visual systems physiology 15hunnid

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1

What are the structural components of the eye?

Optical portion of the eye: Regulation of
light amount entering eye, refraction
• Iris
• Cornea
• Lens
• Ciliary muscles and zonular fibers

Neural portion of the eye:
Phototransduction and signal integration
• Retina (Fovea, optic disc) -
• Optic nerve

Intraocular support structures
• Choroid: Contains blood vessels)
• Sclera: Protective connective tissue
• Aqueous and vitreous humor: Fill anterior, posterior
eye cavities
• Ciliary body: Generation of aqueous fluid
• Canal of Schlemm: Drainage of aqueous fluid

2

Choroid:

Contains blood vessels)

3

• Sclera:

Protective connective tissue

4

• Aqueous and vitreous humor:

Fill anterior, posterior
eye cavities

5

• Ciliary body:

Generation of aqueous fluid

6

• Canal of Schlemm:

Drainage of aqueous fluid

7

Fibrovascular structure with embedded
circular and radial muscles and central pupil
(hole)

iris

8

What is the function of the iris?

: Fibrovascular structure with embedded
circular and radial muscles and central pupil
(hole)
• Contraction of circular muscles constricts pupil
• Contraction of radial muscles dilate pupil
• Iris muscles under control of a reflex arc: Bright
light leads to contraction of circular muscles,
pupillary constriction and reduction of light
entering eye
• Conversely, low light leads to contraction of radial
muscles and pupillary dilation

9

Contraction of circular muscles constricts .................

pupil

10

Contraction of .............. dilate pupil

radial muscles

11

Iris muscles under control of a ...............: Bright
light leads to contraction of circular muscles,
pupillary constriction and reduction of light
entering eye

reflex arc

12

Conversely,................leads to contraction of radial
muscles and pupillary dilation

low light

13

• What are functions of cornea and lens?

Light passes through the front of the eye (cornea) to the lens. The cornea and the lens focus the light rays onto the back of the eye (retina). The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.

14

Lens (25%) and cornea (75%) focus
images on retina

Refraction:

15

Adjustment of lens
convexity by ciliary muscles. Images of
nearby as well as distant objects can be
focused onto retina.

Accommodation:

16

What are frequent pathologies of the optical portion of the eye?

Refractive errors
Myopia (shortsightedness; eye too long to allow
lens to focus distant objects on retina)
Hyperopia (farsightedness; eye too short to allow
lens to focus near objects on retina)
Presbyopia (loss of lens elasticity with age)
Astigmatism (irregular curvature of cornea or lens)

17

Myopia

(shortsightedness; eye too long to allow
lens to focus distant objects on retina)

18

Hyperopia

(farsightedness; eye too short to allow
lens to focus near objects on retina)

19

Presbyopia

(loss of lens elasticity with age)

20

Astigmatism

(irregular curvature of cornea or lens)

21

Cataract

(opaque lens)

22

concave lens corrects..........

myopia or near sightedness

23

a convex lens corrects ............

far sight or hyperopic sight

24

What are the sensory receptors and higher-order sensory neurons in the retina?

Photoreceptors

Cones: Color vision (mainly concentrated in fovea)
Rods: Vision under low illumination levels

Higher-order sensory neurons

Bipolar cells: Excited or inhibited by either rods or cones,
graded potentials and NT release
• Ganglion cells: Excitatory input from bipolar cells, generate
action potentials. Only neurons to project beyond retina. Die in
glaucoma and macular degeneration.
• Inhibitory interneurons:
Horizontal cells between photoreceptors,
amacrine cells between bipolar and ganglion cells:
Lateral inhibition and further processing.

25

Photoreceptors

Cones: Color vision (mainly concentrated in fovea)
Rods: Vision under low illumination levels

26

Higher-order sensory neurons

Bipolar cells: Excited or inhibited by either rods or cones,
graded potentials and NT release
• Ganglion cells: Excitatory input from bipolar cells, generate
action potentials. Only neurons to project beyond retina. Die in
glaucoma and macular degeneration.
• Inhibitory interneurons:
Horizontal cells between photoreceptors,
amacrine cells between bipolar and ganglion cells:
Lateral inhibition and further processing.

27

Excited or inhibited by either rods or cones,
graded potentials and NT release

Bipolar cells:

28

Excitatory input from bipolar cells, generate
action potentials. Only neurons to project beyond retina. Die in
glaucoma and macular degeneration.

