Months after tonya's left index finger was severed in a factory accident, she noticed an increase in the sensitivity of the fingers near the missing one. the increased sensitivity is caused by:

This is caused by somatosensory cortex area that used to receive input for the index finger which severed and as a result that area is now receiving input from the other nearby fingers casing the sensitivity to increase.

Neurons are the carriers of the feelings, pain, sensitivity etc to somatosensory cortex and the served finger part's sensory feelings are being received by nearby fingers.