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Page 144

		2.2.8— The Beating of Heart Cells

		Glass et al. maintained chick heart cells alive in culture. They measured the electrical voltages produced by these cells. Since the voltage changes when the cells beat, they could determine the timing between beats. They also electrically stimulated the cells to beat.

		Even when these cells are not stimulated, they beat spontaneously.

		When they electrically stimulated the cells at a fast rate, then the cells could not recycle fast enough to beat in response to each stimulation. However, the cells did beat in response to every other stimulation.

		When they electrically stimulated the cells at a slower rate, then the cells beat in response to each stimulation.

		When they electrically stimulated the cells at an even slower rate, then there was enough time for the cells to beat between the stimulations. In the time during which 2 stimulations were given, the cells beat 3 times.

		The number of stimulations was coupled to the number of beats in the integer ratios of 2:1, 1:1, and 2:3. Such an integer coupling that depends on the stimulus frequency is a hallmark of a nonlinear response. Linear systems may respond with different phase lags when stimulated at different frequencies, but the type of response, here the number of beats, does not vary with frequency.

 

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