Download Seventeen

The Hunan Nervous System: An Ahatonical Viewoint,
Mlnay L. Bar and John A. Kieman. J.B,
Lippincott Company, Philadelphia, O 1993.
Seventeen
Sixth Edition,
e
The olfactory receptor cells are unique in being neurons located in an
epithelium and in being regularly replaced from a population of precursor
cells.
The unmyelinated axons of the olfactory neurosensory cells constitute
about 20 olfactory nerves on each side. These nerves pass through the
cribriform plate o[the ethmoid bone and end in the overlying olfactory bulb.
A fracture of the cribriform plate is likely to be followed by cerebrospinal fluid
rhinorrhea.
The principal neurons of the olfactory bulb have axons that form the
olfactory tract. This follows the ventral surface of the frontal lobe and ends in
the olfactory trigone, anterior (rostral) to the anterior perforated substance.
Most of the axons of the olfactory tract follow the lateral olfactory stria and
end in the lateral olfactory area, which comprises the corte>< of the uncus,
the limen insulae, the entorhinal area, and the corticomedial nuclei of the
amygdaloid body.
Smaller numbers of olfactory tract fibers end in the anterior olfactory
nucleus and in various nuclei in the region of the anterior perforated
substance. Some of these cell groups give rise to fibers that pass centrifugally in the olfactory tracts and terminate in the olfactory bulbs of both sides,
providing a mechanism for modulation of the input from the olfactory
apparatus.
The regions in which fibers of the olfactory tract terminate are connected,
directly and indirectly, with the limbic system, hypothalamus, and reticular
formation of the brain stem. These connections provide for visceral and
behavioral responses to different odors.
The olfactory system consists of the olfactory
epithelium, olfactory nerves, olfactory bulbs,
and olfactory tracts, together with functionally associated cerebral cortex and subcortical
structures.
Lower vertebrates and many mammals
relyheavily onthe sense of smell. They are said
to be macrosmatic; in the mammalian class,
the dog is a familiar example. Humans are
microsmatic, with smell being much less important than the other senses, especially sight
and hearing. The study of comparative anatomy contributes much to an undetstanding of
those parts of the brain involved in olfaction,
which constitute the rhinencephalon, Thus,
in macrosmatic animals, the rhinencephalic
269
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27(l
Regional Anatomy of the Central Nervous System
in
humans, they are small by comparison with
the remainder of the brain. Even in humans,
however, olfaction is a significant sense that
structures are large and prominent, whereas
conjures up memories and arouses emotions,
Smell also contributes to alimentary pleasures.
Those who have lost their sense of smell complain of impairment of taste, stating that everything is bland and tastes alike, and they may be
unaware of their inability to smell. Much of
our enjoyment of taste is in fact an appreciation of aromas through the olfactory system.
Some chemical stimuli, notably those from
foods with "hot" flavors, excite general sensory fibers of the trigeminal nerve in the nose
and mouth. The olfactory, gustatory, and general sensory responses to chemical stimuli in
the nose may be integrated in the insula,
where the primary cortical areas for the three
systems are in proximity.
Olfactory Epithelium
and Olfactory Nerves The olfactory epithelium is derived from an
ectodermal thickening, the olfactory placode, at the rostral end of the embryonic head,
The cells of this placode give rise to the cells of
the epithelium, the glial cells of the olfactory
nerves, and probably some of the glial cells of
the most superficial layer of the olfactory bulb.
In the adult, the olfactory ep]thelium (Fig.
17- I ) covers an area of 2.5 cnit" in the roof of
each nasal cavity and extends for a short dis-
tance on the lateral wall of the cavity and the
nasal septum. The olfactory sensory cells are
contained in a pseudostratified columnar
epithelium, which is thicker than that lining
the respiratory passages elsewhere, Olfactory
glands (glands of Bowman) beneath the
epithelium bathe the surface with a layer of
Olfactory vesicles
with cilia
Supporting
CCIIS
Oltactory
neu rosensory
cell (position
of nucleus)
lntermediate
stage between
basal cell
and olfactory
neu rosetlsory
cel
Basal
Olf actory
IBowman's)
ceils
grano
1t
metnbrane
Figure 17-1. Olfactory epithelium.
Filum of olfactory
nerve in lamina
propfla
I
Chapter 17: Olfactory System
mucous fluid, inwhich odoriferous substances
are dissolved. The olfactory neurosensory
cells (also known as primary olfactory neu_
rons or simply as olfactory cells) are bipolar
neurons that serve as sensory receptors as well
as conductors of impulses. The major mod_
ification consists of specialization of the dendrite; this process extends to the surface of the
epithelium, where it ends as an exposed
bulbous enlargement known as an olfictory
vesicle, bearing cilia that are exceptional in
that they may be up to 100 pm long. R minority of human.neurosensory cells have apical
tufts of microvilli and resemble the sensory
neurons of the vomeronasal organ, a chemical
sense organ of lower mammals and submam_
malian vertebrates (see end pf this chapter) .
