  Extensive ARPES and low-energy inelastic neutron scattering studies of
cuprate superconductors can be successfully described using a weak-coupling
theory in which quasiparticles on a square lattice interact via scalar and
spin-dependent effective interactions. In this article we point out that in
Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_8$ (Bi2212) both probes are consistent with
dominant near-neighbour Heisenberg interactions. We discuss the implications of
this finding for the mechanism of high-$T_c$ superconductivity.
