General Properties of Receptorscf.linnbenton.edu/mathsci/bio/waitea/upload/Lecture_10_Vision.pdf · Pathway of light Pathway of signal output • Outer 1/3 of retina – Choroid •

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Introduction

Vision

The Special Senses• Specialized structures designed to respond to specific

stimuli

• Variable complexity

Receptors

• Transducers

– Stimulus energy is converted to an AP

• 2 types of graded potentials generated by receptors

– Receptor potential

• Occurs in a separate receptor cell (not the neuron)

• Example: Rods and cones use phototransduction

– Generator potential

• Occurs when either:

– The neuron is the receptor

» Example: neurons in olfactory epithelium

– The receptor releases NT and stimulates the sensory neuron

General Properties of Receptors

Stimulus causing receptor potentials

⇓

Generator potential in afferent neuron

⇓

Nerve impulse

General Properties of Receptors

Sensation and Perception

Stimulatory input

Conscious level = perception

Detection of stimulus = sensation

• Information conveyed by receptors

– Modality

• Type of stimulus or sensation – hearing, vision, taste, etc.

– Location

• Brain refers pain to its point of origin

– Intensity

• Determined by impulse transmission rate, variations in sensory

thresholds in a group of sensory neurons

– Duration

• Encoded by changes in the pattern of impulse transmission over time

General properties of receptors

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Sensory Adaptation

• Reduction in rate of impulse transmission when stimulus is prolonged

• Occurs in all receptors except those for pain

• Let’s try it…

• Stimulus Modality

� Chemoreceptors

� Thermoreceptors

� Nociceptors

� Mechanoreceptors

� Photoreceptors

Classification of Receptors


� Chemoreceptors

� Respond to chemical

concentrations

� Odors, tastes, dissolved

chemicals, etc

� Thermoreceptors

� Nociceptors


� Photoreceptors



� Chemoreceptors

� Thermoreceptors

� Respond to heat and cold

� Nociceptors


� Photoreceptors



� Chemoreceptors

� Thermoreceptors

� Nociceptors

� Free nerve endings

� Respond to tissue damage

� Give the perception of pain


� Photoreceptors



� Chemoreceptors

� Thermoreceptors

� Nociceptors


� Respond to physical

deformity

� Touch pressure, stretch,

vibration, tension

� Photoreceptors


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� Chemoreceptors

� Thermoreceptors

� Nociceptors


� Photoreceptors

� Respond to light

� Rods, cones, certain

retinal ganglion cells


• Origin of stimuli

� Exteroceptors

� Interoceptors

� Proprioceptors



� Exteroceptors

� Respond to stimuli that originate

outside the body

• Examples: light, smell, sound

� Interoceptors

� Proprioceptors



� Exteroceptors

� Interoceptors

� Respond to stimuli that originate

inside the body

• Examples: pressure, hunger, thirst

� Proprioceptors



� Exteroceptors

� Interoceptors

� Proprioceptors

� Respond to stimuli that

originate due to the

positions and tensions

of joints and muscles


• Vision

• Hearing

• Olfaction

• Gustation

Special senses

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• 70% of all sensory receptors are in the eye

• Nearly half of the cerebral cortex is involved in processing

visual information

• Optic nerve is one of body’s largest nerve tracts

Vision Introduction

• The eye is a photoreceptor organ

• Refraction

– Light bends when it passes from one medium to another

• Conversion (transduction) of light into AP’s

• Information is interpreted in cerebral cortex

Vision Introduction

Figure 15.1a

Eyelashes

Site where

conjunctiva

merges with

cornea

Lateral

commissure

Medial

commissure

Eyelid

Eyelid

Eyebrow

Palpebral

fissure

(a) Surface anatomy of the right eye

Figure 15.1b

(b) Lateral view; some structures shown in sagittal section

Orbicularis oculi muscle

Eyebrow

Palpebral conjunctiva(covers inside of eyelids)

Cornea

Eyelashes

Bulbar conjunctiva(covers outer surface of eye)

