Fisiologi Mata

Nurfitri Bustamam, SSi, MKes, MPdKed.

Introduction

Lens cornea

Aperture of the camera pupil

diameter

Film retina

More sophisticated

Protective casing

Nervous system

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Learning Objectives Describe the various parts of the eye and list the

functions of each

Explain how light rays are brought to a focus on the retina

Explain common defects of the optical system of the eye

Describe production of nerve impulses in the eyes & trace the visual pathways to their destinations in the brain.

Define and explain dark adaptation & visual acuity

Describe the eye movement

Explain how we are able to distinguish colors & perceive depth.

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Anatomy of the Eye

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Lapisan mata:

Sklera (protektif, putih, tidak dpt dilewati cahaya) & cornea (di anterior, transparan). Sklera dilapisi membran mucosa yg jernih (conjuctiva)

Choroid (jaringan pembuluh darah)

Retina; melapisi 2/3 bag dlm choroid, berupa jar. saraf yg mengandung fotoreseptor

Lensa kristal; disangga oleh lens ligamen (zonula) yg terhubung dg badan ciliaris yg punya otot sirkuler & longitudinal

Iris: berada di depan lensa, bagian berwarna dari mata tdd serat otot sirkular konstriksi dan serat otot radial dilatasi (mengatur cahaya ke bola mata).

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Vitreous humor: materi gelatinosa jernih yg mengisi ruang antara lensa & retina.

Aqueous humor: cairan jernih, dibuat oleh corpus cilaris dg cara difusi dan transpor aktif mengalir melalui pupil utk mengisi ruang anterior bola mata. Aqueous humor direabsorpsi oleh Canal of Schlemm yaitu vena-vena antara iris dan cornea.

Glaucoma: peningkatan tekanan intraokuler akibat obstruksi pada Canal of Schlemm. Hal itu dapat merusak retina. Tekanan normal bola mata: 10-20 mmHg.

Penyebab glaucoma: infeksi, perdarahan akumulasi debris di Canal of Schlemm.

Terapi: diberi obat utk mengurangi sekresi atau meningkatkan absorpsi aqueous humor atau dioperasi.

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Aqueous humor dibentuk rata-rata 2-3 l/menit

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Rods & cones (fotoreseptor).

Interneuron (inhibisi/eksitasi): sel bipolar, sel horisontal, sel amacrine, dan keluar mata via sel ganglion yg aksonnya membentuk serat n. opticus.

Pigmen epitel berperan utk mengabsorpsi cahaya agar tdk mengalami pemantulan/refleksi kembali shg bayangan tdk kabur.


N. opticus meninggalkan bola mata mll optic disk (karena tdk ada reseptor maka bagian ini merupakan bintik buta).

Macula lutea (pigmen kuning) pusatnya disebut fovea centralis yg tdk mengandung rods tetapi hanya cones yg sangat padat dan berperan dalam ketajaman penglihatan (acuity). Age-related macular degeneration. Gerakan bola mata sebenarnya berusaha menempatkan bayangan tepat di fovea centralis.

Oftalmoskopi- melihat permukaan interior di belakang lensa (fundus) tdd retina, optic disc, macula, fovea, arteriole & vein in superficial layers of retina


Pada fovea, setiap cone

berhub. dg satu sel bipolar

dan langsung ke n. opticus

(penting utk ketajaman

penglihatan).

Di luar fovea byk tdpt rods.

Rods : cones = 120 juta : 6

juta

N opticus tdd 1,2 juta serat

Rasio sel bipolar : sel

ganglion = 105 : 1 (ada

konvergensi)


Comparison of Photoreceptors

Characteristic Rods Cone

Photopigment Rhodopsin Blue, green & red

sensitive pigmen (cone

pigment)

Acuity Low High

Threshold illumination Low High

Type of vision Black/white Color

Most concentrated Peripheral Central (fovea centralis)

Wave peak sensitivity 505 nm Blue (445 nm), Green

(535 nm), Red (570 nm)

Estimated total

number

120 milion 7 milion


Kemampuan mata beradaptasi terhadap cahaya sangat tinggi (10-7 107 mm Lambert) krn ada 2 fotoreseptor & ada pengaturan pupil

Rod vision (scotopic vision)

Transition zone

Cone vision: rhodopsin terurai

Rod vision: rhodopsin terbentuk & dirombak seperlunya

Diperlukan waktu utk menumpuk rhodopsin yg terurai saat terang


Buta senja (Nyctalopia)

Semakin byk rhodopsin,

fotoreseptor semakin

sensitif terhadap cahaya.

