1. Somatic and Special Senses

2. Receptors and Sensations
Types of Receptors
Chemoreceptors – Stimulated by changes in the chemical concentration of substances
Pain Receptors – stimulated by tissue damage
Thermoreceptors – stimulated by changes in temperature
Mechanoreceptors – stimulated by changes in pressure or movement
Photoreceptors – stimulated by light energy

3. Sensations (perceptions)
A sensation is a feeling that occurs when the brain interprets sensory impulses.
Sensations depend on which region of the brain receives an impulse.
At the same time a sensation forms, the cerebral cortex causes the feeling to seem to come from the stimulated receptors: this process is called projection because the brain projects the sensation back to its apparent source.
Projection allows a person to pinpoint the region of stimulation, thus, the eyes seem to see and the ears seem to hear.

4. Sensory Adaptations
Sensory adaptation is an adjustment made when sensory receptors are continuously stimulated.
As receptors adapt, impulses leave them at decreasing rates, until finally, these receptors may stop sending signals.
Impulses can be triggered only if the stimulus strength changes.
A person entering a room with a strong odor experiences sensory adaptation.
At first the sense seems intense, but it becomes less and less noticeable as the smell (olfactory) receptors adapt.

5. Somatic Senses

6. Touch and Pressure Senses
Sensory nerve fibers – common in epithelial tissues where their free ends are between epithelial cells; associated with the sensation of touch and pressure

7. Touch and Pressure Senses
Meissner’s corpuscles – small, oval masses of flattened connective tissue cells within connective tissue sheaths
Two or more sensory nerve fibers branch into each corpuscle and end within it as tiny knobs.
Abundant in the hairless portions of the skin, such as the lips, fingertips, palms, soles, nipples and external genital organs.
Respond to the motion of objects that barely contact the skin, interpreting impulses from them as sensation of light touch.

8. Touch and Pressure Senses
Pacinian corpuscles – relatively large structures composed of connective tissue fibers and cells
Common in the deeper subcutaneous tissues and in muscle tendons and joint ligaments.
Respond to heavy pressure and are associated with the sensation of deep pressure.

9. Touch and Pressure Senses
Ruffini’s corpuscle – deeply located in the dermis and are variant’s of Meissner’s corpuscles with a more flattened capsule.
Mediate sensations of crude and persistent touch.
Slow adapting and permit the fingers to remain sensitive to deep pressure for long periods. (ex. Grasping a steering wheel for a long time.

11. Temperature Senses
Heat receptors – most sensitive to temperatures above 77oF and become unresponsive at temperatures above 113oF (temperatures near or above this stimulate pain receptors, producing a burning sensation)

12. Temperature Senses
Cold receptors – most sensitive to temperatures between 50oF and 68oF ( temperatures below 50oF stimulate pain recpetors, producing a freezing sensation)

13. Temperature Senses
Both heat and cold receptors rapidly adapt. Within about a minute of continuous stimulation, the sensation of heat or cold begins to fade.

14. Sense of Pain
Sensing pain consists of free nerve endings that are widely distributed throughout the skin and internal tissues, except in the nervous tissue of the brain, which lacks pain receptors.
Functions as protection.
Adapts poorly, if at all.
Once a pain receptor is activated, it may send impulses into the central nervous system for some time.
It is believed that injuries promote release of certain chemicals that build up and stimulate pain receptors.
Deficiency of oxygen-rich blood (ischemia) in a tissue or stimulation of certain mechanoreceptors also trigger pain sensation

15. Visceral pain
Pain receptors are the only receptors in viscera whose stimulation produces sensations.
Pain may feel as if it is coming from some part of the body other than the part being stimulated (referred pain).
Referred pain arises from common nerve pathways that carry sensory impulses from skin areas as well as viscera.

