Transcript 20. Olfactory & Vestibulocochlear pathway

Olfactory Pathway
Dr. Zeenat Zaidi
Olfactory Pathway
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The receptors located in an
area, covering about 5 cubic
centimeters, in the superior
part of the nose.
There are about 10 million to
20 million receptor cells for
smell.
Each receptor is a
specialized, ciliated neuron.
Their axons form numerous
small fasciculi, the olfactory
nerves, that pass through
tiny openings in the
cribriform plate of the
ethmoid bone to reach the
cranial cavity.
Olfactory Pathway
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In the cranial cavity, the
olfactory neurons
synapse with neurons in
the olfactory bulb.
The olfactory bulbs (right
and left) are club-like
structures that contain
interneurons and large
mitral cells.
The axons of the mitral
cells form the olfactory
tract, which passes
backward on the basal
surface of frontal lobe.
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Just before reaching the level of the optic chiasma, the tract
divides into lateral, intermediate and medial stria
Fibers in the
medial stria cross
in the midline (in
anterior
commissure)
and terminate in
the opposite
olfactory bulb
Fibers in the
lateral stria
terminate in the
primary olfactory
cortex of the
uncus
Small
intermediate
stria
terminates
into the
anterior
perforated
substance
Olfactory Pathway cont’d
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Adjacent to
the uncus,
the anterior
part of the
parahippocampal
gyrus, or
entorhinal
area,
constitutes
the olfactory
association
cortex
The primary and association
cortices are also collectively
referred to as the pyriform cortex
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The olfactory projection is unique in that it
consists of a sequence of two neurons between
the sensory receptors and cerebral cortex, and
does not project via the thalamus
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The neurons from the olfactory bulb take the
impulses to many areas of the brain, specifically
to those responsible for memory (amygdala) and
emotions (limbic system). This is the reason why
smell can evoke memories of incidences that
occurred many years ago.
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Unlike most other neurons, the olfactory neurons
are constantly replaced every few weeks. The
renewal is regulated by special growth factors.
Clinical Anatomy
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Damage to the olfactory nerves lead to anosmia
 The delicate olfactory nerves are prone to
damage by head injury (e.g., whiplash) where
the head is jolted in an anteroposterior
direction. Here, the brain tends to move
forward/backward, thus damaging the
neurons entering vertically through the
cribriform plate.
 Tumors of meninges may invade the olfactory
nerves
Vestibulo-Cochlear Pathways
Dr. Zeenat Zaidi
Vestibular Pathway
Vestibular Pathway
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The receptors are the hair cells
located in the membranous
labyrinth
Primary afferent neuron make
dendritic contact with hair cells.
Their cell bodies are located in
the vestibular ganglion. Their
central processes:
 Mostly end up in the
vestibular nuclei (lateral,
medial, inferior and lateral)
 Some fibers go to the
cerebellum through the
inferior cerebellar peduncle
The efferents from the
vestibular nuclei project
to:
 Ipsilateral flocculonodular
lobe of cerebellum
through inferior
cerebellar peduncle
 Motor nuclei of cranial
nerves through medial
longitudinal fasciculus
 Spinal cord as lateral &
medial vestibulospinal
tracts.
 Bilaterally to ventral
posterior nucleus of
thalamus, which in turn
project to the cerebral
cortex.
Vestibular Cortex
The cortical region responsible for conscious
awareness of vestibular sensation is
uncertain but is probably:
adjacent to head area of the sensory cortex
in the parietal lobe
or
adjacent to the auditory cortex in the
temporal lobe
Medial Longitudinal Fasciculus
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Extends through out the brain stem
Continues into the spinal cord as
the medial vestibulospinal tract
Projects bilaterally
Has two components:
 The ascending component
establishes connections with the
nuclei of the 12th, 6th, 4th, & 3rd
cranial nerves for the
coordination of head and eye
movements
 The descending component
extends into the spinal cord as
the medial vestibulospinal tract
Vestibulospinal Tracts
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Vestibulospinal fibers influence
the activity of spinal motor
neurons concerned with the
control of body posture and
balance
Two tracts: lateral & medial
Lateral arises from lateral
vestibular (Deiter’s) nucleus,
descends ipsilaterally
Medial is the descending part of
the medial longitudinal
fasciculus, projects bilaterally
Cochlear (Auditory) Pathway
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We really don't hear
with our ears - we
hear with our
brains!
Sound vibrations from
the outside world are
conveyed through this
system until they
reach the brain, and
we hear the sound in
the cortex
Cochlear (Auditory) Pathway
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Multisynaptic pathway
The receptors are the
hair cells of the organ
of Corti
Primary afferent
neurons make
dendritic contact with
hair cells. Their cell
bodies are located in
the spiral ganglion.
Their central
processes terminate
in the dorsal and
ventral cochlear nuclei
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2nd order neurons ascend
into the pons, where:
 Some fibers run
ipsilaterally and terminate
in the superior olivary
nucleus
 Some fibers cross the
midline in trapezoid body
and terminate in the
nucleus of trapezoid
body or in the
contralateral superior
olivary nucleus
From the superior olivary
nuclei, ascending fibers
comprise the lateral
lemniscus, which runs
through tegmentum of pons
and terminate in the inferior
colliculus of the mdibrain
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Some axons within lateral
lemniscus terminate in
small nucleus of the lateral
lemniscus
The inferior colliculus
project to medial geniculate
nucleus of thalamus
The axons originating in
the medial geniculate
nucleus (auditory radiation)
pass through sublentiform
part of the internal capsule
to the primary auditory
cortex (Brodmann’s areas
41, 42) located in the
dorsal surface of the
superior temporal gyrus
(Heschl’s gyri)
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The region of temporal
lobe surrounding the
primary auditory cortex
is known as the
auditory association
cortex or Wernick’s
area (Brodmann’s areas
22)
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Wernick’s area is
related to processing of
language by the brain
Cochlear (Auditory) Pathway cont’d
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The tonotopic pattern
in the auditory area is
such that fibers for
sounds of low
frequency end in the
anterolateral part,
whereas fibers for
sounds of high
frequency go to
posteromedial part
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Superior olivary nucleus sends olivocochlear fibers
to end in organ of Corti through the
vestibulocochlear nerve. These fibers are inhibitory
in function and serve to modulate transmission to
the cochlear nerve
Superior olivary nucleus & the nucleus of the
lateral lemniscus establish reflex connections with
motor neurons of trigeminal and facial motor nuclei
mediating contraction of tensor tympani and
stapedius muscles in response to loud noise
Inferior colliculi establish reflex connections with
motor neurons in the cervical spinal segments
(tectospinal tract) for the movement of head and
neck in response to auditory stimulation
Clinical Notes
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Disturbnce of vestibular nerve functions
 Vertigo
 Nystagmus
Disturbnce of cochlear nerve functions
 Deafness and tinnitis
The representation of cochlea is essentially bilateral at all
levels rostral to the cochlear nuclei
Lesions anywhere along the pathway usually have no
obvious effect on hearing.
Deafness is essentially only caused by damage to the
middle ear, cochlea, or auditory nerve.
Acoustic neuroma: a benign tumour of 8th nerve leads
to compression of the nerve leading to attacks of
dizziness, and profound deafness and ataxia