A larger number of synapses
When scientists compared the brains of rats raised in an enriched environment with the brains of rats raised in an impoverished environment, the rats raised in an enriched environment had a larger number of synapses.
Synapses are structures that allow the transmission of an electrical or chemical signal between two neurons or between a neuron and the target effector cell. Brains raised in an enriched environment have higher rates of synapse formation (synaptogenesis) which can result in increased synapse activity and therefore, cause increased brain activity.