AUDITORY PATHWAY
INTRODUCTION
Higher center for hearing is in temporal
lobe. The fibers of auditory pathway (Vestibulocochlear Nerve, VIII Cranial
Nerve) terminates in temporal lobe.
RECEPTORS
The
hair cells in organ of corti are the receptors of the auditory sensations.
Two types of hair cells outer and inner
hair cells. Each hair cells are innervated by afferent and efferent nerve
fibers.FIRST ORDER NEURONS
The bipolar
cells form the spiral ganglion situated in the modiolus of cochlea represents
the first order neurons.
These neurons
enters the medulla oblongata. After entering the medulla oblongata, the fibers
divide into two groups which end on ventral cochlear nucleus and dorsal
cochlear nucleus of the same side in medulla oblongata. SECOND ORDER NEURONS
The neurons of
the dorsal and ventral cochlear nuclei in the medulla oblongata form the second
order neurons of auditory pathway.
The axons of the
second order neurons pass through a complex pathway, which runs in four different
directions. First Group - Nerve fibers cross the midline and run to the opposite side to form trapezoid body go to the superior olivary nucleus.
Second Group- Second group neurons terminate at the superior olivary nucleus of same side via trapezoid body of the same side.
Third Group - It runs in the lateral lemniscus of the same side and terminate in the nucleus of lateral lemniscus of same side.
Fourth Group - It runs into the reticular formation, cross the midline as intermediate trapezoid fibers and finally join the nucleus of lateral lemniscus of opposite side.
THIRD ORDER NEURONS
Third order
neurons are in the superior olivary nuclei and nucleus of lateral lemniscus.
The third order
order neurons end in medial geniculate body which forms subcortical auditory
center. Fibers from the medial
geniculate body go to the temporal cortex, via internal capsule as auditory
radiation.
The fibers of auditory radiation are
involved in reflex movement of head in response to auditory stimuli.
CORTICAL AUDITORY CENTERS
It is present in the temporal lobe of the cerebral cortex. The auditory areas are-- area 41- It is primary area situated in the superior temporal gyrus.
- area 42 -It is primary area situated in the superior temporal gyrus.
- wernicke’s area- It is secondary auditory area situated posterior to area 41 and area 42.
- Area 22- It is known as audito psychic area and is related with analysis and interpretation of sounds.
FUNCTIONS OF CORTICAL AUDITORY CENTERS
Perception of auditory impulses (Area 41 & Area 42).
Analysis of pitch and intensity of sound. (Wernicke’s area)
- Determination of source of sound. (Wernicke’s area)
Mechanism of Hearing
Mechanism of Hearing
Acoustic energy, in the form of sound
waves, is channeled into the ear canal by the pinna. Sound waves strike the
tympanic membrane, causing it to vibrate like a drum, and changing it into
mechanical energy. The malleus, which is attached to the tympanic membrane,
starts the ossicles into motion. (The middle ear components mechanically
amplify sound). The stapes moves in and out of the oval window of the cochlea
creating a fluid motion. The fluid movement within the cochlea causes membranes
in the Organ of Corti to shear against the hair cells. This creates an
electrical signal which is sent via the Auditory Nerve to the brain, where
sound is interpreted!
Taste
(gustatory) pathway
Receptors
Taste buds on tongue, lips, palatal arch
and soft palate. Each “bud” contains several cell types in microvilli (taste
hairs) that project through taste pore. Gustatory receptor cells communicate
with cranial nerve axon endings to transmit sensation to brain. Cranial Nerves
of taste.
- Anterior 2/3 tongue: chorda tympani→ Facial
nerve
- Posterior 1/3 tongue: Glossopharyngeal
nerve
- Most posterior part of the tongue: Vagus
nerve
First
order neuron:
The first order neuron in the pathway
is the geniculate ganglion of the facial nerve and inferior ganglia of the
glossopharyngeal and vagus nerves.
Second
order neuron:
The second order neuron is the
nucleus solitarius and its upper part enlarged and called the gustatory
nucleus. The axons of the cells of the nucleus ascend to end in the posteromedial ventral nucleus of the thalamus via medial lemniscus.
Third
order neuron:
The third order neuron is the
posteromedial ventral nucleus of the thalamus. The axons of the cells pass
through the sensory radiation to the gustatory area in the superior wall of the
posterior ramus of the lateral sulcus.
Visual
Pathway
The visual pathway includes the
interneurons of the retina, retinal ganglion cells whose axons project via the
optic nerve, chiasma, and optic tract to the lateral geniculate nucleus (LGN)
and neurones within the LGN which project via the optic radiation to the
primary visual cortex.
