The nerves in the spine can be pressed on by herniated discs and bone spurs.  However, they can also be crushed by the intervertebral foramen at the time of impact and injured.  This can lead to radiculitis that isn't detectable on MRI, but still causes a lot of problems.  Another way nerves can be irritated is by an unstable segment.  This also won't show up on MRI, but may show up on flexion-extension views.   Finally, the disc can be injured and leak substances which cause a chemical radiculitis.  Since the false negative rate on MRI is so high (about 1/3 of injuries to the disc are proably missed), this condition is frequently missed by the technology.