Low resistance junctions in crayfish. I. Two arrays of globules in junctional membranes

Low resistance junctions between axons of crayfish ganglia are studied by freeze-fracture and negative staining. In freeze-fracture, fracture planes that go through a junctional membrane expose two faces, both internal, called face A and face B. Face A belongs to the internal membrane leaflet and faces the gap. Face B belongs to the external membrane leaflet and faces the axoplasm. Face A displays pits, 60-100 A in diameter, arranged in a hexagonal array with a unit cell of approximately 200 A. An approximately 25 A bump is frequently seen at the center of each pit. Some pits are occupied by a globule approximately 125 A in diameter, which displays a central depression approximately 25 A in size. Face B contains globules also arranged in a fairly regular hexagonal pattern. The center-to-center distance between adjacent globules is most frequently approximately 200 A; however, occasionally certain globules are seen separated by a distance as short as approximately 125 A. The top surface of the globules occasionally displays a starlike profile and seems to contain a central depression approximately 25 A in diameter. In negatively stained preparations of membranes from the nerve cord, two types of membranes are seen containing a fairly regular pattern. In one, globules approximately 95 A in diameter form a hexagonal close packing with a unit cell of approximately 95 A. In the other, globules of the same size are organized in a larger hexagonal array with a unit cell of approximately 155 A (swollen arrangement). Some of the globules forming the swollen arrangement are seen containing six subunits. The six subunits form a hexagon which is skewed with respect to the main rows of hexagons in such a way that the subunits lie on rows which make an angle of approximately 37 degrees with the main rows.