Recovery cycle of the Hoffmann reflex of tobacco smokers and nonsmokers: relationship to plasma nicotine and cotinine levels.

Ten normal adult tobacco smokers and 10 nonsmoking volunteers 20-31 years of age were the subjects of this study. The tobacco smokers all had a history of daily tobacco smoking. They were asked to stop smoking for 12 hours prior to testing. The Hoffmann (H) reflex and its recovery cycle were measured on different days before and just after smoking one nonfiltered 0 mg, low (0.27 mg), or high (2.16 mg) nicotine containing cigarette in a randomized order. Blood samples were drawn immediately after the H reflex recordings in the tobacco smokers. The blood samples were centrifuged, the plasma removed, frozen, and later assayed for nicotine and cotinine levels. Nonsmokers compared to tobacco smokers before smoking only had a tendency for enhanced amplitude of the recovery cycle. After smoking the nicotine containing cigarettes, the tobacco smokers had a depression of the amplitude of the H reflex recovery cycle. The amplitude of the H reflex recovery cycle at 160 ms was reduced. This decreased significantly with increasing plasma nicotine and cotinine concentrations. Individual differences were marked. The data obtained are consistent with evidence in animals that nicotine and tobacco smoke stimulate Renshaw inhibitory neurons in the spinal cord, either directly or indirectly which leads to a skeletal muscle relaxant effect.