Sensory are the parts of the body that change a stimulus into a nerve impulse and response.

receptors
pathways
potentials
effectors

The correct answer for the blank is- Receptors.

Sensory receptors are the parts of the body that detect any chemical or physical change in the environment ( also called as stimuli). They change the stimuli into nerve impulse that is carried to the brain in order to interpret the information and produce an appropriate response.

Example of sensory receptors- Skin (that detects sensation of touch, pain, temperature), eyes ( detect visual stimuli), ears ( detect sound stimuli).

c. heat from the poles back down towards the equator.