Axons of ganglion cells of the retina → optic nerve →
optic chiasm (partial decussation) → optic tract → lateral geniculate nucleus
(thalamic relay nucleus for vision) → corona radiata (optic radiation) →
primary visual cortex
Receptor
Receptors are the rod (cylindrical
processes) and cons (conical processes) of the retina.
First
order neuron:
The axons of the cells synapse with
the dendrites of the bipolar cells. Bipolar cells are the first order neuron in
this pathway. The axons of bipolar cells
synapse with the dendrites of ganglion cells.
Second
order neuron:
The ganglion cells is the second order neuron in this pathway, the axons
of the ganglion cells forming the optic nerve.
The retina can be divided by a horizontal
line bisecting the fovea into 2 halves; temporal and nasal halves. The fibers from
the nasal half cross to the opposite side. While the fibers from the temporal
half pass through the optic chiasma without crossing.
Optic
Nerve
The unmyelinated axons of ganglion cells
collect at the optic disk and exit from the eye, about 3 or 4 mm to the nasal
side of its center, as the optic nerve. They pierce the sclera in a region
called the lamina cribrosa. Here they acquire myelin sheaths and forming the
optic nerve. The optic nerve leaves the orbital cavity through the optic canal
and unites with the optic nerve of the opposite side to form the optic chiasma.
The
optic chiasma
- The optic chiasma is a flattened bundle of
nerve fibers situated at the junction of the anterior wall and floor of the
third ventricle just anterior to the infundibular stalk.
- The anterolateral corners of the chiasma
are continuous with the optic nerves, and the posterolateral corners are
continuous with the optic tracts.
- All fibers from the nasal half of each
retina cross to the contralateral optic tract. All fibers from the temporal
half of each retina pass through the lateral portions of the chiasm without
crossing and enter the ipsilateral optic tract.
Optic
Tract
The optic tract emerges from the optic
chiasma and passes posterolaterally around the cerebral peduncle.
Each optic tract contains:
- The fibres arising in the temporal retina
of the ipsilateral eye (same side)
- The fibres of the nasal retina of the
contralateral eye (opposite side)
Most
of the fibers now terminate by synapsing with nerve cells in the lateral
geniculate body.
Lateral
Geniculate Body
The lateral geniculate body is a small,
oval swelling projecting from the pulvinar of the thalamus. It forms the third
order neuron. It consists of six layers
of cells, on which synapse the axons of the optic tract. The axons of the nerve
cells within the geniculate body leave it to form the optic radiation.
Optic
Radiation
The fibers of the optic radiation are the
axons of the nerve cells of the lateral geniculate body that passes posteriorly
through the retrolenticular part of the internal capsule and terminates in the
visual cortex.
Visual
cortex
The visual cortex (area 17) occupies the
upper and lower lips of the calcarine sulcus on the medial surface of the
cerebral hemisphere.
The visual association cortex (areas 18 and
19) is responsible for recognition of objects and perception of color.
Neurons
of the Visual Pathway
Four neurons conduct visual impulses to the
visual cortex:
(1)
rods and cones, which are specialized receptor neurons in the retina;
(2)
bipolar neurons, which connect the rods and cones to the ganglion cells;
(3) ganglion cells, whose axons pass to the
lateral geniculate body.
(4)
neurons of the lateral geniculate body, whose axons pass to the cerebral
cortex.
Binocular
Vision
In
binocular vision, the right and left fields of vision are projected on portions
of both retinae. The image of an object in the right field of vision is
projected on the nasal half of the right retina and the temporal half of the
left retina. In the optic chiasma, the axons from these two retinal halves are
combined to form the left optic tract. The lateral geniculate body neurons now
project the complete right field of vision on the visual cortex of the left
hemisphere and the left visual field on the visual cortex of the right
hemisphere. The lower retinal quadrants (upper field of vision) project on the
lower wall of the calcarine sulcus, while the upper retinal quadrants (lower
field of vision) project on the upper wall of the sulcus.
Pupillary
Light Reflex
Light directed into one eye cause both pupils
to constrict. The response of the pupil of the illuminated eye is called the direct
pupillary light reflex. That of the other eye is called the consensual
pupillary light reflex.
Impulse from the retina → Optic tract axons (afferent
limb) → lateral root of the optic tract → lateral geniculate body → superior
brachium → pretectal nucleus → Edinger-Westphal nucleus (bilaterally) →
inferior division of the oculomotor nerve (as preganglionic parasympathetic
fibres) → Ciliary ganglion → postganglionic parasympathetic fibres (short
ciliary nerves) → Sphincter pupillae muscle
Both pupils constrict in the consensual
light reflex because the pretectal nucleus sends fibers to the parasympathetic
nuclei on both sides of the midbrain.
