1. Autocrine signaling: Cells release signaling molecules that bind to receptors on the same cell or neighboring cells of the same type.
2. Paracrine signaling: Cells release signaling molecules that bind to receptors on neighboring cells of a different type.
3. Endocrine signaling: Cells release signaling molecules (hormones) into the bloodstream to be carried to distant target cells that have specific receptors for the hormone.
4. Synaptic signaling: Neurons release neurotransmitters at synapses to communicate with neighboring cells (such as other neurons, muscle cells, or gland cells).