The olfactory System and limbic
system
Idara C. Eshiet
The olfactory system ( CN1)
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.
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.
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.
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).
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.
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.
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.
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)
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.
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 can’t 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.
The limbic system
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.
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.
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.
Lesions of the Hippocampus
People with lesions of the temporal lobe including the
Hippocampus can have a disruption of memory.
This deficit is observed only when the damage is bilateral. It can
be sometimes noticed with unilateral lesions.
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.
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 Wernicke’s Encephalopathy.
Wernicke’s 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.
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.
Function
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.
1.
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.
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.
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
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.
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 it’s 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.
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.
Q1.
Fiber bundles of the limbic system includes
a. Fornix
b. Mammillothalamic tract
c. Stria terminalis
d. Cingulum
e. All of the above
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