Neurochemistry

None of the billions of nerve cells, or neurons, in the human brain functions alone. To process information, neurons must form circuits and must communicate with each other rapidly and with great precision. Within a neuron, the electrical impulses that carry information are propagated by rapid changes in membrane potential that arise from the controlled opening and closing of ion channels. These pores in the cell membrane permit the controlled passage of positive or negative ions between the interior and exterior of the cell, and thereby the conduction of electrical impulses along the cell's processes. Additional mechanisms are required at synapses (neuron junctions) to pass signals from one neuron to another. Although a few neurons form electrical synapses, where electrical signals are conducted directly from one neuron to the other through specialized ion channels (gap junctions), most neurons in the mature nervous system communicate via chemical synapses. At chemical synapses, electrical activity in a presynaptic neuron causes the release of a chemical messenger, a neurotransmitter, which diffuses across the narrow synaptic cleft to bind to neurotransmitter receptors on the postsynaptic neuron and elicit changes in the electrical activity of that neuron.