16. Pain nerve fibers Acute pain fibers
relatively thin, myelinated nerve fibers
conduct impulses rapidly and are associated with the sensation of sharp pain, which typically originates from a restricted area of the skin and seldom continues after the pain-producing stimulus stops
usually sensed as coming only from the skin

17. Pain nerve fibers Chronic pain fibers thin, unmyelinated nerve fibers
conduct impulses more slowly and produce a dull, aching sensation that may be diffuse and difficult to pinpoint
pain may continue for some time after the original stimulus ceases
felt from the skin as well as in deeper tissues

18. Pain nerve fibers
An event that stimulates pain receptors usually triggers impulses on both acute and chronic fibers – causes a dull sensation.
Pain impulses that originate from the head reach the brain on sensory fibers of cranial nerves.
All other pain impulses travel on the sensory fibers of spinal nerves, and they pass into the spinal cord by way of the dorsal roots of these spinal nerves.
Within the spinal cord, neurons process pain impulses in the gray matter of the dorsal horn, and the impulses are transmitted to the brain.

19. Regulation of Pain Impulses
Awareness of pain arises when impulses reach the thalamus. The cerebral cortex determines pain intensity, locates the pain source, and mediates emotional and motor responses to the pain.

20. Regulation of Pain Impulses
1. Areas of gray matter in the midbrain, pons, and medulla regulate movement of pain impulses from the spinal cord.
2. Impulses from special neurons in these brain areas descend in the lateral funiculus to various levels of the spinal cord.
3. These impulses stimulate ends of certain nerve fibers to release biochemicals that can block pain signals by inhibiting presynaptic nerve fibers in the posterior horn of the spinal cord.

21. Regulation of Pain Impulses
Inhibiting substances released in the posterior horn include neuropeptides called enkephalins (have morphine-like actions) and the monamine serotonin (stimulates other neurons to release enkephalins).
Endorphins are another group of neuropeptides with pain-supressing, morphine-like actions. Endorphins are in the pituitary gland and the hypothalamus.
Both provide natural control.

22. Special Senses

23. Sense of Smell
Sense of smell is associated with complex sensory structures in the upper region of the nasal cavity.

24. Olfactory receptors
Olfactory receptors are chemoreceptors, which means that chemicals dissolved in liquids stimulate them.
Receptors function closely with taste to aid in food selection

25. Olfactory Organs
yellowish brown masses that cover the upper parts of the nasal cavity, the superior nasal conchae, and a portion of the nasal septum

26. Olfactory Organs
contains olfactory receptors that are neurons surrounded by columnar epithelial cells; hair-like cilia cover tiny knobs at the distal ends of these neuron’s dendrites; the cilia project into the nasal cavity and are the sensitive parts of the receptors; chemicals enter the nasal cavity as gases, but they must dissolve at least partially in the watery fluids that surround the cilia before receptors can detect them

27. Olfactory nerve pathways
Receptors send nerve impulses along axons of the receptor cells to neurons located in enlargements called olfactory bulbs, which lie on either side of the crista galli of the ethmoid bone, where they are analyzed.
As a result, impulses travel along olfactory tracts to the limbic system.
The major interpreting areas (olfactory cortex) for these impulses are located within the temporal lobes and the base of the frontal lobes, anterior to the hypothalamus

28. Olfactory nerve pathways

29. Sense of Taste
Taste Buds – special organs of taste; occur primarily on the surface of the tongue and are associated with tiny elevations called papillae; also found in smaller numbers in the roof of the mouth and walls of the pharynx

30. Sense of Taste
Taste receptors (gustatory cells) – modified epithelial cells that function as receptors; structure is somewhat spherical with an opening the taste pore, on its free surface; tiny projections called taste hairs protrude from the outer ends of the taste cells and extend from the taste pore, and are the sensitive parts of the receptor cells; taste cells are wrapped in a network of nerve fibers; before a chemical can be tasted, it must dissolve in the watery fluid (provided by salivary glands) surrounding the taste buds

31. Taste receptors (gustatory cells)

32. Taste sensations: 1. Sweet – front of tongue 2. Sour – sides of tongue
3. Salty – all the way around the rim of the tongue
4. Bitter – back of tongue

33. Taste sensations:
Flavor results from one of the primary sensations or from combination of two or more. Experiencing flavors involves taste (concentration of stimulating chemicals), as well as the sensations of odor, texture (touch), and temperature. Chemicals in some foods may stimulate pain receptors. Taste receptors undergo sensory adaptation relatively rapidly.