Unmyelinated axons of the olfactory cells
are gathered into about 20 bundles on each
side, which are the olfactory nerves. These
enter the cranial cavity by passing through the
foramina of the cribriformplate of the ethmoid
bone and then enter the olfactory bulb. The
axons form a superficial fibrous layer
in the
olfactory bulb, then continue more deeply,
and termirLate in specialized synaptic conligurations, the glomeruli. The olfactory axon
terminals release an excitatory neurotransmitter that has not yet been identified. (The dipeptide carnosine has been suspected.)
The few neurosensory cells shown in Figure l7-1 represent some 25 million such cells
in
each half of the olfactory epithelium. The
olfactory cells are continuously being produced by uritosis and differentiation of some of
the basal cells of the olfactory epithelium, and
lost by desquamation. Observations in animals
indicate that although some cells die without
reaching maturity, olfactory neurons probably
are lost by wear and tear rather than because of
an innately short life span, In the hurrran nose,
each receptor neuron survives probably for
about 2 months, Consequently there are always new axons growing along the olfactory
nerves and into the olfactory bulbs.
The olfactory system is exquisitely sensitive
to minute amounts of excitants in the air. Direct stimulation of the receptors, convergence
of many neurosensory cells on the principal
neurons of the olfactory bulb, and facilitation
by neuronal circuits in the bulb are among the
factors responsible for the low threshold, Smell
is a chemical sense, as is taste. For a substance
to be smelled, it must enter the nasal cavity as a
gas or as an aerosol and then dissolve in the
fluid that covers the olfactory epithelium. The
secretory product of Bowman,s glands con_
tains glycoproteins that can bind odoriferous
substances that are not otherwise soluble in
water, for presentation to receptor molecules
on the surfaces of the sensory cilia. That a large
range of odors and aromas can be appreciated
may be due in part to the existence of neu.o_
sensory cells with different chemical speci_
ficities.
The olfactory system adapts rather quickly
to a contrnuous stimulus, so that the odor
becomes unnoticed. A physiological mechanism that allows the receptors to recover is a
cyclic alternation of mucosal blood flow in the
left and right sides of the nose. At any instant,
the side with the higher flow of blood presents
greater resistance to the flow of air because of
swelling of the mucosa. The nasal cavity with
lower air flow consequently receives smaller
amounts of the ambient odoriferous
sub_
stances. Most older people have a reduced acu_
ity of smell, probably caused by a progressive
reduction in the population of neurosensory
cells in the olfactory epithelium.
Ollactory Bulb,
Ttact, and Striae
The olfactory bulb is ventral to the orbital sur_
face of the frontal lobe. It is connected by the
olfactory tract to a central point of attachment
in front of the anterior perforated substance
(see Fig. 17-3).
The olfactory bulb has a characteristic cvto_
architecture in animals that rely heavily on the
sense of smell. There are five layers (Fig. Z-2):
1. Nerve fiber layer (olfactr:ry axons) on the
surface
2. Layer of glomeruli
3. External plexiform
layer
271
272
Regional Anatoffiy of the Central Nerttous Systeu
Fibers'of
Cranule cell
(has no axon)
Periglomerular cell
Layer of
mitral cells
External
plexiform
layer
Tufted cell
Layer
of
glomeruli
Cribriform plate
cel
I
Figure l7-2. Neuronal circuitry of the olfactory bulb'
4. Layer of mitral cells
5. Granule cell layer, which in its deeper
parts also contains the myelinated axons
ihat comprise the medullary center of the
olfactory bulb
The center contains nests of ependymal
cells, which are vestiges of the extension of the
lateralventricle into the bulb in embryonic life'
e
'1
:l
t
.;
.
Chapter 17: Olfactory
neurons with which they synapse, and the transmitter probably is glutamate.
The olfactory bulb contains interneurons of
two types. Periglomerular cells have dendrites
that receive
ctory neuroepithelial
fibers of the
sy
cells
olfactory tract
drodendritic
synapses with the mitral cells. The axons of the
periglomerular cells enter the external plexiform
layer to contact the dendrites of mitral cells
associate
e><cite or i
recePtors
mine, 1-a
peptides. The most numerous interneurons are
the GABAergic granule cells, which have no
axons and are located in the deepest layer ofthe
olfactory bulb. Their dendrites receive axo-
dendritic contacts from mitral 'cells and from
the centrifugal fibers. Other dendrites form
dendrodgndritic synapses with mitral cell dendrites. Sorne of these synaptic arrangements
are shown irr Figure 77 -2.The complex circuitry
of the olfactory bulb recalls that of the retina and
indicates thiat, as is the case with visual images,
sensory data are partially analyzed and edited
before reaching the cerebral olfactory areas.
