@ SSA
@ Physical Structure of Eye Ball
眼球壁主要分三層結構：
1. Corneo-Scleral layer (角膜鞏膜層) = Outer fibrous layer
(1) Sclera 占94%，為六條眼外肌的附著點。
(2) Limbus Sclera與Cornea的接合處，呈凹陷狀，因為此點Sclera與Cornea呈現出不同的曲率。
(3) Cornea 占6%，有五層結構：(a) Stratified squamous epithelium、(b) Bowman’s membrane、(c) Substantia propria、(d) Descemet’s membrane、(e) Endothelium。Cornea呈現透明狀，乃因為：(a) Regularity of its epithelial surface、(b) Its avascularity、(c) Regular pattern of its collagen fibers and ground substance in the substantia propria、(d) Chemical composition and state of hydration of its stroma。
2. Uveal layer (葡萄膜層) = Muscular layer 富含血管組織，可分三個部份：
(1) Choroid layer (脈絡膜) 介於Sclera與Retina之間，功能為：(a) Highly vascularized，因此可提供養分與氧氣給Retina、(b) Richly black-pigmented，因此可吸收穿過Retina之多餘的光線，避免光線反射產生干擾。
(2) Ciliary body (睫狀體) 介於Retina的Ora serrata (鋸齒緣)與Corneoscleral junction之間，內有Ciliary muscle (由circular, radial, and mefidional muscle fibers共同組成)，受Oculomotor nerve之副交感神經支配。功能有三：(a) Accommodation、(b) Production of aqueous humor of the anterior and posterior chambers、(c) Restoration of the mucopolysaccharides of the vitreous body。
## Accommodation：
Ciliary muscle鬆弛 Suspensory ligament緊張 Lens變扁平 視遠物
Ciliary muscle收縮 Suspensory ligament鬆弛 Lens變圓凸 視近物
(3) Iris (虹膜) Þ 它蓋在Lens的前緣，形成了Pupil (瞳孔)。它有兩層上皮乃從Ciliary body延伸而來，但色素分佈不同：
|
|
Ciliary body |
Iris |
|
Surface epithelium |
Nonpigmented |
Heavily pigmented |
|
Deep epithelium |
Pigmented |
Light pigmented myoepithelial cell |
※ Iris含兩種平滑肌：
(a) Sphincter pupillae muscle (瞳孔括約肌) Circular m.，由Parasympathetic composition of the oculomotor nerve所支配，造成Pupillary constriction。
(b) Dilator pupillae muscle (瞳孔擴張肌) Radial muscle，由Sympathetic composition of the superior cervical ganglion所支配，造成Pupillary dilation。此Sympathetic fibers亦可支配Smooth muscle portion of levator palpebrae m. 以及Orbital muscle of Müller (可使眼球在眼眶內維持向前凸出)。
3. Retina layer (視網膜層) = Inner tunica
(a) 胚胎上源自於Optic cup (Two-layered outgrowth of the neural tube)，乃是Forebrain的延伸。Outer layer演變為Pigmented epithelium，Inner layer演變為Neuroretina。而Optic ventricle在胚胎期則與Ventricular system相通。
(b) 可分為三部份：
(i) Neuroretina Þ Ora serrata之後，為Photoreceptive layer + Pigmented epithelium。
(ii) Ciliary retina Þ Unpigmented epithelium over the ciliary body。
(iii) Iridic retina Þ Pigmented epithelium over the posterior iris。
(c) 在Retina的Posterior pole鄰近之處，有三個Concentric regions：
(i) Central part of the retina 直徑6 mm。
(ii) Macula lutea (黃斑) 直徑3 mm。在眼球正後方，視軸通過處。因為神經元中含Carotinoid pigment與Xanthophyll pigment，所以呈黃色。而在Macula lutea的內側有另一凹陷，稱為Optic disk (視盤)或Optic papillae (視乳頭)，此為Optic nerve匯聚正要離開眼球之處，因無Photoreceptive cell的分佈，不能接受光刺激，因而稱為Blind spot (盲點)。另外Optic nerve穿過Sclera之處，稱為Lamina cribrosa (篩板)。
(iii) Central fovea (中央小凹) 直徑0.4 mm，位於Macula lutea的中央。因為此處只含Cone photoreceptive cells，缺乏其他的Retinal layers以及Blood vessels，所以光線刺激較少受到干擾，是光感最敏銳的地方。
@ 眼球的其他構造：
1. Anterior and posterior chambers (前房與後房) 兩者被Iris隔開，但以Pupil相通。內含Aqueous humor (水狀液、眼前房水)，其成分與CSF相似。Aqueous humor由Ciliary body分泌 ® Posterior chamber ® Pupil ® Anterior chamber ® 由Canal of Schlemm (An aqueous vein、Sinus venosus sclerae) 吸收而完成循環。
2. Vitreous body (玻璃狀體) 含Vitreous humor (玻璃狀液、眼後房水)，藉此讓Lens懸浮，並防止Retina塌陷。Vitreous humor由Ciliary body分泌，Turn over非常快速，一半的Vitreous humor可以在10-15 min換新，成份為A transparent gelatinous material which is a hydrated mucoprotein enmeshed in some collagenous fibrils，此外還含有類似Aqueous humor的成份。在Vitreous body內緩慢流動的纖維狀物，被認為是感覺眼前有線或點樣浮動的主要刺激來源。