Conjunctival sac

Orbicularis oculi muscle

• Lacrimal apparatus

– Series of structures that produce tears, direct their flow, and

drain them

• Lacrimal gland and ducts

– Connect to nasal cavity

• Lacrimal secretion (tears)

– Dilute saline solution

» Mucus, antibodies, and lysozyme

– Blinking spreads the tears toward medial commissure

– Drain into the nasolacrimal duct

Accessory Structures

Figure 15.2

Lacrimal gland

Excretory ducts

of lacrimal glands

Lacrimal punctum

Lacrimal canaliculus

Nasolacrimal duct

Inferior meatus

of nasal cavity

Nostril

Lacrimal sac

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• Six extrinsic eye muscles

Accessory Structures

Figure 15.3a

Inferior rectus

muscle

Inferior oblique

muscle

Superior oblique

muscle

Superior oblique

tendon

Superior rectus

muscle

Lateral rectus

muscle

(a) Lateral view of the right eye

Figure 15.3c

(c) Summary of muscle actions and innervating cranial nerves

Lateral rectus

Medial rectus

Superior rectus

Inferior rectus

Inferior oblique

Superior oblique

Moves eye laterally

Moves eye medially

Elevates eye and turns it medially

Depresses eye and turns it medially

Elevates eye and turns it laterally

Depresses eye and turns it laterally

VI (abducens)

III (oculomotor)

III (oculomotor)

III (oculomotor)

III (oculomotor)

IV (trochlear)

Muscle ActionControlling

cranial nerve • Wall of eyeball contains three layers (tunics)

–Fibrous

–Vascular

–Sensory (retinal)

the Eyeball

• Three layers

1) Fibrous tunic

• Sclera – “white” of the eye

• Cornea – clear continuation over the iris and pupil

the eyeball

Figure 15.4a

Sclera

Cornea

Fibrous Tunic

Scleral venous sinus

(AKA canal of Schlemm)

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• Cornea must be actively dehydrated to remain

transparent

• Sodium pumps remove sodium from the

endothelium – water follows

the eyeball

Corneal Edema

• Three layers

2) Vascular tunic (uvea) – 3 components

–Choroid

–Ciliary body

– Iris

the eyeball

• Three layers


–Choroid

» Highly vascular

» Darkly pigmented

- Absorbs stray

light

–Ciliary body

– Iris

the eyeball

• Three layers

2) Vascular tunic (uvea)

–Choroid

–Ciliary body

» Ciliary processes

- Arranged in a ring

around the lens

» Ciliary muscle

- Used to focus the

eye

– Iris

the eyeball

• Three layers


–Choroid

–Ciliary body

– Iris

- Radial muscle fibers contract = pupil dilation

- Circular muscle fibers contract = pupil constriction

the eyeball

Figure 15.4a

Choroid

Sclera

Ciliary bodyCiliary zonule

(suspensory

ligament)

IrisPupil

Lens

Scleral venous

sinus

VascularTunic

Cornea

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• Three layers

3) Retina (innermost layer)

• Photoreceptor neurons

– Rods and Cones

• Bipolar neurons

• Ganglion neurons

– Axons form the optic nerve

– Exit eyeball at optic disc

the eyeball

• Rods and cones

– Specialized photoreceptors

– Rods = black and white vision

– Cones = daylight color vision

– Fovea centralis

• Cones most concentrated here

• Point of highest visual acuity

• Other retinal layers are displaced (forms a depression)

the eyeball

Figure 15.4a

Central artery

and vein of

the retina

Optic disc

(blind spot)

