WHAT GIVES NERVE CELLS THEIR SPECIAL ABILITY to communicate with one another rapidly and with such great precision? We have already seen how signals are propagated within a neuron, from its dendrites and cell body to its axonal terminals. With this chapter, we begin to consider the signaling between neurons through the process of synaptic transmission. Synaptic transmission is fundamental to the neural functions we consider in this book, such as perception, voluntary movement, and learning.
Neurons communicate with one another at a specialized site called a synapse. The average neuron forms several thousand synaptic connections and receives a similar number of inputs. However, this number can vary widely depending on the particular type of neuron. Whereas the Purkinje cell of the cerebellum receives up to 100,000 synaptic inputs, the neighboring granule neurons, the most numerous class of neurons in the brain, receive only around four excitatory inputs. Although many of the synaptic connections in the central and peripheral nervous systems are highly specialized, all neurons make use of one of the two basic forms of synaptic transmission: electrical or chemical. Moreover, the strength of both forms of synaptic transmission is not fixed, but can be enhanced or diminished by neuronal activity. This synaptic plasticity is crucial for memory and for other higher brain functions.
Electrical synapses are employed primarily to send rapid and stereotyped depolarizing signals. In contrast, chemical synapses are capable of more variable signaling and thus can produce more complex interactions. They can produce either excitatory or inhibitory actions in postsynaptic cells and initiate changes in the postsynaptic cell that last from milliseconds to hours. Chemical synapses also serve to amplify neuronal signals, so even a small presynaptic nerve terminal can alter the response of large postsynaptic cells. Because chemical synaptic transmission is so central to understanding brain and behavior, it is examined in detail in the next four chapters.
Synapses Are Predominantly Electrical or Chemical
The term synapse was introduced at the beginning of the 20th century by Charles Sherrington to describe the specialized zone of contact at which one neuron communicates with another. This site had first been described histologically at the level of light microscopy by Ramón y Cajal in the late 19th century.
All synapses were initially thought to operate by means of electrical transmission. In the 1920s, however, Otto Loewi discovered that a chemical compound, most likely acetylcholine (ACh), conveys signals from the vagus nerve to slow the beating heart. Loewi’s discovery provoked considerable debate in the 1930s over whether chemical signaling existed at the fast synapses between motor nerve and skeletal muscle as well as synapses in the brain.
Two schools of thought emerged, one physiological and the other pharmacological. Each championed a single mechanism for all synaptic transmission. Led by John Eccles (Sherrington’s student), the physiologists argued that synaptic ...