The Human Eye: Structure, Function, and Vision

The Human Eye

Structure and Function

The Retina

The retina is a structure at the back of the eye that contains light-sensitive cells called rods and cones.

• Rods: Detect different light intensities, see in black and white, sensitive to low light levels, used to see in dim light.
• Cones: Different types sensitive to different colors (especially primary colors), need bright light to work, cannot see in dim light.

The central part of the retina has a lot of cone cells and no rod cells. The outer part of the retina has no cone cells and a lot of rod cells.

Blind Spot

The blind spot is the point where the optic nerve leaves the eye. There are no receptor cells in this area.

Cornea

The cornea is the transparent front part of the eye. It protects the eye and is responsible for most of the refraction of light (not the lens).

Sclera

The sclera is a tough, fibrous layer that forms the supporting wall of the eyeball and is continuous with the cornea.

The Pupil Reflex

The amount of light entering the pupil of both eyes and focused on the retina is controlled by the pupil reflex. If too much light enters, the photosensitive cells in the retina can be damaged.

Antagonistic Pairs

Antagonistic pairs of muscles work together, with one muscle relaxing while the other contracts (e.g., the iris).

Iris

The iris has two sets of muscles: circular and radial muscles. They work antagonistically. When the radial muscles relax, the circular muscles contract.

Stimulus: Bright room, high light intensity.

Process: Photosensitive cells detect light intensity - electrical signal sent to sensory neuron - relay neuron - motor neuron - radial muscles relax, circular muscles contract (effector).

Response: Pupil contracts (vice versa for a dark room).

Accommodation

Accommodation is the process that allows the eye to focus light from both near and distant objects into a sharp image on the retina.

• Distant objects: Light rays are parallel.
• Near objects: Light rays are diverging.

The lens changes shape to accommodate these differences.

Ciliary muscles and suspensory ligaments are responsible for these changes.

Lens Power

Thicker lens: Shorter focal length, more powerful, able to focus on divergent rays (needed for near objects).

Thinner lens: Longer focal length, less powerful, able to focus on convergent rays (needed for distant objects).

The Endocrine System (Adrenaline)

Note: This section is about the endocrine system, not the eye.

Adrenaline causes several effects on the body:

• Heart: Heart rate increases, more blood carrying glucose and oxygen around the body, increasing aerobic respiration.
• Lungs: Lungs inflate, more oxygen in the blood, removal of carbon dioxide from increased respiration.
• Eyes: More light enters the eye, more information can be processed by the brain about the surroundings.
• Muscles: Increased blood flow, more oxygen and glucose, muscle cells can respire more in response to "fight, flight, or freeze" and are prepared to move.
• Liver: More glycogen converted into glucose, more glucose in the blood for aerobic respiration.
• Brain: Increased blood supply, increased glucose and oxygen to the brain, enabling faster decision-making for survival.