To demonstrate another catalog of reasons why the brain-as-digital-computer metaphor breaks down, Adams described three types of electrical activity that distinguish nerve cells and their connections from passive wires. One type of activity, synaptic activity, occurs at the connections between neurons. A second type, called the action potential, represents a discrete logical pulse that travels in a kind of chain reaction along a linear path down the cell's axon (see Box 9.1). The third type of activity, subthreshold activity, provides the links between the other two.
Each type of electrical activity is caused by "special protein molecules, called ion channels, scattered throughout the membrane of the nerve cells," said Adams. These molecules act as a sort of tiny molecular faucet, he explained, which, when turned on, "allows a stream of ions to enter or leave the cell (Figure 9.2). The molecular faucet causing the action potential is a sodium channel. When this channel is open, sodium streams into the cell," making its voltage positive. This voltage change causes sodium faucets further down the line to open in turn, leading to a positive voltage pulse that travels along the neuron's axon to the synapses that represent that cell's connections to other neurons in the brain. This view of "the action potential," he said, "was established 30 years ago and is the best-founded theory in neurobiology."
The other types of activity, synaptic and subthreshold, "are more complex and less well understood," he said, but it is clear that they, too, involve ion channels. These ion channels are of two types, which either allow calcium to stream into the cell or allow potassium to leave it. Adams has explored the mechanisms in both types of channel. Calcium entering a neuron can trigger a variety of important effects, causing chemical transmitters to be released to the synapses, or triggering potassium channels to open that initiate other subthreshold activity, or producing the long-term changes at synapses that underlie memory and learning.
The subcellular processes that govern whether—and how rapidly—a neuron will fire occur on a time scale of milliseconds but can be visualized using a scanning laser microscope. Activated by additional channels that allow double-charged calcium ions (Ca++) to rapidly enter the cell are two types of potassium channels. One of these channels quickly terminates the sodium inrush that is fueling the action potential. The other one responds much more slowly, making it more difficult for the cell to fire spikes in quick succession. Yet other types of potassium channels are triggered to open, not by calcium, but rather in response to subthreshold voltage changes. Called "M" and "D," these channels can either delay or temporarily prevent