Action potentials in a giant algal cell: a comparative approach to mechanisms and evolution of excitability

Abstract

The giant alga Chara corallina generates action potentials (APs) in response to mechanical stimulation, injury, or direct electrical stimulation. Students examine the waveform characteristics of these APs using standard intracellular recording techniques. Intracellular recording is easier than with neurons because of the large size of the Chara cell. Students observe very negative resting potentials (up to -250 mV), large AP amplitudes with depolarizing peaks approaching 0 mV, AP durations of seconds, and refractory periods up to several minutes. Students calculate Nernst potentials for the ions distributed across the Chara cell membrane to hypothesize the ions responsible for the resting potential and for the depolarizing phase of the AP. These calculations suggest that K(+) is responsible for the resting potential and that Ca(2+) influx and Ca(2+)-activated Cl(-) efflux are responsible for depolarizing phases of the AP, which they are. Comparison of the Chara AP characteristics with animal neuron and muscle APs reinforces understanding of mechanisms of excitability in animals, demonstrates that multiple solutions exist for action potential generation, and leads to discussion of the evolution of ion channels and excitability.

Keywords: Chara; action potential; evolution; excitability.

Figure 1

Strands of Chara corallina. Each strand consists of large cells connected at nodes. The marked cell is approximately 7 cm long.

Figure 2

Cutaway diagram of a Chara cell. Organelles, mainly nuclei, circulate around the vacuole by cytoplasmic streaming. The vacuole is actually larger, and the cytoplasm and chloroplast layer much thinner than shown.

Figure 3

Recording chamber. The two outer wells are for the stimulating wires; the shallower center well is layered with Sylgard and supports the cell for recording. The narrow slits connecting the compartments are filled with Vaseline after the cell is positioned. This prevents the stimulus current from traveling to ground through the saline. The dimensions are only suggested, and not critical.

Figure 4

Repeated stimulation. Note the long relative refractory period between the first two APs. With the fourth stimulus, only a sub-threshold membrane response is seen.

Video 1

Recording and cytoplasmic streaming. This is one frame of a video showing the recording procedure and results. The electrode comes from the top; the arrow indicates a nucleus. See http://crawdad.cornell.edu/chara.video.html for the full video.