34. Taste nerve pathways
Sensory impulses from taste receptors in the tongue travel on fibers of the facial, glossopharyngeal, and vagus nerves into the medulla oblongata. From there, the impulses ascend to the thalamus and are directed to the gustatory cortex, which is located in the parietal lobe, along a deep portion of the lateral sulcus.

36. Sense of Hearing
The ear is the organ of hearing, as well as functioning in the sense of equilibrium.

37. External Ear
Aurical – outer, funnel-like structure that collects sound waves traveling through the air
External auditory meatus –
S-shaped tube; leads inward through the temporal bone for about 2.5 cm

38. Middle Ear
tympanic cavity – air filled space in the temporal bone

39. Middle Ear
eardrum (tympanic membrane) – semitransparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside; has an oval margin and is cone-shaped, with the apex of the cone directed inward; pressure of sound waves moves it back and forth in response and thus produces the vibrations of the sound wave source

40. Auditory ossicles
3 bones attached by tiny ligaments to the wall of the tympanic cavity, and they are covered by mucous membranes; bridge the eardrum and the inner ear, transmitting vibrations between these parts

41. Auditory ossicles
malleus (hammer)– attaches to the eardrum; when the eardrum vibrates, the malleus vibrates in unison and causes the incus to vibrate

42. Auditory ossicles
incus (anvil)– receives vibrations from the malleus and transmits them to the stapes

43. Auditory ossicles
stapes ( stirrup)– receives vibrations from the incus; held by ligaments to an opening in the wall of the tympanic cavity called the oval window, which leads to the inner ear; vibrations of the stapes at the oval window moves fluid within the inner ear, which stimulates the hearing receptors

44. Auditory ossicles
The auditory ossicles also help to increase (amplify) the force of vibrations as they pass from the eardrum to the oval window.

45. Auditory Tube (Eustachian tube)
connects each middle ear to the throat; conducts air between the tympanic cavity and the outside of the body by way of the throat (nasopharynx) and mouth; helps maintain equal air pressure on both sides of the eardrum, which is necessary for normal hearing; this function is noticeable during rapid changes in altitude

46. Auditory Tube (Eustachian tube)

47. Inner Ear
The Inner ear is a complex system of communicating chambers and tubes called a labyrinth.
Each ear has two such structures: the osseous labyrinth, a bony canal in the temporal bone, and a membranous labyrinth, a tube that lies within the osseous labyrinth and has a similar shape.
Between the osseous and membranous labyrinths is a fluid called perilymph that cells in the wall of the bony canal secretes.
The membranous labyrinth contains another fluid called endolymph.

48. Inner Ear Cochlea – functions in hearing
* contains a bony core and a thin, bony shelf that winds around the core like the thread of a screw
* the shelf divides the osseous labyrinth of the cochlea into two compartments, the upper (scala vestibule) leads from the oval window to the apex of the spiral and the lower (scala tympani) extends from the apex of the cochlea to a membrane-covered opening in the wall of the inner ear called the round window

49. Cochlea
The portion of the membranous labyrinth within the cochlea is called the cochlear duct and lies between the two bony compartments and ends as a closed sac at the apex of the cochlea
It is separated from the scala vestibule by a vestibular membrane (Reissner’s membrane) and from the scala tympani by a basilar membrane which contains many thousands of stiff, elastic fibers, whose lengths progressively increase from the base of the cochlea to its apex

