Craig C. Freudenrich
When two electrical wires meet, they are connected with solder; the electrical current passes directly from one wire to the other. A nerve cell also sends a signal to another nerve cell but does so indirectly. The cells come extremely close to each other in what is called a synapse. They do not touch, but are separated by a tiny gap called a synaptic cleft, which is about 12 to 25 nanometers wide; that's about 4,000 to 8,000 times smaller than the width of a human hair. One cell, called a presynaptic cell, sends a chemical signal to the other (postsynaptic cell) across the gap. This process is called synaptic transmission and goes something like this:
1. An electrochemical signal (action potential) travels down the presynaptic cell to the synapse.
2. At the terminal end, called a bouton, are packages inside the cell filled with the chemical message called a neurotransmitter (a presynaptic cell uses only one type of neurotransmitter).
3. When the action potential reaches the bouton, it depolarizes the bouton membrane, which allows sodium ions and calcium ions to enter the bouton from the outside through channels in the membrane.
4. Calcium ions cause the packages to fuse with the membrane and dump the neurotransmitter into the synaptic cleft.
5. The neurotransmitter travels across the synaptic cleft.
6. The neurotransmitter binds to specific proteins called receptors on the membrane of the postsynaptic cell.
7. Excitatory neurotransmitters affect channels on the postsynaptic membrane. It the transmitter is excitatory, it opens sodium channels, which causes an action potential in the postsynaptic cell. If the transmitter is inhibitory, it opens potassium channels, which prevents an action potential from forming in the postsynaptic cell.
8. Any unbound neurotransmitters in the gap must be removed so that
the synapse can reset to transmit later messages. It can diffuse, be taken up by the presynaptic cell or be destroyed by enzymes within the cleft, depending on the type of synapse.
Synaptic transmission is entirely one way. Different neurons have different neurotransmitters; some are excitatory and others are inhibitory.
Many drugs, such as antidepressants, local anesthetics and poisons like puffer fish toxin, curarine and snake venom, work by altering synaptic transmission. Some block the postsynaptic receptors, some bind to a neurotransmitter and others prevent unused neurotransmitters from being destroyed.
Nerve cells connect to each other in what science calls "neural pathways," circuits that are much like wires in your house's electrical system. The nerve cells don't actually touch; they just come close together at synapses. At the synapse, a small gap, called a synaptic cleft, separates any two nerve cells, which use neurotransmitters to send messages from a presynaptic cell (the neuron that sends the message) to a postsynaptic cell (the neuron that receives it). This process is called synaptic transmission.
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