Fill in the missing word (science)

Light enters the eye through the transparent cornea. The Iris controls the amount of light entering the pupil. As it travels through the lens light is focused on to the retina lining the back of the eye. The retina transforms the image into an ____________ impulse. Which travels along the optic nerve to the brain. The spaces either side of the lens is filled with fluid called the vitreous humor. This fluid is under slight pressure and helps the eye keep its shape.

I have filled in all of it but one could you guys please help.

Thanks

electrical

ELECTRICAL..
11-cis retinal
light→ ↓
metarhodopsinII
↓
Activation of transducin
↓
Activation of cGMP photodiestarase
↓
cGMP → 5’-GMP
↓
Decrease in concentration of cGMP
↓
Closure of sodium channels
↓
Hyperpolarisation in receptor cell(*in nerve fibres depolarization occurs)
↓
Decrease in release of glutamate
↓
Response in bipolar and ganglionic cells