Is sciatica usually a problem in the central or peripheral nervous systems?

Peripheral: It is caused by pressure on the sciatic nerve which is a part of the peripheral nervous system.

Usually central as a: Result of a disc herniation or spinal stenosis compressing the thecal sac or nerve roots leading to pain down the leg(s). However, the peripheral nerves can be compressed by extra spinal causes such as a pyriformis syndrome or intrapelvic lesion.

Generally Peripheral: Nervous System, since sciatica affects the lower motor neurons exiting out of spinal canal. Sciatica is not associated with Central Nervous System disorders even though pain can travel into the sciatic nerve from these areas, however its not called sciatica.