Chemicals that carry messages between cells. The first cell releases small bubbles filled with neurotransmitters into the space between itself and its neighbor. This space is called a synapse. The molecules cross the gap. Then, they bind to docking-station molecules called receptors. These receptors then release the signal into the receiving cell.
In a sentence
Every time a nerve needs to make a muscle move, it releases a burst of neurotransmitters.
neuron The impulse-conducting cells that make up the brain, spinal column and nervous system.
neurotransmitter A chemical released at the end of a neuron to carry a message to a neighboring cell. This chemical travels across the space between two cells, and then binds to molecules on a neighboring cell to transmit a message. Neurotransmitters are released from neurons, and can bind to neurons or to other types of cell, including those that make up muscles or glands.
receptor (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.
synapse The junction between neurons that transmits chemical and electrical signals.