visual system

Sclera has blood supply, but not in ____.

cornea

chamber of the eye

anterior chamber and posterior chapter

iris-pupil

pupillary light reflex—> pretectal nucleus

lens

by ciliary muscles

vascular layer

choroid

rod and cone cells

retina

vitreous body

a transparent jellylike substance filling the interior of the eyeball

visual pathway begins with cells in the retina that convert

light into neural signals

signals are processed w/in the retina and are conveyed to the

retinal output cells

retinal output is conveyed by the axons that travel in the

optic nerve, optic chasm, and optic tract; synapses in the lateral geniculate nucleus of the thalamus

vision is a _ neuron pathway

two

first neuron in visual pathway

-optic nerve is the bundle of axons that pass from the retina to the optic chiasm

-R&L optic nerves merge at the optic chiasm, where some axons cross midline

-optic tract conveys visual info from chiasm to the lateral geniculate

(lateral geniculate body is thalamic relay nucleus)

second neuron in visual pathway

optic radiations

post synaptic neuron travels from the lateral geniculate body to the primary visual cortex

second neuron in visual pathway

size up a visual scene in conjunction with other senses like hearing

dorsal stream

"action" pathway- once we know what we see, we know what to do next (ex: reach/grab object)

dorsal stream

recognizes objects, patterns, textures, and shapes

ventral stream

visual library, aiding in recognition and memoryv

ventral stream

collateral branches of the optic nerve convey

signals to midbrain

collateral branches of the optic nerve that convey signals to the midbrain: these signals elicit

-reflexive control of the pupils

-orientation of the eyes and head

CN III has parasympathetic neurons that innervate the

intrinsic muscles of the eye

Pupillary light reflex pathway

1st neuron: parasympathetic nucleus of oculi motor nerve (in the midbrain) to the ciliary ganglion (behind the eyeball)

2nd neuron: to the pupillary sphincter and ciliary muscle

elicited by shining a bright light into one eye

pupillary light reflex

shining light into one eye causes

pupil constriction in the eye directly stimulated by bright light

pupillary light reflex pathway consists of neurons that sequentially connect:

-the retina to the pretectal nucleus in the midbrain

-the pretectal nucleus to the parasympathic nuclei of the oculomotor nerve

-parasympathetic nuclei of the oculomotor nerve to the ciliary ganglion

-ciliary ganglion to the pupillary sphincter muscle

consists of adjustments to view a near object

the near triad

the pupils constrict, the eyes converge, and the lens becomes more complex

the near triad

the accommodation reflex requires activation of the

visual cortex and an area in the frontal lobe of CC (frontal eye field)

ciliary muscles contract when

looking at near objects

increases curvature of lens

ciliary muscle

visual system provides

- sight

- processing of visual information: recognition and location of objects

- control of eye movement

- information used in postural and limb movement control

information from the nasal 1/2 of each retina:

Crosses the midline in the optic chiasm and projects to the contralateral visual cortex

information from the temporal 1/2 of each retina:

- remains ipsilateral in the optic chiasm

- projects to ipsilateral visual cortex

information from the right or left visual field delivered to

contralateral visual cortex

information reaching the primary visual cortex stimulates neurons that discriminate the

shape, size, or texture of objects

information conveyed to the visual association cortex (secondary visual cortex) is analyzed for

colors and motion

a stream of visual information that flows dorsal and used to direct movement

action stream

a stream of visual information that flows ventrally and used to recognize objects

perception stream

collateral branches from optic tract convey

visual signals to midbrain

visual signals sent to midbrain involved in

reflexive response of the pupils and orienting of head and eyes

processing or nonconscious visual information in the midbrain are

superior colliculus and pretectal area

increases refraction of light rays so that the focal point will be maintained on the retina

accommodation

two muscles control pupillary size

pupillary sphincter and pupillary dilator

parasympathetic innervation

pupillary sphincter

lesion results in pupillary dilation and impairment in pupillary constriction in response to bright light or looking at near objects

pupillary sphincter

nucleus is designer-westphal nucleus (vulnerable to compression

pupillary sphincter

pupillary sphincter innervated by

CN III

sympathetic innervation

pupillary dilator

elevates eyelid, innervates sweat glands above eyebrow

pupillary dilator

pathway begins in hypothalamus, goes to T1, then back up to eye

pupillary dilator

consequences of damage along the retinogeniculocortical pathway vary according to the

