What does the nervous system need to determine the strength or intensity of a stimulus?

The nervous system needs two sources of information to determine the strength or intensity of a stimulus. 

The first source of information is the rate at which a neuron fires. The faster the rate at which a neuron fires, the stronger the intensity stimulus. 

The second source of information is the number of neurons that fire at any given moment. When numerous neurons fire simultaneously or in rapid succession, it is an indication of a stronger stimulus. 

In general, the nervous system uses the speed at which a neuron fires to determine the strength or intensity of a stimulus. The faster the neuron fires, the more intense the stimulus. For example, if you sip a cup of very hot beverage, numerous sensory neurons in the mouth will respond or fire rapidly, providing the nervous system with valuable information about the intensity of the original stimulus.