  We have studied Bi$_{2}$Sr$_{1.6}$Nd$_{0.4}$CuO$_{6+\delta}$ using Angle
Resolved Photoemission Spectroscopy in the optimal and overdoped regions of the
phase diagram. We identify a narrow crossover region in the electronic
structure between the nodal and antinodal regions associated with the deviation
from a pure d-wave gap function, an abrupt increase of the quasiparticle
lifetime, the formation of Fermi arcs above T$_c$, and a sudden shift of the
bosonic mode energy from higher energy, $\sim${60meV}, near the nodal
direction, to lower energy, $\sim${20meV}, near the antinodal direction. Our
work underscores the importance of a unique crossover region in the momentum
space near E$_F$ for the single layered cuprates, between the nodal and
antinodal points, that is independent of the antiferromagnetic zone boundary.