Three srnall groups of nerve cells make up
the anterior olfactory nucleus. One is situated ar
the transition between the olfactory bulb and
olfactory tract; the others are deepr to the lateral
and medial olfactory striae described in the next
paragraph. Collateral branches of axons of mitral and tufted cells terminate in this nucleus.
Fibers that originate in the anterior olfactory
nucleus pass through the anterior commissure
to the contralateral olfactory bulb. This is only
one of the populations of centrifugal fibers that
project to the olfactory bulb. Centrifugal fibers
synapse principally with the dendrites of the
interneurons.
Impulses from the olfactory bulb are conveyed to olfactory areas for subjective appreciation of odors and aromas. These areas also
establish connections with other parts of the
brain for emotional and visceral responses to
olfactory stimuli. The olfactory tract expands
into the ollactory trigone at the rostral margin of the anterior perforated substance. Most
of the axons of the tract pass into the lateral
olfactory stria (Fig. 17-3), which goes to rhe
System
lateral olfactory area. Other axons of the olfactory tract, traditionally named the intermediate olfactory stria, leave the olfactory
tdgone to enter the anterior perforated sub_
stance, which is part of the intermediate olfactory area. The name "medial olfactory stria,,is
applied to a ridge that was once thought to
carry olfactory fibers to the septal area. It is
now known that no such connection exists.
Olfactory Areas
of the Cerebral
Hemisphere
R.HINENCEP LON
The "nose brain" was once thought to include
a much larger proportion of the forebrain than
that now known to be devoted to the sense of
smell. The term is now restricted to those re-
gions that receive afferent fibers from the ol_
factory bulbs. The lateral olfactory area
receives afferents from the olfactory bulb
through the lateral olfacrory stria (Fig. 1z-+;
see also Fig. 17-3). The area consists of the
paleocortex of the uncus, cortex of the entorhinal area (the anterior part of the parahip-
pocampal gyrus)
in the
temporal lobe, and
cortex in the region of the limen insulae (see
Fig. 17-3). The uncus, entorhinal area, and
limen insulae are collectively known as the
pyriforrn cortex (or lobe) because the homologous area has a pear-shaped outline in
macrosmatic animals. part of the
amygdaloid
body (amygdala) also is included in the lateral
olfactory area; the uncus is its landmark on the
medial sudace of the temporal lobe. The dorsomedial part of the amygdala consists of the
corticomedial group of nuclei, It receives
olfactory fibers, whereas the larger ventro-
lateral portion, a component of the limbic system, is considered in Chapter lg. The lateral
olfactory area is the principal region for awareness of olfactory stimuli and is, therefore, the
prirnary olfactory area.
The anterior perforated substance, situated
between the olfactory trigone and the optic
tract (see Fig. 17 -3) , derives its name from the
penetration of many small blood vessels into
the brain in this region. It contains several
27t
274
Regional Anatorny of the Central Nenous System
Olfactory
bulb
Olf actorY
trigone
Lateral
"Medial
olf actory
olfactory
stria"
stfla
Anterioi
perforated
substance
Limen
insulae
Metal retractor
Figure l7-3. Some components of the olfactory system seen on the
,
ventral surface of the brain. The right temporal Pole has been cut away to
give a clear view of the olfactory trigone, anterior perforated substance, and
limen insulae. (x 1)
groups of neurons that receive fibers from the
olfactory trigone and together constitute the
intermediate olfactory area. The diago'
nal band of Broca, immediately in front of
the optic tract and beneath the gray matter,
connects the ventrolateral portion of the
amygdala with the septal area and is, therefore, a fiber bundle of the limbic system, The
adjacent nucleus of the diagonal band,
however, is a major source of centrifugal fibers
to the olfactory bulb, the other source being
the contralateral anterior olfactory nucleus.