Optic nerve

Fovea centralis

Macula luteaRetinaChoroid

Sclera

Ciliary body

Ciliary zonule

Cornea

Iris

Pupil

Lens

Scleral venous

sinus

Retina

Figure 15.6b

Pigmented

layer of retina

(b) Cells of the neural layer of the retina

Amacrine cellHorizontal cell

• Rod

Photoreceptors

• Cone

Bipolar

cellsGanglion

cells

Pathway of lightPathway of signal output

• Outer 1/3 of retina

– Choroid

• Inner 2/3 of retina

– Central artery and vein

Retinal Blood Supply

Figure 15.6a

(a) Posterior aspect of the eyeball

Neural layer of retinaPigmented

layer of

retina

Central artery

and vein of retina Optic

nerve

Sclera

Choroid

Optic disc

Pathway of light

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Blood supply to the retina

• Photoreceptors

– Rods

• More numerous at peripheral region

• Dim light

– Indistinct, fuzzy, non-color peripheral vision

• About 100 million per eye

The Eyeball

• Photoreceptors

– Cones

• Red, blue, and green

– Highest density in macula lutea

» Concentrated in fovea centralis

• Operate in bright light

• High-acuity color vision

The eyeball

Figure 15.7

Macula

lutea

Centralarteryand veinemergingfrom theoptic disc

Optic disc

Retina

• Lens

– Biconvex, transparent, flexible

– Attached to ciliary body by suspensory ligaments

– Allows precise focusing of light on the retina

– Forms a partition

• Creates an anterior and posterior cavity

The eyeball

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Figure 15.14 (1 of 3)

Focal

plane

Focal point is on retina.

Emmetropic eye (normal)• Two Cavities

– Anterior cavity

• Filled with aqueous humor

– Modified plasma

– Constantly produced, in constant circulation

– Accumulation may lead to glaucoma

The eyeball

Figure 15.8

Sclera

Bulbar

conjunctiva

Scleral venoussinus

Posteriorchamber

Anteriorchamber

Anteriorsegment(containsaqueoushumor)

Corneal-scleral junction

Cornea

Cornea

Corneal epithelium

Corneal endothelium

Aqueous humor

Iris

Lens

Lens epithelium

Lens

Posteriorsegment(contains vitreoushumor)

Ciliary zonule

(suspensory

ligament)

Ciliary

processes

Ciliary

muscle

Ciliary body

1 Aqueous humor is formed by filtration from the capillaries in the ciliary processes.

2 Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber. Some also flows through the vitreous humor (not shown).

3 Aqueous humor is reabsorbed into the venous blood by the scleral venous sinus.

1

2

3

• Two Cavities

– Posterior cavity

• Filled with vitreous humor

– Gelatinous mass

– Avascular

– Very few cells

» A few phagocytes that remove debris from the field of vision

– Responsible for eye holding its spherical shape

The eyeball

• Visual acuity – ability to discern detail

– Ratio of distances

• 20/20

• 20/40

– Less visual acuity

• 20/15

– More visual acuity

The Physiology of Vision

• 5 processes produce a visual image

1. Refraction

2. Accommodation

3. Pupil constriction

4. Convergence

5. Photoreception

The Physiology of vision

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• The bending of light rays

Refraction

• Most of the light we see is reflected off of objects

Refraction

• This light begins as a single point, but spreads as

distance from the object increases

Refraction

• The eye must collect and re-bend the light to a single

point in order to construct an image

• Since focal length is fixed, the eye changes the shape

of the lens to accomplish this

Refraction

Figure 15.14 (1 of 3)

Focal

plane

Focal point is on retina.

Emmetropic eye (normal)

Focal length is fixed in the eye

• Light passing through a convex lens (as in the eye) is bent so

that the rays converge at a focal point

• The image formed at the focal point is upside-down and

reversed right to left

Refraction

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Figure 15.13a

Lens

Inverted

image

Ciliary zonule

Ciliary muscle

Nearly parallel raysfrom distant object

(a) Lens is flattened for distant vision. Sympathetic

input relaxes the ciliary muscle, tightening the ciliary

zonule, and flattening the lens.

Sympathetic activation

• Refracting media

• Light is refracted by

– Cornea

– Aqueous humor

– Lens

– Vitreous humor

• Change in lens curvature

– Allows for fine focusing of an image

Refraction

• Eye is adapted for distance vision

• Ciliary muscle is relaxed = lens is flat

� As light moves closer greater curvature required

• Ciliary muscle contracts = lens is curved

Accommodation

Figure 15.13a

Lens

Inverted

image

Ciliary zonule

Ciliary muscle

Nearly parallel raysfrom distant object

(a) Lens is flattened for distant vision. Sympathetic

input relaxes the ciliary muscle, tightening the ciliary

zonule, and flattening the lens.

