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Idara C. Eshiet The olfactory System and limbic system
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The olfactory system ( CN1)
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Introduction The sense of smell is much less essential than
vision, audition or the somatic senses but olfactory dysfunction is
considered and important diagnostic sign and will be therefore
reviewed. Special Visceral Afferent (SVA) The sense of smell and
taste are intimately related and are therefore classified as
Special Visceral Afferent (SVA). It is the only sensation that is
transmitted directly to the cortex without relay in the Thalamus.
Olfactory cells are also regularly replaced from precursor cells.
The olfactory nerve ( CN 1) is composed of unmyelinated axons of
receptor cells located in the olfactory mucosa in the nasal
cavity.
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The first order neuron in the olfactory pathway is also the
sensory receptor. The olfactory receptor cells are scattered among
supporting cells. Each receptor cell consists of a small bipolar
nerve cell with a coarse peripheral process that pass to the
surface of the membrane and a fine central process. From the
peripheral processes a number of short cilia arise, the olfactory
hairs which project into the mucus covering the the surface of the
mucus membrane. Odorants bind to receptor proteins on the
cilia.
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The fine central processes form the olfactory nerve fibers.
Bundles of these fibers pass through foramina in the cribiform
plate of ethmoid to constitute 20 olfactory fila on each side. Most
of the fibers terminate on mitral cells in the olfactory bulb. The
nerve fibers form a synaptic connection with the dendrites of the
mitral cells called Glomeruli. Smaller cells called granule and
periglomerular cells synapse also with the mitral cells (
Inhibitory).
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Olfactory tract The relay from the mitral cells constitute the
olfactory tract. It runs from the posterior end of the olfactory
bulb beneath the inferior surface of the frontal lobe of the brain.
As it reaches the anterior perforated substance, it divides into
medial and lateral olfactory striae. lateral olfactory striae The
lateral olfactory striae carries axons to the olfactory areas of
the cerebral cortex namely, the parts of the orbital surface of the
frontal lobe,as well as the uncus, entorhinal area and medial
amygdaloid nucleus of the temporal lobe. medial olfactory striae
The medial olfactory striae carries the fibers that cross the
median plane in the anterior commissure to pass to the olfactory
bulb of the opposite side.
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Olfactory Connections Olfactory connections are said to reach
numerous structures including the Hippocampus, regions in the
reticular formation including the Salivatory nuclei, Dorsal motor
nucleus of Vagus, Dorsomedial nucleus of the thalamus, medial
amygdaloid nucleus. Through these complex interactions, the sense
of smell influences visceral functions( salivation, gastric
secretions, peristalsis), social interaction and reproductive
behavior. Some of the pathways involved in transmitting olfaction
include the medial forebrain bundle ( hypothalamus), striae
medullaris thalami ( to dorso medial nucleus), Striae terminalis (
from amygdala), Dorsal longitudinal fasciculus ( from hypothalamus
to brain stem visceral centers)
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The Uncus It is of clinical significance because: 1. Seizures
often originate in this area ( uncinate fits). These seizures are
often preceded by hallucinations of disagreeable odors. 2. When the
volume of the temporal lobe is increased due to tumors, hemorrhage
or edema, the uncus can press against the brainstem and cranial
nerves with serious consequences( Uncal herniation). The herniating
uncus and adjacent part of the parahippocampal gyrus push the
brainstem to the opposite side, resulting in damage by pressure
against the taut free margin of the tentorium cerebelli. 3. The
brainstem damage is usually contralateral to the side of the
herniation.
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Anosmia Loss of smell is term Anosmia. It is usually bilateral
caused by common cold or allergic rhinitis. Unilateral anosmia
could result from disease affecting the olfactory nerve, bulb or
tract. A unlateral lesion cant produce complete anosmia because of
the bilaterality of the olfactory pathway. Fractures of the
cribiform plate of ethmoid cause avulsion of the olfactory nerves.
Cerebral tumors can produce anosmia by pressing on the olfactory
bulb or tract.
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The limbic system
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Introduction Limbic is a Latin term which means border. Like
the familiar word limbo, it means an intermediate or transitional
state, which is a border. In this case, the border is between the
neocortex and the subcortical structures (diencephalon). The limbic
system includes the hippocampal formation, amygdala, septal nuclei,
cingulate cortex, entorhinal cortex, perirhinal cortex, and
parahippocampal cortex. These last three cortical areas comprise
different portions of the temporal lobe. (Some experts would also
include parts of the hypothalamus, thalamus, midbrain reticular
formation, and olfactory areas in the limbic system.) These
structures are sometimes called the Papez circuit after the
neuroanatomist that pointed out their interconnections.
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The Hippocampus This is a phylogenetically old part of the
cerebral cortex located within the temporal lobe. It typically
refers to the dentate gyrus, the hippocampus proper (i.e., cornu
ammonis), and the subicular cortex. A hippocampal formation is
located in the temporal lobe of each cerebral cortex, medial to the
inferior horn of the lateral ventricle. Hippocampus means seahorse
in Greek. Each hippocampus looks like a seahorse due to the way it
is folded during development.
