Exam 3 Flashcards
What is the sclera
tough outer layer of the eye
thins in front to become the cornea
what is the cornea
Performs 2/3 of the bending of light
Innervated by many sensitive nerve endings to protect it
What is the corneal reflex
Involuntary reflexive blinking of the eye when cornea is stimulated
Detected by afferents of CNV
what is blinking mediated by
motor efferents of CNVII
What is the lens
Performs 1/3 of bending light
Focuses images on retina
Shape is controlled by smooth ciliary muscles
what is eye accommodation
contraction of the ciliary muscles changes the shape of the lens to focus it at different distances
What is the Iris
Controls the amount of light entering the eye through the pupil
what controls the diameter of the pupil
pupillary constrictor
pupillary dilator
what is the anterior chamber
space between cornea and iris
filled with aqueous humor
what is the posterior chamber
space between iris and lens
filled with aqueous humor
what is the vitreous chamber
space between the back of the lens and the surface of the retina
filled with vitreous humor
what are the characteristics of the retina
part of the CNS, not the periphery
Derived from the neural tube
Layered structure ~250 micrometers thick
what is the pigment epithelium
layer of non neuronal cells that form the innermost layer of the retina
maintains rod and cone cells
what is the main types of retinal cells
photoreceptors
local circuit neurons
projection neurons
what are photoreceptors
rods and cones
what are local circuit neurons
bipolar, horizontal, and amacrine cells
what are the projection neurons
ganglion cells
what is the outer segment
near back of retina, embedded in pigment epithelium
phototransduction occurs here
what is the inner segment
closer to cell body
No phototransduction
what are the synaptic endings
release glutamate onto bipolar and horizontal cells
what are the physiological characteristics of photoreceptors
Rods and Cones do not generate APs
Amount of NT released is graded and inversely proportional to light levels
What does it mean that NT is inversely proportional to light level
More light = less glutamate released
What are Rods
Outer Segment is larger than cones
More photopigment
More sensitive to light (can be activated in low light)
what is the opsin molecule for rods
rhodopsin
what are cones
outer segment is smaller
less photopigment
activated at higher light level
what are the opsin molecules of cones
photopsins (red, green, blue)
what is rhodopsin
found in rods
changes conformation when light is absorbed by retinal
responds to a range of wavelengths in the middle of the visible spectrum
what are photopsins
found in cones
change in conformation when light is absorbed by retinal
respond to specific ranges of light that peak at blue, green, and red wavelengths
what is CNII
axons of retinal ganglion cells that project to the thalamus via the optic chiasm
What is the optic disk/papilla
location where axons of ganglion cells come together to exit the eye and form the optic nerve
forms a blind spot bc there are no photoreceptors
what is the macula
area near the center of the retina surrounding the fovea
High acuity
what is acuity
describes the clarity or sharpness of vision
what is the fovea
center of the macula
mostly cones, no rods
location of highest acuity in retina
why does the fovea have the highest acuity in the retina
high density of photoreceptors
no blood vessels
cell bodies moved aside to let light in
what is the distribution of retinal cells in the periphery
low acuity
lower density of photoreceptors
larger number of rods than cones
describe the convergence of rods in the periphery
high convergence of rods and bipolar cells onto ganglion cells
poor spatial resolution
describe the convergence of cones in the fovea
low convergence of cones and bipolar cells onto ganglion cells
high spatial resolution
what is the visual field
any location in space from which light could reach a cell in the visual system
what is a receptive field
the part of the visual field from which light can change the activity of a given cell in the visual system
what is the ON center of cells
activated (depolarized) when light is in the center of the RF
what is OFF center of cells
activated (hyperpolarized) when light is in the surrounding of the RF
what are the properties of photoreceptors
graded response to light
constant release of glutamate in the dark
hyperpolarize in response to light
round receptive field
what are bipolar cells
graded response to light
constant release of glutamate in the dark
center-surround receptive field
ON center and OFF center
what are ganglion cells
generate APs
axons project to the thalamus
center surround receptive fields
ON center and OFF center
What are the responses of OFF BPs and GCs
PR hyperpolarize in response to light
BPs have depolarizing glutamate receptors
GCs have depolarizing glutamate receptors
What are the responses of ON BPs and GCs
PR hyperpolarize in response to light
BPs have hyperpolarizing glutamate receptors
GCs have depolarizing glutamate receptors
what are horizontal cells
graded response to light
release baseline of GABA
depolarized by glutamate from PR
generate LATERAL INHIBITION
what is the pathway from the retina to the primary visual cortex
Retinal Ganglion cells –>
Optic chiasm via optic nerve –>
thalamus via optic tract –>
V1 via optic radiations
what are M cells
Large cells bodies, dendritic trees and receptive fields
Specialized for:
large objects
sharp contrasts in light
objects moving rapidly across visual field
what are P cells
small cell bodies, dendritic trees, and receptive fields
specialized for:
small objects
fine details
objects moving slowly across visual field
what is the lateral geniculate nucleus of the thalamus
six layers
each layer has a retinotopic map
where is V1 located
along the calcarine fissure/sulcus in the occipital lobe
Describe the retinotopic map in V1
Contralateral visual field
what are the receptive fields in RGC and LGN thalamus
circular receptive fields, each with a center and surrounding region
either region can be “on” or “off”
what are the receptive fields of the V1
