I know that introverts have a higher sensitivity to dopamine, for example. Are there are other chemical and neuronal differences?
If you are interested in a subfacet of extraversion, i.e. agentic extraversion, a trait covering positive incentive motivation, positive emotionality, and social dominance (among others), I recommend Depue & Collins' paper Neurobiology of the structure of personality: Dopamine, facilitation of incentive motivation, and extraversion, Behavioral and Brain Sciences, 1999, as a starting point.
Furthermore, I'd point to papers such as Wacker, Chavanon, and Stemmler (2006): Investigating the dopaminergic basis of extraversion in humans: a multilevel approach, Journal of personality and social psychology, and related papers, which provide evidence concerning behavioral and electrophysiological indices of agentic extraversion sensitive to pharmacological interventions (DRD2 antagonist sulpiride).
These papers will, however, challenge your statement that introverts are more sensitive to dopamine. Just so you know.
On a more general level and as argued convincingly by Depue & Collins (1999) it is a more fruitful approach to study subfacets of personality traits, rather than broad traits such as broad extraversion. This is imaginable when considering that broad extraversion covers not only positive incentive motivation (likely related to dopaminergic neurotransmission) but also affiliative extraversion (feeling close to others etc., likely mediated by oxytocin). Thus, there is not necessarily an 'extravert' that shows high activity/levels/etc. of this or that specific neurobiological variable such as dopaminergic neurotransmission, because a person might also be considered an extravert (and score high on, let's say the Big Five Inventory Extraversion scale when experiencing tight bonds and affection for others, while he or she is not very agentically extraverted. However, it is more likely that you will measure certain levels of a specific neurobiological variable (such as dopamine-related measures) when studying a more specific and less "fuzzy" trait (such as agentic extraversion).
Introverts have higher cortical arousal than extroverts: http://io9.com/the-science-behind-extroversion-and-introversion-1282059791
Several decades ago, German psychologist Hans Eysenck came up with a more biologically based model for E/I. According to Eysenck's theory, the behaviors of introverts and extroverts are due to differences in cortical arousal (the speed and amount of the brain's activity). Compared with extroverts, introverts have naturally high cortical arousal, and may process more information per second.