I. Sensory Receptors 
   Sensory receptor cells convert stimuli into electrical energy
   A. Classification by Location 
       1.exteroceptors - affected by stimuli from outside of the body 
       2. interoceptors (visceroceptors) - affected by stimuli inside the body 
       3. proprioceptors - respond to stimuli in the muscles, joints, and associated tissues 
   B. Classification by Stimulus Type 
       1. mechanoreceptors - respond to mechanical forces, such as touch, pressure, vibrations, and stretching 
       2. thermoreceptors - respond to temperature 
       3. electromagnetic receptors – photoreceptors in the eye respond to light (the visible section of the electromagnetic spectrum); some animals can detect electric fields and possibly magnetic fields
       4. chemoreceptors - respond to chemicals, such as taste and smell 
       5. nociceptors - respond to pain 

II. TASTE
 A. Taste is detected by taste buds
 B. We have about 10,000 of them, mostly on the tongue
 C. They are usually in the form of peglike projections in the tongue mucosa called papillae
 D. Gustatory hairs protrude from the taste bud
 E. For a chemical to be tasted, it must dissolve in saliva, diffuse into taste pores, and contact gustatory hairs
 F. There are 4 basic taste sensations
  1. salty - best sensed at the tip of the tongue
  2. sweet - best sensed at the front of the tongue
  3. sour - best sensed at the sides of the tongue
  4. bitter - best sensed at the back of the tongue

III. SMELL
 A. The olfactory nerve ends in an olfactory bulb, located on the cribriform plate above the nasal cavity.
 B. Olfactory receptor cells extend from the bulb, through the plate, to the top of the nasal cavity
 C. Olfactory hairs, at the bottom of the receptor cells, pick up chemicals in the air that you breathe
 D. Olfactory glands, located around the receptor cells, secrete a mucus that "captures" and dissolves airborne odor molecules
 E. We can distinguish about 10,000 different smells
 

Certain animals have chemoreceptors in other parts of their bodies, such as outside the mouth or on the limbs.
 

IV. VISION
 A. Accessory parts of the eye
  1. Eyebrows
  2. Eyelids
  3. Lacrimal Apparatus
   a. Lacrimal glands - located above and to the lateral side of each eye; contain tears
   b. Lacrimal secretion - "tears"; a dilute salty solution that moistens and cleans the surface of the eye
   c. when the tears reach the medial edge of the eye, they enter holes called lacrimal punctum
   d. the tear fluid goes into the lacrimal punctum and into lacrimal canals
   e. the fluid from the canals drains into the lacrimal sac, which drains into nasal cavity
  4. Extrinsic eye muscles
   a. control eye movement
   b. There are two basic eye movements
    i. saccades - small, jerky movements
    ii. scanning movements
 B. The Eye (eyeball)
  1. Posterior eye
   a. the outer layer is sclera, which is white and opaque
   b. the middle layer is choroid; located beneath the sclera, it is dark brown and filled with blood vessels
   c. the inner layer is the retina, which contains the nerve endings which register light
    i. cones - register color; 7 million
    ii. rods - register B&W; 125 million
    iii. blind spot - place where the retina enters the optic nerve; also called the optic disc
    iv. fovea centralis - pit in the back of the retina
   d. the posterior chamber of the eye is filled with vitreous humor (a clear jelly)
  2. Front of the eye
   a. cornea - clear outside layer
   b. aqueous humor - clear jelly-like fluid between the cornea and the lens
   c. lens - focuses the light onto the retina
   d. ciliary bodies - hold the lens in place; muscles pull or relax to change the shape of the lens so that the image focuses properly onto the retina
 C. Eye problems
  1. Nearsighted (myopic)
   a. the lens focuses the object in front of the retina
   b. corrected with a concave lens
  2. Farsighted (hyperopic) 
   a. lens focuses the object behind the retina
   b. corrected with a convex lens
  3. astigmatism - points of light are focused as lines, leaded to blurry vision
D. All vertebrates have single-lens eyes.
E. The simplest type of photoreceptor is the eye cup.  In planarians (a type of flatworm), they are used to detect intensity and direction of light (but not to form images).
F. Many invertebrates (insects, crustaceans) have compound eyes made up of many ommatidia (light-detecting units).  The animal’s ‘brain’ merges the many separate images into one.
 

V. HEARING
A. External Ear - consists of 3 parts
 1. the auricle, or pinna, which is constructed of cartilage; it focuses sound into the ear canal
 2. the external auditory canal 
  a. it is about one inch long by 1/2 inch in diameter
  b. it is skin lining has ceruminous glands, which produce cerumen (earwax) to trap foreign 
   bodies and repel insects
 3. the tympanic membrane (eardrum)
  a. located at the medial end of the ext. auditory canal
  b. it vibrates as sounds hit it
B. Middle Ear (tympanic cavity)
 1. it is filled with air and lined with mucus
 2. there is a tube (pharyngotympanic, or auditory) that connects the middle ear with the nasopharynx
 2. it contains three bones (ossicles)
  a. malleus
  b. incus
  c. stapes
 3. as sounds vibrate the tympanic membrane, it vibrates the malleus, which vibrates the incus, 
  which vibrates the stapes
C. Inner Ear
 1. consists of a bony labyrinth with 3 parts
  a. vestibule
   i. central part of the labyrinth
   ii. contains the saccule and utricle
   iii. the stapes attaches to the oval window on the vestibule's lateral wall
   iv. the vibrating stapes transmits the vibrations to the vestibule, which transmits them to 
    the cochlea via the cochlear fluid
  b. cochlea 
   i. attaches to the vestibule
   ii. houses the spiral-shaped organ of Corti, the receptor organ for hearing
   iii. cochlear hair cells in the organ of Corti, vibrate as the cochlear fluid vibrates; these 
    vibrations are registered as nerve impulses and sent to the brain via the 
    vestibulocochlear nerve
  c. semicircular canals
   i. attaches to the posterior of the vestibule
   ii. there are 3 tubes, one in each plane (x,y,z)
D. Ear Problems
 1. Deafness - any hearing loss, no matter how small
  a. common deafness
   i. something prevents the sound vibrations from getting to the inner ear fluid
   ii. can be caused by: impacted earwax, middle ear inflammation, otosclerosis of the ossicles (ear bones), etc.
  b. sensorineural deafness
   i. caused by damage to the nerves associated with hearing
   ii. this can result from loud noises, aging, tumors, etc.
 2. Tinnitus
  a. ringing or clicking sound in the ears 
  b. often a symptom of cochlear nerve degeneration
  c. can be caused by inflammation of the middle or inner ear
  d. can also be a side effect of some medications, like aspirin
E. Equilibrium and balance
 1. the equilibrium receptors of the inner ear are called the vestibular apparatus
 2. the receptors for static equilibrium are the maculae of the saccule and utricle
  a. the maculae consists of hair cells in an overlying otolithic membrane
  b. movement causes the membrane to pull on the hair cells, sending impulses to the brain
 3. the receptors for dynamic movement are in the 3 semicircular ducts
  a. each responds to angular or rotary movements in its plane (there are 3)
  b. it consists of hair cells whose microvilli are embedded in the gelatinous cupula
  c. movement causes the cupula to bend, which sends impulses to the brain