Lecture Notes on Smooth Muscle

Introduction

• Smooth Muscle Location: Found in walls of hollow organs except the heart, which has cardiac muscle.
• Cell Structure: Spindle-shaped, short, thin cells with one nucleus, no striations.
• Connective Tissue: Lacks connective tissue but has endomysium.
• Blood Vessels: Most blood vessels contain smooth muscle, except the smallest ones.

Structure and Function

• Arrangement in Organs: Two major layers - circular and longitudinal.
• Peristalsis: Allows for wave-like motion to propel contents through organs (e.g., digestive tract).
• Circular Layer: Surrounds the lumen.
• Longitudinal Layer: Runs lengthwise.

Innervation and Neurotransmission

• Autonomic Nerve Fibers: No neuromuscular junctions, contains varicosities that release neurotransmitters.
• Neurotransmitter Variety: Different types unlike skeletal muscle which uses acetylcholine.

Microscopic Anatomy

• No Sarcomeres/Myofibrils: Smooth muscle doesn't have sarcomeres or T-tubules.
• Sarcoplasmic Reticulum: Less developed; relies on extracellular calcium via Calvillo lay.
• Calcium Channels: Found in membrane infoldings.
• Gap Junctions: Allow action potential spread between cells.

Filament Arrangement

• Dense Bodies: Link with intermediate cytoskeleton for contraction.
• Thick Filament Ratio: Lower than skeletal muscle.
• Calcium Binding: Uses calmodulin instead of troponin.
• Contraction: Diagonal filament arrangement allows corkscrew contraction.

Comparison with Other Muscle Types

• Skeletal Muscle: Long, striated, voluntary, attaches to bones.
• Cardiac Muscle: Branched, involuntary, has striations, high interaction with gap junctions.
• Smooth Muscle: Short, no striations, dense with endomysium.

Excitation and Contraction

• Excitation: Involuntary, autonomic neuron control.
• Calcium Source: Extracellular fluid and sarcoplasmic reticulum.
• Pacemaker Cells: Found in cardiac and smooth muscle, self-excitatory.

Contraction Mechanism

• Similarity to Skeletal Muscle: Calcium triggers, ATP required.
• Differences: Calcium from extracellular source, calmodulin involvement.
• Efficient Energy Use: Slow contraction, latch state maintains tone.

Regulation of Contraction

• Neural and Chemical Control: Neurotransmitters can stimulate inhibitory or excitatory responses.
• Chemical Sensitivity: Responsive to CO2, pH, hormones.

Types of Smooth Muscle

• Unitary Smooth Muscle: Found in hollow organs, has gap junctions for action potential spread.
• Multi-Unit Smooth Muscle: Found in large airways, arteries, more precise control.

Conclusion

• Dynamic Capabilities: Smooth muscle adapts to chemical, neural, stretch stimuli.
• Functional Diversity: Suitable for various organs due to its unique properties and control mechanisms.