Electrical activity of motor nerve terminals was recorded with focal extracellular electrodes under visual location with Nomarski optics in the intercostal muscle of the mouse. Following ionophoretic applications of tetrodotoxin (TTX) to the last three nodes of Ranvier, a nerve impulse jumped across the three inexcitable nodes and excited the heminodal region. When excitation at the last node of Ranvier and the heminodal region was blocked by TTX, the distal terminal revealed a small inward current in response to nerve stimulation. This inward current disappeared when TTX was applied to the distal terminal, indicating the presence of active inward currents in this region under normal conditions. When excitation at the last node, heminode and distal terminals was blocked by TTX, the transmitter release was still observed by nerve stimulation, suggesting substantial passive depolarization of the terminal by excitation of up-stream nodes of Ranvier. Local hyperpolarization of the distal terminal caused an increase in the amplitude of the TTX-sensitive inward current at the terminal, and the reverse effects were exerted by depolarization of the terminal. It is concluded that the mouse motor terminal is electrically excitable and that the TTX-sensitive inward current at the terminal is largely masked by passive depolarization due to excitation of up-stream nodes of Ranvier and the heminode under normal conditions.
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