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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.

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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.

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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:
① joinning 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,
② joinning the vestibulospinal tract to the cells of the anterior horn of the spinal cord,
③ entering the cerebellum via the inferior cerebellar peduncle,
④ connecting with the reticular formation of brain stem, vagus and glossopharyngeal nuclei,
⑤ connecting with the temporal, parietal and frontal cortex of the hemisphere.