Polarity reversal of the optical rotation signals with change in direction of impulse conduction along the lobster nerve.

1. The optical rotation signal of nerve associated with excitation was recorded from peripheral nerve taken from a walking leg of a spiny lobster and its properties were analysed. 2. The polarity of the optical rotation signal was reversed when the site of stimulation was changed with reference to the site of optical recording, so that the direction of impulse conduction was reversed, in most of the preparations. 3. Apart from the main response, which is associated with the conducted impulse, a pre-response was found to exist, which manifested itself on anodic stimulation, in a tetrodotoxin-treated nerve, or during the refractory period of the nerve, when the site of stimulation was close to the site of optical recording. The polarity of the pre-response was also reversed when the site of stimulation was changed with reference to the site of optical recording. 4. When the nerve was inclined from the horizontal level, so that the angle of incidence of light to the nerve was changed, the main response changed its amplitude and sometimes its polarity, whereas the pre-response remained practically unchanged. Thus the dependence on the angle of incidence was different between the pre-response and the main response. 5. It is suggested that the dependence of amplitude and polarity of the main response on the angle of incidence of light cannot be explained by the change in molecular axes of the membrane macromolecules, but can only be explained by their conformational change; and therefore the main response can be used as a monitor for the molecular conformation.