Endogenous electric field around muscle fibres depends on the Na+-K+ pump.

We describe here experiments which reveal a new physiological specialization in the endplate (synaptic) region of skeletal muscle fibres. Using a vibrating microelectrode which can detect small currents flowing in extracellular fluid, we have found that the membrane in the endplate region behaves as though a steady positive current is generated in this location. Current re-enters the fibre in the extrajunctional region. Further experiments show that this current is dependent on the activity of the sodium pump. The electric field created by this current may be important for long-term interactions between muscle and nerve.