  We calculate the hypercharge and flavor singlet axial couplings related to
the spin of the nucleon in a constituent quark model. In addition to the
standard one-body axial currents, the model includes two-body axial exchange
currents. The latter are necessary to satisfy the Partial Conservation of Axial
Current (PCAC) condition. For both axial couplings we find significant
corrections to the standard quark model prediction. Exchange currents reduce
the valence quark contribution to the nucleon spin and afford an interpretation
of the missing nucleon spin as orbital angular momentum carried by nonvalence
quark degrees of freedom.
