Smooth Muscle Types and Functions - EASY

• Smooth & Cardiac Muscle Topics

Smooth Muscle Composition and Size

• Composed of smaller fibers, typically 1 to 5 micrometers in diameter and 20 to 500 micrometers in length.
• Skeletal muscle fibers are significantly larger, about 30 times greater in diameter and hundreds of times longer.

Organ Systems Involving Smooth Muscle

• Gastrointestinal tract.
• Cardiovascular system.
• Renal system, specifically the urinary bladder.
• Genital system.
• Respiratory tract.
• Integumentary system, involving the arrector pili muscles of the skin.
• Sensory system.

Smooth Muscle Functions Overview

• Smooth muscle exists throughout the body with a variety of functions.
• Aids in digestion and nutrient collection in the stomach and intestines.
• Functions in the urinary system.
• Plays a vital role in regulating blood pressure and tissue oxygenation.

Types of Smooth Muscle & Function

• Distinction Factors: physical dimensions, organization, response to stimuli, innervation, and function.
• Two major types: multi-unit and unitary.
• Innervated by a single nerve ending.
• Each fiber can contract independently.
• Contraction from one cell does not spread to the next; it is confined to the originally stimulated cell.
• Observed in large airways to the lungs, large arteries, arrector pili, and internal eye muscles.
• Visceral: most common in the human body, forming the walls of hollow organs.
• Produces slow, steady contractions facilitating movement of substances, like food.

Physiology Of Smooth Muscle

• Dense Bodies: analogous to the Z-discs in skeletal muscle.
• Calcium ions are supplied primarily from the extracellular environment.
• T-tubules are absent, instead having small indentations called calveoli.
• Sarcoplasmic reticulum: present but less developed compared to skeletal muscle.
• Actin and Myosin: Same interaction as in skeletal muscle (no troponin complex)
• Activated by Ca++ ions, and ATP is degraded to ADP and phosphate for energy.
• Controlled by Ca⁺⁺, contraction duration, the required energy amount, excitation-contraction coupling, and physical organization.

Differences in Muscle Tissue

• Striated muscle tissue includes skeletal and cardiac muscle and has alternating light and dark striations.
• Nonstriated muscle tissue lacks the alternating light and dark striations.

Action of Contraction

• Cross-bridge formation: Regulated by calmodulin.
• Smooth muscle stimulation: External Ca++ ions through opened calcium channels and additional Ca++ from the sarcoplasmic reticulum.
• Calcium binds: In cytoplasm forming Ca++-calmodulin.
• Myosin Head activation: Activated by MLCK by phosphorylating them, converting ATP to ADP.
• Thin Filaments slide: Pulling dense bodies.
• Muscle fiber bulges: Ends are pulled toward the center.

Muscle Tone

• Muscle contraction: Continues until calcium pumps transport Ca++ ions or back into the sarco. reticulum
• Low calcium concentration: Remains in the sarcoplasm and helps maintain muscle tone.
• Calcium concentration: Used to maintain blood pressure.

Smooth Muscle vs Skeletal Contraction:

• Both use attractions between myosin and actin, though smooth muscles arranged differently.

Contraction Summary by Steps:

• Ca++ calmodulin joins/activates myosin kinase.
• Regulatory chain phosporylated.
• Myosin head can bind to actin, entire process with intermittent pulls when chain is phosphorylated.

Smooth Muscle Membrane Potentials: Key Aspects

• Typical smooth muscle action potential: spike potential is elicited by an external stimulus.
• Slow waves lead to action potential
• Treshold must be met by slow wave
• Called pacemaker waves.
• Visceral Smooth Muscle: Stretched sufficiently where spontaneous action potentials are usually generated/
• Action potentials Result from the normal slow wave potentials and increase of negativity.
• Response: Gut wall stretch, contraction automatically and rhythmically, peristaltic waves that move the contents distally

Smooth Muscle Contraction Without Action Potentials:

• Multi-unit smooth muscle: Contracts with the use of nerve stimuli.
• Nerve endings secrete acetylcholine.
• Transmitter depolarization (junctional potential): of the membrane elicits contraction.
• Action potentials do not develop.

Cardiac Muscle

• Heart: Pulsatile two-chamber pumps composed of an atrium and a ventricle.

Atrial and Ventricular Compositions

• Heart's Composition: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers.
• Muscle Contraction compared to Skeletal: atrial and ventricular contract the same way, duration much longer.
• Myocardial Conducting Cells: 1% of cells form conduction system
• Purkinje: Smaller and few nyofibrils.
• Cells: initiate/propagate travel throughout the heart.

How Heart Beats:

• Contractions- initiated at set rate and spread rapidly from cell to trigger contractile mechanism.
• Property of the cell known as Autorhythmicity.

Cardiac Muscle Components

• Striated in appearance
• Functional unit called sarcomere
• Connect to one another at intercalated discs
• Contain gap junctions.
• Nuclei are centrally located
• Abundant mitochondria
• Less abundant than in skeletal muscle, greater in density
• Specialized ion channels
• Sarcomere Shortening/lengthening allowed by anchored ends

Dark Cell Membranes

• Dark blue areas: intercalated discs.
• Cell membranes separating cells connected w parallel fibers.
• Gap junctions: intercellular fluid, along axes of cardiac muscle fibers. Action potentials travel across intercalated discs: all cells excited.

Membrane Junctions

• Desmosomes.
• Gap junctions.

Cardiac Rhythm

• Electrical potential initiated
• Property of autorhythmicity

Cardiac Cells

• Myocardial contractile cells
• Myocardial conducting cells
• Impulse Conducting: responsible for heart contraction.