The results of Monte-Carlo modelling of volume and spark ignition of spherical laser targets are systematized and the critical parameters are obtained both of isobaric and of isochoric central ignitors. It is shown that the target thermonuclear (TN) burning is independent on ignitor presence if the ignitor dimension and temperature are lower than the critical ones. In the opposite case TN flash results in effective burning with gain G to approximately 100, and the TN energy release is practically independent on the further increase of ignitor parameters. The intermediate gains 1<G<100 are obtained in rather narrow range of ignitor parameters near the critical. It is shown that the values of critical ignitor parameters are very sensitive to the way of ignition. The critical dimensions of isobaric ignitors are greater by several fold than ones of isochoric ignitors. On the contrary the overcritical target gain is practically independent on ignition origin and may be evaluated with a good accuracy by the simple asymptotic expression. The isochoric spark ignition associated with the fast ignition scheme is considered in detail and evaluations of minimum energy absorbed by extra laser pulse in terms of energy absorbed by basic driver are presented.