The extracellular environment of motoneurones in the cat spinal cord in vivo was altered by means of local perfusion of the central canal. Intracellular recordings were made to determine the effects of raised extracellular Ca2+or Mg2+concentration on the monosynaptic afferent excitatory postsynaptic potential (EPSP). Raised extracellular Mg2+concentration reversibly reduced the EPSP amplitude, whereas raised extracellular Ca2+concentration produced extremely large increases in the monosynaptic EPSP amplitude, up to almost an order of magnitude. In some cases, a reduction in amplitude of the EPSP and a delay in its onset were also observed, following raised extracellular Ca2+concentration. This effect was thought to be due to a divalent cation block of the presynaptic action potential. A major conclusion from this study is that group Ia afferent terminals have a much greater transmitter release capacity than suggested by previous studies at this connection. © 1991.
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