No current flows through the connection AB. Current flows in loops, and there is no other route by which the current could return to A. In order to sustain a current in wire AB there would have to be a source of electric charge at A and a sink to absorb electric charge at B.
The 4V cell maintains a potential difference of 4V between B and A, with A being more +ve.
In the loop at A, the 1V source is divided equally between the 5$\Omega$ resistors, so that M is 0.5V higher than A.
In the loop at B, the 3V source is divided in proportion between 3, 2 and 1$\Omega$ resistors, so there is a 0.5V drop across the 1$\Omega$ resistor. The potential of point N is -3+0.5 = -2.5V below that at point B.
So M is 2.5+4.0+0.5 = 7.0V above N.