Connected

The human brain contains an estimated 86 billion neurons, each forming on average between 1,000 and 10,000 synaptic connections, resulting in a total connectivity on the order of 100 to 500 trillion synapses in a typical adult.

These numbers vary widely by brain region—cerebellar granule cells have extremely high synapse densities, while some inhibitory interneurons form fewer, more specialized connections.

Outliers exist at both ends of the spectrum: individuals with enriched synaptic density may exhibit heightened plasticity and learning capacity, while conditions like Alzheimer’s disease, schizophrenia, or certain developmental disorders can reduce overall synaptic count and efficiency.

Synaptic pruning during adolescence also shapes connectivity, optimizing network efficiency by removing weaker or redundant synapses.

Advanced imaging and connectomics studies show that while the average connectivity is immense, small variations in specific circuits—such as those governing working memory or sensory integration—can have outsized effects on cognitive performance and behavior.