The neuron is the fundamental unit of the nervous system. Its major characteristic is the ability to communicate within the nervous system, with other parts of the body, and with the environment. With billions of multisynaptic connections, the nerve cells serve higher mental functions that include memory, thinking, reasoning, calculation, and language. Neuroglial cells, which support and protect nerve cells, are important in tissue repair and participate in phagocytizing cellular debris. Nerve cells communicate with one another through nerve impulses that represent all neuronal activity. The nerve impulses have a chemical component that underlies the electric potential of the cells. A neurotransmitter is a chemical substance release at a synapse that transmits signals across neurons. There are two types of transmitters in the nervous system: small molecules and large molecules (peptides). Small-molecules neurotransmitters include acetylcholine, dopamine, norepinephrine, serotonin, glutamate, and GABA. They are known to have short-lasting effects. Large-molecule peptides produce long-lasting effects on postsynaptic nerve cells.