A ligand gated channel is a series of passageways in the cell membrane that are regulated by the direct binding of a ligand on the cell membrane. This operates across the cell membrane in direct response to the binding of a ligand to the cell membrane. This method contrasts to the indirect method used by second messenger gateways in the cell membrane. Many of the ligands used in this manner act as neurotransmitters, receiving stimuli from the exterior of the cell and eliciting direct responses from the cell membrane itself. The purpose of this type of cell communication is to help assist the cell membrane in the regulation of substance transportation back and forth across the cell membrane. The cell membrane has a quality called "selective permeability", which means it will allow some substances to cross into the interior of the cell, while it will not allow other substances the same liberty.