3. Lens (晶體) Þ 胚胎發生上源自於Ectoderm，由於受Optic vesicle (源自Neural tube，會演變成Optic cup) 的Cell-to-cell interactions影響，Optic vesicle上覆的Ectoderm會陷進來形成Lens pit，再融合形成Lens vesicle，然後脫離表面並沉入Optic cup而形成Lens。Lens的結構為：
(1) Lens capsule Þ A thick epithelial basement membrane which is connected via the suspensory ligament to the ciliary body。
(2) Epithelium Þ 位於前表面者為單層細胞，位於兩側者則有分生能力。
(3) Lens fibers Þ 為Posterior pales的細胞變長並失去細胞核，含有Crystalline protein，以前後方向充滿了晶體中央腔室。
@ Clinical Correlation
1. Swollen optic disk or Choked disk = Papilledema (視盤或視乳頭水腫) 由於CSF pressure或Intracranial pressure增高所致。
2. Retina detachment (視網膜剝離) Retina的Pigment cell layer與Layer of rods and cones之間為Congenital weak point，容易產生剝離，造成部份視盲。
3. Glaucoma (青光眼) 為Aqueous humor分泌過多或引洩不良所致。會造成Intraocular pressure增高，若高過Normal 15-20 mmHg時，則嚴重傷害Retina。
4. Cataracts (白內障) 隨著年齡Lens protein的分子量增大，或例如Diabetes patient的不正常糖代謝，都容易引發此病。
@ Layers of Inverted Retina 由內至外的層次構造，但在Central fovea、Optic disk、Peripheral margins near the ora serrata等處除外。
1. Layer of the pigment epithelium 功能是：(1) Absorb (mop up) the light rays which were not successful in activating the photopigments in the rods and cones、(2) Provide metabolic (e.g. active transport of ions) and functional support for the rods and cones、(3) Act as a phagocyte to remove debris from the rods and cones。
2. Layer of the outer and inner segments of the rods and cones
3. Outer limiting membrane Tight junctions between Müller cells (a supporting cell, resembling astrocyte) with rods and cones。組織染色下，呈現嗜伊紅的膜狀物。
4. Outer nuclear layer Cell bodies and fibers of rods and cones。
5. Outer plexiform layer (Trial synaptic layer) Synapses among rods, cones, bipolar cells, and horizontal cells。
6. Inner nuclear layer Cell bodies of bipolar cells, horizontal (anaxonic) cells, amacrine (anaxonic) cells, interplexiform cells, and Müller cells。
7. Inner plexiform layer Synapses among bipolar cells, amacrine cells, and ganglion cells。
8. Ganglion cell layer Cell bodies of ganglion cells and synapses as in the above layer (7)。
9. Nerve fiber layer Axons of ganglion cells。
10. Inner limiting membrane Junctional complexes of expanded ends of Müller cells at the surface of vitreous body。
@ Photoreceptor Cells 每一細胞可分為五部份：(1) Outer segment、(2) Neck、(3) Inner segment、(4) Cell body、(5) Synaptic base【此部份在Cones稱Pedicle，在Rods則稱為Spherule。】
1. Rod cell (桿狀細胞) React to low intensities of illumination and subserve twilight and night vision，亦即只感受黑白兩色。Rod cells在Retina中的分佈不均，周邊多而中央少，亦即Ora serrata處最多而Central fovea處則無，此分佈可以解釋Peripheral vision現象，即在光線不足時，用眼睛餘光斜看物體較清楚。其Outer segment有雙層膜盤狀物，內含Visual purple or rhodopsin (視紫質)，是由Opsin和Retinal所組成。這種Photopigments可以經常更新，並可產生Photochemical reaction，激發Generator potential。
2. Cone cell (錐狀細胞) Þ Have a higher threshold of excitability and are stimulated by light of relatively high intensity (Color vision)。Cone cells在Retina中的分佈相反於Rod cells，周邊少而中央多。在Outer segment內的Rhodopsin，其Cone opsin有三種型式，分別可以吸收紅、藍、綠光，因此人眼可以看到這些色彩的互補顏色。
@ Response of Retina to Stimulation by Light
1. In the dark or resting state, the receptor cells are continuously releasing an excitatory neurotransmitter that depolarizes bipolar cells.
