1. Chapter 8
The Special Senses

2. Senses Somatic senses Special senses
Involve receptors from more than one place in the body
Help coordinate muscle movement
Maintain body temperature
Special senses
Extremely sensitive receptors
Supply us with detailed information about the world around us
Sight
Sound
Smell
Taste

3. Special Senses Special senses include Sensory adaptation
Photoreceptors for vision (most acute sense)
Mechanoreceptors for hearing (2nd most acute sense) and balance
Chemoreceptors for smell and taste
Sensory adaptation
The perception that sense simply decreases to the point where we are not aware of it any longer
Get used to an unchanging taste, smell or sight

4. Olfaction Smelling When a receptor binds its specific odor molecule
Occurs in the upper chambers of the nasal passages, take deep breaths to flood upper chambers
Sensory receptors respond to chemicals dissolved in the mucus lying over them
When a receptor binds its specific odor molecule
Olfactory cells: modified neuron that extend from the olfactory bulb through the cribiform plate into the mucus lining of the nasal cavity ending in 6-12 olfactory cilia, bearing at least one of thousands of specific olfactory receptors
A sensory impulse is sent to the olfactory bulb and on to the brain

6. Olfaction
Neural stem cells give rise to new olfactory neurons every 40 days (one of few sites where neurons formed in adults)
Neural connections between the olfactory bulb and the limbic system explain why smells trigger memories and emotion

7. Gustation Taste The sense of taste is linked with the sense of smell
Subtle difference in taste due to involvement of olfaction- hole in back of oral cavity connects to the nasal cavity, closes during swallowing
Flavor of food due to: texture (tongue), taste (papillae), and odor (olfactory epithelium)

8. Gustation Receptors for taste are in roughly 10,000 taste buds
Most of which are on the tongue in small bumps called papillae
Taste buds can distinguish five categories of taste
Sweet, sour, salty, bitter, and umami (savory/meaty)
When stimulated, taste bud receptor cells send information on to the brain
Babies: born with more taste buds than adults, even on inside of the cheeks, initially sweet and sour with salty coming within the first year
Number decreases as we age, especially after 50

10. Hearing The ear has three functional parts The outer ear
Composed of the pinna and external auditory canal (lined with ceruminous glands- secrete ear wax)
FXN: capture sound waves and funnel them to the middle ear
The middle ear
Ear drum (tympanic membrane) marks the beginning of the middle ear
Compression waves in the air (sound) cause the membrane to vibrate, converting sound into mechanical motion
Attached to the inside of the tympanic membrane is the malleus (ossicle)
The vibrating tympanic membrane moves the malleus, which in turn moves the incus (ossicle) through a synovial joint

11. Hearing
The stapes (ossicle) is the final small bone of the middle ear
Ossicles: smallest bones in body (malleus, incus and stapes)
Dampen or amplify the movement of the tympanic membrane
Extremely loud noises dampened by muscles tightening around the joints between ossicles, loosen when the noise is soft
The inner ear
Beyond the stapes lies the oval window
A membrane that functions like the tympanic membrane
The oval window bounces in response to movement of the stapes creating “fluid waves” in the inner ear

13. Hearing
The entire middle and inner ear are within a hollow portion of the temporal bone
This area is filled with air and communicates with the external environment through the eustachian tube (auditory tube), equalizes air pressure on both sides of the tympanic membrane (necessary to hear), “pop”- opening of tube
The cochlea (coiled tube) of the fluid-filled inner ear has three compartments
1) Vestibular canal (uppermost) and the 2) tympanic canal (tip of snail shell, ends at round window- converts to fluid waves- perilymph)- form a U-shaped fluid-filled passage for the pressure waves generated at oval window

14. Hearing
and 3) organ of Corti (center- converts mechanical vibration into sensory input)
The tectorial membrane lies on top of the organ of Corti and rests on hair cells, which are the sensory receptors of the auditory system
Hair cells do not have hairs - rather, they have organelles called stereocilia which look like hairs - stereocilia are sound-sensing organelles
As the stapes pushes air against the membrane, the stereocilia move and a nerve impulse is created in the neuron of that particular hair cell
Each part of the tectorial membrane is sensitive to a different pitch

15. Tympanic membrane vibrates- ossicles move, stapes pulls oval window in and out creating fluid waves in inner ear
Pressure waves pass through cochlea, create enough energy to deform the chochlear duct
Tectorial membrane gets distorted
Stereocilia bend
Bending stimulates nerve impulse

16. Equilibrium Equilibrium is the sense of balance
The vestibule and semicircular canals of the inner ear are responsible for the two types of equilibrium
Static equilibrium (also called gravitational equilibrium)
The physical response to gravity that tells us which direction is down
Utricle and saccule: located in the vestibule, initiate a nerve impulse when the hairs in them bend, contain 2 blobs at right angles to each other = maculae (contain tiny pieces of bone called otoliths that respond to gravity)
Maculae held in by hair cells that are stuck in gelatin- respond to movement

17. Static Equilibrium

18. Dynamic Equilibrium
Dynamic equilibrium (also called rotational equilibrium)
Detects acceleration or deceleration of your head
3 semicircular canals lying in different planes that are filled with fluid (X-horizontal, Y-vertical, and Z-transverse)
Base of each = ampulla which houses the dynamic equilibrium receptor- flame-shaped cupula of gel with hairs embedded

19. Dynamic Equilibrium

20. Vision The eye has three layers Sclera (shape and protection)
Sclera (fibrous layer)
Choroid (vascular layer)
Retina (nervous layer)
Sclera (shape and protection)
Dense connective tissue forming the white sclera and clear cornea (admits and refracts light)
Protected by eyelids, eyelashes and eyebrows: keep dust and particles out of eyes
Six extrinsic muscles connect eyeball to bony orbit: lateral, medial, superior and inferior rectus muscles move eye up, down, left and right and superior and inferior oblique
Bathed by lacrimal gland secretions, drain in holes near nasal cavity

21. Vision Choroid Dark, pigmented
Houses blood supply to eye and melanin to absorb light so it does not bounce around in the eye but strikes the retina only once
Visible as the iris: colored portion of the front of the eye, muscular diaphragm that regulates the amount of light entering the eye by closing or opening the pupil
Color reflection of the amount of melanin in choroid
Ciliary body: holds the lens in place to accommodate near and far vision and secretes aqueous humor

22. Vision
Aqueous humor: bathes lens and cornea supplying oxygen and nutrients, constantly filtered from the blood by the canal of Schlemm
If constricted by glaucoma- increase in pressure that can destroy the light sensitive cells in retina
Nervous layer
Retina: receives light and converts it to nerve impulses
Lens: refracts light
Vitreous humor: maintains shape of eyeball and keeps the retina flat against the choroid

24. Visual Accuity and Lens
Visual acuity requires the eye to focus entering light onto the retina at the back of the eyeball
The changing of lens shape to view nearby objects is called accommodation (lens bulges)
Accommodation gets more difficult with age Because the lens continues to add layers over time
Making the lens thicker and stiffer

25. Visual Impairments
Nearsightedness and farsightedness are both caused by the lens' inability to accommodate light properly
Nearsightedness (myopia)
The eye is too long for the lens to focus the light rays on the retina, focal point in the vitreous humor instead of on the retina
Results in forming an image that is spread out and fuzzy when it hits the retina
Use concave lens to spread rays farther before entering the eye
Farsightedness (hyperopia) is the opposite of nearsightedness
lens focuses the image behind the retina, use convex lens to correct by focusing light rays before entering eye
Astigmatism
The cornea is imperfectly shaped, resulting in an uneven pattern of light hitting the retina
Some areas of the image are in focus, but not others

27. Retina Optic disk: blind spot, no photoreceptors, optic nerve exits
Composed of: rods, cones, bipolar cells and ganglionic cells
Macula lutea: directly behind pupil, high concentration of cones
Fovea: center of macula, only cones

28. Retina Composed entirely of layers of neurons
Rods and cones are neurons that detect light - the photoreceptors
The cones respond to bright light, providing color vision and resolution
Cones are concentrated near the center of the retina, where incoming light is strongest
The rods function in low levels of light, providing only vague images
Rods are spread across the periphery of the retina
The bipolar cells and ganglionic cells are interneurons that carry the action potential generated by the photoreceptors to the brain
Photoreceptors: oriented toward brain therefore light must pass through the retina before stimulating cells (indirect retina)

30. Rods
Rods
When a photon of light hits a rod, a neural response is initiated via the chemical rhodopsin
Rhodopsin is a visual pigment that responds to low levels of white light (dark)
Light splits it into retinal and opsin, which causes a series of events that generate an action potential
Easily bleached (falls apart due to increase in light, cannot regenerate until light is reduced)

31. Cones
Cones
Use the visual pigments retinal and opsin, readily regenerate in the cones unlike the rods, function in bright light
There are three types of cones, each of which is sensitive to different wavelengths of light
Red
Green
Blue

33. Path of Vision
Visual nerve impulses first pass toward the front of the eye
from the photoreceptors to the bipolar neurons
The bipolar neurons transmit the impulse to the ganglionic cells
in the anterior of the retina
Ganglionic cells collect impulses from a small cluster of bipolar cells
and pass them to the brain via the optic nerve

34. Path of Vision
Vision occupies more space in the brain than any other special sense
Visual impulses travel along the optic nerve, through the thalamus to the occipital lobe of the brain
Some impulses cross to the opposite side of the brain at the optic chiasma
The view from the right eye is partially projected on the left side of the visual cortex of the cerebrum, and that from the left is partially projected on the right side
The image reaching the occipital lobe is upside down and inverted
The brain must flip and invert the image before it makes sense to us

35. Sensory Loss Vision and hearing can diminish with age
Some hearing loss is due to mechanical malfunctions
Conduction deafness
Sound is poorly conducted from the outer ear to the inner ear
Hearing aids can help those suffering from conduction deafness by increasing the amplitude of sound that enters the ear
Nerve deafness
Cannot be resolved by a hearing aid
Sound is either not detected by the cochlear nerves or the nerve impulse is not transmitted to the brain
Cochlear implants convert sound vibrations into electrical impulses and have shown some promise in treating nerve deafness