1. Anatomy Ch. 8
Special Senses

2. The Eye and Vision
Of all the sensory receptors in the body 70% are in the eyes.
Accessory Features
Extrinsic eye muscles
6 muscles attached to the outer surface of the eye that function in moving the eye
Eyelids
Function in protection
Have eyelashes which function in protection
Glands associated with the eyelids called tarsal glands produce an oily secretion that lubricates the eye.

3. Conjunctiva Lacrimal apparatus
Membrane that secretes mucus that lubricates and moistens the eyeball
Lacrimal apparatus
Produces tears that clean the eyeball
Secretions contain lysozyme which destroys bacteria
Secretions empty into the nasal cavity

4. Internal structure of the eyeball
The eyeball is a hollow sphere
The wall of the eyeball is composed of 3 layers (tunics)
The interior of the eyeball is filled with fluids called humors that help the eye maintain shape
The lens is the main focusing apparatus of the eye
The eye contains 2 chambers

5. Layers of the Eyeball Wall
Fibrous Layer
Outermost layer
Contains the protective sclera and the transparent cornea
The sclera is the white of the eye
The cornea is the area where light enters the eye
Well supplied with nerves
Most exposed part of the eye
Can be damaged easily but can repair itself
Can be transplanted with no possibility of rejection due to lack of blood vessels

6. Vascular layer
Middle layer
3 regions
1) Choroid
Blood rich
Contains a dark pigment that prevents light from scattering in the eye
2) Ciliary body
Smooth muscle
Attaches to the lens

7. 3) Iris
Colored region of the eye
Opening in the center is called the pupil that allows light to pass through
The iris is composed of smooth muscle which regulates the amount of light that enters the eye by changing the size of the pupil
Bright light or close vision: pupil constricts
Dim light or distant vision: pupils dilate

8. Sensory layer
Inner most
Contains the 2 layered retina
1) Pigmented layer
Outer layer
Absorbs light and prevents scattering
Has cells that act as phagocytes

9. 2) Neural layer
Inner layer
Contains photoreceptors called rods and cones
Signals pass through the optic nerve to the occipital lobe

10. Photoreceptors are found all over the retina except where the optic nerve leaves the eyeball.
This area is called the optic disk or blind spot.

11. Rods and Cones
Rods
The majority of rods are found at the edge of the retina
The number decreases as you move to the center
Allow us to see in gray tones in dim light
Provide peripheral vision

12. Cones
The majority of cones are found in the center of the retina
The number decreases as you move to the edges
Allow us to see in detail in color in bright light conditions
The fovea centralis is located next to the blind spot. It is loaded with cones and is the area where we have the sharpest vision (macula lutea)

13. There are 3 types of cones:
Responds to blue light
Responds to green light
Range that includes green and red
Different colors are caused by simultaneous impulses that are mixed and interpreted in the brain.

14. Lens
Biconvex shape
Focuses light on the retina
Held in position by a ligament that attaches to the ciliary body
Divides the eye into 2 chambers

15. The anterior segment is filled with a watery fluid called aqueous humor
The posterior segment is filled with a gel like substance called vitreous humor. Vitreous humor helps prevent the eye from collapsing
Both humors help maintain the pressure inside the eye

17. Physiology of Vision
Light rays are refracted or bent as they encounter the cornea, aqueous humor, lens, and vitreous humor
The refractive power of the cornea and humors does not change but the refractive power of the lens can change by changing shape
The more the lens bulges the more it bends light and vice versa.
In order to make close vision possible the lens must bulge more. This process is called accommodation.

18. The image formed on the retina is called a real image.
A real image is reversed from left to right, upside down, and smaller
The image is placed in the correct orientation by the occipital lobe.

19. Visual Pathway to the Brain
Axons from the eye bundle together to form the optic nerve
At a point in the brain called the optic chiasma fibers from the medial (inner) side of each eye cross over to the opposite side of the brain.
Fibers from the lateral (outer) side of each eye stay on the same side of the brain.
Fibers on each side of the brain join together and are called optic tracts.

20. Each tract contains fibers from both eyes
The optic tracts pass through the thalamus and enter the occipital lobe where seeing occurs
Each side of the brain receives input from both eyes. This causes binocular vision.

