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Q23 - Nerve Conduction and Volume Conductor

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Nervous System Physiology

When a battery is placed in an aqueous solution containing salts similar to those found in human body fluids, charged particles flow from the anode to the cathode. The location of the anode from which the electrical current flows is called the “source” (starting point), and the location of the cathode into which it flows is called the “sink” (end point, or absorber).

The same principle can be used to measure the extracellular current generated when cells are electrically excited. When excitation is initiated locally in a resting cell, it generates a movement of ions at the site of the excited membrane. This is true of nerve fibres.

Note: the locations of the source and sink change rapidly, i.e. a phenomenon occurring in milliseconds during the conduction of action potentials. When electrodes are placed along a nerve fibre (positive closer to the cell body than negative), the excitation generated in the cell body will be observed as a signal. This signal can be recorded as a graph.

Figure 1: Diagrams showing (A) battery source/sink in salt solution, (B) electrode placement on nerve fibre, and (C) the recorded signal waveform

True or False?

Q23.1. In a resting cell, an excitation initiated locally causes an influx of Na+ to the cell.
Q23.2. In the repolarization phase of an action potential, an efflux of Na+ is observed.
Q23.3. When the resting region of the cell acts as the source, the excited region acts as the sink.
Q23.4. Figure 1 C presents a signal registered by the experimental setup shown in Figure 1 B.
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Question reproduced from IBO 2023, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF