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Synaptic Pruning

A young brain grows far more connections than it needs, then keeps the ones that get used and removes the rest. Press play and watch 20 years go by. At Normal speed, one second is about half a year.

Age: 0.0 years
Synapse overproduction

Synapse Count

Now: 0 Peak: 0 Pruned: 0 Now vs. peak: –

Legend

Neuron
Synapse
Tagged Synapse
Microglia
Signal

Experiences

Show

What's happening?

  1. Overproduction. In the first years of life, neurons form synapses much faster than they will need them. Synapse density in human cortex peaks in early childhood, well above adult levels.
  2. Use strengthens. When one neuron repeatedly helps the next one fire, the synapse between them gets stronger. Here, each experience (Vision, Language, Music) drives one pathway again and again.
  3. Weak synapses get tagged. Synapses that rarely carry signals weaken. Complement proteins (C1q and C3) mark them for removal. Tagged synapses turn into red dashed lines, and can still be rescued if they get used.
  4. Microglia remove them. Microglia, the brain's resident immune cells, find tagged synapses and engulf them.

Try this

Uncheck Music before age 5 and watch its orange pathway fade, get tagged, and get eaten. Check it again later. Does the pathway come back?

Keep in mind

This is a simplified model. In it, removed synapses never regrow. Real brains still form new synapses throughout life, just far fewer. A human cortex has around 100 trillion synapses, and different regions prune on different schedules: visual areas peak within the first year, while prefrontal cortex keeps pruning into the twenties.

Artificial neural networks get pruned too. After training, deleting the smallest weights often costs little accuracy.

Embed: <iframe src="https://nelurad.github.io/neurosims/synaptic-pruning/" width="100%" height="1100" style="border:0" title="Synaptic Pruning"></iframe>

References

  • Huttenlocher, P. R., & Dabholkar, A. S. (1997). Regional differences in synaptogenesis in human cerebral cortex. Journal of Comparative Neurology, 387(2), 167–178.
  • Petanjek, Z., et al. (2011). Extraordinary neoteny of synaptic spines in the human prefrontal cortex. PNAS, 108(32), 13281–13286.
  • Stevens, B., et al. (2007). The classical complement cascade mediates CNS synapse elimination. Cell, 131(6), 1164–1178.
  • Schafer, D. P., et al. (2012). Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner. Neuron, 74(4), 691–705.