Ganglion cells:

29

• Inhibitory interneurons:

Horizontal cells between photoreceptors,
amacrine cells between bipolar and ganglion cells:
Lateral inhibition and further processing.

30

..............between photoreceptors,
.............between bipolar and ganglion cells:
Lateral inhibition and further processing.

Horizontal cells

amacrine cells

31

How are photoreceptors distributed in the retina?

...

32

contains only cones
• High density of photoreceptors
• Lack of bipolar, ganglion cell layer
• No convergence

Fovea

33

Peripheral retina

contains mostly rods
• Density of cones decreasing with increasing
distance from fovea
• Convergence between rods and bipolar cells,
bipolar cells and ganglion cells

34

Optic disc

• Region where retinal ganglion cell axons bundle to
form optic nerve
• No photoreceptors: blind spot

35

What are photopigments?

Photoreceptors have membrane stacks called
disks in outer segment

Disks contain photopigment: Proteins with
covalently bound light-absorbing retinal (cofactor)
• Different photopigments: Rods have rhodopsin;
cones have either of three different opsins
absorbing light at different (overlapping)
wavelengths

36

......... have membrane stacks called
disks in outer segment

Photoreceptors

37

•.........contain photopigment: Proteins with
covalently bound light-absorbing retinal (cofactor)

Disks

38

Different photopigments: Rods have ...........
cones have either of three different opsins
absorbing light at different (overlapping)
wavelengths

rhodopsin;

39

................have either of three different opsins
absorbing light at different (overlapping)
wavelengths

cones

40

Photopigments confer light sensitivity to photoreceptors-explain

Light leads to isomerization of retinal
(conformational change from cis to trans), and
activates the opsin. The opsin initiates a signalling
cascade
• Light-activated trans-retinal dissociates from
opsin and is converted back to cis-retinal before
re-binding opsin

41

Phototransduction: How do photoreceptors transduce light into changes in
neurotransmitter release?

Darkness:
cGMP constantly generated
- cGMP-gated cation channels open
- persistent depolarization
-continuous neurotransmitter release
Light:
Conformational change of retinal
in opsin or rhodopsin
- activation of transducin (G protein)
- degradation of cGMP
-closure of cGMP-gated cation channels
- membrane hyperpolarization
- reduction of neurotransmitter release

42

How is photoreceptor activation relayed to retinal ganglion cells?

...

43

Visual processing in the retina

Retina has two kinds of bipolar cells: ON bipolar cells have
inhibitory metabotropic glutamate receptors, OFF bipolar
cells excitatory ionotropic glutamate receptors
• Light leads to depolarization of ON bipolar cells,
hyperpolarization of OFF bipolar cells
• ON bipolar cells synapse onto ON-center ganglion cells,

44

Retina has two kinds of bipolar cells: ..............have
inhibitory metabotropic glutamate receptors, ...........
cells excitatory ionotropic glutamate receptors
• Light leads to depolarization of ..............,
hyperpolarization of OFF bipolar cells
• ON bipolar cells synapse onto ON-center ganglion cells.

ON bipolar cells

OFF bipolar

ON bipolar cells

45

ON bipolar cells synapse onto ON-center ...............

ganglion cells.

46

• Retina has two kinds of bipolar cells: ON bipolar cells have
inhibitory metabotropic glutamate receptors, .............
cells excitatory ionotropic glutamate receptors
• Light leads to depolarization of ON bipolar cells,
hyperpolarization of OFF bipolar cells
• ON bipolar cells synapse onto ON-center ganglion cells,
OFF bipolar cells onto .................. ganglion cells

OFF bipolar

OFF-center

47

Receptive fields of ganglion cells: Due to lateral inhibition,
light in the receptive field periphery inhibits ...............
ganglion cells and stimulates .............. ganglion cells

ON-center

OFF-center

48

How does information from the retina reach the visual cortex?

Retinal ganglion cell axons form optic nerve & tract →
neurons in thalamus → neurons in visual cortex
• Decussation of axons from ganglion cells in nasal half of
retinas at optic chiasm: Right half of visual field represented in
left visual cortex

49

• To midbrain:

Control of changes in pupil size in response to
illumination

50

: Control of eye movements, movements of head
and neck

• To midbrain

51

Control of biological clock by light

To hypothalamus:

52

• What is sound?

• Sound = Waves of compressed and expanded
air

53

....................determined by magnitude of
pressure differences (wave amplitude)

Loudness

54

...............determined by the speed with which
pressure changes oscillate (wave frequency)

Pitch

55

How is sound transmitted in the outer and middle ear?

