Conduction pathways that cross one or more of the central nervous system level boundaries; e.g. Tr.spino-thalamicus lateralis which connects the spinal cord to the midbrain, Tr cortico-spinalis which connects the cerebible cortex to the spinal cord.
A projection path is either ascending or descending, i.e. the signal flow begins at a lower level and ends at a higher level, respectively, begins at a higher level and ends at a lower level.
A projection path is also either uncrossed or crossed, i.e. the signal flow begins and ends on the same side of the mean line and begins on one side and ends on the other.