A neuron that just fired needs a moment before it can fire again — the ion gates have to reset. That tiny dead time (~1–2 ms) sets a hard ceiling on how fast a neuron can shout: past a point, more input doesn’t mean more spikes, just spikes jammed against the limit. The brain’s built-in rate cap. Slide the input drive up and watch the spike train saturate.
After each action potential a neuron is refractory: an ABSOLUTE period (~1–2 ms) during which sodium channels are inactivated and NO spike is possible, then a relative period of raised threshold. The absolute period τ imposes a maximum firing rate of 1/τ (~500–1000 Hz) — spikes cannot pack tighter than τ apart however hard you drive the cell. A fail-loud self-check throws unless the rate saturates exactly at 1/τ for large drive. ◆ real cellular neuroscience, node-verified.
Absolute-refractory hard cap (τ=2 ms → 500 Hz); real neurons add a soft relative-refractory tail and adaptation that lower the sustainable rate — the 1/τ ceiling is the exact upper bound.