Near
Reflex (accommodation reflex)
When visual attention is directed to a nearby
object 3 things happen in a reflex manner:
- Convergence of eyes, so that the image of
the object falls on both foveae;
- Contraction of the ciliary muscle and a
resultant thickening of the lens, so the image of the object is in focus on the
retina;
- Pupillary constriction, which improves the
optical performance of the eye by reducing certain types of aberration and by
increasing its depth of focus.
Normal visual pathway → primary visual cortex → visual
association cortex --> superior colliculus and/or pretectal area or (visual
eye field in the medial side of the frontal lobe through the superior
longitudinal bundle) → Corticobulbar fibers → Edinger-Westphal nucleus →
ciliary ganglion → short ciliary nerves → sphincter pupillae muscle and ciliary
muscle (contraction of the ciliary muscle increase the thickness of the lens)
with convergance of both eyes by the contraction of medial rectus in both side.
Corneal
Reflex
Light touching of the cornea or conjunctiva
results in blinking of the eyelids. Afferent impulses from the cornea or
conjunctiva travel through the ophthalmic division of the trigeminal nerve to
the sensory nucleus of the trigeminal nerve. Internuncial neurons connect with
the motor nucleus of the facial nerve on both sides through the medial
longitudinal fasciculus. The facial nerve and its branches supply the
orbicularis oculi muscle, which causes closure of the eyelids.
Olfactory
pathways
Olfactory
Receptors
The olfactory receptors are embedded in
mucous membrane of the upper part of the nasal cavity above the superior
concha. The fine central processes of bipolar receptor nerve cells form the
olfactory nerve fibers. Bundles of these
nerve fibers pass through the openings of the cribriform plate of the ethmoid
bone to enter the olfactory bulb.
Olfactory
Bulb
This ovoid structure have several types of
nerve cells; the mitral cells, tufted cells and granular cells. The incoming
olfactory nerve fibers synapse with the dendrites of the mitral cells and form
synaptic glomeruli. The olfactory bulb, in addition, receives axons from the
contralateral olfactory bulb through the olfactory tract.
Olfactory
Tract
This narrow band of white matter runs from
the posterior end of the olfactory bulb under the inferior surface of the
frontal lobe of the brain. It consists of the central axons of the mitral and
tufted cells of the bulb and some fibers from the opposite olfactory bulb.
As the olfactory tract reaches the anterior
perforated substance, it divides into medial and lateral olfactory striae.
- The lateral
stria carries the axons to the olfactory area of the cerebral cortex.
- The medial olfactory stria carries the
fibers that cross the median plane in the anterior commissure to pass to the
olfactory bulb of the opposite side.
Anterior commissure is a small commissure
that connects the two halves of the olfactory system.
Olfactory
Cortex
It includes the following regions;
Primary
olfactory cortex:
uncus, limen insulae (apical
region of the insula) and corticomedial part of the amygdaloid body.
Secondary
olfactory cortex:
Secondary olfactory cortex is the entorhinal area (anterior part of the
parahipocampalgyrus that lies behind the uncus.
The olfactory cortex collectively called piriform
lobe. Note the olfactory cortex is the one area of cortex that receives direct
sensory input without an inter posed thalamic connection.
- Anterior 2/3 tongue: chorda tympani→ Facial nerve
- Posterior 1/3 tongue: Glossopharyngeal nerve
- Most posterior part of the tongue: Vagus nerve
First order neuron:
The first order neuron in the pathway is the geniculate ganglion of the facial nerve and inferior ganglia of the glossopharyngeal and vagus nerves.
Second order neuron:
The second order neuron is the nucleus solitarius and its upper part enlarged and called the gustatory nucleus. The axons of the cells of the nucleus ascend to end in the posteromedial ventral nucleus of the thalamus via medial lemniscus.
Third order neuron:
The third order neuron is the posteromedial ventral nucleus of the thalamus. The axons of the cells pass through the sensory radiation to the gustatory area in the superior wall of the posterior ramus of the lateral sulcus.
Visual Pathway
The visual pathway includes the interneurons of the retina, retinal ganglion cells whose axons project via the optic nerve, chiasma, and optic tract to the lateral geniculate nucleus (LGN) and neurones within the LGN which project via the optic radiation to the primary visual cortex.
Axons of ganglion cells of the retina → optic nerve → optic chiasm (partial decussation) → optic tract → lateral geniculate nucleus (thalamic relay nucleus for vision) → corona radiata (optic radiation) → primary visual cortex
Receptor
Receptors are the rod (cylindrical processes) and cons (conical processes) of the retina.
