Human Senses and Eye Anatomy

The Senses

• The human body has five basic senses: touch, taste, smell, sight, and hearing
• Touch is further divided into tactile, temperature, pressure, and pain
• Sensory receptors for each of these senses are specialized

The Eye and Vision

• The eye is responsible for sight
• The human eye contains over a million nerve fibers
• The eye is protected by the bony orbit
• A cushion of fat surrounding the eye provides additional protection

Accessory Structures of the Eye

• The eyelids, eyelashes, ciliary glands, conjunctiva, and lacrimal apparatus protect and lubricate the eye
• The eyelids brush particles out of the eye and cover the eye
• Eyelashes trap airborne particles
• Ciliary glands are modified sweat glands that lie between eyelashes
• The conjunctiva lines the eyelids and connects to the surface of the eye, secreting mucus to lubricate the eye

Conjunctivitis

• Inflammation of the conjunctiva
• Caused by bacterial or viral infections
• Highly contagious

Lacrimal Apparatus

• The lacrimal apparatus is responsible for producing and draining tears
• The lacrimal gland produces tears
• Lacrimal canals drain tears from the eyes
• The lacrimal sac provides passage for tears to the nasal cavity
• The nasolacrimal duct empties tears into the nasal cavity

Function of the Lacrimal Apparatus

• Tears are a dilute salt solution
• Tears contain antibodies and lysozyme which help protect the eye from foreign substances like bacteria
• Tears protect, moisten, and lubricate the eye

Extrinsic Eye Muscles

• Extrinsic eye muscles attach to the outer surface of the eye
• These muscles are responsible for eye movements

Structure of the Eye

• The eye is composed of three layers: the fibrous tunic, the choroid, and the sensory tunic
• The fibrous tunic is the outermost layer
• The choroid is the middle layer
• The sensory tunic is the innermost layer

The Fibrous Tunic

• The sclera is the tough, white connective tissue layer of the fibrous tunic
• The sclera can be seen as the “white of the eye”
• The cornea is the transparent, central, anterior portion of the fibrous tunic
• The cornea allows light to pass through the eye and bends light

Choroid Layer

• The choroid is a blood-rich layer that nourishes the eye
• The choroid contains pigment to prevent light scattering
• The choroid is modified internally to form the ciliary body and the iris

Sensory Tunic (Retina)

• The sensory tunic contains the light-sensitive receptor cells
• The sensory tunic also contains ganglion cells and bipolar cells
• Signals travel from the retina to the brain through the optic nerve

Visual Pigments

• Rhodopsin, a visual pigment found in rods, is composed of opsin and retinal proteins
• Light hitting rhodopsin causes it to bleach

Rhodopsin & Opsin

• Rhodopsin is a pigment in rod cells that absorbs light
• Rhodopsin is composed of opsin and retinal
• Retinal is a vitamin-A derivative
• Retinal changes shape when it absorbs light
• Rhodopsin straightens out and breaks down into opsin and retinal
• There are three other opsins besides rhodopsin:
  • Photopsin I: absorbs yellowish-green light
  • Photopsin II: absorbs green light
  • Photopsin III: absorbs bluish-violet light

Neurons of The Retina Overview

• The retina is a light-sensitive layer at the back of the eye
• The retina contains specialized neurons, called photoreceptor cells, that detect light
• There are two main types of photoreceptor cells: rods and cones

Rod Cells

• Rods are most abundant towards the edges of the retina
• Rods allow for dim light vision and peripheral vision
• Rods are more sensitive to light; they do not respond in bright light
• Rods are involved in black and white (grayscale) vision

Cone Cells

• Cones allow for detailed color vision
• Cones are densest in the center of the retina
• The fovea centralis is an area of the retina with only cones
• Cones respond best in bright light

Optic Disk

• The optic disk is the point where the optic nerve leaves the eye
• There are no photoreceptor cells at the optic disk, resulting in a blind spot

Cone Sensitivity

• There are three types of cones:
  • Red cones: sensitive to long wavelengths
  • Green cones: sensitive to medium wavelengths
  • Blue cones: sensitive to short wavelengths
• Each cone type is most sensitive to a different wavelength of light
• Color blindness occurs from a lack of one or more cone type
• Most common forms of colorblindness are red-green, due to a lack of red receptors

How We See Colors

• The brain compares the input from different cone cells to determine color
• Red and green cone signals are compared by opponent cells in the retina
• Opponent cells compute the balance of red and green light from a specific visual field location
• Other opponent cells compare blue cone signals with combined red and green cone signals

Color Blindness

• Color blindness is an inherited trait
• The lack of one or more cone types causes color blindness
• Colorblindness is sex-linked, meaning it is more common in males
• Color blindness is caused by a gene on the X chromosome, of which males have only one