How many afferent nerve fibers?

Factual Questions
1

Can anyone give a decent estimate of how many afferent* nerve fibers there are going in to the human brain?

(For the full grade, show your work. :p)

*Sensory, ingoing, for those who don’t know. But if you don’t know that you certainly aren’t going to know the answer.

2

Not counting the cranial and vagus nerves, the number I’ve seen in a few places is on the order of 10 million afferent spinal neurons.

3

Thanks. That is a good start, but I was hoping to include the cranial nerves too. (Isn’t the vagus normally counted as one of them?)

4

Oh, I just found this on Wikipedia:

So, since the optic nerve is usually considered entirely afferent in humans,* that gets us up to about 12.4 million.

*Actually I have my doubts about this, since efferent fibers have been discovered in the optic nerves of pretty much all vertebrate species where they have specifically been looked for, and there is at least some, indirect, evidence for efferent optic nerve fibers in humans, but it seems to remain the conventional wisdom that they don’t exist. At any rate, they are at best only a small proportion of the fibers.