The Nervous System: Sensory Systems

Three layers of the eye
- Outermost layer
- Sclera: The white outer coat of the eyeball.
- Cornea: The transparent front part of the eye that covers the iris, pupil, and anterior chamber.
- Middle layer
- Choroid: Contains blood vessels and connective tissue.
- Ciliary body: Contains zonular fibers and ciliary muscle, helps in lens shape adjustment.
- Iris: The colored part of the eye, containing the pupil.
- Inner layer
- Retina: Contains photoreceptors, fovea, and optic disk (blind spot).

Anterior segment
- Anterior chamber
- Posterior chamber
Posterior segment (vitreous chamber)
- Contains vitreous humor, a gel-like substance.

Optic disk: No photoreceptors present, leading to a blind spot.
This spot is not noticeable when both eyes are open.
Blind spot test: A diagnostic test where individuals cover one eye and focus on a point.

Light energy: Ranges from 1 nm (gamma rays) to 1012 nm (radio waves).
Electromagnetic waves: Different types include TV waves, microwaves, infrared, visible light, ultraviolet, X-rays, and cosmic rays with different wavelengths (e.g., visible light: 350-750 nm).

Reflection: Light waves reflect off objects allowing us to see them.
Refraction: Bending of light waves when passing through media of different densities.
The degree of refraction depends on:
- Differences in densities
- Angle of incidence of light waves
- Curvature of cornea and lens

Cornea: Provides fixed curvature; contributes 3x more refraction than the lens.
Lens: Adjustable curvature allows focusing on objects at various distances.

Accommodation: The ability of the lens to change shape to focus on near and far objects.
Increasing lens curvature is necessary to focus on near objects.

Parasympathetic control: Involves contraction of the ciliary muscle, resulting in:
- Decreased tension on zonular fibers
- Lens becoming thicker/rounder
- Increased refractive index
Far Vision:
- Ciliary muscle relaxed
- Lens flat
- Pupil open
Close Vision:
- Parasympathetic stimulation
- Ciliary muscle contracted
- Lens rounded
  -Pupil constricted

Emmetropia: Normal vision; light focuses correctly on the retina for both near and distant objects.
Myopia (Near-sightedness):
- Lens of the eye is too strong for the length of the eyeball.
- Corrected with a concave lens to decrease overall refractive power.
Hyperopia (Far-sightedness):
- Lens is too weak for the length of the eyeball.
- Corrected with a convex lens to increase overall refractive power.
Presbyopia: Hardening of the lens making it difficult to focus on close objects.
Cataracts: Opacity of the lens, resulting in blurred vision.
Glaucoma: Increased volume of aqueous humor leading to degeneration of the optic nerve.
Astigmatism: Irregularities in the lens or cornea causing distorted vision.

Size of the Pupil: Changes in response to light intensity.
Iris: Composed of two layers of smooth muscle:
- Inner circular muscle: Functions as a constrictor managed by parasympathetic control.
- Outer radial muscle: Functions as a dilator managed by sympathetic control.

Three layers of Neural Tissue:
- Outer Layer: Contains photoreceptors (rods and cones).
- Middle Layer: Consists of bipolar cells, amacrine cells, and horizontal cells.
- Inner Layer: Contains ganglion cells.
Fovea: Area of the retina with a high concentration of cones, responsible for sharp central vision.
Macula lutea: Surrounds the fovea and is critical for detailed vision.

Function of Photoreceptors: Rods and cones communicate with bipolar cells, which then communicate with ganglion cells.
Axons of ganglion cells form the optic nerve.
Horizontal and Amacrine Cells: Provide lateral modulation of signal flow.

Dry and Wet Types: Dry macular degeneration is typically less severe; wet is characterized by abnormal blood vessel growth in the retina (angiogenesis).
Diabetic Retinopathy: Specifically affects blood vessels in the retina leading to vision loss.

Phototransduction: The process of converting light energy into nerve signals.
Photopigments: Each consists of a retinal and opsin.
Types of Opsins:
- Rods: Contain rhodopsin for black-and-white vision.
- Cones: Three types, enabling color vision
- L for long-wave (red)
- M for middle-wave (green)
- S for short-wave (blue)

Light Activation
- Retinal and opsin dissociate, activating transducin.
- Phosphodiesterase gets activated, leading to hydrolysis of cGMP.
- This process causes sodium and calcium channels to close, stopping the release of neurotransmitter (NT) to bipolar cells.
Dark State
- Retinal and opsin are connected.
- Transducin is not active, and phosphodiesterase is not activated.
- cGMP is released, opening sodium and calcium channels, leading to neurotransmitter release to bipolar cells.

Circadian Rhythms: Daily rhythms that run slightly longer than 24 hours without light.
Special photoreceptors: Melanopsin is linked to the rhythm-generating center of the brain.

Ganglion Cell Receptive Fields: The receptive field properties of bipolar cells are continuous to those of ganglion cells.
Ganglion Cells: Transmit action potentials; their axons form the optic nerve.

Pathway Components:
- Ganglionic Cells
- Optic Nerve (Cranial Nerve II)
- Optic Chiasm: Where nasal retina axons cross to contralateral side, while temporal retina axons remain ipsilateral.
- Optic Tract: Carry visual information to the brain.

Lateral Geniculate Body of Thalamus: Site of synapse for ganglion cell axons.
Optic Radiations: Pathway to the visual cortex in the brain.
Cortex Mapping: The right visual field corresponds to the left cortex and vice versa.

Processing Pathways: Different types of visual information are processed through separate pathways.
- Neurons: Specific neurons transmit information for color, shape, and movement.
- Higher Cortical Areas: Integrate visual information from various pathways.

Visual Fields: Encompasses monocular and binocular visual fields providing two perspectives.
Brain Construction: The brain constructs a three-dimensional image based on input from both eyes.