1) Sound waves funneled by pinna and external
auditory canal onto tympanic membrane,
which starts to vibrate
2) The tympanic membrane vibration causes
movement of connected auditory ossicles
(middle ear bones)
3) Auditory ossicles couple vibrations to oval
window (= membrane separating middle ear
and inner ear). Leverage of auditory ossicles
causes additional amplification of sound
(=vibration amplitude)

56

How is sound transmitted in the inner ear?

4) Vibrations of the oval window causes
pressure waves in the perilymph
(fluid) in scala vestibuli. and scala
tympani.
5) Pressure waves deform the walls of
the scala vestibuli and cause pressure
waves in endolymph which fills
cochlear duct
6) Pressure waves in endolymph lead to
vibrations of the basilar membrane.
Depending on frequency of the
pressure waves, i.e. sound pitch,
different regions of the basilar
membrane vibrate.

7) Resting on basilar membrane is Organ of Corti
with inner hair cells (= sensory receptors). Inner
hair cells have stereocilia (=protrusions) on apical
tip which are embedded in a structure called the
tectorial membrane. When basilar membrane
vibrates, stereocilia are bent

57

Auditory transduction: How do hair cells transduce sound into neurotransmitter release?

8) Stereocilia contain mechanically
gated cation ion channels
connected by tip link proteins.
Bending of stereocilia opens or closes
ion channels, leads to K+ influx and
membrane depolarization
9) Membrane depolarization causes
Ca2+ influx and graded release of
neurotransmitter glutamate
10) Glutamate depolarizes primary
sensory neuron, which elicits action
potentials

58

• How are sound pitch and intensity encoded in the auditory information?

• Sound pitch determines which hair cells
depolarize (dependent on location on
basilar membrane): high-pitched sounds
activate hair cells at base of cochlea, lowpitched sounds hair cells at apex
• Sound amplitude (loudness) determines
amount of hair cell depolarization, glutamate release, and ultimately action potential
frequency of first-order auditory neuron

59

How does information from the cochlea reach the auditory cortex?

• Auditory pathway polyneuronal, with relays in
brainstem (cochlea nuclei, inferior olivary
nucleus), midbrain (inferior colliculus) and
thalamus (medial geniculate nucleus) before
terminating in primary auditory cortex
• Inferior olivary nucleus important in sound
localization: Detects differences in timing of
action potentials arriving from two ears

60

How is the primary auditory cortex organized?

Primary auditory cortex (temporlal lobe):
Conscious awareness of sound
• Information from base and apex of cochlear
reaches opposite ends of auditory cortex:
Tonotopic organization
• Further processing in auditory association area
(feature recognition, integration with other sensory
input)

61

What is the structure and function of the vestibular apparatus?

Vestibular apparatus: Series of fluid-filled tubes
in the inner ear. Together with cochlea often also
called labyrinth.
Consists of semicircular canals and otolith
organs, utricle and saccule
Function: Detection of rotational and linear
accelerations of the head

62

• What is the function of the semicircular canals?

• Detect rotational acceleration along three
perpendicular axes.
• Have hair cells in ampullae (= bulges) of each
canal with stereocilia in cupula (=gelatinous
mass)
Head rotation:
Endolymph exerts pressure on cupula, causing
bending of stereocilia
- Cation channels are either opened or closed
(depending on direction of rotation)
- De- or hyperpolarization of hair cells
- In- or decrease neurotransmitter release
- NT release causes in- or decrease in firing
frequency of primary sensory neurons

63

• What is the function of the otolith organs?

• Detect linear accelerations of head in horizontal or
vertical directions
• Utricle: Respond to accelerations in the horizontal plane
• Saccule: Respond to accelerations in the vertical plane
• Stereocilia of hair cells in utricle and saccule are
ensheathed in gelatinous substance containing otoliths.
Otoliths respond to gravitational forces and cause bending
of hair cell stereocilia.

64

• What are the vestibular efferents and their functions?

...

65

Nystagmus (jerky back-and-forth movement of eyes) is a pathology

results when Brainstem:
Neurons
controlling
eye muscles
Fixation of gaze with
head movement are not functioning well.

66

Spinal cord is responsible for Postural adjustment
of head and body, what pathology result from malfunction

(usually compensation
through proprioceptive,
visual input)

67

the Cortex (via thalamus) has a main function of Perception of body
orientation and acceleration; symptoms of this vestibular pathology or trauma include.......

Vertigo
Mismatch of vestibular
and visual input:
Motion sickness