This is the part of the acquisition system that actually interacts with the brain. Tetrodes are a specific type of electrode, made up of four wires that are twisted and heated to seal them together. Tetrodes are usually held in place in a tetrode drive such as the Shuttle Drive, which allows them to be lowered into the brain to target the cells of interest.
The ends of each wire are not insulated, and pick up changes in electric field strength, caused by movement of charges such as during an action potential or synaptic input (see: extracellular recordings). These signals travel through the electrode, via the EIBs (Electrode Interface Boards & Adapters )to the headstage, where they are amplified and digitized before being sent to the acquisition board. For clarity, only one tetrode is shown here, but the mouse can hold 16 tetrodes (64 channels).
Why use tetrodes?
Each tetrode has four conducting tips, which are all at slightly different places in the tissue. If a nearby neuron spikes, this action potential is likely to be picked up by multiple channels, each at a different distance to the cell. Having the same spike recorded on 4 channels gives us a 4-dimensional ‘description’ of a spike, making it easier to distinguish spikes from different individual cells, increasing the number of single units we can record from in one go.
Tetrode tips are electroplated, often with gold, which stacks on to the tips and greatly increases their surface area (see insert on right, from S.A. Desai et al. Front. Neuroeng., 06 May 2010). This plating reduces tip resistance and results in less voltage attenuation (and precious signal lost) across the electrode, before it reaches the amplifier in the headstage.
At the other end of the tetrode, the four wires are separated out into different holes (vias) on the EIB (electrode interface board), and held in place using gold pins (and a layer of epoxy once you’ve gold-plated!). The signal from each via is routed to a different pin in the connector, which links to the headstage.
When the assembly of the shuttledrive is complete, you’ve gold-plated and you’re sure you won’t have to replace tetrodes, you can attach the EIB to the shuttleDrive body. Gently twist the EIB when you place it on so that all the tetrode wires are safely inside the drive body. Make sure the EIB is glued on well to the drive body.