Smooth Muscle

Overview and Location

Smooth muscle is a type of involuntary muscle tissue found primarily in the walls of hollow organs throughout the body. Unlike skeletal and cardiac muscle, smooth muscle is not under conscious control and plays a crucial role in regulating the movement of substances within organ systems.

• Location: Found in the walls of the respiratory, digestive, urinary, reproductive, and circulatory systems (except the heart and smallest blood vessels).

• Organization: Most smooth muscle is organized into sheets of tightly packed fibers.

Layered Structure of Smooth Muscle in Organs

Most hollow organs contain two layers of smooth muscle sheets, with fibers oriented at right angles to each other. This arrangement allows for coordinated contractions that facilitate the movement of substances.

• Longitudinal layer: Fibers run parallel to the long axis of the organ. Effect: Contraction causes the organ to shorten.

• Circular layer: Fibers run around the circumference of the organ. Effect: Contraction causes the lumen (internal space) of the organ to constrict.

• Function: Alternating contractions and relaxations of these layers mix and propel substances through the lumen of hollow organs.

• Example: Peristalsis in the digestive tract is driven by coordinated contractions of longitudinal and circular smooth muscle layers.

Structural Differences Between Smooth and Skeletal Muscle

Cellular Characteristics

Smooth muscle fibers differ significantly from skeletal muscle fibers in their shape, size, and internal organization.

• Shape: Spindle-shaped, thin, and short compared to the wider and longer skeletal muscle fibers.

• Nucleus: Each smooth muscle fiber contains a single, centrally located nucleus.

• Striations: Smooth muscle lacks striations due to the absence of sarcomeres.

• Connective Tissue: Contains only endomysium; lacks the connective tissue sheaths found in skeletal muscle.

Innervation and Junctions

• Innervation: Smooth muscle is innervated by the autonomic nervous system (involuntary control).

• Varicosities: Nerve fibers form bulbous swellings called varicosities that release neurotransmitters into wide synaptic clefts known as diffuse junctions.

• Gap Junctions: Smooth muscle fibers are electrically connected via gap junctions, allowing depolarization to spread from cell to cell. Skeletal muscle fibers are electrically isolated.

Internal Structures

• Sarcoplasmic Reticulum (SR): Less elaborate than in skeletal muscle; lacks T tubules.

• Caveolae: Pouch-like infoldings of the sarcolemma containing numerous Ca2+ channels for rapid influx of extracellular calcium.

• Filament Arrangement: Thick and thin filaments are arranged diagonally, causing contraction in a corkscrew manner.

• Intermediate Filaments and Dense Bodies: Noncontractile intermediate filaments form a lattice-like network anchored to dense bodies (analogous to Z discs in skeletal muscle), which anchor filaments to the sarcolemma.

Comparison Table: Smooth vs. Skeletal Muscle

Mechanism of Smooth Muscle Contraction

Similarities to Skeletal Muscle Contraction

Smooth muscle contraction shares several fundamental mechanisms with skeletal muscle contraction.

• Sliding Filament Mechanism: Actin and myosin interact to produce contraction.

• Calcium Trigger: Increased intracellular Ca2+ initiates contraction.

• ATP Requirement: ATP is required for the sliding process.

• Relaxation: Contraction stops when Ca2+ is no longer available.

Differences from Skeletal Muscle Contraction

• Calcium Source: Most Ca2+ comes from the extracellular space via voltage-gated or non-voltage-gated channels, with some from the SR.

• Regulatory Proteins: Ca2+ binds to calmodulin (not troponin).

• Activation Cascade: Activated calmodulin activates myosin light chain kinase (MLCK).

• Phosphorylation: MLCK phosphorylates myosin heads, activating myosin ATPase activity.

• Cross Bridge Formation: Activated myosin forms cross bridges with actin, leading to contraction.

• Relaxation: Requires Ca2+ detachment from calmodulin, active transport of Ca2+ out of the cytosol, and dephosphorylation of myosin.

Key Equations

• Phosphorylation of Myosin:

• Calcium Activation:

Regulation of Smooth Muscle Contraction

Neural Regulation

Smooth muscle contraction is regulated by nerves, hormones, and local chemical changes.

• Neurotransmitter Binding: Can cause either graded (local) potentials or action potentials, leading to increased Ca2+ concentration in the sarcoplasm.

• Response Variability: The effect depends on the neurotransmitter released and the type of receptor present; the same neurotransmitter can be stimulatory in one organ and inhibitory in another.

Hormonal and Local Chemical Regulation

• Spontaneous Depolarization: Some smooth muscle cells lack nerve supply and can depolarize spontaneously or in response to chemical stimuli.

• Chemical Factors: Hormones, high CO2, pH changes, and low oxygen can influence contraction by binding to G protein-linked receptors.

• Dual Regulation: Some smooth muscles respond to both neural and chemical stimuli.

Summary Table: Smooth Muscle Contraction Steps

Additional info: Smooth muscle contraction is slower and more sustained than skeletal muscle contraction, allowing for prolonged control of organ diameter and movement of substances.