Matlab low pass filter design -removing noise
To answer your direct question, the parameters are defined by the following diagram, taken from the Mathworks documentation



But, as JRE pointed out, your "spike" signal will also be highly distorted by any low-pass filter that has a significant effect on your noise.

This is not a simple problem to solve. If your signal always has the same general form in the time domain, you might consider a "matched filter". You also might want to look at various kinds of nonlinear filters.