50. Cochlea

51. organ of Corti
contains hearing receptors; located on the upper surface of the basilar membrane and stretches from the apex to the base of the cochlea; its receptor cells (hair cells) are organized in rows and have many hair-like processes that project into the endolymph of the cochlear duct; above these hair cells is a tectorial emebrane attached to the bony shelf of the cochlea, passing over the receptor cells and contracting the tips of the hairs

52. Cochlea

53. organ of Corti

54. Steps in the Generation of Sensory Impulses from the Ear
1. Sound waves enter external auditory meatus.
2. Waves of changing pressure cause eardrum to reproduce vibrations coming from sound wave source.
3. Auditory ossicles amplify and transmit vibrations to end of stapes.
4. Movement of stapes at oval window transmits vibrations to perilymph in scala vestibule.
5. Vibrations pass through vestibular membrane and enter endolymph of cochlear duct.

55. Steps in the Generation of Sensory Impulses from the Ear
6. Different frequencies of vibration in endolymph stimulate different sets of receptor cells.
7. As a receptor cell depolarizes, its membrane becomes more permeable to calcium ions.
8. Inward diffusion of calcium ions causes vesicles at the base of the receptor cell to release neurotransmitter.
9. Neurotransmitter stimulates ends of nearby sensory neurons.
10. Sensory impulses are triggered on fibers of cochlear branch of vestibulocochlear nerve.
11. Auditory cortex of temporal lobe interprets sensory impulses.

56. Auditory Nerve Pathways
Nerve fibers associated with hearing enter the auditory nerve pathways, which pass into auditory cortices of the temporal lobes, where they are interpreted.
Some fibers cross over, so that impulses arising from each ear are interpreted on both sides of the brain.
Consequently, damage to a temporal lobe on one side of the brain does not necessarily cause complete hearing loss in the ear on that side.

57. Sense of Equilibrium

58. Inner Ear
semicircular canals - provide a sense of equilibrium

60. Static Equlibrium
Senses the position of the head, maintaining stability and posture when the head and body are still.
The organs for static equilibrium are located within the vestibule, a bony chamber between the semicircular canals and cochlea.
The membranous labyrinth inside the vestibule consists of 2 expanded chambers: a utricle and a saccule. Each of these chambers has a tiny structure called a macula on its anterior wall.
Macula contain numerous hair cells, which serve as sensory receptors.

62. Static Equlibrium
The head bending forward, backward, or to the side stimulates hair cells. The movement tilts the gelatinous masses of the macula bending the hair cells, which signal the nerve fibers associated with them.
The resulting impulses travel into the CNS on the vestibular branch of the vestibulocochlear nerve.
These impulses inform the brain of the head’s position.
The brain responds by sensing motor impulses to skeletal muscles, which contract or relax to maintain balance.

63. Dynamic Equilibrium
The three semicircular canals detect motion of the head and aid in balancing the head and body during sudden movement.
The canals lie at right angles to each other and each corresponds to a different anatomical plane.
Suspended in the perilymph of the bony portion of each canal is a membranous canal that ends in a swelling called an ampulla.
The ampulla contains the sensory organs of the semicircular canals, crista ampullaris.
Each crista ampullaris contains a number of sensory hair cells and supporting cells. The hair cells extend upward into a dome-shaped gelatinous mass called the capula.

65. Dynamic Equilibrium
Rapid turns of the head or body stimulate the hair cells of the crista ampularis.
The semicircular canals move with the head or body, but the fluid inside the membranous canals remains stationary. This action bends the capula in one or more of the canals in a direction opposite that of the head or body movement. The hairs in the capula also bend stimulating the hair cells to signal their associated nerve fibers, sensing impulses to the brain.
Parts of the cerebellum are important in interpreting impulses from the semicircular canals and helping to maintain balance.

