Neurotransmitters, also known as chemical messengers, are endogenous chemicals that enable neurotransmission....
Yo! :D So basically if somebody LACKS neurotransmitters, it means that the brain will not receive certain messages and the function of the brain is going to change by what messages it gets.
Why would it change the function?
Answer: Because the brain only reacts to the messages it receives by neurotransmitters. :)