The septal area, on the medial surface of the
frontal lobe ventral to the rostrum of the
corpus callosum, was formerly known as the
"medial olfactory area" , but it does not receive
any fibers of the olfactory tract. The septal area
is a component of the limbic system of the
brain and can no longer be assigned a role in
olfaction as well.l
Olfactory stimuli induce visceral responses
by modulating the activities of the autonomic
nervous system. Examples are salivationwhen
there are pleasing aromas from the prepara-
t The nomenclature of the olfactory areas of the human
brain is decidedly unsatisfactory. Even though a "medial"
olfactory area is no longer recognized in the mammalian
brain, we have retained the old "intermediate olfactory
area" for want of a better term. A detailed account of its
components is beyond the scope of the present text.
i,
Chapter 17: Oflactory System
Sources of centrifugal
fibers of olfactory tract:
anterior olfactory nucleus
and nucleus of diagonal band
Intermediate
Olfactory
bulb
olfactory area
Olfactory tract
Lateral
olfactory areA
Entorhinal area
Olfactory
epithelium
Figure l7-4. Components of the olfactory tract.
tion of food, and nausea or even vomiting
evoked by an ofllensive stench, The olfactory
system shares the entorhinal cortex with the
limbic system, and the limbic system has extensive connections with the septal area and
the hypothalamus. Most of the fibers that connect the septal area'and hypothalamus with
Clinical Coraideraiions
Fractures of the floor of the anterior fossa of the
autonomic nuclei are in the medial forebrain bundle. This bundle, which contains
fibers projecting rosftally as well as caudally,
traverses the lateral part of the hypothalamus.
Descending fibers from the hypothalamus
proceed to autonomic nuclei in the brain stem
and spinal cord. Other descending fibers of the
medial forebrain bundle end in raphe reticular
nuclei and in the solitary nucleus.'
2
The autonomic nuclei are associated with cranial nerves
III, VII. IX, and X (see Ch. 8) and with spinal nerves TI-L2
and S2-S4 (see Ch. 5). The organization and functions
ofthe autonomic nervous system are discussed in Chapter 24.
cation with the external environment is dangerous because it provides a route whereby
bacteria.may enter and attack the meninges
and the brain.
tory loss is likely to be unilateral.
An irritating lesion that affects the lateral
olfactory area may cauEe
acterized by an imaginary
involuntary movements o
275
276
Regional Anatomy of the Central Neruous System
and often by other features ol'disturbed function of the temporal lobe (seer Ch. 1B).
rminal and
meronasal Nerves
-
TWo small cranial nerves associated with the
olfactory system were discov,:red after the l2
main cranial nerves were given their numbers.
The terminal nerve (nervus terminalis) is present, although of microscopic size, in the adult
human brain. Sometimes it is called cranial
nerve zero because it is medial (and therefore
perhaps rostral) to the olfactory nerves. The
vomeronasal system appears on-ly transiently
in human embryonic developrnent, but in
most other terrestrial vertebrates, it has important functions in adult life.
Terminal Nerue
The fibers of the tiny terminal nerve lie along the
medial side of the olfactory irulb and olfactory
tract. Bipolar neuronal cell bodies are present in
small ganglia along the coutrse of the nerve.
Their distal processes pass through the cribriform plate and are distributed to the nasal septum. In animals, the proximal procesSes have
been traced experimentally to the septal and
preoptic areas.
Vomeronasal Sgstem
In most mammals other than humans and in
many submammalian vertebrates, there is a
vomeronasal organ, which is a blind-ended
tube lined by sensory epithelium in the ventral
part of each side of the nasal septum. The re-
teptor neurons are similar to those in the olfactory epithelium, but they have microvilli rather
than cilia at their apical poles. Vascular connective tissue with sympathetic vasomotor innervation separates the vomeronasal epithelium
from the rigid cartilaginous wall of the tube' The
neurosensory cells of the epithelium give rise to
axons that constitute the vomeronasal nerve.
This passes through the cribriform plate alongside the olfactory neryes an,l ends in a part of
the forebrain called the accessory olfactory
bulb. In lower mammals arrd reptiles, the entrance to the vomeronasal organ is immediately
dorsal to the nasopalatine foramen, through
which the oral and nasal cavities communicate
behind the incisor teeth. Activity of the sympathetic nervous system causes constriction of the
blood vessels of the vomeronasal organ, making the connective tissue layer thinner because
it contains less blood. The resulting enlargement of the lumen sucks in tiny drops of liquid
that have been either sniffed into the nostrils or
deposited by the tongue into the nasopalatine
foramen.
The vomeronasal system is for the detection
of chemical messages from other members of
the same species. The compounds (pheromones) used as sexual attractants and for
marking territory may be secreted by specialized sweat or sebaceous glands, or they may be
in the urine. ln animals with welldweloped vomeronasal systems, such as snakes
and rodents, transection of the vomeronasal
nerves impairs reproductive behavior but does
not interfere with feedino.
excreted
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Doucette R: PNS-CNS Tlansitional zone of the first
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Bemstein JJ: Neurogenesis of sensory neurons
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vomeronasal system. Annu Rev Neurosci l0:
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Chapter 17: OlfactorY SYstem
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