Pada defisiensi vit. A,

pembentukan rhodopsin

lambat.

Mengapa disebut buta

senja? Bagaimana

perubahan intensitas

cahaya pada senja hari?


Adaptasi Gelap Perub. terang ke gelap: retina

buta, perlahan mampu/terbiasa melihat dlm gelap. Penurunan ambang visual adaptasi gelap yg lamanya 20-30 menit. Terdapat 2 komponen adaptasi gelap: Adaptasi gelap cones: cepat,

sdkt/kecil, hanya di daerah fovea & sekitarnya.

Adaptasi gelap rods: lambat, lbh besar, lbh lama, utk daerah perifer retina.

Adaptasi Gelap & Adaptasi Terang


Kacamata merah akan memperpendek masa adaptasi gelap karena gelombang sinar merah hanya sedikit merangsang rods, tetapi cones tetap mampu berfungsi seperti biasa (studio foto/kamar gelap, radiologist, pilot terbang malam)

Masa adaptasi gelap adalah masa pembentukan rhodopsin (jika kena sinar rusak). Diperlukan jumlah rhodopsin cukup utk mampu melihat dalam gelap.

Dari gelap ke terang: silau, cahaya terlalu terang, tetapi lama kelamaan terbiasa dalam terang, adaptasi cahaya (light adaptation) lamanya 5 menit.


Semakin kuat cahaya semakin besar respons hiperpolarisasi

Neurotransmiter yg dihasilkan fotoreseptor bersifat inhibisi

Incident light Structural changes in the

retinene of photopigment

Metarhodopsin II

Activation of transducin

Activation of photodiesterase

Decreased intracelular cGMP

Closure of Na+ channels

Hyperpolarization

Decreased released of

synaptic transmitter Response in bipolar cells

& other neural elements

Phototransduction in rods & cones


Teori YOUNG HELMHOLTZ: manusia mempunyai 3 cones

Sensasi warna ditentukan oleh frekuensi relatif & impuls dari tiap jenis kerucut.

Ke-3 sel kerucut dapat terangsang dg intensitas yg berbeda atau hanya 1 jenis kerucut yg terangsang sehingga dapat melihat berbagai kombinasi warna.

MEJIKUHIBINIU (ROY G BIV)

Melihat Warna


Colorblindness (buta warna)

Test: Ishihara charts

Buta warna: tdk dpt membedakan warna tertentu. Anomaly: thd warna tertentu lemah, tdk

cerah

Anopia: buta warna, protanopia (buta warna merah), deuteranopia (buta warna hijau), trianaopia (buta warna biru).

Trichromat (punya 3 cones), dichromat (2 cones), monochromat (hanya punya 1 macam cones)


Prinsip Optik & Pembentukan Bayangan di Retina

Berkas sinar yg melewati pusat lensa tdk

dibiaskan/refraksi

Besarnya bayangan ditentukan oleh sudut visual

(visual angle)

Bayangan di retina terbalik (terbiasa sejak lahir)


Titik fokus lensa bergantung radius. Kekuatan refraksi = 1/jarak fokus dalam meter (Dioptri)

Jarak fokus = 0,5 m kekuatan refraksi = 1/0,5 = 2 Dioptri


M. ciliaris relaksasi jika melihat jauh ( > 6 meter)

Jika melihat dekat terjadi refleks akomodasi/near response (lensa mata mencembung karena kontraksi m. ciliaris, pupil konstriksi, & konvergensi mata).

Pada umur 40-45 th daya akomodasi mulai menurun (presbyopia) karena lensa mata mulai mengeras & melemahnya m. ciliaris akibat proses menua.


Akomodasi

Refleks

Stimulus: titik potong sinar di belakang retina

Reseptor: cone & rod (bayangan kabur)

Aferen: n. opticus

Eferen: n. oculomotor

Proses: ciliary muscle kontraksi

diameter mengecil ligamen

mengendor lensa mencembung.


