Special Senses

The term “special senses refers to the four more traditional senses known as: smell, taste, sight, and hearing.

EYE

Anatomy of the eye:

  • the adult eye is a sphere with a diameter of about 2.5 cm

  • anteriorly, the eye is protected by eyelids

    • projecting from the eyelids are the eyelashes

  • there are a few modified sebaceous glands that are associated with the eyelid edges:

    • meibomian glands - produce oily secretion that lubricates eye

    • ciliary glands - modified sweat glands between the eyelashes

  • lining the eyelids is the conjunctiva which secretes mucus

    • helps to lubricate the eyeball and keep it moist

  • PINK EYE - also known as conjunctivitis

    • it is essentially inflammation of the conjunctiva

    • highly contagious and it is caused by bacteria or viruses

    • results in reddened, irritated eyes

Internal structures

  • the wall of the eye is composed of tunics (coats) and the interior is filled with humors (fluids) that help maintain shape

  • the lens is supported upright and splits into two cavities

    • the anterior segment contains aqueous humor

      • similar to blood plasma and is continuously secreted by a special area of the choroid

      • helps maintain a intraocular pressure

      • reabsorbed into the venous blood through canal of schlemm

    • the posterior segment contains vitreous humor similar

      • helps prevent eye from collapsing inward by reinforcing it internally

  • tunics:

    • sclera: outer most tunic

      • white of the eye

    • sclera contains the cornea - where light enters the eye

    • choroid - middle tunic

      • contains pigment that prevents light from scattering inside the eye

      • the choroid is modified to form two smooth muscle structures:

        1. ciliary body - which the lens is attached to

        2. iris

      • the iris has a rounded opening which is the pupil - where light passes

    • retina - inner most sensory tunic

      • contains millions of photoreceptor cells: rods and cones

        • these photoreceptor cells are distributed over the entire retina except for where the optic nerve leaves the eyeball. This site is called the optic disk but can also be referred to as “the blind spot”

        • lateral to each blind spot is the fives centralis. this is a pit that contains only cones, making it the point of sharpest vision

        • the rods are most dense at the edge of the retina while the cones are most dense at the center

        • rods allow us to see in gray tones and in dim light and they provide for our peripheral vision

        • cones are discriminatory receptors that allow us to see the details of our world in color under bright light conditions

        • there are three types of cones that are sensitive to different wavelengths of light

          1. green cones - respond to green light

          2. blue cones - respond to red light

          3. red cones - respond to a variety of light including red and green

Visual pathway of light

  1. cornea

  2. aqueous humor

  3. pupil

  4. lens

  5. vitreous humor

  6. retina

  7. optic nerve

HEARING

anatomy of the ear:

outer (external ear)

  • the outer ear is composed of the pinna or auricle

    • collects and directs sound waves into the auditory canal but in humans its function is lost

  • external auditory canal - short, narrow chamber carved into the temporal bone of the skull

    • skin-lined walls are called ceremonious gland - secrete a yellow waxy substance called earwax

  • tympanic membrane - also referred to as eardrum

    • sound waves hit this eventually and cause it to vibrate

Middle ear:

  • sometimes called the tympanic cavity

  • flanked laterally by eardrum and medially by two openings:

    • oval window

    • round window

  • auditory tube - runs obliquely downward to link the middle ear cavity with the throat

    • normally flattened and closed however swallowing or yawning can open it briefly to equalize the pressure in the middle ear cavity with the external pressure

  • tympanic cavity is spanned by three smallest bones in the body: ossicles - transmit the vibratory motion of the eardrum to the fluids of the inner ear

    1. hammer (malleus) - moves with the eardrum transferring vibrations to the anvil

    2. anvil (incus) - passes vibrations to stirrup

    3. stirrup (stapes) - presses on the oval window of the inner ear (eventually leads to the excitement of hearing receptors

inner ear:

  • maze of bonds chambers called osseous or bony labyrinth

  • three subdivisions of the inner ear:

    1. cochlea

    2. vestibule

    3. semicircular canals

  • the bony labyrinth is filled with a plasma-like fluid called perilymph

    • suspended within the perilymph is a membranous labyrinth - a system of membraneous sacs that more or less follows the shape of the bony labyrinth

      • within the membranous labyrinth is a thicker fluid called endolymph

Mechanism of hearing

within the membranes of the snail like cochlea is the organ of corti

  • the organ of corti contains the hearing receptors otherwise known as hair cells

    • these receptors are called mechanoreceptors

CHEMICAL SENSES: taste and smell

  • receptors used for taste and smell are classified as chemoreceptors

    • they respond to chemicals in a solution

  • the receptors for smell and taste compliment each other and respond to many of the same stimuli

  • there are 4 main taste sensations:

    1. sweet receptors - respond to substances such as sugars, saccharine, and some amino acids

    2. sour receptors - respond to hydrogen ions or the acidity of a solution

    3. bitter receptors - respond to alkaloids

    4. salty receptors - respond to metal ions in a solution

  • at birth, the least developed sense is sight and the first sense is hearing.