In some ways, neurons act like computers. That is, they receive messages, process those messages, and send out the results as new messages to other cells. In the case of neurons, the message consists of chemicals that interact with the outer surface of the cell membrane. This chemical interaction with the cell membrane causes chemical changes within the receiving neuron. Once inside, those chemicals can be processed-broken down or changed into other chemicals. Chemicals can also leave the cell and function as the cell's output.
One name for the constant exchange of chemical messages between neurons is neurotransmission. Neurotransmission involves three basic steps. In this section you'll find an overview of each step.
1. Neurons release neurotransmitters
A resting neuron has a negative charge. That is, there are more negative ions inside the axon than outside the axon. (Ions are molecules with an electric charge.) In contrast, the fluid outside the axon has a positive charge. Because the outside and inside of the axon have different charges, the axon is said to be polarized.
When a neuron is excited or fires, several events take place to create an electrical impulse. Sodium ions, which have a positive charge, enter the axon. This depolarizes the axon-that is, changes the electrical charge inside the axon from negative to positive. This change starts at one end of the axon and continues all the way to the other end. In response to this electrical impulse (called an action potential), the vesicles swarm to the very edge of the axon and release neurotransmitters into the synapse.
After the neurotransmitters are released, potassium ions flow out of the axon. Potassium ions have a positive charge, so their absence restores the negative charge inside the axon. The neuron is again polarized and at rest, waiting to fire another impulse.
2. Neurotransmitters bind to receptors
Neurotransmitters float across the synapse until they hit the dendrites of the next neuron. On each dendrite, neurotransmitters find molecules that are set to receive them. These molecules are called receptors. Neurotransmitters recognize specific receptors and "grab"" on to them, a process called binding. (The neuron that originally released the neurotransmitter is the "sending" neuron; the neuron that binds the neurotransmitter is the "receiving" neuron.)
Each receptor accepts only certain neurotransmitters, much like a lock accepts only a certain key. After binding is done, receptors let go of the neurotransmitters. At that point, several things can happen. Some neurotransmitters are destroyed by enzymes. In other cases, proteins transport neurotransmitters back to the axon from which they originally came, a process called reuptake. Reuptake allows neurotransmitters to use the same neurotransmitters over again-a kind of "recycling."
Binding causes a set of chemical reactions within the receiving neuron. Those reactions start up the same kind of impulse that was fired in the sending neuron. In this way, the original impulse is conducted through the sending neuron-and through the rest of the neurons in a nerve pathway. Eventually, the impulse reaches its final destination, such as a muscle, gland, or organ. The result is a change in the way we think, feel, or behave.
Binding passes on the message.
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Schematic animation 1.4 MB|
Neurotransmission 3.2 MB
The chemical reactions inside the receiving neuron are called second messengers. Second messengers pass along the original message from the neurotransmitter. In fact, neurotransmitters are sometimes called first messengers.
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