location of lesion

visual losses are described by referring to the

visual field deficit

Complete interruption of the optic nerve results in

ipsilateral blindness and loss of pupillary light reflex

optic nerve is entirely myelinated by

oligodroglia and is frequently affected by MS

interrupts the efferent limb of the light reflex, resulting in pupillary dilation and loss of both direct and consensual response in the ipsilateral eye

oculomotor lesion

complete lesion of the retina or of the optic nerve results in

total loss of vision in ipsilateral eye

loss of information in both temporal visual fields

bitemporal hemianopia

lesion at center of optic chiasm results in

bitemporal hemianopia

The loss of visual information from the same visual field, right or left, in both eyes

homonymous hemianopia

results from complete lesion of visual pathway posterior to chiasm (CVA)

homonymous hemianopia

The person has no awareness of any visual information due to a lesion in the brain

cortically blind

The ability of a cortically blind individual to orient to, point to, or detect movements of visual objects

blindsight

requires intact function of the retina and pathways from the retina to the superior colliculus and lateral geniculate nucleus

blindsight

coordination of the 2 eyes is maintained via synergistic action of the

extraocular muscles

signals conveyed by the MLF coordinate

head and eye movements by providing bilateral connections among vestibular and ocular motor nuclei in the brainstem and spinal accessory nerve nuclei in the SC

lesions affecting the CNs that innervate extraocular muscles (CN III, IV, VI) or the MLF cause

misalignment of the eyes (dysconjugate eye movements)

acute disorders affecting cranial nerves cause

double vision

chronic disorders of cranial nerves cause

misalignment and may suppress vision from one eye and the person will lose depth perception

complete lesion of the oculomotor nerve causes

-Severe ptosis (drooping of eyelid)

-Ipsilateral eye is aimed outward and down

-Diplopia (double vision)

-Deficits in moving ipsilateral eye medially, downward, and upward

-Loss of direct (ipsilateral) pupillary light reflex

-Loss of constriction of pupil in response to focusing on a near object

lesion of the trochlear nerve prevents a

activation of superior oblique muscle

ipsilateral eye cannot look downward and inward

trochlear nerve lesion

people with lesions of trochlear nerve complain of

double vision, difficulty reading, and visual problems when descending stairs

complete lesion of abducens nerve will cause eye to

deviate inward

a person with this lesion will be unable to voluntarily abduct eye and will have double vision

abducens nerve lesion

lesions affecting the MLF produces

internuclear ophthalmoplegia by interrupting signals from abducens nucleus to the OM nerve

when connection between the abducens nucleus and the OM nucleus is interrupted, the eye contralateral to the lesion moves

normally, but the eye ipsilateral to the lesion cannot adduction past midline when contralateral eye moves laterally

precise control of eye position is achieved by

-MLF

-vestibulo-ocular reflexes

-cerebral centers

superior colliculus coordinates

reflexive orienting movements of the eyes and head via the MLF

protects the eye surface

tears and blinking

light compensation; sympathetic response

pupils

moves eyes onto a target (saccades, smooth pursuit, vergence)

gaze and lens curvature

2 objectives of eye movements

Keep position of eyes stable during head movements

Direct gaze at visual targets

gaze stabilization (aka visual fixation) during head movements is achieved by

VOR and optokinetic nystagmus

fast movements to switch gaze from one object to another

saccades

eye movements that follow a moving object

smooth pursuits

movement of the eyes toward (convergence) or away from midline (divergence) to adjust for different distances between the eyes and the visual target

vergence movements

stabilize visual images during head movements; prevents them from bouncing or jumping around

VOR

vestibular receptors for the VOR are in 3 fluid-filled tubes inside each inner ear called

semicircular canals

stimulating a pair of semicircular canals induces

eye movements in roughly the same plane as the canals

VOR moves the eyes in the opposite direction of the

head movement

Adjusts eye position during slow head movements

optokinetic nystagmus

elicited by moving visual stimuli

optokinetic nystagmus

allows the eyes to follow large objects in the visual field

optokinetic nystagmus

normal response than can be elicited in an intact nervous system by optokinetic stimulation, rotation of the head, temperature stimulation of the semicircular canals, or by moving eyes to extreme horizontal position

physiologic nystagmus

sign of nervous system abnormality

pathologic nystagmus

things that may influence eye movements

-auditory info

-VOR

-visual stimuli

-sensory info from extraocular muscles

-emotion system

Quickly switch vision from one object to another

saccades

control of diagonal saccades requires adjusting relative levels of activity of the

pontine gaze center and midbrain reticular formation

saccades can be generated

voluntarily or elicited by a variety of stimuli (e.g., visual, tactile, auditory, nociceptive)

moving visual stimulus is essential for the production of

smooth pursuit movements