Galen and hippocrates believed in the 4 humours that were responsible for particular personalities: Blood: causes the sanguine or cheerful temperament. Black bile: causes the melancholic or depressed temperament. Yellow bile: causes the choleric or angry temperament. Phlegm: causes the calm or phlegmatic temperament. Biochemical substance that is involved in the communication among neurons. One neuron transmits a message to another neuron. The electrical impulse travels through the axon terminal and cause the neurotransmitter substance to be released in the synapse. The neurotransmitters are absorbed by the dendrite of the other neuron. Electrical impulse travels along the axon towards the axon terminal. Depending on which neurotransmitter is involved, the molecules can either encourage the neuron to send the electrical impulse to the next neuron or discourage it from sending the message. Central nervous system is composed of the brain and spinal cord. The three neurotransmitters active in this system are dopamine, serotonin. Facilitates the transmission of signals of reward.