The type of cell most responsible for relaying information about external stimuli like heat to the brain are sensory neurons.

Here's why:

* Sensory neurons are specialized nerve cells that detect and respond to specific types of stimuli from the environment.

* Thermoreceptors are a specific type of sensory neuron that are sensitive to temperature changes.

* When heat is detected, the thermoreceptors send signals along their axons to the brain.

* The brain processes these signals and interprets them as the sensation of heat.

Let me know if you'd like to learn more about specific types of sensory neurons or how they work!