Trends in neurodevelopment are, at least to me, a bit counterintuitive. It is surprising that there would be the most synaptic connections in humans at ~ 8 months after birth rather than, say, 18 years. But following the logic of synaptic pruning, this is the world we live in.
Using light and electron microscopy, a new study sheds some light on these processes. The authors provide quantitative measurements of the trade-off to large numbers of synapses in newborn mice, which is that each individual axon and synapse is smaller.
They study the motor axons of neuromuscular junctions, but presumably the same patterns of redistribution generalize to elsewhere in the nervous system. Some of their findings:
- At birth, the main branch of the motor axons entering muscles had an average diameter of 1.48 ±0.03 μm, compared to 4.08 ±0.07 μm at 2 weeks old
- In the cleidomastoid, at birth each motor axon innervated an average of 221 ±6.1 different muscle fibers, compared to 18.8 ±3.0 at 2 weeks old
- At embryonic day 18, each terminal axon branch covered an average of 14.2 ±11.4% of the muscle’s acetylcholine receptors, compared to ~100% by single axons in adults
These results and others in the paper show that although there are fewer total synapses in later stages of development, each axon/synapse is bigger and more specific.
Tapia JC, et al. Pervasive synaptic branch removal in the Mammalian neuromuscular system at birth. 2012 Neuron, PMID: 22681687.