Eye Anatomy and Diseases 1

Anatomy and Functions of the Eye

• The iris is responsible for giving color to the eyes.
• The optic nerve transmits visual information from the retina to the brain.

Layers of the Eye

• The eye has three layers: the sclera (outer), the choroid (middle), and the retina (inner).

Visual Disorders

• Presbyopia is a condition where the eye's lens loses elasticity, leading to difficulty in focusing on close objects.
• A cataract is characterized by clouding of the eye's lens, often causing blurred vision.
• Glaucoma is a group of eye diseases that damage the optic nerve, often due to increased intraocular pressure.
• Strabismus refers to misalignment of the eyes, leading to double vision or vision issues.
• Diplopia is the medical term for double vision, typically arising from strabismus or other eye issues.

Function of the Retina

• The retina converts light into neural signals and processes visual information before sending it to the brain.
• The pinhole principle describes how a small aperture allows light rays to converge, improving focus.

Retinal Cells

• The majority of ganglion cells in the retina are known as midget ganglion cells.
• Rods and cones are the two types of photoreceptor cells; rods are more sensitive to light, while cones are responsible for color vision.
• The peripheral region of the retina has a higher ratio of rods to cones, making it more sensitive in low light.

Horizontal and Amacrine Cells

• Horizontal cells integrate input from multiple photoreceptors and help refine visual perception.
• Amacrine cells receive input from bipolar cells and can connect to ganglion cells, bipolar cells, and other amacrine cells.

Visual Pathway and Brain Integration

• The hypothalamus plays a role in regulating circadian rhythms and some visual reflexes.
• The optic chiasm is where optic nerve fibers cross, allowing visual information from both eyes to be processed by both hemispheres of the brain.
• The lateral geniculate nucleus (LGN) in the thalamus relays retinal inputs to the visual cortex.

Midbrain Functions

• The primary visual cortex (also called V1 or Brodmann's area 17) receives input from the LGN and processes visual information.
• The superior colliculus in the midbrain is involved in eye movements and visual reflexes.
• The midbrain's pretectum controls pupil size in response to light exposure.

Neuronal Projections and Streams

• Layer IVCα of the striate cortex primarily receives input from magnocellular neurons.
• The dorsal stream analyzes visual motion and is associated with the visual control of action.
• The ventral stream is responsible for object recognition and perception.
• The parvocellular pathway (part of the ventral stream) is responsible for the perception of color.

Calcium and Visual Adaptation

• Calcium plays a crucial role in light adaptation by modulating phototransduction reactions in photoreceptor cells.