Neuroscience in Medicine

4 Synaptic Transmission (p. 95-96)

Henrique von Gersdorff

OVERVIEW

The nervous system is composed of specialized cellular circuits that allow an animal to perform tasks essential for survival. Neurons are organized to form these circuits, and they transmit electrical and chemical signals among themselves to process sensory input, initiate behavioral responses, and regulate an animal's internal physiology. The critical link between neurons that permits communication and establishes the foundation for neuronal circuitry is called the synapse, and this chapter will discuss fundamental synaptic properties.

Synapses are sites of close cellular contact where fast, highly localized transmission of chemical and electrical signals can occur. The human brain has approximately 1011neurons that form about 1015 synapses. By comparison, the simple nematode worm C. elegans has exactly 320 neurons with only about 7600 synapses. The capacity of the human brain to form such an astronomical number of synapses has surely contributed to the success of our species and its vast repertoire of behaviors. In order to understand how synapses confer such complexity of neuronal circuitry, it is important to explore the details of information transfer at the synapse.

The process of communication between neurons, termed synaptic transmission, is also key to developing better medical treatments of neurologic conditions for several reasons. The causes of severalmental disorders and neuromuscular diseases can be traced to dysfunctional synapses. Synapses are also the locus of action for several neurotoxins and psychoactive drugs (some of which can cause debilitating and life-long addictions).

Finally, determining how synapses transmit signals and how neuronal circuits are remodeled and modulated at the synaptic level will eventually allow us to understand the basis of neuronal learning and memory.

Synapses vary widely in shape, size, and functional capability. Presumably, such architectural and functional diversities are tailored for the specialized information transfer and processing needs of individual neurons and circuits. For example, many synapses function as high-fidelity relay stations. The connection between motor neurons and muscle fibers (termed the neuromuscular junction), the giant synapses in the mammalian and aviary auditory systems involved in sound localization, and the squid giant synapse, which allows a rapid escape behavior, are all examples of high-fidelity relays. These are synapses where reliability is at a premium, and the synaptic architecture is designed as a fail-safe mechanism for information transfer.

Other synapses, such as the bouton-type synapses of the cortex and hippocampus, often fail to transmit signals and are thus considered comparatively unreliable. These bouton synapses, however, have the capacity to become more fail-safe with repetitive use. This type of change in synaptic strength is an example of plasticity and is thought to underlie the long-lasting storage of information acquired through repetitive use of an associated neuronal circuit. In other words, the specific strengthening of a particular set of synaptic connections may form the basis for some types of learning and memory. Equally important may be the weakening of synaptic connections, a process that could either cause the loss of certain synaptic memory or endow the freedom for retasking a particular neuronal circuit. Thus, synapses must be considered as highly dynamic and plastic structures that can adapt their output to match the demands imposed by their current information processing needs.