pl1 = Plot3D[1/(1 + E^(-a - c)), {a, -8, 8}, {c, 0, 8},
Mesh -> {{0}, {0}}, PlotStyle -> None,
MeshStyle -> Directive[Thick, Red]];
pl2 = Plot3D[1/(1 + E^(-a - c)), {a, -8, 8}, {c, 0, 8},
MeshFunctions -> {#1 + #2 &}, Mesh -> {{0}, {0}},
PlotStyle -> None, MeshStyle -> Directive[Thick, Red]];
pl3 = Plot3D[1/(1 + E^(-a - c)), {a, -8, 8}, {c, -8, 8},
Mesh -> {{0}, {0}}, MeshFunctions -> {#2 &}, PlotStyle -> None,
MeshStyle -> Directive[Thick, Red]];
pl4 = Plot3D[1/(1 + E^(-a - c)), {a, -8, 8}, {c, -8, 8},
PlotStyle -> Directive[Yellow, Specularity[White, 20]],
MeshStyle -> Opacity[.5], PlotPoints -> 40];
Show[pl3, pl1, pl2, pl4]
