Final answer:
Olfactory receptor neurons use cilia to trap odorant molecules and increase the surface area for olfactory receptor proteins, which detect specific odorants and send signals to the brain.
Step-by-step explanation:
The cellular structure that allows olfactory receptor neurons to increase the surface area of exposed dendritic processes, which contain olfactory receptor proteins, are extensions known as cilia. These hair-like cilia are receptor-laden and essential for trapping odorant molecules.
The variations in the amino acid chains of the sensory receptors on the cilia make them sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, dedicated to detecting specific odorants.
When odorant molecules are inhaled and dissolve in the mucus of the olfactory epithelium, they bind to these receptors, which are G protein-coupled, and this binding initiates a signal to the olfactory bulb of the brain.