Saliva's produced throughout the day and night. The production of saliva is under the control of the autonomic nervous system. This activity occurs without your having to think about it. These parasympathetic cells are located in the vicinity of the substantia nigra . The parasympathetic cells are located below the substantia nigra.
Parasympathetic cells occur in the lowermost or sacral spinal cord.
Paralleling the parasympathetic cells are other autonomic calls called sympathetic cells. These sympathetic cells are also near the substantia nigra but are located above the substantia nigra. The unknown process that causes the substantia nigra cells to die also affects the sympathetic cells. The parasympathetic and sympathetic cells, nerve fibers, and the relationship of the cells and fibers to the organs they regulate differ. And, in some instances the parasympathetic and sympathetic nervous systems may act synergistically.