66. Sense of Equilibrium
Other sensory structures aid in maintaining equilibrium.
Certain mechanoreceptors, particularly those associated with the joints of the neck, inform the brain about the movements.
The eyes detect changes in posture that result from body movements.
Such visual information is so important that even if the organs of equilibrium are damaged, a person may be able to maintain normal balance by keeping the eyes open and moving slowly.

67. Vertigo
Vertigo is the feeling that you or your environment is moving or spinning. It differs from dizziness in that vertigo describes an illusion of movement. When you feel as if you yourself are moving, it's called subjective vertigo, and the perception that your surroundings are moving is called objective vertigo. Unlike nonspecific lightheadedness or dizziness, vertigo has relatively few causes.

68. Causes of Vertigo

69. Sense of Sight

70. Visual Accessory Organs
Visual accessory organs are housed within the pear-shaped orbital cavity of the skull.
This orbit contains fat, blood vessels, nerves, and connective tissue.

71. Eyelid - has four layers
1. skin – the thinnest skin of the body; covers the lid’s outer surface and fuses with its inner lining near the margin of the lid
2. obicularis oculi muscle – acts as a sphincter and closes the lids when it contracts
3. levator palpebrae superioris muscle – raises the upper lids and thus helps open the eye
4. conjunctiva – mucous membrane that lines the inner surfaces of the eyelids and folds back to cover the anterior surface of the eyeball (except for its central portion [cornea])

72. Lacrimal apparatus
1. lacrimal gland – located in the orbit and secretes tears continuously; moistens and lubricates the surface of the eye and the lining of the lids; tears exit through tiny tubules and flow downward and medially across the eye; tears contain an enzyme (lysozyme) that is an antibacterial agent, reducing the risk of eye infections
2. superior and inferior canaliculi – two small ducts which collect tears that flow into the lacrimal sac, which lies deep in a groove of the lacrimal bone, and then into the nasolacrimal duct, which empties into the nasal cavity

75. Extrinsic Muscles
Extrinsic Muscles arise from the bones of the orbit and insert by broad tendons on the eye’s outer surface.
6 extrinsic muscles move the eye in various directions:

76. Extrinsic Muscles
1. superior rectus – rotates eye upward and toward midline; oculomotor nerve III
2. inferior rectus – rotates eye downward and toward midline; oculomotor nerve III
3. medial rectus – rotates eye toward midline; oculomotor nerve III

77. Extrinsic Muscles
1. lateral rectus – rotates eye away from midline; abducens nerve VI
2. superior oblique – rotates eye downward and away from midline; trochlear nerve IV
3. inferior oblique – rotates eye upward and away from midline; oculomotor nerve III

79. Structure of the Eye

80. Outer Tunic (fibrous tunic)
Cornea
comprises the anterior sixth of the outer tunic and bulges forward
window of the eye and helps focus entering light rays
composed largely of connective tissue with a thin layer of epithelium on its surface
transparent because it contains few cells and no blood vessels and its cells and collagenous fibers form regular patterns

81. Outer Tunic (fibrous tunic)
Sclera
a. continuous with the cornea
b. white portion of the eye
c. makes up the posterior 5/6th of the outer tunic
d. opaque due to many large, disorganized, collagenous, and elastic fibers
f. protects the eye and is an attachment for the extrinsic muscles
g. in the back of the eye, the optic nerve and certain blood vessels pierce the sclera

83. Middle Tunic (vascular tunic)
Choroid coat
posterior 5/6th of the globe of the eye
is loosely joined to the sclera and is honey combed with blood vessels, which nourish surrounding tissue
contains many pigment-producing melanocytes, which produce melanin to absorb excess light and thus keep the inside of the eye dark

84. Middle Tunic (vascular tunic)
Ciliary body
a. thickest part of the middle tunic
b. extends forward from the choroid coat and forms an internal ring around the front of the eye
c. contains many radiating folds called ciliary processes and groups of muscle fibers that constitute the ciliary muscles

