In chemical signaling, a cell may target itself (autocrine signaling), a cell connected by gap junctions, a nearby cell (paracrine signaling), or a distant cell (endocrine signaling). Paracrine signaling acts on nearby cells, endocrine signaling uses the circulatory system to transport ligands, and autocrine signaling acts on the signaling cell. Signaling via gap junctions involves signaling molecules moving directly between adjacent cells.

Cells communicate by both inter- and intracellular signaling. Signaling cells secrete ligands that bind to target cells and initiate a chain of events within the target cell. The four categories of signaling in multicellular organisms are autocrine signaling, direct signaling across gap junctions, paracrine signaling, endocrine signaling. Autocrine signals are received by the same cell that sent the signal or other nearby cells of the same kind. Gap junctions allow small molecules, including signaling molecules, to flow between neighboring cells. Paracrine signaling takes place over short distances. Endocrine signals are carried long distances through the bloodstream by hormones.

Internal receptors are found in the cell cytoplasm. Here, they bind molecules that cross the plasma membrane. These complexes move to the nucleus and interact directly with cellular DNA. Cell-surface receptors transmit a signal from outside the cell to the cytoplasm. Ion channel-linked receptors, when bound, form a pore through the plasma membrane through which certain ions can pass. Enzyme-linked receptors transmit a signal from outside the cell to a membrane-bound enzyme. Binding causes activation of the enzyme. Small hydrophobic molecules (like steroids) are able to penetrate the plasma membrane and bind to internal receptors.

Cell signals control metabolism.

1. Autocrine signals involve a cell signaling itself. These signals are very common in embryonic development, during viral infections,  inflammation and pain cascades in humans.
2. Gap junctions are water-filled channels that allow calcium ions [Ca₂+] to pass between cells. The passage is too small for proteins and DNA. This signal is useful for quick and easy transport.
3. Paracrine signals provide a quick response across a distance for nerve cells.  This signal can involve enzymes, as shown below.

The distance between the presynaptic cell and the postsynaptic cell—called the synaptic gap—is very small and allows for rapid diffusion of the neurotransmitter. Enzymes in the synapatic gap degrade some types of neurotransmitters to terminate the signal.

4. Endocrine signals function for distant cells. This includes hormones used by our thyroid and pituitary for a slower but longer lasting signal.