Transcript Olfactory pathway

Olfactory pathway
1- first neuron :
It is formed of the olfactory bipolar cells
placed in the mucosa of the roof of the nose
and upper part of the nasal cavity . The axon
of these cells form 12 -20 olfactory nerve
fibers ( non –myelinated ) which pass through
cribriform plate of ethmoid to synapse around
the cells of the olfactory
bulb.
2- Second neuron
:
It is formed of the cells of the olfactory bulb.
( interneurones and large mitral cells) Axons
from the mitral cells ( myelinated nerve fibers )
form the olfactory tract . The olfactory tract
passes backwards on the basal surface of the
frontal lobe. On reaching the level of optic
chiasma , most olfactory tract fibers form the
lateral olfactory stria. These fibers pass into
the depth of lateral fissure .
They terminate in the primary olfactory cortex
of the uncus. Also, in the amygdala which is
responsible for conscious appreciation of
sense of smell .
The uncus is present on inferomedial aspect of
the temporal lobe .
Adjacent to the uncus is the entorhinal area
(the anterior part of the parahippocampal gyrus)
which constitutes the olfactory association
cortex .
The primary and association areas are referred
as the pyriform cortex .
The medial root crosses the midline through the
anterior commissure and joins the uncrossed
lateral root of the opposite side . So, it connects
the olfactory centers of the 2 cerebral
hemisphere together .
N.B.
The olfactory projection is consists only from 2
neurones between the sensory receptors and
cerebral cortex. Also, it does not project via the
thalamus.
In a depression on either side of
the crista galli lie the cribriform
plates of the ethmoid which
accommodate the olfactory bulbs.
Anosmia
Follows damage to the olfactory nerves . There is loss in
the sense of smell and also of the flavour of foods.
It can occur when tumours of the meninges
( meningiomas ) invade the olfactory nerves .
Or follows head trauma .
N.B.
Elementary aspects of taste e.g. sweet ,salt , bitter and
sour are preserved .
Hearing pathway
1- first neurone :
The cell bodies of the cochlear nerve fibers
lie within the cochlea and are called spiral
ganglion .
The dendrites carry hearing sensation from
hair cells of the organ of Corti in the inner ear
( cochlea ) .
The axons form the cochlear nerve which
enters the brain stem at the junction of the
pons and medulla oblongata to end around
the cells of the ventral and dorsal cochlear
nuclei in sections at the pontomedullary
junction.
2- second neurone :
It is formed by the cells of the cochlear
nuclei. Few axons ascend in the lateral
lemniscus of the same side. Most of the
axons form the trapezoid body which crosses
the midline of the caudal pons to reach the
opposite side where it is joined by the few
fibers from the cochlear nuclei of the same
side and both groups of fibers ascend as
lateral lemniscus of the opposite side.
Therefore each lateral lemniscus contains
cochlear fibers and carries hearing
stimuli from both ears mostly from the
opposite ear .
Each lateral lemniscus ascends to midbrain
where the fibers end in 2 site :
A. Some fibers end directly by synapsing
around the cells of the medial geniculate
body , third order neuron ( lower center
of hearing ) (metathalamus ).
B. Other fibers end directly by synapsing
around the cells of the inferior colliculus
which send their axons through inferior
brachium quadrigeminum to the medial
geniculate body , third neuron.
The final step in the pathway consists of
axons pass through the internal capsule
to the primary auditory cortex of the
temporal lobe in Heschl’s gyri which are
situated on the dorsal surface of the
superior temporal gyrus and are largely
hidden within the lateral fissure .
The region of the temporal lobe surrounding
the primary auditory cortex is called auditory
association cortex or Wernicke’s area . In the
dominant hemisphere , it used for
understanding of the spoken word and has
important connections with other language
areas .
N.B.