85. Ciliary body
d. many strong, delicate fibers, called suspensory ligaments, extend inward from the ciliary processes and hold the transparent lens in position
e. the body of the lens lies directly behind the iris and pupil and is composed of differentiated epithelial cells called lens fibers
f. the cytoplasm of the lens fibers is the transparent substance of the lens

86. Ciliary body
g. the lens capsule is a clear, membrane-like structure composed largely of intercellular material whose elastic nature keeps it under constant tension
h. the suspensory ligaments attached to the margin of the capsule are also under tension and pull outward, flattening the capsule and the lens inside
i. when the fibers contract, the choroid coat is pulled forward and the ciliary body shortens relaxing the suspensory ligaments

87. Ciliary body
j. the lens thickens in response and is now focused for viewing closer objects than before
k. the lens thickens in response and is now focused for viewing closer objects than before
l. to allow focus on more distant objects, the ciliary muscles relax, tension on suspensory ligaments increases, and the lens becomes thinner and less convex again
m. this ability of the lens to adjust shape to facilitate focusing is called accommodation

88. Iris
a. thin diaphragm composed mostly of connective tissue and smooth muscle fibers
b. from the outside, the iris is the colored portion of the eye
c. extends forward from the periphery of the ciliary body and lies between the cornea and lens dividing the space (anterior cavity)
d. separated into an anterior chamber (between the cornea and iris) and a posterior chamber (between the iris and vitreous body, and containing the lens)

89. Iris
e. the epithelium on the inner surface secretes a watery fluid called aqueous humor into the posterior chamber
f. the aqueous humor circulates from this chamber through the pupil, a circular opening in the center of the iris, and into the anterior chamber
g. aqueous humor fills the space between the cornea and lens to nourish and aid in maintaining shape of the front of the eye
h. aqueous humor leaves the anterior chamber through veins and a special drainage canal, the scleral venous sinus (canal of Schlemm) located in its walls

90. Iris
the smooth muscle fibers of the iris are organized into two groups:
1. circular set – acts as a sphincter; when it contracts, the pupil gets smaller, and the amount of light entering decreases
2. radial set – when these muscles contract, the pupil’s diameter increases, and the amount of light entering increases

92. Inner Tunic
Retina
a. contains the visual receptor cells (photoreceptors)
b. nearly transparent sheet of tissue that is continuous with the optic nerve in the back of the eye and extends forward as the inner lining of the eyeball and ends just behind the margin of the ciliary body

93. Retina c. has a number of distinct layers:
1. macula lutea – yellowish spot in the central region which has a depression in its center called the fovea centralis (region of the retina that produces the sharpest vision
2. Optic disk – medial to the fovea centralis; where nerve fibers from the retina leave the eye and join the optic nerve; a central artery and vein also pass through the optic disk; these vessels are continous with the capillary network of the retina, and with vessels in the underlying choroid coat which supply blood to the inner tunic; since the optic disk region has no receptor cells, it is commonly known as the blind spot

94. Retina
d. the space bounded by the lens, ciliary body, and retina is the largest compartment of the eye and is called the posterior cavity and is filled with a transparent, jelly-like fluid called vitreous humor
e. vitreous humor with collagenous fibers comprise the vitreous body, which supports the internal parts of the eye and helps maintain its shape

98. Light Refraction
Light refraction is the bending of light rays due to focusing.
When light waves pass at an oblique angle from a medium of one optical density into a medium of a different optical density light refraction occurs.
This process occurs at the curved surface between the air and the cornea and the curved surface of the lens itself.
A lens with a convex surface (such as in the eye) causes light waves to converge.

99. Light Refraction
The convex surface of the cornea refracts light waves from outside objects.
The convex surface of the lens, and to a lesser extent, the surfaces of the fluids within chambers of the eye then refract the light again.
If eye shape is normal, light waves focus sharply on the retina.
The image that forms on the retina is upside down and reversed from left to right.
The visual cortex interprets the image in its proper position.

