Neurotransmitters help the body regulate your heartbeat, blood pressure, breathing, muscles, thoughts, memory, sleep, stress, hormone response, senses, and digestion. These neurotransmitters are composed of three main parts: a cell body, an axon, and an axon terminal. They transmit one of three signals in their messages (excitatory, inhibitory, modulatory). Excitatory neurotransmitters cause the neuron to fire off the message to the next cell (e.g. glutamate, epinephrine, and norepinephrine). Inhibitory neurotransmitters block/stop the message from being passed along to other cells (e.g. GABA, glycine and serotonin). Modulatory neurotransmitters influence other chemical messengers and their effects. They adjust how cells communicate with one another at the synapse. They are also able to affect large amounts of neurons at the same time. Certain neurotransmitters affect an individual's mood, such as norepinephrine, serotonin, and dopamine.