Barrier cell - Indestructible cells that are gray.

Target Barrier cells - Destructible versions of barrier cells.

Diode Barrier cells - Barrier cells that have a passageway to go through (1-way only).

These neurons are distinct from other neurons with spatial firing properties, such as grid cells, border cells, barrier cells, conjunctive cells, head direction cells, and spatial view cells.

The arachnoid is composed of an outermost portion (arachnoid barrier cell layer), with tightly packed cells and no extracellular collagen, that is why it is considered to represent and effective morphological and physiological meningeal barrier between the cerebrospinal fluid and subarachnoid space and the blood circulation in the dura.