100. Visual Receptors
Visual receptors are modified neurons that are located in a deep portion of the retina and are closely associated with a layer of pigmented epithelium.
The epithelial pigment absorbs light waves not absorbed by the receptor cells, and together with the pigment of the choroid coat, keeps light from reflecting off surfaces inside the eye.
Visual receptors are stimulated only when light reaches them.
A light image focused on an area of the retina stimulates some receptors, and impulses travel from them to the brain.

101. Visual Receptors
The impulse leaving each activated receptor provides only a fragment of the information required for the brain to interpret a total scene. There are two distinct kinds of receptors:
1. Rods – long, thin projections at their ends; hundreds of times more sensitive to light, therefore can provide vision in dim light; produce colorless vision; provide general outlines of objects give less precise images because nerve fibers from many rods converge, their impulses are transmitted to the brain on the same nerve fiber
2. Cones – have short, blunt projections; detect color; provide sharp images

103. Visual Receptors
The fovea centralis, the area of sharpest vision, lacks rods but contains densely packed cones with few or no converging fibers. Also in the fovea centralis, the overlying layers of the retina and the retinal blood vessels are displaced to the sides, more fully exposing receptors to incoming light. Consequently, to view something in detail, a person moves the eyes so that the important part of the image falls on the fovea centralis.

104. Visual Pigments
Both rods and cones contain light-sensitive pigments that decompose when they absorb light energy.

105. Visual Pigments
Rods – contain the light-sensitive biochemical called rhodopsin (visual purple).
In the presence of light rhodopsin breaks down into molecules of a colorless protein called opsin and a yellowish substance called retinal (retinene) that is synthesized from vitamin A.
Decomposition of rhodopsin alters the permeability of the rod cell membrane, as a result, a complex pattern of nerve impulses originate in the retina and then travel along the optic nerve into the brain where they are interpreted as vision.
In bright light there is more decomposition.
In dim light there is less decomposition and more regeneration of rhodopsin

106. Visual Pigments
Cones – the light-sensitive pigments are similar to rhodopsin in that they are composed of retinal combined with protein.The protein, however differs.
Three different sets of cones each contain an abundance of one of the three different visual pigments.
The wavelength of light determines the color that the brain perceives from it.
For example, the shortest wavelengths are perceived as violet, and the longest as red.

107. Visual Pigments The three cone pigments are:
1. erythrolabe – sensitive to red light waves
2. chlorolabe – sensitive to green light waves
3. cyanolabe – sensitive to blue light waves

108. Visual Pigments
The color a person perceives depends on which set of cones or combination of sets the light in a given image stimulates.
If all three sets of cones are stimulated, the person senses the light as white, and if none are stimulated, the person senses black.
Different forms of colorblindness result from lack of different types of cone pigments.

109. Visual Nerve Pathways
The axons of the retinal neurons leave the eyes to form the optic nerves.
Just anterior to the pituitary gland, these nerves give rise to the X-shaped optic chiasma, and within the chiasma, some of the fibers cross over.
The fibers from the nasal (medial) half of each retina cross over, but those from the temporal (lateral) sides do not.
Fibers from the nasal half of the left eye and the temporal half of the right eye form the right optic tract, and fibers from the nasal half of the right eye and the temporal half of the left eye form the left optic tract.
Just before the nerve fibers reach the thalamus, a few of them enter nuclei that function in various visual reflexes.
Most of the fibers, however, enter the thalamus and synapse in its posterior portion.
From this region, the visual impulses enter nerve pathways called optic radiations, which lead to the visual cortex of the occipital lobes.

110. Visual Nerve Pathways
Just before the nerve fibers reach the thalamus, a few of them enter nuclei that function in various visual reflexes.
Most of the fibers, however, enter the thalamus and synapse in its posterior portion.
From this region, the visual impulses enter nerve pathways called optic radiations, which lead to the visual cortex of the occipital lobes.