Muscle Physiology: Smooth Muscle

Characteristics and Classification of Smooth Muscle

• Definition and General Characteristics of Smooth Muscle:     * Smooth muscle is categorized as non-striated, involuntary muscle tissue.     * It is primarily located in the walls of hollow organs, blood vessels, glands, and structures such as the eye and skin.     * Its primary role is to produce contractions that regulate internal physiological functions and secretions.     * The internal physical arrangement of the fibers differs significantly from skeletal muscle.     * Each organ's smooth muscle is distinctive based on several factors:         * Physical dimensions.         * Organization into specific bundles or sheets.         * Response to various stimuli.         * Characteristics of innervation.         * Specific functions.

• Types of Smooth Muscle:     * Multi-unit Smooth Muscle:         * Composed of discrete, separate smooth muscle fibers.         * Each muscle fiber operates and contracts independently.         * Each fiber is independently innervated.         * Specific Locations: Ciliary muscles of the eyes, iris muscle of the eyes, base of hair follicles, smaller airways of the lungs, and the walls of large blood vessels.     * Single-unit (Visceral/Unitary/Syncytial) Smooth Muscle:         * Fibers are organized into sheets or bundles.         * Cell membranes adhere to one another so that force generated in one fiber is transmitted to adjacent fibers.         * These fibers contract together as a single functional unit.         * Cell membranes are joined by numerous gap junctions.         * Specific Locations: Gastrointestinal (GI) tract, bile ducts, ureters, uterus, and many blood vessels.

Microscopic Structure and Organization

• Individual Fiber Structure:     * Smooth muscle cells are small and spindle-shaped.     * They are non-striated and contain a single, centrally located nucleus.     * They lack the coarse connective tissue coverings (epimysium, perimysium) characteristic of skeletal muscle.     * Cells are typically arranged into sheets.

• Role of Gap Junctions in Single-unit Smooth Muscle:     * Gap junctions allow for the passage of small water-soluble molecules, specifically ions and glucose, between cells.     * The physical gap is very narrow, measuring approximately $2 - 4\,nm$.     * They are critical in tissues with electrically excitable cells as they allow action potentials to spread rapidly from cell to cell.     * Examples of this rapid spread include cardiac contraction and peristaltic movements of the intestines.     * Gap junctions are also found in connective tissue, epithelial tissue, cardiac muscle, and certain neurons.

• Dense Bodies and Myofilament Arrangement:     * Large numbers of actin filaments are attached to DENSE BODIES, which serve as anchorage points similar to Z-disks in skeletal muscle.     * Distribution of Dense Bodies:         * Some are attached to the cell membrane (sarcolemma).         * Some are bound together by intercellular protein bridges.         * Most are dispersed throughout the cytoplasm inside the cell.     * Filament Composition:         * Smooth muscle contains no troponin-tropomyosin complex.         * Myosin filaments are intercalated among the actin filaments.         * Intermediate filaments provide structural support within the cell.

Mechanical and Structural Adaptations

• Side-Polar Cross-Bridges:     * Unlike skeletal muscle, smooth muscle myosin filaments have side-polar cross-bridges.     * Cross-bridges on one side of the filament bend in one direction, while those on the opposite side bend in the reverse direction.     * This configuration allows myosin to pull actin filaments simultaneously in opposite directions.     * Resulting Efficiency: Smooth muscle can contract up to $80\,\%$ of its length, whereas skeletal muscle is limited to approximately $30\,\%$.

• Sarcoplasmic Reticulum (SR) and Caveolae:     * The SR is only slightly developed in smooth muscle and is not the major source of calcium ($Ca^{2+}$). The extracellular fluid (ECF) serves as the primary reservoir.     * The SR may not even be present in all smooth muscle fibers; when present in large cells, it lies near the cell membrane.     * Caveolae: These are small invaginations of the cell membrane that touch the surface of the SR. They represent a rudimentary version of T-tubules and are believed to trigger calcium release from the adjacent SR.     * Relationship to Speed: There is a direct correlation between the extent of the SR and the speed of contraction; a more extensive SR results in faster contraction.

Physiological Stimulation and Innervation

• Factors Initiating Contraction:     * Contraction is initiated by receptors that can either stimulate or inhibit the process.     * Primary stimuli include nervous stimulation, hormonal stimulation, local tissue chemical factors, self-excitation, and mechanical stimulation (stretching).     * Stimulation involves depolarization toward a threshold through the opening of sodium ($Na^+$) and calcium ($Ca^{2+}$) ion channels.

• Nervous Stimulation:     * Smooth muscle is innervated by the Autonomic Nervous System (ANS), which controls involuntary internal functions.     * The ANS consists of the Sympathetic Nervous System (SNS - "fight or flight") and the Parasympathetic Nervous System (PNS - "rest and digest").     * Varicosities: Nerve fibers generally do not make direct contact with the muscle membrane. Instead, terminal axons have multiple swellings called varicosities which store and secrete neurotransmitters into the matrix coating.     * Neurotransmitters:         * Acetylcholine (ACh): Typically associated with the PNS.         * Norepinephrine (NE): Typically associated with the SNS.         * ACh and NE usually have opposite effects on target tissues.

