Page 353 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Sense of Smell olfactory bulb. The glomeruli therefore func-
tion as convergence centers that integrate and
The neuroepithelium of the olfactory region relay signals from the same sensor type. Their
7
contains ca. 10 primary olfactory sensor cells respective sensor protein also determines
(! A1) which are bipolar neurons. Their den- which glomerulus newly formed sensor axons
drites branch to form 5–20 mucus-covered will connect to. Periglomerular cells and granu-
cilia, whereas the axons extend centrally in lar cells connect and inhibit mitral and bristle
bundles called fila olfactoria (! A1,2). Ol-
cells (! A2). Mitral cells act on the same recip-
Central Nervous System and Senses endings (trigeminal nerve) in the nasal mucosa lar cells which, on the other hand, are inhibited
factory neurons are replenished by basal cell
rocal synapses (! A, “+/–”) in reverse direction
to activate the periglomerular cells and granu-
division in 30–60-day cycles. Free nerve
by efferents from the primary olfactory cortex
also react to certain aggressive odors (e.g., acid
and contralateral anterior olfactory nucleus
or ammonia vapors).
(! A2, violet tracts). These connections enable
Olfactory sensors. Odorant molecules (M r
15–300) are transported by the inhaled air to
the cells to inhibit themselves or nearby cells
the olfactory region, where they first dissolve
(contrast), or they can be disinhibited by
in the mucous lining before combining with
higher centers. The signals of the axons of mi-
tral cells (1) reach the anterior olfactory nu-
receptor proteins in the cilial membrane. These
genes distributed in most chromosomes),
commissure) to the mitral cells of the con-
tralateral bulb and (2) form the olfactory tract
whereby probably one olfactory sensor cell
projecting to the primary olfactory cortex (pre-
only expresses one of these genes. Since only a
12 are coded by a huge family of genes (500–750 cleus. Its neurons cross over (in the anterior
part of the sequence of about 40% of these
piriform cortex, tuberculum olfactorium, nu-
genes is expressed, humans have roughly 200– cleus corticalis amygdalae). The olfactory
400 different sensor cell types. Olfactory re- input processed there is relayed to the hy-
ceptors couple with G s-proteins (G olf proteins; pothalamus, limbic system (see also p. 330),
! B and p. 274ff.) that increase the conductiv- and reticular formation; it is also relayed to the
ity of the sensor cell membrane to cations, neocortex (insula, orbitofrontal area) either
+
thereby increasing the influx of Na and Ca 2+ directly or by way of the thalamus.
and thus depolarizing the cell. Thresholds. It takes only 4 ! 10 -15 g of
methylmercaptan (in garlic) per liter of air to
Sensor specificity (! A3). Olfactory sensor cells rec-
ognize a very specific structural feature of the trigger the vague sensation of smell (percep-
odorant molecules they are sensitive to. The cloned tion or absolute threshold). The odor can be
receptor 17 of the rat, for example, reacts with the properly identified when 2 ! 10 – 13 g/L is pres-
aldehyde n-octanal but not to octanol, octanoic acid, ent (identification threshold). Such thresholds
or aldehydes which have two methyl groups more or are affected by air temperature and humidity;
less than n-octanal. In the case of aromatic com- those for other substances can be 10 10 times
pounds, one sensor recognizes whether the com- higher. The relative intensity differential
pound is ortho, meta or para-substituted, while threshold ∆I/I (0.25) is relatively high
another detects the length of the substituent regard-
less of where it is located on the ring. The different (! p. 352). Adaptation to smell is sensor-de-
molecular moieties of an odorant molecule therefore pendent (desensitization) and neuronal (! C).
activate different types of sensors (! A3, top right). The sense of smell has various functions.
Jasmine leaves and wine contain several dozens and Pleasant smells trigger the secretion of saliva
hundreds of odorants, respectively, so their overall and gastric juices, whereas unpleasant smells
scent is a more complex perception (integrated in warn of potentially spoiled food. Body odor
the rhinencephalon).
permits hygiene control (sweat, excrement),
Olfactory pathway (! A2). Axons of (ca. 10 ) conveys social information (e.g., family,
3
same-type sensors distributed over the ol- enemy; ! p. 330), and influences sexual be-
factory epithelium synapse to dendrites of havior. Other aromas influence the emotional
their respective mitral cells (MC) and bristle state.
340
cells (BC) within the glomeruli olfactorii of the
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.