Obi wan hears the destruction of a planet and all of its people through 'the force'. these sounds are only in his head and are not carried through space, since space has no through which sound could transmit.

after receiving a stimus, the sensory receptors send(s) signals to the appropriate area of the body.  

the must be sensory neurons.  

hope its .

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

oxygen +1 most likely