elongated receptive fields with bands of “on” and “off” regions
sensitive to stimulus orientation and movement of a stimulus in a specific direction
How are neurons grouped together in the V1 cortex
Ocular dominance
Receptive field orientation
color preference or no color preference
what results from damage to ventral stream association cortices
recognition and identification of stimuli
patients are aware of objects in their visual field but cannot identify them
what is Achromatopsia
inability to perceive color
caused by bilateral lesions of area V4
what is visual apperceptive agnosia
impaired ability to group/integrate parts of an object to perceive it as a whole
cannot draw/copy an object or identify what it is
damage to inferior temporal cortex
what is visual associative agnosia
patients can perceive an object but cannot recognize it or describe what it is for
can draw/copy objects but not identify it
damage to anterior temporal lobe
what is Prosopagnosia
patients cannot identify people by their faces though they can identify components of faces and can identify people by other cues
damage to face area of the fusiform gyrus
what results from damage to the dorsal stream association cortices
inability to:
relate objects to their location
visually integrate parts of an object or multiple objects
perceive motion
what is optic ataxia
inability to use visual guidance for accurate reaching to an object
can reach accurately when give auditory cues
damage to parieto-occipital junction
what simultanagnosia
cannot perceive more than one object at a time
due to bilateral damage in the area of the parieto-occipital junction
what is akinetopsia
inability to perceive motion
bilateral lesion to area MT/V5
when is the area MT/V5 activated
tasks that require an individual to pay attention to a moving stimulus
what is contralateral neglect syndrome
tendency to ignore sensory information from and perception of the side of the body contralateral to a lesion
lesions of right parietal cortex
what are the functions of the vestibular system
detects head position and head movements
helps maintain posture and balance
involved in some reflexive eye movements
what is the vestibular apparatus
located in inner ear, near cochlea
what are semicircular canals
detect rotational acceleration of head
what are otolith organs
detect static position of head and linear acceleration
what are the three canals of the semicircular canals
horizontal
superior/anterior
posterior/inferior
what is the canals of the semicircular canals filled with
endolymph
Describe inertia of the endolymph
when you turn your head, the endolymph lags behind head motion
endolymph moves in direction opposite to head rotation
cupula only displaced during change in motion
How does the left horizontal semicircular canal respond to movement to the left
endolymph stays put and pushes the cupula and hair cells inside it to the right (toward the anterior side of the body)
Stereocilia toward kinocilium
Depolarized –> more NT
how does the right horizontal semicircular canal respond to movement to the left
endolymph stays put and pushes the capula to the right (poster side of body)
stereocillia away from kinocilium
hyperpolarize –> Less NT
what do otolith organs detect
static position of the head in space
linear acceleration
what are the two otolith organs
utricle and saccule
two different orientations
what are otoconia
calcium carbonate crystals
embedded in otolithic membrane
describe the utricle
oriented mainly horizontal
linear acceleration in the horizontal plane and head tilt
describe the saccule
oriented mainly vertically
linear acceleration in the vertical plane and head tilt
where does the vestibular nuclei receive projections from
vestibular afferents (cells bodies in vestibular ganglion)
proprioceptive information from muscle spindles in the neck
what is the rectus muscle in the eye
pulls eye in single straight direction (up and down)
what is the oblique muscle in the eye
cause the eye to rotate (torsion)
what does the medial rectus muscle do
adduction toward the nose
what does the lateral rectus muscle do
abduction away from the nose
what does the superior rectus muscle do
elevation
what does the inferior rectus muscle do
depression
what does the superior oblique muscle do
intorsion (top of eye toward nose)
what does the inferior oblique muscle do
extorsion (top of eye away from nose)
Describe CNIII
oculomotor cranial nerve
all eye muscles except superior oblique and lateral rectus (somatic motor)
lens accommodation and pupil dilation (visceral motor)
pure motor
Describe CNIV
trochlear nerve
superior oblique muscle
pure somatic motor
Describe CNVI
Abducens Nerve
Lateral rectus muscle
pure somatic motor
what is the goal of the vestibulo-ocular reflex
keep eyes fixated on the same location in space despite head movement
what are the brainstem nuclei of the vestbulo-ocular reflex
medial vestibular nucleus
abducens nucleus
oculomotor nucleus
what is the pathway for VOR
Vestibular afferents –> Medial vestibular nucleus –> abducens nuclei (ipsilateral inhibitory, contralateral excitatory) –> lateral rectus muscles (CNVI) and oculomotor nuclei –> extraocular muscles (CNII)
what does it mean that VOR circuitry is bilaterally symmetrical
Abducens nuclei receive excitatory input from the contralateral side and inhibitory input from the ipsilateral side
what does no head movement mean for the VOR
No head movement –> input is balanced so motor neurons in the abducens nuclei are not active
What is nystagmus
unilateral damage causes loss of balance of inputs resulting in alternating reflexive eye movements
what is the slow phase of nystagmus
eye move slowly toward the side of lesion (VOR)
what is the quick phase of nystagmus
eyes jerk away from side of lesion (attempting to fixate)
how do you test vestibular function
put warm or cold water in the ear canal
causes movement of fluid in horizontal semicircular canal on that side only
triggers nystagmus
Describe what happens when warm water is in the left ear canal
depolarizes vestibular afferents on LEFT
VOR triggered:
slow phase: eyes to right
quick phase: eyes to left