2. When the bipolar cells are maintained in a depolarized state (in the dark), they release continuously inhibitory neurotransmitter at the bipolar cell-ganglion cell synapse and, thereby, “prevent” the ganglion cells from firing.
3. Light stimulation Þ Conformational changes in opsins Þ Hyperpolarization in the plasma membrane of rods and cones Þ Non-release of excitatory neurotransmitter to bipolar cells Þ Hyperpolarization of bipolar cells Þ Decrease the amount of inhibitory neurotransmitter Þ Increase ganglion cells’ rate of action potential discharge。
@ Cellular Proportion in the Retina Þ 為Convergence方式，由多至少。但在Central fovea處，Photoreceptor：Bipolar：Ganglion = 1：1：1。故Central fovea是光感最敏銳的地方。
1. Rod cells = 120 million。
2. Cone cells = 6-10 million。
3. Bipolar cells = 10-20 million。
4. Ganglion cells = 1 million。
@ Visual Pathway
1. Optic nerve (視神經) Þ 由Ganglion cells的Axons所構成。它們在Retina時為Unmyelinated，由Optic disk離開眼球後才具有Myelin sheath，是Oligodendrocytes所製造的，因此Retina與Optic nerve均屬CNS的結構。
2. Optic chiasm (視交叉) Þ 並非所有的Optic nerve均交叉，它的規則是：Retina與Macular lutea的Temporal half (顳側)之Optic nerve不交叉，而Nasal half (鼻側)之Optic nerve跨過中線至對側的Optic tract。另外，有少數的Optic nerve fibers並不進入Optic tract，它們在Optic chiasm處直接進入Hypothalamus的Suprachiasmatic nucleus，因而與Neuroendocrine regulation、光周期以及松果體的機制有關。
3. Optic tract (視徑) Þ Optic tract中的有些神經纖維並不進入LGB，而是直接進入Pretectal nucleus；或進入LGB後，再間接進入Superior colliculus，因而與眼睛的光反射有關。
4. Lateral geniculate body (LGB) (外側膝狀體) Þ 為Thalamus中Pulvinar之一小卵圓狀突起物。由腹側至背側可分為六層，第1、4、6層接受Contralateral Projections，而第2、3、5層接受Ipsilateral projections。這些投射為Point to point projection。
5. Optic radiation = Geniculocalcarine tract (視放線、膝狀體禽距徑) Þ 可分二途徑走：
(1) Geniculocalcarine fibers for lower visual field：起於LGB Þ 經過Retrolenticular part of internal capsule Þ 繞著Lateral ventricle向後走 Þ 止於Visual cortex。
(2) Geniculocalcarine fibers for upper visual field：起於LGB Þ 先向前繞過Temporal horn of the lateral ventricle Þ 形成Meyer’s loop or temporal loop Þ 止於Visual cortex。【Meyer’s loop是投射到Calcarine sulcus下側，當此Loop受損時會造成〝對側上視野缺限〞，臨床稱為〝Pie in the sky〞。】
6. Visual cortex = Area 17 (視皮質) Þ 位於Calcarine sulcus (禽距溝)的上、下兩側，是屬於Heterotypical (Granular) cortex，因有一條很清晰的帶狀紋─Line of Gennari，故又名Striate area (紋狀區)。Point to point projection如下：
(1) 下視野 « Retina之上1/4 « LGB之內側 « Calcarine sulcus上側
(2) 上視野 « Retina之下1/4 « LGB之外側 « Calcarine sulcus下側
(3) 中央視野 « Macula lutea « LGB之後半部 « Visual cortex之後1/3
7. Visual association area = Areas 18 and 19 (視覺聯合區)
@ Visual Reflexes
1. Pupillary light reflex
2. Accommodation-convergence reflex
3. Pupillary skin reflex
4. Corneal reflex
5. Visual body reflex
@ Visual Defect Caused by Interruption of Visual Pathway
1. Circumferential blindness (環狀視盲)
2. Total blindness of right eye (右眼全盲)
3. Bitemporal hemianopsia (兩顳側半盲)
4. Right nasal hemianopsia (右鼻側半盲)
5. Left homonymous hemianopsia (左同側半盲)
6. Left homonymous hemianopsia except for macula lutea (黃斑除外的左同側半盲)