Axons perform two major functions that underlie the electrical activity of neurons. They generate action potentials in the axon initial segment and enable propagation of action potentials to the presynaptic terminal to trigger chemical signaling to postsynaptic neurons. The action potential waveform can be modulated by intrinsic factors such as axon geometry or biophysical properties that eventually enhance neurotransmitter release. This view has been extended by new evidence showing that extrinsic signals arising from astrocytes also control the action potential waveform and influence synaptic strength. This plasticity is independent of the tripartite structure formed by astrocytes with the neuronal pre- and postsynaptic elements. By shaping axonal action potential waveform, astrocytes act as extrinsic instructors of glutamatergic transmission in the hippocampus.