21. Eye Reflexes
Intrinsic eye muscles
Autonomic
Affect lens curvature
Affect muscles of the iris which controls pupil size
Extrinsic eye muscles
Controls eye movement and allows the eyes to follow moving objects
Allows for convergence which allows us to view close objects
Controlled by cranial nerves III, IV, and VI

22. Photopupillary reflex
Constricting of the pupils when exposed to bright light
Accommodation pupillary reflex
Constricting of the pupils when we view close objects allowing for more acute vision

23. Development of Vision
Generally speaking, vision is the only special sense that is not fully functional when the baby is born.

24. Homeostatic Imbalances of the Eye
Color blindness occurs when one or more types of cones are missing.
The most common is the lack of red or green receptors
Because genes regulating color vision are on the X chromosome, color blindness is sex linked.
This causes colorblindness to be more common in males than females.

25. Cataracts Occurs when the lens becomes hard and opaque
Causes vision to become hazy and distorted and eventually causes blindness
Common causes include diabetes, exposure to intense sunlight, and heavy smoking

26. Glaucoma Occurs when drainage of the aqueous humor is blocked.
Pressure in the eye increases which compresses the retina and optic nerve.
Causes pain and possible blindness
Progresses slowly and may have no symptoms at first.
Once symptoms do occur the damage has already been done.

27. The Ear and Hearing and Balance
Structure of the Ear
External (outer) ear
Auricle (Pinna)
Outer visible portion of the ear
External acoustic meatus (auditory canal)
Short chamber that enters the temporal bone
Contains ceruminous glands that secrete earwax

28. The tympanic membrane or eardrum is found at the end of the canal
It separates the outer and middle ear
Sound waves cause the eardrum to vibrate

29. Middle Ear (tympanic cavity)
Area of the ear enclosed by the eardrum and oval window
The auditory tube connects the middle ear to the throat
This tube is normally flat but swallowing or yawning can open it to equalize pressure
The middle ear contains the 3 smallest bones in the body called ossicles which transmits vibrations from the eardrum to the fluid of the inner ear.

30. The 3 ossicles
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
Sound waves vibrate these bones in order with the last vibrating the oval window which vibrates fluid in the inner ear.

31. Internal (Inner ear)
Maze of bony chambers called the bony labyrinth
The 3 regions of the labyrinth are the cochlea, vestibule, and semicircular canals
The bony labyrinth is filled with a fluid called perilymph
Suspended in the perilymph is a membranous labyrinth
The membranous labyrinth contains a fluid called endolymph

32. Equilibrium
The equilibrium sense responds to movements of the head
The equilibrium receptors are found in the inner ear
There are 2 types of equilibrium: static and dynamic

33. Static equilibrium
Occurs in the vestibule of the bony labyrinth
Receptors called maculae report on changes in position of the head when the body is not moving
The maculae help us keep our head erect
Movements of the maculae send messages to the vestibular nerve which sends a message to the cerebellum.

34. Dynamic Equilibrium
Occurs in the semicircular canals of the bony labyrinth
Responds to angular or rotation of the head rather than straight line movements
Receptor regions are called crista ampullaris
Messages are sent to the vestibular nerve which sends the message to the cerebellum

35. Hearing
Within the cochlea is the spiral Organ of Corti which contains hair cells
Sound waves reach the cochlea through vibrations of the eardrum, ossicles, and oval window
The vibrations set fluids in the cochlea in motion
The force of sound waves is increased as it passes through the ossicles

36. The Organ of Corti is a fluid filled structure that is divided by a basilar membrane.
The basilar membrane contains fibers called hair cells that are bent by movements of another membrane called the tectorial membrane.
Fibers attached to the membrane of this region have different lengths
High pitched sounds affect short fibers
Low pitched sounds affect long fibers
The hair cells transmit impulses along the cochlear nerve to the auditory cortex of the temporal lobe where hearing occurs

37. Homeostatic Imbalances of the Ear
Deafness
Hearing loss of any degree
There are 2 types
Conduction
Results when something interferes with the conduction of sound vibrations to the fluids of the inner ear.
Sensorineural
Degeneration or damage to receptors in the organ of Corti, cochlear nerve, or neurons in the auditory cortex.
Often results from loud sounds.