Sympathetic activation

Figure 15.13b

Divergent raysfrom close object

(b) Lens bulges for close vision. Parasympathetic

input contracts the ciliary muscle, loosening the

ciliary zonule, allowing the lens to bulge.

Invertedimage

Parasympathetic activation

• Near vision

– Light rays from near objects require extensive refraction

– Accommodation

• Ciliary muscles contract

• Lens shape changes

• Eye strain

Accommodation

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• Near point

– Closest point at which an object can be brought into focus

• Typically 25cm

– Maximum bulge

– Presbyopia - Aging

Accommodation

• Close vision requires

–Accommodation

–Pupil constriction• Prevents most divergent light rays from entering the eye

–Convergence• Medial rotation of the eyeballs toward the object being viewed

(binocular vision)

Close Vision

• Contraction of the circular muscles = constriction

• Contraction of the radial muscles = dilation

Pupil Manipulation

• Sensory transduction

– Light energy is converted into nerve impulses

• Takes place in retina

– Photoreceptors

Photoreception

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• Rods and cones

–Outer segment of each contains visual pigments

• Molecules that change shape as they absorb light

Photoreception

Figure 15.15a

Process of

bipolar cell

Outer fiber

Apical microvillus

Discs containing

visual pigments

Melanin

granules

Discs being

phagocytized

Pigment cell nucleus

Inner fibers

Rod cell body

Cone cell body

Synaptic terminals

Rod cell body

Nuclei

Mitochondria

Connecting

cilia

Basal lamina (border

with choroid)

The outer segmentsof rods and cones are embedded in the pigmented layer of the retina.

Pig

mente

d l

ayer

Oute

r se

gm

ent

Inner

se

gm

ent

Figure 15.15b

Rod discs

Visualpigmentconsists of• Retinal• Opsin

(b) Rhodopsin, the visual pigment in rods, is embedded in

the membrane that forms discs in the outer segment.

Note that they

are connected

to each other

here

Figure 15.18 (1 of 2)

Na+

Ca2+

Ca2+

Photoreceptor

cell (rod)

Bipolar

cell

Ganglion

cell

In the dark

In the dark, cGMP opens sodium

channels and depolarizes

photoreceptors

Inhibitory neurotransmitter is released

(glutamic acid)

Bipolar cells are

constantly inhibited

in the dark

Figure 15.16

11-cis-retinal

Bleaching of

the pigment:

Light absorption

by rhodopsin

triggers a rapid

series of steps

in which retinal

changes shape

(11-cis to all-trans)

and eventually

releases from

opsin.

1

Rhodopsin

Opsin and

Regeneration

of the pigment:

Enzymes slowly

convert all-trans

retinal to its

11-cis form in the

pigmented

epithelium;

requires ATP.

Dark Light

All-trans-retinal

Oxidation

2H+

2H+

Reduction

Vitamin A

2

11-cis-retinal

All-trans-retinal

Figure 15.18 (2 of 2)

1 cGMP-gated channels

are closed, so Na+ influx

stops; the photoreceptor

hyperpolarizes.

Voltage-gated Ca2+

channels close in synaptic

terminals.

2

No neurotransmitter

is released.

3

Lack of IPSPs in bipolar

cell results in depolarization.

4

Depolarization opens

voltage-gated Ca2+ channels;

neurotransmitter is released.

5

EPSPs occur in ganglion

cell.

6

Action potentials

propagate along the

optic nerve.

