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No. 27
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Sensory nervous pathways (2)
Ⅲ) The Visual Pathways and
Pupillary Reflex Route
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1. The visual pathway
(1) First neurons
The first order neurons of the visual
pathway are the bipolar cells in middle
layer of the retina.
Their peripheral processes synapse with
the rod and cone cells (photoreceptors)
in the outer layer of the retina.
Their central processes form synapse with
the ganglionic cells.
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(2) Second neurons
The second neurons of the route are the
ganglionic cells in the inner layer of the retina.
The axons of the second neurons aggregate at
the optic disc to form the optic nerve, which
enters the cranial cavity through the optic canal
and unite to form the optic chiasma, beyond
which, they continue as the optic tract.
Within the optic chiasma fibers (ganglion cell
axons) from the medial (nasal) half of each retina
cross to the opposite side, but the fibers from the
lateral (temporal) half remained uncrossed.
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As a consequence of the crossing of the axons in
the chiasma, the left optic tract consists of
fibers (ganglion cell axons) from the lateral
(temporal) half of the retina of the left eye and
the medial (nasal) half of the retina of the right
eye. Both of these fibers groups carry visual
information from the right visual field.
Conversely, the right optic tract consists of
axons from the lateral (temporal) half of the
retina of the right eye and the medial (nasal) half
of the retina of the left eye. Both of these fiber
groups carry visual information from the left
visual field.
On each side, the optic tract courses outward and
backward, encircling the cerebral peduncle. Most
of the fibers terminate in the lateral geniulate
body.
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(3) Third deurons
The third neurons are located in the lateral
geniculate body of the dorsal thalamus.
The third neurons then give rise to the optic
radiation which passes through the posterior
limb of the internal capsule and terminates in the
visual areas on both banks of the calcarine
sulcus of cerebrum.
A small portion of the optic tract continues as the
brachium of superior colliculus to the
pretectal area and the superior colliculi, from
which the fibers forming the tectospinal tract,
which is concerned with optic reflex.
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Visual field: You can see the space range when
your eyeballs are fixed. As the effect of dioptric
apparatuses, the images of objects in the visual
field of nasal half are projected on the temporal
half of the both retinas.
While the images of objects in the visual field of
temporal half are projected on the nasal half of
the both retinas.
The images of objects in the visual field of upper
half are projected on the lower half of the retina,
and the images of objects in the visual field of
lower half are porjected on the upper half of the
retina.
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Loss of vision for one half of the visual field is
known as hemianopia.
① The interruption of the optic nerve in one side
causes the complete blindness on the same side.
② The interruption of the fibers crossing in the
optic chiasma (sometimes caused by pituitary
tumors) results in the blindness in the temporal
halves of the visual fields of both eyes
(bitemporal hemianopia).
③ The damage to the uncrossed fibers of the
chiasma in one side produces the blindness in the
nasal half of the visual field in the corresponding
eye.
④ Lesions in one side of the optic tract, optic
radiation or optic center produce the
homonymous hemianopia in the both visual field
of the opposite side to the lesion (For example,
lesions in the right side of the optic tract, optic
radiation or optic center produce the binasal
hemianopia of visual field of right eye and
bitemporal hemianopia of visual field of left eye).
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2. Pupillary reflexes
Light shone on the retina of one eye causes both
pupils to constrict.
The response in the eye stimulated is called the
direct pupillary light reflex, while that in the
opposite eye is known as the indirect pupillary
light reflex.
Axons of retinal ganglion cells which pass via the
optic nerve, optic tract and brachium of superior
colliculus to the pretectal area,
Axons of pretectal neurons which terminate
bilaterally in Edinger-Westphal nucleus (accessory
nucleus of oculomotor nerve),
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Preganglionic fibers from the E-W nucleus course
with fibers of the oculomotor nerve and synapse
in the ciliary ganglion,
Postganglionic fibers from the ciliary ganglion
project to the sphincter of the iris to regulate the
contraction of the pupil.
Retina→optic nerve→optic chiasma→bilateral
optic tracts→brachium of superior
colliculus→pretectal area→bilateral accessory
nuclei of oculomotor nerve→oculomoter
nerve→ciliary ganglion→postganglionic
fibers→sphincter of the iris→pupils to constriction
of pupils.
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Light shone on the retina of one eye
whose optic nerve is injured, does not
cause both pupils to constrict, but light
shone on the healthy one cause both
pupils to constrict.
The pupil on the side of which the
oculomotor nerve is damaged, does not
constrict when light is shone on either
pupil.
Ⅳ) The Acoustic (or auditory)
Pathway
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1. First neurons
The first neurons are bipolar cells in
the cochlear ganglion.
Their peripheral processes run to the
spiral organ (of Corti) in the internal ear.
Their central processes join the cochlear
nerve and pass through the internal
acoustic meatus to the cochlear nuclei.
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2. Second neurons
The second neurons are in the cochlear nuclei.
Some axons of the second neurons course
medially along the ventral border of the pontine
tegmentum to form the trapezoid body which
passes through or ventrally to the medial
lemniscus. They next cross the raphe to form a
longitudinal ascending bundle known as the
lateral lemniscus. The fibers of the lateral
lemniscus terminate directly or indirectly in the
medial geniculate body.
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3. Third neurons
The third neurons are the cells of the
medial geniculate body.
Their axons join the acoustic radiation.
The fibers of the acoustic radiation course
via the posterior limb of the internal
capsule (the inferior thalamic radiation) to
the transverse temporal gyri.
Because the acoustic center on one side
receives fibers from the bilateral cochlear
nuclei, damage to the ipsilateral acoustic
paths does not cause a hearing defect.
Ⅴ) The Equilibratory Pathway
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The first neurons of this pathways are
bipolar neurons situated in the vestibular
ganglion, whose peripheral processes
innervate the cristae ampullares, maculae
of the utricle and saccule, and whose
central processes join the vestibular nerve
and terminate in the vestibular nucleus of
the pons.
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The vestibular nucleus contains the second
neurons whose axons are grouped into five
courses:
①joinning the medial longitudinal fasciculus
whose fibers end in the oculomotor, abducent and
trochlear nuclei, and the motor cells of the
anterior horn of the upper cervical cord,
②joinning the vestibulospinal tract to the cells of
the anterior horn of the spinal cord,
③entering the cerebellum via the inferior
cerebellar peduncle,
④connecting with the reticular formation of brain
stem, vagus and glossopharyngeal nuclei,
⑤connecting with the temporal, parietal and
frontal cortex of the hemisphere.