All the IO pins contain a pair of clamping diodes to protect against over-voltage and negative-voltage situations:
As long as the current flowing through one of those diodes doesn't exceed its maximum forward current all will be well and the Arduino won't be damaged. If too much current flows then you will damage the pin.
Since you have 10KΩ resistors, at -5V the maximum current that could flow (assuming a 0.7V drop on the diode) would be (-5 + 0.7) / (10,000 / 2) = -860µA (negative because it's flowing backwards out of the pin from ground to your negative voltage source, not into the pin to ground).
Unfortunately the datasheet doesn't provide a current limit specification for those diodes, but I would expect such a small amount of current to be safe enough.