title: Caffeine increases spinal excitability in humans.
abstract: The Hoffman reflex (H reflex) has long been established as a measure of spinal excitability. Caffeine is one of the most widely consumed drugs in the world. Because it is known to increase excitatory neurotransmission, we hypothesized that caffeine would increase spinal excitability and thus alter the H reflex by increasing its amplitude. Seven subjects each attended the laboratory on 2 days. Caffeine (6 mg/kg) was administered on one day and a placebo was administered on the other. The tibial nerve was stimulated at incremental intensities to create an H-reflex recruitment curve prior to capsule administration (pretest) and 1 h later (posttest) on each day. The slope of H-reflex recruitment curve normalized to that of the M wave (H(slp)/M(slp)) was compared (pretest to posttest). Caffeine increased spinal excitability 43 +/- 17% (P < 0.05). Thus, caffeine may be used to safely increase spinal excitability in electrophysiological studies of the human neuromuscular system. Our results also suggest that caffeine intake should be controlled when the H reflex is used in diagnostic and experimental situations.
authors.affiliation: School of Kinesiology and Health Science, Faculty of Pure and Applied Science, York University, Toronto, Ontario, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Walton
authors.forename: J
authors.initials: J
authors.lastname: Kalmar
authors.forename: E
authors.initials: E
authors.lastname: Cafarelli
mesh_terms: D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000328:Adult; D002110:Caffeine / Q000494:pharmacology*; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D004558:Electric Stimulation; D004576:Electromyography / Q000187:drug effects; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D006181:H-Reflex / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects* / Q000502:physiology; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology; D015854:Up-Regulation / Q000187:drug effects* / Q000502:physiology
chemical_list: D000697:Central Nervous System Stimulants; D002110:Caffeine