First order neuron:
The axons of the cells synapse with the dendrites of the bipolar cells. Bipolar cells are the first order neuron in this pathway. The axons of bipolar cells synapse with the dendrites of ganglion cells.
Second order neuron:
The ganglion cells is the second order neuron in this pathway, the axons of the ganglion cells forming the optic nerve.
The retina can be divided by a horizontal line bisecting the fovea into 2 halves; temporal and nasal halves. The fibers from the nasal half cross to the opposite side. While the fibers from the temporal half pass through the optic chiasma without crossing.
Optic Nerve
The unmyelinated axons of ganglion cells collect at the optic disk and exit from the eye, about 3 or 4 mm to the nasal side of its center, as the optic nerve. They pierce the sclera in a region called the lamina cribrosa. Here they acquire myelin sheaths and forming the optic nerve. The optic nerve leaves the orbital cavity through the optic canal and unites with the optic nerve of the opposite side to form the optic chiasma.
The optic chiasma
- The optic chiasma is a flattened bundle of nerve fibers situated at the junction of the anterior wall and floor of the third ventricle just anterior to the infundibular stalk.
- The anterolateral corners of the chiasma are continuous with the optic nerves, and the posterolateral corners are continuous with the optic tracts.
- All fibers from the nasal half of each retina cross to the contralateral optic tract. All fibers from the temporal half of each retina pass through the lateral portions of the chiasm without crossing and enter the ipsilateral optic tract.
Optic Tract
The optic tract emerges from the optic chiasma and passes posterolaterally around the cerebral peduncle.
Each optic tract contains:
- The fibres arising in the temporal retina of the ipsilateral eye (same side)
- The fibres of the nasal retina of the contralateral eye (opposite side)
Lateral Geniculate Body
The lateral geniculate body is a small, oval swelling projecting from the pulvinar of the thalamus. It forms the third order neuron. It consists of six layers of cells, on which synapse the axons of the optic tract. The axons of the nerve cells within the geniculate body leave it to form the optic radiation.
Optic Radiation
The fibers of the optic radiation are the axons of the nerve cells of the lateral geniculate body that passes posteriorly through the retrolenticular part of the internal capsule and terminates in the visual cortex.
Visual cortex
The visual cortex (area 17) occupies the upper and lower lips of the calcarine sulcus on the medial surface of the cerebral hemisphere.
The visual association cortex (areas 18 and 19) is responsible for recognition of objects and perception of color.
Neurons of the Visual Pathway
Four neurons conduct visual impulses to the visual cortex:
(1) rods and cones, which are specialized receptor neurons in the retina;
(2) bipolar neurons, which connect the rods and cones to the ganglion cells;
(3) ganglion cells, whose axons pass to the lateral geniculate body.
(4) neurons of the lateral geniculate body, whose axons pass to the cerebral cortex.
Binocular Vision
In binocular vision, the right and left fields of vision are projected on portions of both retinae. The image of an object in the right field of vision is projected on the nasal half of the right retina and the temporal half of the left retina. In the optic chiasma, the axons from these two retinal halves are combined to form the left optic tract. The lateral geniculate body neurons now project the complete right field of vision on the visual cortex of the left hemisphere and the left visual field on the visual cortex of the right hemisphere. The lower retinal quadrants (upper field of vision) project on the lower wall of the calcarine sulcus, while the upper retinal quadrants (lower field of vision) project on the upper wall of the sulcus.
Pupillary Light Reflex
Light directed into one eye cause both pupils to constrict. The response of the pupil of the illuminated eye is called the direct pupillary light reflex. That of the other eye is called the consensual pupillary light reflex.
Impulse from the retina → Optic tract axons (afferent limb) → lateral root of the optic tract → lateral geniculate body → superior brachium → pretectal nucleus → Edinger-Westphal nucleus (bilaterally) → inferior division of the oculomotor nerve (as preganglionic parasympathetic fibres) → Ciliary ganglion → postganglionic parasympathetic fibres (short ciliary nerves) → Sphincter pupillae muscle
Both pupils constrict in the consensual light reflex because the pretectal nucleus sends fibers to the parasympathetic nuclei on both sides of the midbrain.
Near Reflex (accommodation reflex)
When visual attention is directed to a nearby object 3 things happen in a reflex manner:
- Convergence of eyes, so that the image of the object falls on both foveae;
- Contraction of the ciliary muscle and a resultant thickening of the lens, so the image of the object is in focus on the retina;
- Pupillary constriction, which improves the optical performance of the eye by reducing certain types of aberration and by increasing its depth of focus.
