Tissues are groups of structurally similar cells that work together as functional units. Based on their structure and function, tissues can be of four types. Epithelial tissues are composed of single or multiple cell layers that cover external body parts, line internal body surfaces and form different glands. Muscle tissues consist of specialized contractile cells that facilitate body movements. Connective tissues contain cells in a viscous matrix of extracellular fibers. They provide structural support, transport nutrients, and serve as fat reservoirs. Nervous tissues consist of neurons that respond to stimuli by generating electrical signals, which help coordinate various body activities. Different tissue types organize to form organs and carry out specialized functions. For instance, food ingestion activates the nervous tissue, stimulating the stomach's epithelial lining to secrete digestive juices. Smooth contractile muscles and the adjoining connective tissues on the stomach wall then churn the food to release nutrients to be absorbed throughout the body.