Electrophysiology Methods

Transcript

0:00 – 0:30  [Basic Definition]  Electrophysiology methods include diverse techniques that are used to measure the electrical activity of live neurons.  Three common approaches are used to study electrophysiology: intracellular recordings in which an electrode is placed inside a cell, extracellular recordings in which electrodes measures the activity of nearby cells, and patch clamp recordings, in which an electrode is placed against the cell membrane to measure the activity of individual ion channels.  These approaches allow for the measurement of voltage, current, and resistance, and these approaches can be used in live animals, or in vitro preparations of tissue slices or individual cells.

0:30-2:30 

Voltage-gated channels and synapses are the natural sources of current that change transmembrane potentials and ultimately generate neural activity. However, to understand these processes electrophysiological approaches have been developed, which allow us to manipulate levels of current and voltage using microelectrodes. In current-clamp, microelectrodes inject fixed amounts of current and monitor changes in the transmembrane voltage.

During voltage-clamp, electrodes are instead used to step to fixed voltages, and then monitor the resulting changes in current. Voltage-clamp experiments alternate between a holding potential, which is typically negative to de-inactivate or reset any voltage-gated channels, and a test potential, which is used to probe the activation of voltage-gated channels or postsynaptic receptors.

It’s pretty easy to distinguish which mode you are using, since in current clamp the current values are stable while in voltage-clamp the voltage values are stable. These two approaches allow you to assess the impact of current and voltage on neuronal processing. For instance, in current-clamp you can inject current to examine where threshold is and how neurons would respond synaptic current. In voltage-clamp, you can hold the membrane potential at different voltages and examine the impact on current flowing through voltage-gated channels or post-synaptic receptors.

A more recent method called ‘patch-clamp’ allows you to isolate currents from individual ion channels, by isolating of a ‘patch’ of membrane and performing voltage-clamp. Collectively, these unitary, microscopic currents contribute to the neuron-wide macroscopic currents that ultimately govern neuronal activity.

In addition to recording intracellular electrical signals, we can use extracellular electrodes to monitor electrical activity from the outside. In this case the polarity is reversed, since you are now measuring the drop in voltage caused by sodium flowing in and the increase in voltage caused by potassium flowing out. Extracellular recordings are useful for recording many neurons in deeper parts of the brain, where intracellular recordings are more challenging.

2:30-3:00  [Parallel Vocabulary] In the broadest sense, electrophysiology includes any technique used to measure the electrical activity of cells, for example, EEG measures whole brain activity and EKG measures cardiac activity.  In the context of neuroscience however, the term electrophysiology is generally used in a very precise way to mean the study of electrical activity in individual neurons or muscles.  In this sense, electrophysiology is an umbrella term that includes many approaches such as intracellular recordings, extracellular recordings, patch clamp recordings, voltage clamp recordings and current clamp recordings.

3:00-4:00  [Here’s a real world example]  Electrophysiology has played and continues to play an important role in the study of neurobiology. For example, did you know that more than half a century before the 3D structure of the voltage-gated sodium channel was first visualized, electrophysiology was used to reveal many details about the structure and function of voltage-gated sodium channels, as well as their contribution to the action potential. Alan Hodgkin and Andrew Huxley were awarded the Nobel Prize for experiments in which they used the voltage-clamp method to study neural signals in squid.  By using the voltage-clamp method, Hodgkin and Huxley were able to measure sodium and potassium currents across the neural membrane at a range of voltages.  These experiments revealed many characteristics of a typical action potential such as the contribution of sodium currents to the rising phase, the inactivation of sodium currents, and the contribution of sodium currents to the refractory period. Importantly, the experiments that demonstrated sodium currents, lead to mathematical models that predicted the anatomy and physiology of voltage-gated sodium channels.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) A Guide to Research Techniques in Neuroscience (Carter and Shieh)

• Chapter 4: “Electrophysiology”

2) Neuroscience Exploring the Brain (Bear)

• Chapter 4 “The Action Potential”

Media attributions

Field potential schematic by Synaptidue is licensed under a Creative Commons Attribution Sharealike 3.0 license (CC-by-SA 3.0).

Patch Clamp by Peter Wolber is licensed under a Creative Commons Attribution Sharealike 3.0 Unported license (CC-by-SA 3.0).