Patch voltage clamp of squid axon membrane

Summary

A small area (patch) of the external surface of a squid axon can be “isolated” electrically from the surrounding bath by means of a pair of concentric glass pipettes. The seawater-filled inner pipette makes contact with the axon and constitutes the external access to the patch. The outer pipette is used to direct flowing sucrose solution over the area surrounding the patch of membrane underlying the inner pipette. Typically, sucrose isolated patches remain in good condition (spike amplitude >90 mV) for periods of approximately one half hour. Patches of axon membrane which had previously been exposed to sucrose solution were often excitable. Membrane survival of sucrose treatment apparently arises from an outflow of ions from the axon and perhaps satellite cells into the interstitial cell space surrounding the axolemma. Estimate of the total access resistance (electrode plus series resistance) to the patch is about 100 kΩ (7 Ω cm²). Patch capacitance ranges from 10–100 pF, which suggests areas of 10⁻⁴ to 10⁻⁵ cm² and resting patch resistances of 10–100 MΩ. Shunt resistance through the interstitial space exposed to sucrose solution, which isolates the patch, is typically 1–2 MΩ. These parameters indicate that good potential control and response times can be achieved on a patch. Furthermore, spatial uniformity is demonstrated by measurement of an axoplasmic isopotential during voltage clamp of an axon patch. The method may be useful for other preparations in which limited membrane area is available or in special instances such as in the measurement of membrane conduction noise.