  A description of the generalized Gerasimov-Drell-Hearn sum rules for proton,
neutron and deuteron is suggested. The proton consideration confirms the
earlier conjecture, that the structure function $g_T$ features a smooth
$Q^2-$dependence, while the structure function $g_2$ is changing rapidly, due
to the elastic contribution to the Burkhardt-Cottingham sum rule. The neutron
case features the close connection to the Bjorken sum rule. The deuteron case
reveals the crucial role of photodesintegration channel, resulting in the
strong nuclear correction at very low $Q^2$. The new approximate sum rules for
this channel reflecting the delicate interplay between nucleon and nuclear
structure are suggested.