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The hippocampus projects by way of the fornix to the mammillary
bodies of the hypothalamus and septal nuclei. The fornix is a large
fiber bundle that follows a C-shaped course from the hippocampus to
its target areas. The fibers destined for the fornix collect on the
surface of the Hippocampus as a thin sheet that converges into a
bundle of fibers called the Fimbria. The fimbria leaves the
posterior end of the hippocampus forming the Crus of the fornix.
The two crura from both sides join together forming the Body of
fornix that heads to the mammillary bodies in the
Hypothalamus.
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Lesions of the Hippocampus People with lesions of the temporal
lobe including the Hippocampus can have a disruption of memory.
bilateral. This deficit is observed only when the damage is
bilateral. It can be sometimes noticed with unilateral lesions. a
loss of Recent memories Anterograde Amnesia This deficit is a loss
of Recent memories. This means that memory of recent events is
lost. This is termed Anterograde Amnesia. A common cause of
bilateral Hippocampal damage is anoxia from interruption of blood
or oxygen supply. The Hippocampus is one of the first sites to be
irreversibly damaged by transient ischemia or anoxia.
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Korsakoff Syndrome Patient with this syndrome also present with
Anterograde amnesia. Patients could also present with retrograde
amnesia and confabulate( make up stories) to make up for past
memories. It is mainly seen in alcoholics who have thymine
deficiency and follows an acute presentation of Wernickes
Encephalopathy. Wernickes Encephalopathy presents with Ocular
palsies, confusion and gait ataxia. It is also related to Thymine
deficiency. In Wernicke- korsakoff Syndrome, lesion are always
found in the mammillary bodies and the Dorsomedial nucleus of the
Thalamus.
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Amygdala This is a large nuclear complex located immediately
rostral to the Hippocampus, within the temporal lobe. It is almond
shaped. It is divided into different nuclei. It is usually seen as
part of the Basal ganglia. It is involved in emotional expession
and visceral functions. The amygdala receives inputs from all
senses as well as visceral inputs. Since the amygdala is very
important in emotional learning it is not surprising that visceral
inputs are a major input source.
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Function 1. Control of emotions 2. Control of sexual behavior
3. Control of food and water intake. The mechanisms for mediating
these behavioral patterns are not well understood.
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Afferents to the amygdala Visceral inputs come from the
hypothalamus, septal area, orbital cortex. Olfactory sensory
information comes from the olfactory bulb. Auditory, visual and
somatosensory information comes from the temporal and anterior
cingulate cortices.
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Efferents from the Amygdala 1. Ventral amygdalofugal pathway
projects to the olfactory nucleus, anterior perforated substance,
piriform cortex and ventral striatum( parts of the caudate, putamen
and nucleus accumbens septi). The ventral amygdalofugal pathway is
important because it is a link whereby motivation and drives,
through the limbic system, can influence responses. It is also a
link whereby responses are learned. In this case, this is the link
whereby associative learning takes place.
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2. Stria terminalis to the Hypothalamus The stria terminalis
overlaps with the ventral amygdalofugal pathway in that it also
connects to the septal nuclei and hypothalamus and thus forms a
loop. 3. Connects directly to the hippocampus 4. Directly to the
entorhinal cortex 5. Directly to the dorsomedial nucleus of the
thalamus
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Kluver- Bucy Syndrome Bilateral damage to the rostro- ventral
portion of the temporal lobes that includes the hippocampal
formation, amygdala can lead to this syndrome. a. Patients become
Docile- previously violent patients displayed little or no emotion.
b. Anterograde amnesia c. Hyperphagia d. Hyperorality e.
Hypersexuality f. Visual Agnosia- patients do not recognize complex
visual patterns g. Psychic blindness- Objects in the visual vield
are treated inappropriately.
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Septal Area It is a component of the limbic system located
anterior to the hypothalamus. It consists of the Septal nucleus,
the septum pellucidum, the small portion of the neocortex that
forms part of the limbic lobe. Because of its connections with the
hippocampus and hypothalamus it is considered as part of the limbic
system. The hippocampus sends connections to the septal
nuclei(area) by way of the columns of fornix. Its function is not
well understood.
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Cingulate Gyrus Is the portion of the limbic lobe that overlies
the corpus callosum. It receives connections from the anterior
nucleus of the thalamus. This puts it in close communication with
the hypothalamus by way of the mammillothalamic tract. The
cingulate gyrus is connected to the parahipocampal gyrus by way of
the cingulum. It is believed to be involved in the generation of
emotional and visceral responses but it is not quite clear
how.
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Q1. Fiber bundles of the limbic system includes a. Fornix b.
Mammillothalamic tract c. Stria terminalis d. Cingulum e. All of
the above
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Q2. Kluver bucy syndrome results due to a. Unilateral temporal
lobe damage b. Bilateral temporal lobe damage c. Damage to the
cingulate cortex d. Damage to the mammillary bodies