7

Photoreceptor

cell (rod)

Bipolar

cell

Ganglion

cell

Light

Ca2+

In the light

In the presence of light

bipolar cells are not

inhibited

Breakdown of

rhodopsin causes closing

of sodium channels

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• Excitation of cones

– Method of excitation is similar to that of rods

– There are three types of cones

• Named for the colors of light absorbed: blue, green, & red

• Light absorbed depends upon opsins present

Photoreception

• Light adaptation– Occurs when moving from darkness into bright light

• Large amounts of pigments are broken down instantaneously

– Produces glare

• Pupils constrict

• Dramatic changes in retinal sensitivity

– Rod function ceases

• Cones and neurons rapidly adapt

– Visual acuity improves over 5–10 minutes

Photoreception

• Dark adaptation

– Occurs when moving from bright light into darkness

• The reverse of light adaptation

• Cones stop functioning in low-intensity light

• Pupils dilate

• Rhodopsin accumulates in the dark

– Retinal sensitivity increases within 20–30 minutes

Photoreception

• Duplicity theory

– Why do we have 2 kinds of photoreceptors?

• Neural circuits for night are not suited for day

– Rods:Bipolar cell

• 600:1

• Spatial summation of large field = poor detail

– Cones:Bipolar cell

• 1:1

• Best for detail in small receptive fields

Photoreception

• Axons from medial portion of each retina cross at optic

chiasma

– Continue as optic tracts

• Optic radiation fibers connect to the primary visual cortex in

the occipital lobes

– Visual input from both eyes is interpreted by brain

• Distance

• Depth perception

Visual Pathway to the brain

Figure 15.19a

Pretectal nucleus

Fixation point

Optic radiation

Optic tract

Optic chiasma

Uncrossed (ipsilateral) fiber

Crossed (contralateral) fiber

Optic nerve

Lateral geniculatenucleus ofThalamus – primary connection of optic nerve to occipital lobe

Superior colliculus Occipital lobe (primary visual cortex)

The visual fields of the two eyes overlap considerably.Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma.

Suprachiasmatic

nucleus

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• Myopia

(nearsightedness)

– Close objects seen clearly

– Image is focused in front

of the retina

– Correction = concave lens

Abnormalities of Vision

• Hyperopia

(farsightedness)

– Distant objects seen

clearly

– Image is focused behind

the retina

– Correction = convex lens

Abnormalities of Vision

• Astigmatism

• Detached retina

• Conjunctivitis

• Glaucoma

• Cataract

• Diplopia

• Night blindness

• Color blindness

• Macular Degeneration

Abnormalities of vision


• Astigmatism

– Abnormal curvature of eye

– Light rays don’t meet at a common focus

– Causes visual distortion

• Detached retina

– Retina becomes separated from underlying choroid

– Causes loss of vision in affected area


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• Conjunctivitis

– Inflammation of the conjunctiva

– Causes by bacteria, viruses, irritants, allergies

– “Pink eye”


• Glaucoma

– Increased pressure in the eyeball

– Leads to optic nerve damage

– Causes gradual loss of sight

– Damage is irreversible, but may be slowed or stopped


• Cataract

– Lens becomes progressively opaque

– Results in blurred vision

– Can be surgically removed and replaced with an artificial

lens


• Diplopia

– Double vision

– At least 20 different causes

• Cranial nerve III palsy

• Aneurysm

• High blood pressure

• Diabetes

• Inflammatory conditions

• Guillain-Barre syndrome

• Problems with the lid,

pupil, or eye muscles

• Etc.


• Nightblindness (nyctalopia)

– Loss of low light vision

– Most common cause: retinitis pigmentosa

• Rods lose ability to respond to light


• Colorblindness

– Defect in perception of colors

– Defect in cones

– Can be acquired; usually genetic


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• Macular degeneration

– Damage to macula and retina

– Risk factors

• Genetic

• Lifestyle


• Bonus: amblyopia

– “Lazy eye”

– Decreased vision in eye that appears normal

– Developmental problem of the brain

– Brain doesn’t recognize image from one eye


Documents

General Properties of Receptorscf.linnbenton.edu/mathsci/bio/waitea/upload/Lecture_10_Vision.pdf · Pathway of light Pathway of signal output • Outer 1/3 of retina – Choroid •