Page 317 - Small Animal Clinical Nutrition 5th Edition

324 Small Animal Clinical Nutrition

turbance odors that become airborne if such odors are volatile
VetBooks.ir Box 18-1. Genes and Olfaction: or from the ground, which could include both individual odors
and contact/disturbance odors (Hepper and Wells, 2005).
People vs. Dogs.
Scents tend to be concentrated at their source. Odors spread
and become less concentrated, forming a scent cone. When air
Olfactory receptors constitute the largest gene family in verte-
brates. Species that have highly developed olfactory senses are is stagnant around the source of an odor, scent pools can form.
referred to as macrosmatic (e.g., dogs), whereas species with a Factors such as wind, terrain, air temperature, humidity and soil
weak sense of smell are termed microsmatic (e.g., people). temperature can affect the scent cone or scent pool, including
There are likely several reasons dogs are much better at odor where and how far it is dispersed (Jones et al, 2004). Hunters
detection than people. The surface area of the canine olfactory using dogs generally agree that the best scent conditions
epithelium is as much as 20 times greater than that of people. include moderate humidity and moderate ambient tempera-
The density of neuronal cells and the number of olfactory recep- tures (early morning and late afternoon on warm sunny days)
tors that are expressed on their surface as well as the size of the (Holloway, 1961). Wind can be channeled by obstructions or
olfactory bulb have to be taken into consideration when compar- terrain and can rapidly disperse scent in unexpected directions
ing the olfactory capabilities of different species. The canine
olfactory epithelium can express up to 20 times more olfactory (Jones et al, 2004).Thus, odors do not disperse in a linear con-
receptors than that of people. This contributes to the ability of tinuous gradient but result in odor fragments or patches.These
dogs to detect odorant molecules at a much lower concentra- pockets of odors vary in concentration and are separated by
tion. The binding affinity of odorant molecules for their related areas of clean air where no odor is present.Thus, dogs are often
olfactory receptor is also likely to be an important variable that exposed to variable and intermittent odor signals. In the face of
could explain differences in sensing abilities between people this complexity, dogs and many other animals are able to deter-
and dogs. The range of types of olfactory receptors of dogs is mine the direction of a trail and find the source of an odor.
around 30% larger than for people, which could contribute to Furthermore, they can be trained to identify specific odors
the wider range of odorant molecules that dogs can detect. (Hepper and Wells, 2005). Human handlers who are aware of
factors affecting odor dynamics can improve detection success
The Bibliography for Box 18-1 can be found at by directing the activity of a scent-detection dog accordingly
www.markmorris.org.
(Table 18-2).
Olfactory System
The olfactory system consists primarily of the nose, nasophar-
ynx (including the vomeronasal organ and olfactory epitheli-
um), olfactory and vomeronasal nerves and the olfactory bulb
(Figure 18-1). The pulmonary system and oro- and nasophar-
ynx facilitate olfaction by moving and directing air over the
olfactory epithelium.
The olfactory epithelium contains the olfactory receptors.
This is where odor molecules interact to stimulate an olfactory
sensation. Chemicals that are best detected by the olfactory sys-
tem are volatile and both water and lipid soluble. Such mole-
cules are readily made airborne and dissolve in the mucus that
covers the olfactory epithelium. After this occurs, they bind to
specific substances called G-protein-coupled-receptors, which
are on the cilia of the olfactory receptor neurons. As a result, a
signal transduction of smell is initiated (Jones et al, 2004). Box
18-1 discusses gene-based differences between the olfactory
capabilities of people and dogs.
For olfaction to occur, odorants need to contact the olfactory
receptors in the olfactory epithelium. Thus, air from the envi-
ronment must pass into the nasal cavity to reach the olfactory
epithelium. The course taken by air as it passes through the
nasal cavity is difficult to predict because of the complex anato-
Figure 18-1. Drawing of a mid-sagittal section of a canine head my of the nasal cavity. There are distinctly different routes of
showing olfactory and upper respiratory anatomy. airflow during normal inspiration and during sniffing. Sniffing
draws considerably more air over the olfactory epithelium as
als, and as a result, these materials release odors. When follow- opposed to normal inspiration. During normal inspiration, air
ing scent, dogs may be detecting airborne odors arising direct- tends to route below the olfactory epithelium, a more efficient
ly from air moving past an animal or person, from contact/dis- route for pulmonary function (Becker and King, 1957).