Human Physiology, 14th edition (2016)

Sensory Physiology 277

Up to 50 G-proteins may be associated with a single

receptor protein. Dissociation of these G-proteins releases

many G-protein subunits, thereby amplifying the effect many

times. This amplification could account for the extreme sen-

sitivity of the sense of smell: the human nose can detect a

billionth of an ounce of perfume in air. Even at that, our

sense of smell is not nearly as keen as that of many other

mammals.

Once the action potential has been produced, it must be

conducted into the brain to convey the olfactory sense. Each

bipolar olfactory neuron has one unmyelinated axon, which

projects through the holes in the cribriform plate of the eth-

moid bone into the olfactory bulb of the cerebral cortex, where

it synapses with second-order neurons. Therefore, unlike other

sensory modalities that are first sent to the thalamus and from

there relayed to the cerebral cortex, the sense of smell is trans-

mitted directly to the cerebral cortex.

Figure 10.9 The neural pathway for olfaction. The olfactory epithelium contains receptor neurons that synapse with
neurons in the olfactory bulb of the cerebral cortex. The synapses occur in rounded structures called glomeruli. Secondary neurons,
known as tufted cells and mitral cells, transmit impulses from the olfactory bulb to the olfactory cortex in the medial temporal lobes.
Notice that each glomerulus receives input from only one type of olfactory receptor, regardless of where those receptors are located in
the olfactory epithelium.

Olfactory bulb

Olfactory tract

Olfactory

bulb

Nasal cavity

Cilia

Columnar

epithelium

Olfactory

receptor

neurons

Cribriform

plate of

ethmoid bone

Glomeruli

Tufted cell

(secondary neuron)

Mitral cell

Interneurons (secondary neuron)

Figure 10.10 Colorized transmission electron

micrograph of olfactory receptor. Note the cilia projecting into

the nasal cavity from the sensory dendrite.