There are three main factors that cause the inside of a neuron to be negatively charged: 

1. The sodium-potassium pump expels three Sodium (Na+) ions from the cell for every two Potassium (K+) ions that it lets into the cell. 
2. There is an abundance of negatively charged proteins inside the cell but a low concentration of negatively charged proteins outside the cell. 
3. Neuronal membranes are more permeable to K+ than to Na+, leading to a net loss of K+ ions from the neuron, which significantly contributes to the negative charge inside. This is the most significant factor that contributes to the negative charge inside of a neuron.