Near point: titik jelas terdekat, pada usia

10 tahun 9 cm, 60 tahun 83 cm.


Emmetropia; mata normal

Myopia; rabun jauh

Hyperopia; rabun dekat


Astigmatism: 90% karena curvatura

cornea & 10% karena curvatura lensa tdk

uniform bayangan di retina kabur.

Etiologi kelainan refraksi

Genetik

Perilaku

Macam2:

- Diameter bola mata (aksis bola mata)

- Kurvatura kornea/lensa

- Indeks refraksi

- Posisi lensa



Visual Acuity

Visual acuity: distinctness or discrimination of objects

All assessments of acuity depend on some estimation of the angle subtended at the eye by two line which can just be seen to be separated from each other.

The greater that angle, the less acuity

The standard of reference used is the best possible discrimination made by normal young subjects in bright light; this requires that the distinguishable line subtend an angle of 1 minute at the eye.

The commonest way of measuring a persons acuity is to display rows of letters or symbols of different sizes (Snellens chart) at standard distance. The components of 6 or 20 row of letters are of a width each subtend an angle of 1 minute at the observers eye when he is 6 m (20 feet) away.


A person whose vision rated 20/20 is seeing details at a distance of 20 feet as clearly as a normal individual could.

Vision noted as 20/15 is better than average, for at 20 feet the person is able to see details that would be clear to normal eye only at a distance of 15 feet.

Conversely, a person with 20/30 vision must be 20 feet from an object to discern details that a person with normal vision could make out at a distance of 30 feet.


Visus

V = 6/6 V < 6/6

Emetropia/hipermetropia

Lensa +

Lebih

jelas

Visus

mengecil

hipermetropia emetropia

muda tua

miopia presbiopia


Refleks Pupil


Refleks Pupil

Pupil konstriksi utk mengurangi sinar yg masuk, pupil sebelahnya ikut konstriksi (consensual light reflex).

Jalur reflex pupil berbeda dg akomodasi

Pupil Argyll-Robertson: akomodasi intact tetapi refleks pupil hilang sebab adanya lesi di midbrain (misalnya syphilis)

Lengkung refleks: n opticus brachium of the superior colliculus pretectal nucleus ipsilateral & contralateral Edinger-Westphal nucleus ciliary ganglion (N. III) sphincter pupilae (otot sirkularis).


Lengkung Refleks Pupil


Visual Pathway Stimulus (ligth)

Rods & Cones

Bipolar Cells

Gangglion Cells

Lateral Geniculate Nucleus

Visual Cortex

Visual Association Cortex

Retina

Thalamus

Cortex

Optic nerve

Pathway


Pusat Penglihatan


Visual Fields

The technique of perimetry allows precise mapping out visual fields

The visual fields are determined by the position of the orbits, the extent and shape of the retina in each eye and the external obstruction to the incidence of light (nose & brow).


Proyeksi lapang pandang dapat digunakan utk menentukan lokasi kerusakan di jaras penglihatan


Monocular & Binocular Vision


Depth Perception

If you close your right eye then open it and close your left, you will notice that the picture changes somewhat.

Each eye receives a slightly different image, because their foveas are 2-3 inches apart. For one thing, each eye provides some visual information that is unavailable to the other because your nose and eye socket block its view of the opposite side.

Each eye sees the world from a slightly different angle. The association & integrative areas of the cortex compare these views and use them to provide us with depth perception.


Stereoscopic Vision


Otot-otot bola mata

Eye Movements

1. Saccades movement: melihat dg cepat & meloncat-loncat

2. Smooth persuit movement: mengikuti gerak objek

3. Convergence movement

4. Vestibular movement



Eye Movement Disorders

Strabismus: uncoordinated extrinsic eye muscles, primary symptom: diplopia

Diplopia (double vision), results when the contraction of the extrinsic muscles is not coordinated. The eyes focus on different area of the visual field & two images are seen.

Nystagmus: involuntary unilateral or bilateral rhytmic movement of the eyes.

Paralysis of extraocular muscle: ptosis: dropping of the eyelid

Fisiologi Mata

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