Technical Challenges and Advances What Are Tetrodes and Microdrives

Action potentials of single cells are recorded by bringing small-diameter wires close to the soma of neurons (see Figure 37.1). Because the pyramidal cells of the hippocampus are so tightly packed, a single 13-micron diameter electrode will detect action potentials from many neurons. This is good, except that with a single electrode, one cannot differentiate action potentials from individual neurons. To solve this problem, hippocampal researchers twist several electrodes together into a tetrode with four electrodes (Gray et al., 1995; McNaughton et al., 1983). Tetrodes bring four electrode tips to the hippocampal pyramidal cell layer, and therefore triangulation can be used to discriminate between active cells. The amplitudes of the waveforms that the wires record are largest for cells that are nearest to a particular electrode tip. More distant electrodes will have waveforms of smaller amplitude for that cell. This is illustrated in the waveforms of Figure 37.2. The action potentials of each neuron produce their own unique signature of tetrode waveforms. This allows researchers to distinguish between different neurons that are recorded simultaneously. With this technological advance, it is now possible to record simultaneously from ten or more well-isolated neurons from one tetrode.

Another important technological development has been the ability to advance many tetrodes individually down to the cell layers. Because the position of the electrodes must be precisely in the pyramidal cell layer during recordings, the electrodes are not advanced to the hippocampal layers when the rat is anesthetized during surgery. Instead, the tetrodes are implanted to the overlying cortex and slowly advanced (50 microns/day) so that the rat is awake and potentially ready for recordings when the cell layer is found (see Figure 37.1). To do this, each tetrode is attached to its own microdrive, enabling precise adjustments in recording location to be made so that each tetrode will find the ideal layer and stay there for stable recordings over at least several days. Using these demanding techniques, today laboratories are able to record from twelve tetrodes simultaneously, which allows recordings to distinguish between as many as 100 cells at a time. Recording from such a network of activity with stable cells over one week allows investigation of the information processing of neurons during the formation and retrieval of memories.

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Blood Pressure Health

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