How do cells communicate?

Cells communicate via four types of chemical signaling pathways. The main difference between the four categories of cellular signaling is the distance that the signal travels to reach the target cell. 

Autocrine Signaling - Autocrine signaling occurs when a cell responds to its own signaling molecule. This plays a key role in regulating pain sensation and inflammatory responses. Cells also signal themselves to undergo programmed death when they are infected with a virus, killing themselves along with the virus in the process. Autocrine signaling is more common during early development of an organism. It ensures that cells develop into the correct tissues. 

Paracrine Signaling - Paracrine signaling takes place between two cells that are in close proximity to each other but are not connected. In this type of cellular communication, the signaling molecules move across the short distance by diffusion through the extracellular matrix. Paracrine signals generally elicit quick responses that are short-lived.  

Endocrine Signaling - Signals that travel long distances via the bloodstream from the originating cells to the target cells are called endocrine signals. These signals, known as hormones, originate from endocrine cells in endocrine glands such as the pituitary gland, the hypothalamus, and thyroid gland. Because the movement through the bloodstream is relatively slow, endocrine signals produce a slower response. However, the effects are longer lasting. 

Direct Signaling - Direct signaling facilitates a coordinated response by a group of cells to a signal that only one of the cells may have received. This is commonly seen in gap junctions in animals and plasmodesmata in plants, which are small water-filled channels that occur between the plasma membranes of neighboring cells. These channels allow small signaling molecules such as calcium ions to diffuse between the two cells but block larger molecules such as proteins and DNA from passing through. The specificity of the channels preserves the independence of the cells while still allowing them to transmit signals quickly and easily.