Life Without Chemistry

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Chemical Messengers
Action potentials are transmitted along axons to specialised regions called synapses, where the axons contact the dendrites of other neurons. These consist of a presynaptic nerve ending, separated by a small gap from the postsynaptic component which is often located on a dendritic spine. The electrical currents responsible for the propagation of the action potential along axons cannot bridge the synaptic gap. Transmission across this gap is accomplished by chemical messengers called neurotransmitters. around the synaptic cleft. Some of these have miniature vacuum cleaners at the ready, called transporters, whose job is to suck up the transmitter in the cleft. This clears the chemical messengers out of the way before the next action potential comes. But nothing is wasted - these glial cells then process the transmitter and send it back to be stored in the storage vesicles of the nerve endings for future use. Glial-cell housekeeping is not the only means by which neurotransmitters are cleared from the synapse. Sometimes the nerve cells pump the transmitter molecules back directly into their nerve endings. In other cases, the transmitter is broken down by other chemicals in the synaptic cleft.

Messengers that open ion channels
The interaction of neurotransmitters with receptors resembles that of a lock and key. The attachment of the transmitter (the key) to the receptors (the lock) generally causes the opening of an ion channel; these receptors are called ionotropic receptors (see Figure). If the ion channel allows positive ions (Na+ or Ca++) to enter, the inflow of positive current leads to excitation. This produces a swing in the membrane potential called an excitatory post-synaptic potential (epsp). Typically, a large number of synapses converge on a neuron and, at any one moment, some are active and some are not. If the sum of these epsps reaches the threshold for firing an impulse, a new action...