Page 604 - Fundamentals of anatomy physiology

Chapter 17 The Special Senses 591

Figure 17–1 The Olfactory Organs.

Olfactory Pathway to the Cerebrum Basal cell: divides to To
replace worn-out Olfactory olfactory
Olfactory Olfactory Olfactory Olfactory Central
epithelium nerve bulb tract nervous olfactory receptor cells gland bulb
fibers (N I) system

Cribriform Olfactory
plate nerve fibers

Lamina
propria

Developing
olfactory
receptor cell

Olfactory Olfactory
epithelium receptor cell

Cribriform Supporting cell
plate Mucous layer
Knob

Superior Substance being smelled 17Olfactory dendrites:
nasal concha
surfaces contain
a The olfactory organ on the right receptor proteins
side of the nasal septum. (see Spotlight
Figure 17–2)

b An olfactory receptor is a modified neuron with
multiple cilia-shaped dendrites.

with the binding of an odorant to a G protein-coupled receptor bone to reach the olfactory bulbs of the cerebrum, where the first
in the plasma membrane of an olfactory receptor cell dendrite synapse occurs (Figure 17–1). Efferent fibers from nuclei else-
(Spotlight Figure 17–2). where in the brain also innervate neurons of the olfactory bulbs.
This arrangement provides a mechanism for central adaptation
Between 10 and 20 million olfactory receptors fill an area or facilitation of olfactory sensitivity. Axons leaving the olfac-
of roughly 5 cm2 (0.8 in.2). If we take into account the exposed tory bulb travel along the olfactory tract to the olfactory cortex,
dendritic surfaces, the actual sensory area probably approaches the hypothalamus, and portions of the limbic system.
that of the entire body surface. Yet, our olfactory sensitivities
cannot compare with those of other vertebrates such as dogs, Olfactory stimulation is the only type of sensory informa-
cats, or fishes. A German shepherd dog sniffing for smuggled tion that reaches the cerebral cortex directly. All other sen-
drugs or explosives has an olfactory receptor surface 72 times sations are relayed from processing centers in the thalamus.
greater than that of the nearby customs inspector! Certain smells can trigger profound emotional and behavioral
responses, as well as the memories, due to the parallel dis-
Olfactory Pathways tribution of olfactory information to the limbic system and
hypothalamus. The perfume industry understands the practical
The olfactory system is very sensitive. However, the activation implications of these connections and works to develop odors
of an afferent fiber does not guarantee an awareness of the that trigger sexual responses.
stimulus. Considerable convergence takes place along the ol-
factory pathway, and inhibition at the intervening synapses can Olfactory Discrimination
prevent the sensations from reaching the olfactory cortex of the
cerebral hemispheres. p. 513 The olfactory receptors them- The human olfactory system can make subtle distinctions
selves adapt very little to an ongoing stimulus. Rather, central among 2000–4000 chemical stimuli. No apparent structural
adaptation ensures that you quickly lose awareness of a new differences exist among the olfactory receptor cells, but the
smell but remain sensitive to others. epithelium as a whole contains receptor populations with dis-
tinct sensitivities. At least 50 “primary smells” are known. It
Axons leaving the olfactory epithelium collect into 20 or is almost impossible to describe these sensory impressions
more bundles that penetrate the cribriform plate of the ethmoid