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NERVOUS SYSTEM

Action at the Synapse

When a nerve impulse gets to the axon terminal, it triggers the release of a special neurotransmitter chemical from tiny synaptic vesicles into the synaptic cleft. As "packets" of neurotransmitter spread throughout the synaptic cleft, some transmitter molecules come into contact with receptor molecules in the postsynaptic membrane of the next neuron and merge into a transmitter-receptor complex. This complex then changes its shape and permits an inflow of sodium ions in some neurons and an outflow of potassium ions in others. More ions now flow through the postsynaptic membrane, causing a voltage change. When sodium-ion permeability is increased, incoming positively charged sodium ions depolarize the neuron by raising its internal voltage. By contrast, when potassium-ion permeability is increased, outgoing potassium ions hyperpolarize the neuron by lowering its internal voltage. These chemically induced voltage changes are called postsynaptic potentials (PSPs). The positive-going ones are excitatory postsynaptic potentials (EPSPs), and the negative-going ones are inhibitory postsynaptic potentials (IPSPs). EPSPs can produce action potentials. IPSPs oppose their production. Simple addition allows information to pass from one neuron to another. If an EPSP occurs just before a prior one had died away, it simply adds to the tail of the earlier one. By contrast, IPSPs subtract from the sum of EPSPs.

A typical neuron might receive both kinds of PSPs from a thousand other neurons, all of which are producing about ten action potentials a second. A neuron's voltage at any given moment, therefore, reflects all the summation activities of a thousand inputs. As the inputs arrive they are rapidly added to or subtracted from the total neuron voltage. If enough EPSPs overcome the IPSPs, the neuron fires an impulse. If IPSPs predominate, it does not. What would normally happen if a depolarizing stimulus acted on a neuron continuously? After the first AP was generated a brief pause would ensue during the refractory period as the ion channels and other nerve-cell operations returned to normal. Then a second AP would form, then another pause, then a third AP, and so on, as long as the stimulus was there. A steady but spaced train of APs would run at a frequency of so many per second, speeding up or slowing down as the intensity of the above-threshold stimulus increased or decreased. This frequency constitutes the code by which commands are sent throughout the body.

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