The introductory text reads, “Messages travel within a neuron using electrical signals. But communication between neurons depends on the movement of chemicals—neurotransmitters. Though they all work in the same way, there are many different types of neurotransmitters, each linked to unique effects on behavior. However, drugs and other substances, known as agonists and antagonists, can alter this process of communication between neurons by boosting or blocking normal neurotransmitter activity.” The top illustration shows two neurons, with a highly-magnified callout of the end of one neuron and start of another. In the main graphic, a flattened teardrop shape is labeled “Sending neuron” and shows a lightning bolt pointing downward. A callout to the lightning bolt reads, “1. Action potential reaches terminal buds.” The cross-section drawing shows several round shapes filled with colored dots in the flattened end. A callout to the round shapes reads, “2. Action potential triggers vesicles to release neurotransmitters into synaptic gap.” Two of the round shapes open at the bottom of the teardrop, and several colored dots are sprinkled in the space between the neurons (the space is labeled “Synaptic gap.)” Callouts to the colored dots are labeled “Neurotransmitters.” Below the neuron is the bottom of another flattened teardrop shape labeled “Receiving neuron,” with several green donut shapes on the surface, labeled “Receptor sites.” The callout reads, “3. Neurotransmitters bind to their matching receptor sites on receiving neuron’s dendrite, causing positively charged particles to enter cell. Action potential is created.” A callout to blue dots moving upward to the interior of the upper neuron reads, “Excess neurotransmitter being reabsorbed by the sending neuron.” The callout reads, “4. After binding, neurotransmitters are reabsorbed or diffuse out of synaptic gap.” Several lightning bolts are drawn downward through the lower neuron. A callout at the bottom of the lower neuron reads, “Drugs and other substances can alter normal neurotransmission.” An arrow pointing downward and left is labeled “Boost” and leads to a close-up image of a person smoking a cigarette. The lower left graphic shows transmitting and receiving neuron ends. In the transmitting end, several round vesicles are shown containing blue neurotransmitter dots, with free neurotransmitters floating in the synaptic space. In the receiving neuron, some receptor sites are shown with attached blue neurotransmitter dots, with others binding purple dots labeled “Agonists.” Multiple lightning bolts extend from the receptor sites down the neuron. The text reads, “Agonists boost normal neurotransmitter activity. Nicotine mimics acetylcholine and causes this same activation. More receptors are activated, and more messages are sent.” An arrow pointing downward and right is labeled “Block” and leads to an image of a man with a blow dart gun in his mouth. The lower right graphic shows transmitting and receiving neuron ends. In the transmitting end, several round vesicles are shown containing blue neurotransmitter dots, with free neurotransmitters floating in the synaptic space. In the receiving neuron, one receptor site shows an attached blue neurotransmitter dot, with others binding orange flat shapes labeled “Antagonists.” One lightning bolt extend from the receptor sites bound to a neurotransmitter down the neuron. The text reads, “Antagonists block normal neurotransmitter activity. Curare, the paralyzing poison used on blowgun darts, acts as an acetylcholine antagonist. It blocks acetylcholine receptors, preventing the neurotransmitter from activating them, so fewer messages are sent.