Aging & Senescence · §4

Senescence as a stuck attractor

Cellular senescence is a cell that has crossed into an irreversible arrested R19 basin. Past the spinodal the proliferative basin disappears, so the arrest is one-way — a sub-spinodal pulse is still reversible, but a supra-spinodal crossing is not — and arrested cells accumulate monotonically, reaching a fraction of 0.391 in the run.

Driving a cell past the spinodal annihilates the proliferative basin, making the arrested state an absorbing attractor (return is impossible), while a sub-spinodal pulse relaxes back. Repeated exposure therefore accumulates arrested cells over time, with the arrested fraction rising monotonically to 0.391.

Irreversibility is a saddle-node, not a threshold

Senescence is permanent for a structural reason. Below the spinodal the proliferative and arrested basins coexist, so a perturbation that pushes a cell toward arrest can relax back. Above the spinodal the proliferative basin no longer exists, so the cell cannot return — the arrested state is absorbing.

The run confirms both halves: a supra-spinodal drive is one-way, and a sub-spinodal pulse is reversible. Irreversibility is therefore not an assumption; it follows from which basins exist at a given drive.

Accumulation and SASP

Because each crossing is permanent, arrested cells accumulate. The arrested fraction rises monotonically with continued exposure, reaching 0.391 in the simulation. The senescence-associated secretory phenotype then raises neighbours' crossing hazard, coupling senescence to the risk multiplier of §7.

The irreversibility and the monotone accumulation are graded [V]; the absolute per-year rate is [O].