One mossy fiber and the CA3 pyramidal cell it contacts, in real
reconstructed geometry.
The pulse is not a sweep across the screen. Every point on these
meshes carries its own distance measured along the cell's own skeleton, so the
wavefront follows the real branching and splits wherever the cable splits. On the
fiber it runs from the far end toward the boutons. In the cell it travels from the
thorn down to the soma and then out along the descending arbor, and it does
not run back up the dendrites, because that is not where a signal goes. The
soma sits 130.5 µm of cable from the contacts, which is what sets the
moment it lights.
Six inputs, two attempts
The honest version of the same event. One mossy fiber fires, lands hard on the
thorn, and the cell does not answer. The other five fibers arrive. All six fire
together, and only then does the cell send a signal of its own.
Eighteen seconds. All 165 mossy fiber synapses onto this one cell, from
all six fibers that reach it.
Held on the thorn, then opening out
Tight on the contact through the arrival, then the frame opens as the
signal enters the cell, so the descending arbor has somewhere to run to.
Locked off
No camera movement at all. The whole cell stays in frame from the first
moment to the last.
What one input actually does
A single mossy fiber makes one of the strongest connections in the brain, touching
a CA3 pyramidal cell at dozens of release sites at once. Even so, one incoming spike
is usually not enough to make the cell fire. When the granule cell fires a rapid burst
of a few spikes, the connection strengthens within milliseconds and then reliably
triggers a spike in the CA3 cell.
Two honest limits on what you are seeing. The reconstruction
is cut off at the edge of the tissue block, so this cell's arbor is incomplete and its
measured cable runs shorter than a whole CA3 pyramidal cell would. And the descending
arbor shown carrying the signal is mostly basal dendrite with the axon embedded in
it; at this resolution the two could not be cleanly separated. The direction of
travel is right, but this is not a picture of an isolated axon.