What is the bare minimum amount of sources you need to use?

Can you use outside sources you find on your own?

First person (I and me) is fine to use in this essay. 

The deficiency of this neurotransmitter is associated with Parkinson disease.

Earache:

Farsightedness is more properly called:

This cranial nerve functions in the sense of  smell:

Farsightedness is more properly called:

This portion of the nervous system prepares the body for stressful or emergency situations, commonly          called “fight or flight” responses:

The brain begins as a tube-like structure called