• Hormonal Stimulation:     * Circulating hormones can be excitatory or inhibitory, provided the target tissue has specific receptors.     * Hormones affecting smooth muscle include: Epinephrine, Angiotensin II, Endothelin, Vasopressin, Oxytocin, Serotonin, and Histamine.

Myogenic Response and Local Control

• Local Chemical Factors:     * In small vessels where nerve supply is minimal or absent, smooth muscle is highly sensitive to the surrounding interstitial fluid.     * Factors that cause relaxation and increased blood flow include:         * Lack of oxygen ($O_2$) in local tissues or excess carbon dioxide ($CO_2$).         * Increased hydrogen ions ($H^+$).         * Increase in lactic acid.         * Increase in body temperature.

• Myogenic Response (Mechanical Stimulation):     * This is an intrinsic mechanism in arterioles activated by stretching.     * Mechanism: Stretching opens mechanosensitive cation channels, allowing $Na^+$ and $Ca^{2+}$ influx, leading to depolarization and contraction.     * Effects:         * Increased pressure causes stretching, leading to contraction (vasoconstriction).         * Decreased pressure reduces stretching, leading to relaxation (vasodilation).     * Physiological Role: Autoregulation of blood flow to protect capillaries from pressure surges. This is especially vital in the kidney, brain, and heart.

Electrophysiology: Action Potentials and Junctional Potentials

• Potentials in Multi-unit Smooth Muscle:     * These do not generate true action potentials.     * Stimuli produce junctional potentials that spread across the small fibers to trigger contraction, as the fibers are too small for a propagated action potential.

• Potentials in Single-unit (Visceral) Smooth Muscle:     * 1. Spike Potentials: Similar to skeletal muscle. They can be elicited by extrinsic stimuli or self-excitation.         * Slow Wave Rhythm: APs often arise from a basic slow wave rhythm of the membrane potential. When the wave is strong enough to reach the threshold, an AP initiates (e.g., intestinal rhythmical contraction).         * The cause is potentially the waxing and waning pumping of positive ions ($Na^+\text{, } Ca^{2+}$).     * 2. Action Potentials with Plateau:         * Features a delayed repolarization phase (prolonged depolarization).         * Responsible for prolonged contractions in the ureter, uterus, and vascular smooth muscle.         * Smooth muscle membranes in these areas have many voltage-gated calcium channels but few voltage-gated sodium channels.

Molecular Mechanism of Contraction

• The Role of Calcium:     * The influx of $Ca^{2+}$ is the primary driver for both action potential depolarization and contraction.     * $Ca^{2+}$ binds to Calmodulin (a regulatory protein/multipurpose receptor), rather than troponin.     * Binding causes a conformational change that activates proteins required for cross-bridge cycling.

• The 5 Steps of Smooth Muscle Contraction:     1. Calcium Concentration Increase: Calcium enters the cytosol from the ECF (primary) and the SR (secondary).     2. Binding: Calcium ions bind reversibly with calmodulin.     3. Complex Activation: The Calcium-Calmodulin complex joins and activates MYOSIN LIGHT CHAIN KINASE (MLCK), a phosphorylating enzyme.     4. Phosphorylation: MLCK phosphorylates the REGULATORY LIGHT CHAIN on each myosin head.         * When non-phosphorylated, the myosin is inactive.         * When phosphorylated, it becomes active and can bind with actin for attachment-detachment cycling.     5. Contraction:         * Activated myosin hydrolases ATP into ADP and $P_i$.         * Myosin binds to actin (cross-bridge attachment).         * Phosphate is released, leading to the power stroke and subsequent ADP release.         * Rigor state occurs (myosin and actin bound) until a new ATP binds to release the myosin from the actin.

Mechanisms of Relaxation and Sustained Contraction

• Relaxation Process:     * Requires a decrease in intracellular $Ca^{2+}$ concentration.     * Calcium Pumps: Move $Ca^{2+}$ back to the ECF or SR. These pumps are ATP-dependent and slow-acting, causing smooth muscle contractions to last longer than skeletal muscle.     * Myosin Phosphatase: Located in the cytosol. Once $Ca^{2+}$ levels fall, this enzyme inactivates myosin by splitting the phosphate from the regulatory light chain. This stops the cycle and ends contraction.

• Contractile Particularities:     * Smooth muscle exhibits slow cycling of myosin cross-bridges (slow attachment, release, and reattachment).     * This provides increased force of contraction and very low energy requirements.     * Useful for organs maintaining tonic contraction (intestines, bladder, gallbladder).     * Efficiency: Myosin Light Chain Phosphatase (MLCP) can deactivate myosin while it is still attached to actin, which severely slows the binding of new ATP, enabling sustained contraction with minimal energy expenditure.

Questions & Discussion

• Sphincter Humor and Classification:     * A dialogue occurs between various physiological structures regarding sphincters.     * A character asks: "a sphincter says what?"     * Another character responds: "oh, real mature."     * A capillary sphincter clarifies: "I'M NOT EVEN THAT KIND! I'M A CAPILLARY SPHINCTER!"     * The Iris states: "My IRIS is a SPHINCTER. Does that amuse you?"     * The response given is: "No sir."