Approximately how many ganglion cell axons leave the retina to form the optic nerve?

• A. Ten thousand
• B. Hundred thousand
• C. Ten million
• D. One million

Answer: (D)

The key idea is that the retina’s output to the brain is determined by the number of retinal ganglion cells and their axons. These cells are the final output neurons of the retina, and their axons bundle together to form the optic nerve. In humans, there are about 1 million retinal ganglion cells, so roughly one million axons leave the retina to become the optic nerve. The other options are far from this number: ten thousand and hundred thousand are too small to account for all the ganglion cells, while ten million would be more than the actual count. So the best approximate value is about one million.