Sense of Smell

Sense of Smell

The upper nasal cavity contains smell (olfactory) receptors. The sense of smell works closely with the sense of taste. The olfactory organs are yellow-brown masses of pseudostratified epithelium covering the upper part of the nasal cavity, the superior nasal conchae, and part of the nasal septum. The lamina propria makes up the olfactory organs. Bipolar neurons surrounded by column-like epithelial cells are called olfactory recep-tor cells (FIGURE 15-1) or olfactory ­sensory neurons. The dendrites of these neurons are covered at the distal end by hair-like and mostly stationary cilia, which greatly increase the receptive surface area. Mucus from the olfactory glands helps to capture and dissolve airborne odorants. The sensory neurons are surrounded and cushioned by supporting cells that are columnar in arrangement. Short olfactory stem cells lie at the base of the olfactory epithelium. Unlike many other sensory neurons, those involved in olfaction are often damaged, with a life span of only 30–60 days. They are replaced when olfactory stem cells in the olfactory epithelium differentiate. Odorant molecules stimulate varieties of olfactory receptor proteins to differenti-ate between odors. These molecules must partially condensate from gases to fluids before receptors can detect them. The receptors of both smell and taste are chemoreceptors. These two senses complement each other, responding to different groups of chemicals.

Olfactory receptor cell fibers synapse with neu-rons located in the enlarged olfactory bulbs, which lie on either side of the ethmoid bone. These bulbs analyze impulses, which are transmitted along the olfactory tracts to the limbic system. Most smells are interpreted in the olfactory cortex within the tempo-ral lobes of the brain and at the lower frontal lobes in front of the hypothalamus. Filaments of the olfactory nerves synapse with mitral cells, which are actually second-­order sensory neurons. This occurs in complex olfactory­ glomeruli. Activation of the mitral cells causes impulses to flow from the olfactory bulbs through the olfactory tracts, or local integrators. Odors are consciously interpreted and identified to the part of the frontal lobes just above the orbit, and only some of this information reaches the thalamus. Another path-way reaches the hypothalamus, amygdala, and other limbic system regions. Emotional responses to smells such as gas or smoke may be elicited there, triggering the fight-or-flight response. Salivation and digestive tract actions are stimulated by appetizing odors. Pro-tective choking or sneezing reflexes may occur if an unpleasant odor is detected.

Olfactory stimulation occurs as biochemical pathways are activated, allowing an influx of sodium ions, triggering an action potential. There are several hundred types of olfactory receptor cells. They can bind to several types of odorant molecules and vice versa. Because the olfactory organs are high up in the nasal cavity, faint odors may be difficult to perceive. The sense of smell is more intense with a new odor at first, fading over time. Because the olfactory epi-thelium is located high up in the nasal cavity, it is not as efficient in detecting certain odors as it is in other animals. Sniffing the air pulls more odorant molecules across this epithelium, intensifying olfaction. Olfac-tion occurs when a molecule binds to the surface of an olfactory cilium. Olfactory receptor cells have a short life span, of about 60 days.

A single odor may be made up of hundreds of chemicals. Unlike the sense of taste, the sense of smell is not easily classified with regard to how it works. Our olfactory sensory neurons are stimulated by different combinations of olfactory qualities, which together can allow us to distinguish approximately 10,000 odors. In the human nose, about 400 “olfactory genes” are active, with each gene encoding a unique receptor protein. Each protein is believed to respond to one or more odors, with each odor binding to several different types of receptors. Only one type of receptor protein, however, belongs to each receptor cell. Nasal cavities also contain temperature and pain receptors that are affected by irritants. Sharp, cooling, or spicy irritants create impulses from these receptors, which reach the CNS through trigeminal nerve afferent fibers. To smell a certain odorant, it must be in a gaseous state (vola-tile) as it enters the nasal cavity. It must also dissolve in the fluid that coats the olfactory epithelium. Olfactory receptor cells are the only neurons that are replaced throughout adult life.