23 Questions
Where do the fibers of the optic nerve converge?
On the optic disc
What is the optic nerve comparable to within the central nervous system (CNS)?
A tract
Where does the optic nerve leave the orbital cavity?
Through the optic canal
Where is the optic chiasm situated?
At the junction of anterior wall and floor of 3rd ventricle
What does the optic nerve unite with to form the optic chiasma?
Optic nerve of opposite side
Which fibers cross midline and enter the optic tract of the opposite side?
Fibers from nasal half of each retina, including nasal half of macula
Where do fibers from the temporal half of each retina, including temporal half of macula, pass?
Posteriorly in optic tract of same side
Where do most fibers from the optic tract terminate?
Lateral geniculate body (LGN)
What does LGN stand for in relation to visual pathways?
Lateral Geniculate Nucleus
What is responsible for forming myelin sheaths on fibers of the optic nerve?
Oligodendrocytes
What is another name for the point where fibers from nasal and temporal halves of each retina cross midline?
Optic chiasm
Where does primary visual processing occur?
Visual cortex
What is responsible for object recognition and color perception?
Visual association cortices
Which structure is responsible for pupillary constriction during direct and consensual light reflexes?
Edinger-Westphal nuclei
What occurs when the eyes are directed from a distant to a near object?
Accommodation reflex
Which reflex occurs when the cornea or conjunctiva is touched?
Corneal reflex
What is necessary for coordinated, parallel movements of the eyes to prevent diplopia?
Frontal eye fields
Where does the abducens nucleus, embedded in the PPRF, project for abduction of the eye?
Medial longitudinal fasciculus
What serves as a reflex gaze center for automatic scanning movements of the eyes and head?
Superior colliculi
What results from destruction of the macula?
Central scotoma or blindness in the central part of the visual field
Which condition can result from compression of the optic chiasm?
Pituitary tumor
Where are contralateral defects produced from lesions past the chiasm?
Visual cortex
Which structure contains the superior colliculi and projects to the suprachiasmatic nucleus for circadian rhythms and neuroendocrine function?
Tectum
Study Notes
- Optic radiations refer to the geniculocalcarine tracts, which are the axons of neurons within the lateral geniculate body.
- The tract passes posteriorly and terminates in the visual cortex (area 17), also known as the striate cortex or Brodmann area 17.
- The visual cortex, located on the medial surface of the occipital lobe, is responsible for primary visual processing and occupies the upper and lower lips of the calcarine sulcus.
- Visual association cortices (areas 18 and 19) are adjacent to the visual cortex and are responsible for object recognition and color perception.
- Direct and consensual light reflexes occur when light is shone into one eye, resulting in pupillary constriction on the illuminated eye (direct reflex) and the opposite, non-illuminated eye (consensual reflex).
- Afferent impulses from the eye travel through the optic nerve, optic chiasm, and optic tract. A small number of fibers leave the optic tract and synapse on neurons in the olivary pretectal nucleus in the mid-brain.
- Efferent: Pretectal neurons send axons to parasympathetic nuclei (Edinger-Westphal nuclei) of cranial nerve III on both sides, which synapse and send postganglionic fibers through the short ciliary nerves to the constrictor pupillae muscle of the iris, causing pupillary constriction.
- Both pupils constrict during the consensual light reflex due to pretectal nucleus sending fibers to the parasympathetic nuclei on both sides.
- Fibers that cross the median plane do so close to the cerebral aqueduct in the posterior commissure.
- The accommodation reflex occurs when the eyes are directed from a distant to a near object. Medial recti muscles contract, causing the ocular axes to converge and the lens to thicken. Pupils also constrict to restrict light waves to the thickest central part of the lens.
- Afferent impulses from the eye travel through the optic nerve, optic chiasm, optic tract, and lateral geniculate body to the visual cortex. The visual cortex is connected to the eye field in the frontal cortex.
- Cortical fibers descend from the frontal cortex, through the internal capsule, to the oculomotor nuclei in the midbrain, resulting in the oculomotor nerve traveling to the medial recti muscles.
- Some descending cortical fibers synapse with the parasympathetic nuclei (Edinger-Westphal nuclei) of cranial nerve III on both sides, resulting in pupillary constriction.
- The accommodation reflex is a convergence reflex, with an afferent pathway through the optic nerve, an afferent center in the visual cortex, an efferent center in the oculomotor nucleus, and an efferent pathway through the oculomotor nerve.
- The corneal reflex occurs when the cornea or conjunctiva is touched, resulting in blinking of the eyelids.
- Afferent impulses from the cornea or conjunctiva travel through the ophthalmic nerve to the sensory nucleus of the trigeminal nerve. Internuncial neurons connect the motor nucleus of the facial nerve on both sides through the medial longitudinal fasciculus, allowing the orbicularis oculi muscle to close the eyelids.
- Conjugate horizontal gaze is necessary for the eyes to move in coordinated, parallel movements to ensure the image projects to the same spot on each retina and prevent diplopia.
- The frontal eye fields in each frontal lobe serve as the cortical control center for horizontal gaze. Axons from the frontal eye fields project to the brainstem and decussate to the contralateral paramedian pontine reticular formation (PPRF) in the pons.
- The brainstem control center for ipsilateral horizontal gaze is the PPRF. The abducens nucleus, embedded in the PPRF, projects to the lateral rectus muscle for abduction of the eye and to the medial longitudinal fasciculus for adduction of the contralateral eye.
- Automatic scanning movements of the eyes and head occur when reading and in response to visual stimuli. Protective closing of the eyes and raising of an arm for protection are also visual reflexes.
- Visual impulses follow the optic nerve, optic chiasm, and optic tracts to the superior colliculi, which serve as a reflex gaze center.
- Impulses are relayed to the tectospinal and tectobulbar (tectonuclear) tracts and to neurons of the anterior gray columns of the spinal cord and cranial motor nuclei.
- The tectum, or dorsal part of the midbrain, contains the superior colliculi and projects to the suprachiasmatic nucleus for circadian rhythms and neuroendocrine function.
- Destruction of the macula results in a central scotoma or blindness in the central part of the visual field. Lesions of the optic nerve cause anopsia, or loss of vision, and loss of the sensory limb of the light reflex.
- Compression of the optic chiasm can result from pituitary tumors or meningiomas, leading to loss of peripheral vision in both temporal fields.
- Lesions past the chiasm produce contralateral defects, while lesions of the visual radiations are more common and can result in cortical blindness with intact pupillary reflexes.
- Some causes of lesions include optic neuritis, central retinal artery occlusion, internal carotid artery aneurysm, pituitary tumor, craniopharyngioma, middle or posterior cerebral artery occlusion.
Test your knowledge on the optic nerve, its anatomy, and functions within the visual pathways and reflexes. Explore the role of the optic nerve in conveying visual information from the retina to the brain and its comparison to a tract within the central nervous system.
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