A tissue is a group of cells with similar structures and functions. There are four types of body tissues: epithelial, connective, nervous, and muscle. An epithelial tissue is a sheet of cells that covers the surface of the body and the outside and inside of many internal organs. Because epithelial tissue wraps around organs, it functions as a shield. With its thin walls, this tissue also absorbs, filters, and secretes substances. Connective tissue is composed of cells and an extracellular matrix that binds the cells and organs of the body together, integrating all parts of the body. Every organ is associated with connective tissue. Examples of connective tissues include bone, cartilage, and fat.Muscle tissue and nervous tissue are made up of excitable cells. Excitable cells have a cell membrane specialized to alter the electrical activity of the cell. Molecules inside of cells have a net negative charge compared to the molecules found outside the cell. This charge differential is the membrane potential. Excitable cells allow the inside of the cell to rapidly become more positive (depolarization) and then recover to the original state (repolarization). This depolarization and repolarization of the membrane is known as an action potential, which serves as a form of electrical communication. When an action potential passes through muscle tissue, it generates force that leads to voluntary and involuntary movements of the body. Voluntary movements are under conscious control, such as moving limbs and changing facial expressions. Involuntary movements are under unconscious control, such as the beating of the heart or the churning of food in the stomach. Nervous tissue is used to transmit electrical and chemical signals throughout the body by means of specialized cells called neurons. These signals coordinate body actions and can perceive sensory information, stimulate muscle movement, and control cognitive functions such as memory and emotions. These four tissue types work together to keep the body functioning and healthy.