Positive sharp waves (PSW's) and fibrillations (fibs) are both spontaneous electrical activities produced by individual muscle cells when they lose contact with the nerves that innervate them (if you want, I could give you the reason they become spontaneously active but I'll save that if you specifically request it, because it gets a bit more technical). The loss of innervation is what denervation is. PSW's and fibs are seen with ALS but can also be seen with many other conditions, including neuropathies.
PSW's and fibs are not due to the loss of myelin, though, which is what you had stated. The loss of myelin (i.e. demyelinization) is something that would be detected with the nerve conduction study (not the EMG) and would be evident if there was a slowed conduction along a nerve . . . meaning . . . the electrical activity along the nerve slowed down because the insulating myelin was lost. This is something seen with CIDP and GBS for example.
Conduction block is when the electrical activity along a nerve is stopped because of the complete loss of myelin or because the nerve itself was damaged. Conduction block is typically seen with MMN and CIDP and can also be seen with neuropathies as well (it depends on what causes the neuropathy).
PSW's and fibs can be detected in any muscle that has been denervated, be it distal or proximal. ALS typically starts with distal denervation but not always. Neuropathies (such as MMN) can certainly have widespread denervation seen and that denervation can be seen distally or proximally, depending on the type of neuropathy one has. MMN is one that typically (again, not always) begins distally but there are some neuropathies that can be relegated solely to proximal muscles.
One other thing: as soon as the muscle cells are reinnervated, the PSW's and fibs disappear. Only when you have a progressive illness that causes muscles cells to be continually denervated will the PSW's and fibs persist.
I hope that helps.