09c Visual System: Which cranial nerves are involved? Chapter 12

SensoryMotor Vision: --seeing --moving eyes --opening eyelids --pupil constriction II (Optic)III (Oculomotor) IV (Trochlear) VI (Abducens) Vision, as used for communication involves sight + movement of eye! Vision is an active process, not just passive.

Remind yourself of relationship between vision and communication, as related to clinical practice. How are eyes used in expressive communication? How are eyes used in expressive communication? How are eyes used in receptive communication? How are eyes used in receptive communication? Does receptive communication using eyes involve both sensory pathways to eye, motor pathways to eye, or both? Does receptive communication using eyes involve both sensory pathways to eye, motor pathways to eye, or both?

II: Sensory III, IV, and VI: Motor

Visual sensory: II Visual motor: III, IV, VI

Visual sensory: II (Optic nerve)

C.N. II: Receptors, dendrites and cell body in retina Bipolar cell Bipolar cell Bundles of retinal bipolar cell axons emerge from retina, and make up the two optic nerves (c.n. II) Bundles of retinal bipolar cell axons emerge from retina, and make up the two optic nerves (c.n. II) –One for each eye –Right and left optic nerves join at optic chiasm

Distribution of photosensors Distribution of photosensors –Rods- peripheral retina –Cones- central retina (macula lutea/fovea centralis) Functions of photosensors Functions of photosensors –Sensitive to rays of different wavelengths –Cones- day vision, acuity, color vision Nonfunctional in dark Nonfunctional in dark –Rods - night vision and shades of green Nonfunctional in bright or daylight Nonfunctional in bright or daylight

Transduction Transduction –Absorption of electromagnetic energy by visual pigments –Conversion into neural impulses

Pupillary Aperture Pupillary Aperture –Protective mechanism in intense light –Regulation of light amount entering eyes –Dilator & constrictor muscle fibers of iris Bright light- parasympathetic constriction of pupil Bright light- parasympathetic constriction of pupil –Small amount of light in Dim light- sympathetic dilation; Dim light- sympathetic dilation; –greater amount of light in

Pupillary light reflex

Superior view Optic nerves (c.n. II) are in the PNS. Optic nerves join at the optic chiasm.

Where is the optic nerve (PNS), with reference to the brainstem? Ventral (frontal) viewLateral view

Where does the visual pathway continue after optic nerve? (Trace 1 st order and 2 nd order visual sensory neurons: Superior view) Crossing at optic chiasm Crossing at optic chiasm –Some fibers cross, but not all Optic tract (in CNS) Optic tract (in CNS) Synapse at lateral geniculate body (in thalamus) Synapse at lateral geniculate body (in thalamus) Optic radiation Optic radiation End in primary visual cortex (medial occipital lobe) End in primary visual cortex (medial occipital lobe)

Projections to cortex

Trace 1 st order and 2 nd order visual sensory neurons: Lateral view Optic nerve Optic nerve Optic chiasm Optic chiasm Optic tract (in CNS) Optic tract (in CNS) Synapse at lateral geniculate body/ Synapse at lateral geniculate body/ (nucleus in thalamus) (nucleus in thalamus) Optic radiation Optic radiation End in primary visual cortex (medial occipital lobe) End in primary visual cortex (medial occipital lobe)

Optic nerve (II) coming from eyes; optic chiasm; optic tract coursing back to lateral geniculate body Ventral (frontal) viewLateral view

Which side of the retina of each eye transduces light energy : …from left visual hemifield? …from right visual hemifield?

How is cranial nerve II tested? Testing of visual fields Testing of visual fields –Cut #1: blindness –Cut #2: bitemporal heteronymous hemianop(s)ia –Cut #3 homonymous hemianop(s)ia (same would happen with complete cut across 5&6) –Damage to primary visual cortex: cortical blindness (cortical visual impairment) (brainstem) Horizontal plane

Now, consider the cranial nerves associated with visual motor function: III, IV, VI

Muscles of visual motor system Move eye Move eye Control eyelid opening Control eyelid opening Control pupil dilation Control pupil dilation Superior view, left eyeLateral view Note: For this class, you don’t need to know the names or locations of the muscles involved. Just know the motor functions (listed on left)

Possible movements of the eye, and their associated cranial nerves (For this class, no need to know associated muscles; arrows indicate muscle pull direction) + eyelid opening + pupil constriction III, oculomotor III, oculomotor Eye Eye Elevation (item C) Elevation (item C) Depression (item D) Depression (item D) Adduction (item B) Adduction (item B) Extorsion (item E) Extorsion (item E) Eyelid opening Eyelid opening Pupillary light reflex Pupillary light reflex IV, trochlear: Intorsion (item F) IV, trochlear: Intorsion (item F) VI, abducens: Abduction (item A) VI, abducens: Abduction (item A) ExtorsionIntorsion F DC B A E

IV, trochlear IV, trochlear –Sign: Hard to move eye “down and in” –Symptom: Diplopia VI, abducens VI, abducens –Sign: Eye pulled medially –Symptom: Diplopia Pathology of III, IV, VI (p. 153 of W&A) III, oculomotor III, oculomotor –Signs –Eye abducted and depresssed –Upper eyelid droops (ptosis) –Pupil dilated and non- reactive –Symptoms –Diplopia (double vision)

Sidebar for the curious…. Checking c.n. III after head injury (pupillary light reflex; autonomic) Checking c.n. III after head injury (pupillary light reflex; autonomic) Locked-in syndrome (maladie de l'emmuré vivant ‘walled-in alive disease’) Locked-in syndrome (maladie de l'emmuré vivant ‘walled-in alive disease’) –Complete loss of all voluntary muscles in the body except the eyes –(ventral part of the pons is damaged, e.g., from blockage of basilar artery) –Book and movie: The Diving Bell and the Butterfly (locked-in syndrome, (locked-in syndrome, augmentative movie, NIH) augmentative movie, NIH)