Visual Pathways and Reflexes

Question 1

Describe the passage of the optic nerve

Answer 1

Optic nerve consists of axons of the retinal ganglion; it goes from the eye to the optic chiasm and then the axons pass through the optic chiasm to the optic tract

Question 2

To what areas of the brain do (retinal ganglion) cells from the optic tract project?

Answer 2

Lateral geniculate nucleus (then to visual cortex in occipital lobe) and superior colliculus in the midbrain (optic tectum)

Question 3

Where do the majority of (retinal ganglion) cells, that continue through the optic chiasm, project?

Answer 3

To the visual thalamic relay (LGN) which then sends axons to the optic radiation to the primary visual cortex in the occipital lobe

Question 4

Where is the primary visual cortex located?

Answer 4

Occipital lobe

Question 5

How is visual recognition mediated?

Answer 5

By the lateral geniculate-occipital cortex pathway

Question 6

Damage to what pathway leads to perceptual blindness?

Answer 6

Lateral geniculate-occipital cortex pathway

Question 7

How is object tracking mediated?

Answer 7

Some optic nerve fibres project to the superior colliculus and this part of the brainstem is necessary for the ability to track a moving object

Question 8

Damage to what pathway leads to difficulty fixating on an object or tracking a moving object?

Answer 8

Optic nerve fibres going to the superior colliculus (midbrain)

Question 9

What is another name for the superior colliculus?

Answer 9

Optic tectum

Question 10

Explain how information from one half of the visual field reaches the contralateral visual cortex

Answer 10

The left visual field projects to the right visual cortex (and vice versa). The nasal hemiretina from the left eye and the temporal hemiretina from the right eye both project to the same right visual cortex

Question 11

What are the left and right visual fields?

Answer 11

Left or right of the fixation point

Question 12

What is the nasal hemiretina?

Answer 12

The hemiretina closest to the nose (medial), and this projects to the contralateral visual cortex

Question 13

What is the temporal hemiretina?

Answer 13

The hemiretina furthest from the nose (lateral) and this projects to the ipsilateral visual cortex

Question 14

What drug blocks the pupillary light reflex?

Answer 14

Atropine

Question 15

Describe the physiology of the pupillary light reflex

Answer 15

1. Light activates retinal afferents of optic nerve (CN II) –> pretectal nuclei (just in front of the superior colliculus)
2. Axons from the pretectal nuclei –> Edinger Westphal nuclei and the contralateral connection crosses in the posterior commissure before synapsing in the EW nuclei
3. Preganglionic fibres (parasympathetic) from the EW enter the oculomotor nerve (CN III) and synapse in the ciliary ganglion
4. Postganglionic fibres innervate the constrictor muscle of the iris (sphincter pupillae) –> pupillary constriction

Question 16

Describe the physiology of the accommodation reflex

Answer 16

1. Optic nerve afferents (CN II) travel to lateral geniculate nucleus (LGN)
2. The optic nerve afferents –> primary visual cortex (occipital lobe)
3. Some fibres from occipital lobe go to the cells of accommodation centre (midbrain) near the pre-tectal region
4. From here;fibres travel from the accommodation centre to EW nucleus
5. At EW nucleus parasympathetic stimulation occurs –> ciliary muscle contraction –> shortens the suspensory ligament –> lens relaxation and passive thickening
6. Accommodation centre also stimulates oculomotor efferents –> medial rectus –> convergence of the eyes to a near object

Question 17

What are frontal eye fields?

Answer 17

Specialised parts of the premotor cortex which are dedicated to motor control of the extraocular eye muscles

Question 18

What are the three aspects of the accommodation reflex?

Answer 18

Pupillary constriction, accommodation (lens thickening to see close object) and convergence (eyes moving in together)

Question 19

Describe how lens thickness is controlled

Answer 19

At rest the lens is under tension from the ring of suspensory ligaments which stretch and flatten it away from it’s relaxed shape. The ciliary muscle is a sphincter, and when this contracts it reduces the diameter of the ring of suspensory ligaments and provides slack to the tension –> lens bulges out to its relaxed shape (thickening –> greater refractive power)

Question 20

What is the vestibulo-ocular reflex (VOR)?

Answer 20

This reflex functions to stabilize images on the retinas during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field(s)

Question 21

Describe the physiology of the vestibulo-ocular reflex (VOR)

Answer 21

Sensory input = CN VIII (vistubulocochlear nerve) –> carries signals from semicircular canals
Output = CN VI (abducens nerve) and CN III (oculomotor nerve)

Question 22

What is nystagmus?

Answer 22

Pathology of the VOR and is a combination of an initial slow rotation followed by a fast flick back; direction of pathology is given by the direction of the fast flick

Question 23

What is the function of the blink reflex?

Answer 23

Protects the eyes from foreign bodies

Question 24

Describe the physiology of the blink reflex

Answer 24

Input: sensory nerve endings in the cornea of conjuncive (V1 - ophthalmic branch)
Output: facial nerve (CN VII) motor to the orbicularis oculi muscle to push the eyelids together

Question 25

What is a scotoma?

Answer 25

Like a blind spot and will only appear in the affected eye

Question 26

What is the result of a partial optic nerve lesion?

Answer 26

An ipsilateral scotoma e.g. if there is partial optic nerve lesion in left eye, this causes a blind spot in the left eye

Question 27

What is the result of a complete optic nerve lesion?

Answer 27

Blindness in that eye

Question 28

What is the result of an optic chiasm lesion?

Answer 28

Bitemporal hemianopia i.e. loss of the lateral portion of vison in both eyes

Question 29

What is the result of an optic tract lesion?

Answer 29

Homonymous hemianopia e.g. if there is lesion of optic tract on right side, this will cause the loss of vision on the left side in both eyes

Question 30

What is the role of the medial longitudinal fasciculus?

Answer 30

Links CN III, IV and VI near the midline