In the context of sensory receptors, the intensity of the stimulus is communicated to the brain by varying the of the action potential sent to the brain.

The answer to this question would be: frequency

The higher intensity of the stimulus, the higher frequency of the action potential will be sent. The nerve can't send a higher action potential, so it firing at higher at higher intensity instead. So when you felt an intense pain, that means the pain receptor is firing more stimulus per second compared to low or mild pain.

Not sure what you mean by this question