Muscle Tissue: Structure and Function

Overview of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various bodily functions. There are three main types of muscle tissue, each with distinct characteristics and roles in the body.

• Skeletal muscle: Voluntary, striated, attached to bones, responsible for body movement.

• Cardiac muscle: Involuntary, striated, found in the heart wall, pumps blood.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs, moves substances within the body.

Functions of Muscle Tissue

• Produce body movement (skeletal muscle)

• Stabilize body positions (skeletal muscle): Maintains balance and posture.

• Move substances within the body (smooth muscle): E.g., food through digestive tract.

• Guard body entrances and exits (skeletal muscle): Voluntary control of sphincters.

• Support soft tissues: Provides structural support.

• Produce heat: Muscle contraction breaks down ATP, generating heat.

• Store nutrients: Proteins in muscle can be broken down for energy.

Types of Muscle Tissue

Cardiac Muscle Tissue

• Control: Involuntary

• Location: Heart wall

• Function: Pumps blood

• Striations: Highly striated (visible microscopically)

Smooth Muscle Tissue

• Control: Involuntary

• Location: Walls of hollow internal structures (e.g., stomach, intestines)

• Striations: None (non-striated)

Skeletal Muscle Tissue

• Control: Voluntary

• Function: Produces movement by pulling on bones

• Striations: Visible microscopically

Properties of Muscle Tissue

• Electrical excitability: Ability to produce electrical signals (action potentials) in response to stimuli (electrical or chemical).

• Contractility: Ability to contract (shorten) forcefully.

• Extensibility: Ability to stretch/lengthen without being damaged.

• Elasticity: Ability to return to original length after being stretched.

Skeletal Muscle Structure

Connective Tissue Components

• Epimysium: Dense sheath of collagen fibers surrounding the entire muscle; separates muscle from other tissues/organs and connects to deep fascia.

• Perimysium: Fibrous layer dividing muscle into compartments (fascicles); contains collagen, elastic fibers, blood vessels, and nerves.

• Endomysium: Surrounds individual muscle fibers; made of areolar connective tissue; contains myosatellite (stem) cells for repair.

Muscle Fiber (Cell) Structure

• Myofibrils: Bundles of protein filaments (myofilaments) within muscle fibers; responsible for contraction.

• Myoblasts: Embryonic cells that fuse to form muscle fibers; result in multinucleated cells.

• Sarcolemma: Plasma membrane of a muscle fiber; selectively permeable, initiates electrical signals.

• Sarcoplasm: Cytoplasm of a muscle fiber; contains myoglobin (oxygen-binding protein).

• Transverse (T) tubules: Invaginations of sarcolemma; conduct action potentials into muscle fiber.

• Sarcoplasmic reticulum (SR): Similar to smooth ER; stores and releases calcium ions for contraction.

Myofilaments and Sarcomere Structure

Myofilaments

• Thin filaments: Mostly actin

• Thick filaments: Mostly myosin

Sarcomere: Functional Unit of Myofibril

• Z lines: Define boundaries of a sarcomere

• I band: Region with only thin filaments (actin)

• A band: Region with thick filaments (myosin), includes overlap with thin filaments

• H band: Region with only thick filaments

• M line: Center of sarcomere, holds thick filaments together

Muscle Proteins

• Contractile proteins: Generate force during contraction (actin and myosin)

• Regulatory proteins: Control contraction process (troponin and tropomyosin)

• Structural proteins: Maintain alignment and structure (titin, nebulin)

Structure of Thin Filaments

• F-actin: Filamentous actin, twisted double strand of G-actin subunits

• Nebulin: Holds F-actin strands together

• Tropomyosin: Covers myosin-binding sites on actin

• Troponin: Holds tropomyosin in place; binds Ca2+ to initiate contraction

Structure of Thick Filaments

• Myosin: Composed of two twisted subunits; tails face M line, heads face out

• Titin: Connects thick filaments to Z line, provides elasticity

Sliding Filament Theory

• During contraction, thin filaments slide over thick filaments, shortening the sarcomere and thus the muscle fiber.

• Key changes during contraction:

  • H and I bands get smaller

  • Zones of overlap get larger

  • Z lines move closer together

  • A band width remains unchanged

Excitable Plasma Membranes

Membrane Potentials

• All cells are polarized: inside is slightly negative compared to outside.

• Positive charges: More Na+ outside (ECF), more K+ inside (cytosol)

• Negative charges: Mostly proteins inside cell; cannot cross membrane

• Leak channels: Allow Na+ to enter, K+ to leave

• Sodium-potassium ion pumps: Export 3 Na+, import 2 K+; maintains resting membrane potential

Equation:

Action Potential Propagation

• Neurons and skeletal muscle fibers have electrically excitable membranes.

• Depolarization and repolarization sequence produces an action potential (electrical impulse).

Neuromuscular Junction (NMJ)

Structure and Function

• Location where a motor neuron controls a skeletal muscle fiber.

• One NMJ per muscle fiber; one motor neuron may branch to control multiple fibers.

• Components:

  • Axon terminal: Contains vesicles with acetylcholine (ACh)

  • Motor end plate: Region of muscle fiber membrane with ACh receptors; contains acetylcholinesterase (AChE) to break down ACh

  • Synaptic cleft: Space between axon terminal and motor end plate

Events at the Neuromuscular Junction

1. Electrical impulse (action potential) arrives at axon terminal.

2. ACh is released (exocytosis) into synaptic cleft.

3. ACh diffuses across cleft, binds to receptors, changes Na+ permeability.

4. Na+ influx generates action potential in muscle fiber.

5. Action potential spreads across sarcolemma and down T tubules.

6. SR releases stored Ca2+ into sarcomeres, initiating contraction.

Muscle Fiber Contraction Cycle

1. Resting sarcomere: Myosin heads are energized and cocked.

2. Calcium ions arrive from SR.

3. Active sites exposed: Ca2+ binds to troponin, moves tropomyosin, exposes actin sites.

4. Cross-bridge formation: Myosin heads bind to actin.

5. Power stroke: Myosin head pivots, pulling actin.

6. ATP binds to myosin head: Myosin detaches, ATP hydrolyzed, head re-cocked.

7. Cycle repeats as long as Ca2+ and ATP are available.

Excitation-Contraction Coupling

• Link between action potential in sarcolemma and start of muscle contraction.

• Action potential in motor neuron leads to ACh release, excitation of muscle fiber, Ca2+ release, and contraction.