyes, skin, and ducts; controls flow and movement within organs.

Comparison of muscle tissue types

Properties of Muscle Cells

Muscle cells possess unique physiological properties that enable their function:

Contractility: Ability to shorten and generate force.
Excitability: Ability to respond to stimuli (chemical, mechanical, or electrical).
Conductivity: Ability to conduct electrical signals across the membrane.
Distensibility: Ability to stretch without rupture.
Elasticity: Ability to return to original length after stretching.

Structure of Muscle Cells

General Structure

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

Sarcoplasm: Cytoplasm of a muscle cell.
Sarcolemma: Plasma membrane of a muscle cell.
Myofibrils: Bundles of contractile proteins, making up most of the cell's volume.
Sarcoplasmic Reticulum (SR): Modified smooth endoplasmic reticulum that stores and releases calcium ions.

Generalized cell vs. muscle cell structure

Skeletal Muscle Fiber Structure

Skeletal muscle fibers are long, cylindrical, multinucleated, and striated. They arise from the fusion of embryonic myoblasts.

Size: About 100 µm in diameter and up to 30 cm long.
Striations: Alternating light and dark bands visible under a microscope.
Multiple Nuclei: Result from myoblast fusion.

Skeletal muscle fiber size and shape

Internal Organization

Key features of skeletal muscle fibers include:

Myofibrils: Surrounded by the sarcoplasmic reticulum, which stores and releases calcium.
Transverse Tubules (T-tubules): Inward extensions of the sarcolemma that surround each myofibril and are filled with extracellular fluid.
Terminal Cisternae: Enlarged regions of the SR flanking each T-tubule; together with a T-tubule, they form a triad.

Structure of a skeletal muscle fiber

Myofibril and Myofilament Structure

Types of Myofilaments

Myofibrils are composed of three types of myofilaments:

Thick Filaments: Made of myosin, with globular heads and a tail; responsible for force generation.
Thin Filaments: Composed of actin (with active sites), tropomyosin (covers active sites at rest), and troponin (regulates contraction).
Elastic Filaments: Made of titin, providing elasticity and structural support.

Structure of myofilaments

Sarcomere Organization

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

I Band: Light region; contains only thin filaments.
A Band: Dark region; contains thick filaments (and overlapping thin filaments).
H Zone: Central region of A band; only thick filaments.
M Line: Middle of A band; holds thick filaments in place.
Z-Disc: Anchors thin filaments and marks the boundary of each sarcomere.

Structure and bands of the sarcomere 3D structure of the sarcomere

Structure	Description	Mnemonic
A band	Dark band with thick and thin filaments	A is the dArk band
I band	Light band with only thin filaments	I is the lIght band
H zone	Middle of A band, only thick filaments	"H" is in the A band
M line	Middle of A band, holds thick filaments	M for "middle" or "myosin"
Z-disc	Bisects I band, anchors filaments	Z shape (see figure)

Memory cues for sarcomere bands

Levels of Organization

Muscle structure is organized hierarchically from the whole muscle down to myofilaments:

Muscle → Fascicle → Muscle Fiber → Myofibril → Myofilament

Levels of organization within a skeletal muscle

Sliding-Filament Mechanism of Contraction

Mechanism

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere and generating tension. The I bands and H zone narrow, while the A band remains unchanged.

Hand analogy for sliding-filament mechanism

Membrane Potential and Action Potentials

Resting Membrane Potential

Muscle fibers maintain a resting membrane potential (typically around -90 mV) due to the distribution of sodium and potassium ions across the sarcolemma, maintained by the sodium-potassium pump.

Resting membrane potential Ion gradients maintained by Na+/K+ pump Electrochemical gradient for potassium ions Generation of resting membrane potential No membrane potential Creation of negative membrane potential

Action Potentials

An action potential is a rapid, temporary change in membrane potential, involving depolarization (influx of Na+) and repolarization (efflux of K+). This electrical signal is propagated along the sarcolemma and T-tubules.

Stages of an action potential

Neuromuscular Junction and Muscle Contraction

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber, consisting of the axon terminal, synaptic cleft, and motor end plate. Acetylcholine (ACh) is the neurotransmitter that initiates muscle fiber depolarization.

Structures of the neuromuscular junction Excitation phase at the NMJ

Excitation-Contraction Coupling

This process links the excitation of the sarcolemma to the contraction of the myofilaments via calcium release from the SR.

Excitation-contraction coupling

Contraction Phase: Crossbridge Cycle

Calcium binds to troponin, causing tropomyosin to move and expose actin's active sites. Myosin heads bind to actin, undergo a power stroke, and detach upon ATP binding, repeating the cycle.

Preparation for contraction Crossbridge cycle of the sliding-filament mechanism Crossbridge cycle continued Rope analogy for crossbridge cycle

Muscle Relaxation

Relaxation occurs when ACh is degraded, calcium is pumped back into the SR, and tropomyosin blocks actin's active sites.

Process of muscle relaxation

Rigor Mortis

After death, ATP is depleted, calcium remains in the cytosol, and myosin heads cannot detach from actin, causing muscles to stiffen until proteins degrade.

Rigor mortis Big picture of skeletal muscle contraction

Energy Sources for Muscle Contraction

ATP Regeneration

Muscle fibers regenerate ATP through three main processes:

Creatine Phosphate Reaction: Provides immediate ATP for short bursts of activity.
Glycolytic (Anaerobic) Catabolism: Produces ATP from glucose without oxygen; yields lactic acid.
Oxidative (Aerobic) Catabolism: Occurs in mitochondria, uses oxygen, and provides sustained ATP production.

Immediate energy sources for muscle fibers Glycolytic and oxidative energy sources

Muscle Tension and Contraction Types

Muscle Twitch and Myogram

A muscle twitch is the response to a single action potential, recorded as a myogram with 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 amount of tension a muscle can produce depends on sarcomere length before contraction. Optimal overlap allows maximal crossbridge formation.

Length-tension relationship

Types of Muscle Contractions

Isotonic Concentric: Muscle shortens as it contracts.
Isotonic Eccentric: Muscle lengthens while contracting.
Isometric: Muscle length does not change during contraction.

Types of muscle contractions

Muscle Fiber Types and Motor Units

Classes of Skeletal Muscle Fibers

Slow-Twitch (Type I): Slow, fatigue-resistant, oxidative metabolism, high myoglobin (red muscle).
Fast-Twitch (Type II): Fast, powerful, fatigue quickly, glycolytic metabolism, low myoglobin (white muscle).

Comparison of muscle fiber types

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Recruitment of additional motor units increases force production.

The motor unit

Adaptations and Fatigue

Physical Training

Endurance Training: Increases oxidative capacity and resistance to fatigue.
Resistance Training: Increases muscle fiber size (hypertrophy) and strength.
Disuse: Leads to atrophy and decreased strength/endurance.

Adaptive changes of muscle fibers

Muscle Fatigue

Fatigue results from depletion of metabolites, decreased oxygen, accumulation of chemicals, and environmental factors. Recovery requires restoration of homeostasis and oxygen consumption (EPOC).

Smooth and Cardiac Muscle

Smooth Muscle

Smooth muscle is found in hollow organs and is responsible for peristalsis, sphincter formation, and regulation of flow. It lacks striations and contracts via a different mechanism involving calmodulin and myosin light-chain kinase.

Structure of smooth muscle tissue and cells Contraction of smooth muscle cells

Cardiac Muscle

Cardiac muscle is striated, branched, and connected by intercalated discs. It is autorhythmic due to pacemaker cells and contracts as a unit to pump blood.