Here's a general example on how to do your own ticks
axes %create a figure with one axe
set(gca,'XLim',[0 100]) %change the x axe limit
xt=linspace(1,100,10) %make a vector starting on 1 and ending in 100 with 10 elements
set(gca,'XTick',xt) %set your own ticks from the vector xt
In your case you must do your own ticks vector from the data y you have, maybe something like you did with the threshold levels.
For the character string on the colorbar you can use the text function.