• transmission and reception of information by a signal
• It requires a sender, a signal, a medium (through which the signal is transmitted), and a reciever

Homeostasis

• depends on the ability of every cell to send and recieve communications
• all cells transmit and recieve signals that help maintain the body in good health
• problems with communication can cause disease

• another word for chemical signals because the signals bind to receptors
• ligare= to bind

• Proteins that change the activity of the cell when bound by the chemical signal
• cells to which signals bind

• released by body cells and travel through the bloodstream (the medium) to act on distant cells.
• endocrine secretions- released by endocrines
• neurohormones- released by nerve cells

Neurotransmitters

• special class of paracrine factors
• released by neurons and travel a very short distance across a narrow intercellular space to another neuron or an effector cell to induce an electrical signal
• enduces electrical signals

• a small molecule that transmits a cell surface signal to sites of action in the cytoplasm or nucleus
• secret agents that carry coded messages from outsiders who cannot penetrate the cell's border security

• a liquid that mimics the effect of an endogenous ligand. 

• bind to receptor and block the binding site, preventing the endogenous ligand from binding
• bind to related receptor and decrease the effect of the endogenous ligane even if the endogenous ligand binds to its receptor
• don't turn off the receptor; they just prevent it from turning on or limit its effect.