The resting potential is the electrical potential difference across the membrane of a neuron when it is not conducting an impulse; the inside is negatively charged relative to the outside (polarised), maintained by the sodium-potassium pump keeping more sodium ions outside and potassium ions inside. The action potential (nerve impulse) is the rapid reversal of this charge when a stimulus is applied: sodium ions rush in and the membrane becomes depolarised (positive inside), and this change travels along the axon as the nerve impulse before the resting state is restored.

A synapse is the junction between the axon terminal of one neuron and the dendrite (or cell body) of the next neuron, with a tiny gap called the synaptic cleft between them. When a nerve impulse reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters (such as acetylcholine) into the synaptic cleft. These diffuse across the gap and bind to receptors on the next neuron, generating a new impulse in it. Thus transmission across a synapse is chemical and one-directional.

Insulin is a hormone secreted by the beta cells of the pancreas; it lowers the level of glucose in the blood by promoting the uptake of glucose by cells and its conversion into glycogen in the liver. A deficiency of insulin causes the disease diabetes mellitus. Adrenaline (epinephrine) is secreted by the adrenal medulla; it is the emergency or fight-or-flight hormone that prepares the body for stress by increasing heart rate, blood pressure, breathing rate and blood glucose level.