Neurons generally exchange messages when the sending neuron releases a chemical (neurotransmitter) which activates receptors in the receiving neuron. In most cases, neurons do not actually touch each other; instead, there is a tiny gap, called a synapse. Neurotransmitter molecules may cross the synapse to activate a receptor on the receiving neuron, or may remain in the synapse unused until it is broken down or reabsorbed into the sending neuron to be recycled for later use.