Na+ action potentials in human photoreceptors

Mammalian photoreceptors are hyperpolarized by a light stimulus and are com monly thought to be nonspiking neurons. We used the whole-cell patch-clamp technique on surgically excised human retina to examine whether human photo receptors can elicit action potentials. We discovered that human rod photor eceptors express voltage-gated Na+ channels, and generate Na+ action potent ials, in response to membrane depolarization from membrane potentials of -6 0 or -70 mV. Na+ spikes in human rods were elicited at the termination of a light response that hyperpolarized the potential well below -50 mV. This s erved to amplify the release of a neurotransmitter when a bright light is t urned off, and thus selectively amplify the off response to the light signa l.