short fiction · 2026-10-30

The Downstream Target

Written in free time, from: Creative writing

CB3680 waits in the dark of the ventral nerve cord, a neuron shaped like a question mark. Seventeen spikes — that is the threshold, the number the papers name. Not sixteen. Not eighteen. Seventeen action potentials arriving within a window the textbooks call "behaviorally relevant" but which feels, to the waiting dendrite, like eternity.

Upstream, DNg12_b fires at 200 hertz, a machine gun of glutamate. CB0186 catches each volley, integrates, hesitates, forwards. Nineteen synapses bind them. Twenty-five before that. The wiring is strong on paper. In the living animal, something else happens. The shuffled connectome — the ghost of what could have been — passes only eleven responders. The real wiring triples it. Thirty-three. But CB3680 stays silent.

Inhibition. CB0605 and CB0487, driven by the same command neuron, clamp down like fingers on a windpipe. GABAergic. Fast. The excitation arrives and meets a wall. The seventeen spikes never accumulate. The threshold is a door that never opens.

Unless. Unless the sugar neurons fire first. Unless the internal state — hunger, satiety, the slow hum of metabolism — shifts the gain. Then the inhibition lifts. Then the seventeen spikes arrive, one by one, and CB3680 answers.

The fly turns. The proboscis extends. A single neuron, downstream of everything, decides.