Extracellular Single-Unit Recording Methods*

Since their refinement in the early 195Os, extracellular, singleunit recording methods have been used to obtain a wealth of data about the properties of CNS structures. The applications of the technique have been diverse: extracellular microelectrodes have been used to map the potential fields of single discharging neurons in order to answer fundamental questions about the excitability of CNS dendrites (Frank and Fuortes, 1955; Fatt, 1957; Nelson and Frank, 1964), and more recently they have been used to study the behaviorally related discharge patterns of CNS neurons in the awake, moving animal (e.g., Evarts, 1968; Mountcastle et al., 1975). To an appreciable extent, the exciting new neuroanatomical tracing methods that have been developed over the past decade (cf Jones and Wise, 1977) have supplanted extracellular recording methods as a technique for tracing CNS connectivity patterns. However, the single-unit recording method is the technique of choice for studies of the responses of central neurons to sensory stimuli and of their behaviorally related firing patterns in the alert, moving animal. Moreover, many applications remain for the tracing of functional network connections on a microanatomical scale. It is the purpose of this chapter, therefore, to outline certain basic principles concerning the extracellular potential fields of single, active neurons, and the techniques for recording their action potentials extracellularly. It is hoped that this material will be useful to investigators who are currently using this technique, or who plan to use it, within their own laboratories. Some of the material that is presented here is new, but much of it is similar to that found in a number of previous excellent discussions of the