Smooth Muscle Features and Function

Absence of Classical Neuromuscular Junction

• Smooth muscle does not have a classical neuromuscular junction like skeletal muscle.
• Can be stimulated by various stimuli:
  • Electrical
  • Chemical

Dual Innervation

• Controlled by both sympathetic and parasympathetic nervous systems.
• Regulates contraction and relaxation.
• Discussed in the communication module (Module 6).

Stretch-Activated Channels

• Responsible for myogenic contraction (stretch-dependent contraction).
• Intrinsic ability of smooth muscle; the more it is stretched, the stronger the contraction.

Hormonal and Local Signals

• Histamine: Causes contraction in bronchioles, restricting airflow.
• Nitric Oxide: Causes relaxation, increasing blood vessel diameter.
  • Example: Nitroglycerin is converted to nitric oxide to relieve angina.

Pacemaker Potentials

• Slow Wave Potentials: Multiple sub-threshold stimuli until reaching threshold to initiate action potential.
• Pacemaker Potential: Unstable resting membrane potential leading to periodic depolarization and repolarization.
  • Important for smooth muscle contraction series, e.g., GI tract.
  • Also present in cardiac pacemaker cells.

Chemicals and Contractions

• Pharmaceutical chemicals can initiate contractions without altering resting membrane potential.

Hyperplasia

• Smooth muscle can increase the number of cells through hormones (does not occur in skeletal muscle).
  • Example: Estrogen causes the growth of smooth muscle cells in the uterus during pregnancy.

Comparison with Skeletal and Cardiac Muscle

Cardiac Muscle

• Shares features with both skeletal and smooth muscle.
• Striated: Sarcomere structure similar to skeletal muscle.
• Single or Dual Nuclei: Unlike multinucleated skeletal muscle.
• Branched Fibers: Three-dimensional contraction.
• Graded Contraction: Fine-tuned contraction to meet metabolic needs.
• Gap Junctions: Electrically linked cells for rapid communication.
• Pacemaker Potential: Present in autorhythmic cells to initiate heart contraction.
• Sympathetic and Parasympathetic Control: Adjusts heart rate by influencing pacemaker potentials.