Silelihaskiht

Silelihaskiht is the term used to describe the smooth muscle layer that lines the walls of many hollow organs and blood vessels. In anatomical usage, it refers to the coat composed of smooth muscle cells that underlie the epithelial layers or form part of the muscularis externa in the gastrointestinal tract and the tunica media in vessels. This layer is involuntary and non-striated, consisting of spindle-shaped cells that typically contain a single nucleus. In the GI tract, the smooth muscle layer is organized into circular and, in some regions, longitudinal orientations, contributing to peristalsis and segmentation that move or mix contents through the lumen. In blood vessels, the smooth muscle layer regulates vessel diameter and hence blood flow and pressure.

Histologically, smooth muscle cells lack the sarcomere organization seen in skeletal muscle. Contraction is driven by calcium ions that activate the enzyme myosin light-chain kinase via the calcium-binding protein calmodulin. The actin–myosin interaction occurs along dense bodies and is often coordinated by gap junctions, allowing synchronized contractions in many smooth muscle groups. The tissue typically maintains a baseline tone and is more fatigue-resistant than skeletal muscle, enabling sustained contractions.

Regulation of silelihaskiht is primarily autonomic, with sympathetic and parasympathetic inputs modulating contraction and relaxation. Local factors, hormonal signals, and mechanical stretch can also influence tone. Practical importance lies in its role in controlling lumen diameter, propelling contents in the GI tract, and modulating flow in the vasculature. Dysfunction can contribute to conditions such as intestinal dysmotility, asthma-like bronchial constriction, or vascular abnormalities stemming from abnormal smooth muscle tone.