8: Overview of Synaptic Transmission

Introduction

• Synapses Are Either Electrical or Chemical

• Electrical Synapses Provide Instantaneous Signal Transmission

  • Cells at an Electrical Synapse Are Connected by Gap-unction Channels

  • Electrical Transmission Allows the Rapid and Synchronous Firing of Interconnected Cells

  • Gap Junctions Have a Role in Glial Function and Disease

• Chemical Synapses Can Amplify Signals

  • Neurotransmitters Bind to Postsynaptic Receptors

  • Postsynaptic Receptors Gate Ion Channels Either Directly or Indirectly

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.

The specialized site at which one neuron communicates with another is called a synapse, and synaptic transmission is fundamental to the neural functions we consider later in the book, such as perception, voluntary movement, and learning.

The average neuron forms several thousand synaptic connections and receives a similar number. The Purkinje cell of the cerebellum receives up to 100,000 synaptic inputs. Although many of these connections 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 can be enhanced or diminished by cellular activity. This plasticity of synapses is crucial to memory and 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 behaviors. They can mediate either excitatory or inhibitory actions in postsynaptic cells and produce electrical changes in the postsynaptic cell that last from milliseconds to many minutes. Chemical synapses also serve to amplify neuronal signals, so even a small presynaptic nerve terminal can alter the response of large postsynaptic cells. Not surprisingly, most synapses in the brain are chemical. Because chemical synaptic transmission is so central to understanding brain and behavior, it is examined in detail in the next four chapters.

Synapses Are Either Electrical or Chemical

The term synapse was introduced at the beginning of the twentieth 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 the chemical compound acetylcholine (ACh) conveys signals from the vagus nerve to the heart. Loewi's discovery provoked considerable debate in the 1930s over whether chemical signaling existed at other synapses, including synapses between motor nerve and skeletal muscle as well as synapses in the brain.