Normal visual pathway → primary visual cortex → visual association cortex --> superior colliculus and/or pretectal area or (visual eye field in the medial side of the frontal lobe through the superior longitudinal bundle) → Corticobulbar fibers → Edinger-Westphal nucleus → ciliary ganglion → short ciliary nerves → sphincter pupillae muscle and ciliary muscle (contraction of the ciliary muscle increase the thickness of the lens) with convergance of both eyes by the contraction of medial rectus in both side.
Corneal Reflex
Light touching of the cornea or conjunctiva results in blinking of the eyelids. Afferent impulses from the cornea or conjunctiva travel through the ophthalmic division of the trigeminal nerve to the sensory nucleus of the trigeminal nerve. Internuncial neurons connect with the motor nucleus of the facial nerve on both sides through the medial longitudinal fasciculus. The facial nerve and its branches supply the orbicularis oculi muscle, which causes closure of the eyelids.Olfactory pathways
Olfactory
Receptors
The olfactory receptors are embedded in
mucous membrane of the upper part of the nasal cavity above the superior
concha. The fine central processes of bipolar receptor nerve cells form the
olfactory nerve fibers. Bundles of these
nerve fibers pass through the openings of the cribriform plate of the ethmoid
bone to enter the olfactory bulb.
Olfactory
Bulb
This ovoid structure have several types of
nerve cells; the mitral cells, tufted cells and granular cells. The incoming
olfactory nerve fibers synapse with the dendrites of the mitral cells and form
synaptic glomeruli. The olfactory bulb, in addition, receives axons from the
contralateral olfactory bulb through the olfactory tract.
Olfactory
Tract
This narrow band of white matter runs from
the posterior end of the olfactory bulb under the inferior surface of the
frontal lobe of the brain. It consists of the central axons of the mitral and
tufted cells of the bulb and some fibers from the opposite olfactory bulb.
As the olfactory tract reaches the anterior
perforated substance, it divides into medial and lateral olfactory striae.
- The lateral
stria carries the axons to the olfactory area of the cerebral cortex.
- The medial olfactory stria carries the
fibers that cross the median plane in the anterior commissure to pass to the
olfactory bulb of the opposite side.
Anterior commissure is a small commissure
that connects the two halves of the olfactory system.
Olfactory
Cortex
It includes the following regions;
Primary
olfactory cortex:
uncus, limen insulae (apical
region of the insula) and corticomedial part of the amygdaloid body.
Secondary
olfactory cortex:
Secondary olfactory cortex is the entorhinal area (anterior part of the
parahipocampalgyrus that lies behind the uncus.
The olfactory cortex collectively called piriform
lobe. Note the olfactory cortex is the one area of cortex that receives direct
sensory input without an inter posed thalamic connection.
Olfactory Receptors
The olfactory receptors are embedded in mucous membrane of the upper part of the nasal cavity above the superior concha. The fine central processes of bipolar receptor nerve cells form the olfactory nerve fibers. Bundles of these nerve fibers pass through the openings of the cribriform plate of the ethmoid bone to enter the olfactory bulb.
Olfactory Bulb
This ovoid structure have several types of nerve cells; the mitral cells, tufted cells and granular cells. The incoming olfactory nerve fibers synapse with the dendrites of the mitral cells and form synaptic glomeruli. The olfactory bulb, in addition, receives axons from the contralateral olfactory bulb through the olfactory tract.
Olfactory Tract
This narrow band of white matter runs from the posterior end of the olfactory bulb under the inferior surface of the frontal lobe of the brain. It consists of the central axons of the mitral and tufted cells of the bulb and some fibers from the opposite olfactory bulb.
As the olfactory tract reaches the anterior perforated substance, it divides into medial and lateral olfactory striae.
- The lateral stria carries the axons to the olfactory area of the cerebral cortex.
- The medial olfactory stria carries the fibers that cross the median plane in the anterior commissure to pass to the olfactory bulb of the opposite side.
Anterior commissure is a small commissure that connects the two halves of the olfactory system.
Olfactory Cortex
It includes the following regions;
Primary olfactory cortex:
uncus, limen insulae (apical region of the insula) and corticomedial part of the amygdaloid body.
Secondary olfactory cortex:
Secondary olfactory cortex is the entorhinal area (anterior part of the parahipocampalgyrus that lies behind the uncus.
The olfactory cortex collectively called piriform lobe. Note the olfactory cortex is the one area of cortex that receives direct sensory input without an inter posed thalamic connection.