Some cochlear fibers may synapse around
the cells of
1- superior olive
2- nucleus of trapezoid body
3- nucleus of lateral lemniscus
The superior olivary nucleus and the nucleus
of the lateral lemniscus establish reflex
connections with motor neurons of the
trigeminal and facial motor nuclei , mediating
contraction of the tensor tympani and
stapedius muscles in response to loud
noise.
Throughout the ascending auditory
projection there exists a so – called
tonotopical representation of the
cochlea which is analogous to the
somatotopic
At the cortical level , the organs of
hearing are bilaterally represented so
that unilateral lesions of the primary
auditory cortex cause partial
deafness .
Acoustic neuroma
It is a benign tumour of 8th nerve .
Attacks of dizziness with deafness
occur.
Unilateral or bilateral acoustic
neuromas occur in the inherited
disease neurofibromatosis .
Vestibular pathway
Function : it carries coordinating
stimuli to the eyes ,neck, and body to
maintain equilibrium according to the
position and movements of the head
.
First neurone :
Their cell bodies are located in the
vestibular ganglion in the internal
auditory meatus. The dendrites of
these cells carry stimuli from the hair
cells in the 3 semicircular canals
( crista ampularis ) , utricle and saccule
( maculae ) of membranous labyrinth
.
The axons form the vestibular nerve
which enters the brain stem at the
pontomedullary junction ends at 5
places
a. cerebellum ( direct vestibulocerebellar tract ) though inferior
cerebellar peduncle
B. 4 vestibular nuclei located close together
beneath the lateral part of the floor of the 4th
ventricle ( inferior in the medulla –lateral at
the pontomedullary junction – medial &
superior in the pons ) .
Second neurone :
The axons of vestibular nuclei form 5 tracts that
reach :
1-one tract to the cerebellum:
Indirect vestibulo-cerebellar tract :
It is formed by the axons of the superior and
lateral vestibular nuclei which pass through
the inferior cerebellar peduncle to the
flocculonodular lobe of the cerebellum . It is
concerned with the control of equilibrium.
N.B. The vestibular nuclei receive input from the
labyrinthine and cerebellum. ( Fastigial
Vestibular Pathway ) . The axons end in the
lateral vestibular nucleus on both sides. Also,
some Purkinje cell axons project directly to
lateral vestibular nucleus.
2- One ascending tract to 3, 4 and 6 cranial
nerves
Vestibulo-Ocular tract :
mainly the axons of the superior & medial nuclei
join the medial longitudinal bundle to reach 3,4
and 6 cranial nerve nuclei . It is concerned with
coordination of head and eye movements.
1- stimulation of the right vestibular nuclei causes
conjugate movement of both eyes to the left
side .
2- Vestibular reflexes help to make the eyes able
to remain fixed on stationary objects during
movement of the head & body .
3- When the head is turned slightly to the right
side , the eyes turn to the left side with an equal
distance so that the field of vision remains the
same.
The 3rd and 4th tracts are two descending
tracts:
The lateral and medial vestibulo-spinal
tracts:
The function of these tracts is to increase
the tone of the extensor muscles of the
limbs to support the body against gravity
and to maintain upright posture .
Lateral vestibulospinal tract is formed
by the axons of the lateral vestibular
nucleus ( Deiter’s nucleus ) . The fibers
descend ipsilaterally in the ventral
funiculus. It mediates excitatory
influences upon extensor motor
neurons.
Medial vestibulospinal tract is formed
by the axons of the medial ,lateral and
inferior nuclei. The fibers descend
ipsilaterally in the medial longitudinal
fasciculus and is located adjacent to
ventral median fissure .
Both tracts descend directly to end
around the anterior horn cells of the
spinal cord .
5-Vestibulo - Reticular Connection
The axons of the vestibular nuclei reach
the vomiting and vasomotor centers in the
reticular formation .
Other fibers ascend to contralateral
thalamus ( ventral posterior nucleus )
which project to cerebral cortex . It is
probably adjacent to the head area of the
sensory cortex or adjacent to the auditory
cortex in the temporal lobe .