1. By means of light-beating spectroscopy, a four-fold increase in the modulation of laser light scattered at right angles by a locust (Schistocerca gregaria, Forskal) ganglion is detected when potassium ions replace sodium ions in the Ringer solution. 2. This is interpreted as an increased level of motion of the scattering particles (size 0.3-3.0 mum, if viscosity is taken to be 0.01 poises). 3. The amplitude of the potassium-response is similar at all frequencies in the range 6.3-150 Hz and is reversible on return to normal (Na+) Ringer. 4. Desheathing the ganglion reduces the half-time of the potassium-response by 3–4 times. 5. By means of photon-correlation spectroscopy it was estimated that less than 10% of the tissue contributes to the motion detected. 6. Cyanide (1–2 mM) typically enhances the potassium-response and renders it irreversible, suggesting that the response is thermally rather than metabolically-driven. In addition, the dependence of both the correlation function and the power spectrum on the scattering angle is in the direction predicted for a diffusive process. 7. Cobaltous ions (2–10 mM), which block calcium entry into nerve cells, depress the potassium-response. 8. It is proposed that potassium-depolarization and the resultant calcium entry into the cells causes a partial liquefaction of the cytoplasms which is detected as an increase in the level of Brownian motion. This mechanism could be used in vesicular release or in growth.

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