Nerve Cells Make an Allor None Decision to Generate an Action Potential

At the neuromuscular junction, virtually every action potential in the presynaptic motor neuron triggers an action potential in the postsynaptic muscle cell. The situation at synapses between neurons, especially those in the brain, is much more complex because the postsynaptic neuron commonly receives signals from many presynaptic neurons (Figure 7-47). The neuro-transmitters released from presynaptic neurons may bind to an excitatory receptor on the postsynaptic neuron, thereby opening a channel that admits Na+ ions or both Na+ and K+ ions. The acetylcholine receptor just discussed is one of many excitatory receptors, and opening of such ion channels leads to depolarization of the postsynaptic plasma membrane, promoting generation of an action potential. In contrast, binding of a neurotransmitter to an inhibitory receptor on the postsynaptic cell causes opening of K+ or CP channels, leading to an efflux of additional K+ ions from the cytosol or an influx of Cl-ions. In either case, the ion flow tends to hyperpolarize the plasma membrane, which inhibits generation of an action potential in the postsynaptic cell.

A single neuron can be affected simultaneously by signals received at multiple excitatory and inhibitory synapses. The neuron continuously integrates these signals and determines whether or not to generate an action potential. In this process, the various small depolarizations and hyperpolar-izations generated at synapses move along the plasma membrane from the dendrites to the cell body and then to the axon hillock, where they are summed together. An action potential is generated whenever the membrane at the axon hillock becomes depolarized to a certain voltage called the threshold potential (Figure 7-48). Thus an action potential

▲ EXPERIMENTAL FIGURE 7-47 A fluorescent micrograph of two interneurons reveals that many other neurons synapse with them. These cells, from the hippocampal region of the brain, were stained with two fluorescent antibodies: one specific for the microtubule-associated protein MAP2 (green), which is found only in dendrites and cell bodies, and the other specific for synaptotagmin (orange-red), a protein found in presynaptic axon terminals. The numerous orange-red dots, which represent presynaptic axon terminals from neurons that are not visible in this field, indicate that these interneurons receive signals from many other cells. [Courtesy of O. Mundigl and P deCamilli.]

is generated in an all-or-nothing fashion: Depolarization to the threshold always leads to an action potential, whereas any depolarization that does not reach the threshold potential never induces it.

Direction of action potential

Dendrite

Cell body

Direction of action potential

Dendrite

Cell body

Axon hillock

Postsynaptic cell

Axon hillock

Postsynaptic cell

Electrode to measure electric potential

Membrane potential in the postsynaptic cell

Threshold potential

Action potential

Membrane potential in the postsynaptic cell

Threshold potential

Action potential

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