The nervous and endocrine systems are often considered together because they are both involved with communication and send signals from one part of the body to another. Each system sends its messages by different mechanisms, however, and the two systems have correspondingly different, though complementary, structures and functions. The nervous system is designed for very quick communication and uses electrochemical impulses to transmit information through nervous tissue. The endocrine system performs relatively more slowly, and uses chemical hormones, which usually circulate in the blood, to convey messages from a point of origin to a group of target cells. Together, the two systems act to coordinate and regulate the activities of the body as a whole, including the major functions of receiving, processing, and reacting to sensory stimuli, as well as the maintenance of homeostasis.