Neurons send what are known as electrochemical signals. Once a neuron has been stimulated by some sort of stimulus, it generates an electric potential that travels down the length of the cell. This is the 'electro' part of electrochemical. Once the electric current reaches the axon terminal at the end of the cell, it triggers the release of certain chemical messengers. This is the 'chemical' part of electrochemical.
The primary class of signaling molecules are called neurotransmitters. These chemical messengers allow one neuron to communicate to another, and the response these messages generate depend on factors such as what specific type of messenger was sent, how much of it was sent, how long the message lasted, etc. Between the part of the first neuron that is sending the signal, the axon, and the second neuron that is receiving the signal, the dendrite, there exists a minute gap known as the synapse. Released neurotransmitters must cross this synapse in order to reach their specific receptors on the other side, and then are recycled or broken down after achieving their desired effects.
Common neurotransmitters include:
Acetylcholine, which is heavily involved in communicating to muscles, but also plays roles in attention and arousal.
Dopamine, which has functions including voluntary motor movements, mood, and goal-oriented behavior.
Serotonin, which has effects ranging from emotion to sleep
Glutamate, which is the major excitatory neurotransmitter of the brain and is involved in learning
GABA, which is the major inhibitory neurotransmitter of the brain and is involved in numerous functions
In addition, hormones may have profound interactions with the nervous system. Examples include adrenaline, which controls responses to acute environmental stress, and melatonin which establishes biological rhythms and sleep patterns.
Neurons begin at a resting potential of around -70 milivolts, during which no signals will be transmitted
An adjacent neuron sends neurotransmitters that bind to the dendrites of the neuron
If these neurotransmitters are excitatory, then gated ion channels will open up to permit ambient sodium ions (Na+) into the cell (making the potential more positive). If these neurotransmitters are inhibitory, then gated ion channels will open up to permit internal potassium ions (K+) out of the cell (making the potential more negative)
For an action potential to be generated, enough excitatory neurotransmitters must be present to raise the charge above a certain threshold level. Once this level is exceeded, an all-or-none electrical potential is sent down the length of the axon (depolarization)
After depolarizing, the neuron's ion pumps begin working to reestablish the resting potential. While the neuron is doing so, it enters a refractory period during which no further action potentials can be generated. (hyperpolarization)
Once the potential has been restored, the neuron is ready for a new stimulus