13. Smooth muscle

Introduction

• Introduction : Smooth muscle is present throughout the body, where it serves a variety of functions. It is in the stomach and intestines, where it helps with digestion and nutrient collection. It exists throughout the urinary system, where it functions to help rid the body of toxins and works in electrolyte balance. It is present throughout arteries and veins, where it plays a vital role in the regulation of blood pressure and tissue oxygenation. Without these vital functions, the body would not be able to maintain even its most basic functions.

Morphology :

• At a cellular level, smooth muscle functions as an involuntary non-striated muscle. Smooth muscle contains thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern. On microscopic examination, it appears homogenous. Smooth muscle cytoplasm contains large amounts of actin and myosin. Actin and myosin act as the main proteins involved in muscle contraction. Actin filaments attach to dense bodies spread throughout the cell.
• Characters :
• Lack of Sarcomeres and T-tubules: Smooth muscle does not have the sarcomeres that are typical of skeletal muscle, which are responsible for the striated appearance. It also lacks T-tubules, which in skeletal muscle help spread the action potential and synchronize contraction.
• Absence of Troponin: Instead of troponin, which is part of the regulatory mechanism in skeletal muscle, smooth muscle uses a system involving calmodulin.
• Presence of Calmodulin: In smooth muscle, calcium ions bind to calmodulin, forming a complex. This complex activates Myosin Light Chain Kinase (MLCK).
• Action of MLCK: MLCK phosphorylates the light chain on the myosin head, which is a crucial step for myosin to interact with actin filaments and facilitate contraction.
• Actin to Myosin Ratio: Smooth muscle contains a higher ratio of actin to myosin (approximately 12:1), differing from the equal distribution in skeletal muscle, reflecting its different mode of contraction.
• Dense Bodies: The myosin filaments in smooth muscle are attached to dense bodies, which are analogous to Z-discs in skeletal muscle but dispersed throughout the cytoplasm. Dense bodies are composed of the protein α-actinin.
• Gap Junctions: Smooth muscle cells contain gap junctions, which allow for the direct transfer of ions and small molecules between adjacent cells. This feature enables smooth muscle cells to contract in a coordinated manner across large groups, important for their function in organs like the intestines and blood vessels.