Does MMR only go up/down for a win or loss?

its based on win loss. Riot refuses to release the algorithm though so its hard to say with complete certainty that win/loss is unaccounted for

imagine it like this

-100 -------------- 0 -------------- 100

you start at 0 the more games you win closer to 100 and the higher your MMR becomes lose games and it drops to 0 however if you lose more games than normal you will drop towards -100 and your MMR will decrease ... its purely based on games being won or lost... however i heard that every few weeks if i rememebr correctly theres like a very small soft MMR reset.... which is why if you have 50 more losses than wins you can make a comeback right now i have 15 more losses than wins and i gain only 13 LP per win now :( but theres been times where ive had 50 more losses and im on 18 LP ... thats where the resets have come from :)