Printed in U.S.A. SIZES OF END PLATE COMPARTMENTS, DENSITIES OF ACETYLCHOLINE RECEPTOR AND OTHER QUANTITATIVE ASPECTS OF NEUROMUSCULAR TRANSMISSION’

The area of the postsynaptic membrane and the volume of the synaptic cleft were calculated for the end plates of the diaphragm and sternomastoid of mouse and rat. From these dimensions we were able to extrapolate, from data given by others on acetylcholine (ACh) released during neuromuscular transmission and on ACh receptor per whole end plate, to densities in the postneural compartments. The concentration of ACh in the cleft per nerve impulse was found to be ‘ 10- ’ M and the density of ACh receptor was between 5 and 10 x lO3/p2 of postsynaptic membrane. (This is approximately a factor of 2 to 3 higher than that for acetylcholinesterase at this site.) From these values we conclude that the con-centration of ACh receptor in the plane of the postsynaptic membrane is considerably higher than that of ACh in the cleft during neuromuscular transmission. In addition the ACh itself is present at considerably higher concentrations than necessary to give optimal re-sponse. We calculated that the acytelcholinesterase plus ACh receptor together would oc-cupy about 25 % of the surface area of the postsynaptic membrane. Evidence from physiologic, biochemical and histologic studies has led to the formulation o