chapter 4 anatomy content

eyes, ears, taste

what are special senses

• the special senses are:

  • sight

  • hearing

  • smell

  • taste

• special senses are large receptors, complex sensory organs or localized clusters of receptors

• touch is a mixture of general senses of temperature, pressure, and pain

the eyes and vision

• 70% of all sensory receptors are located in the eyes

• over 1 million nerve fibers carry information from the eyes along the optic tracts to the visual processing center of the brain

the eyes detect light

• vision occurs when light…

  • enters the eye

  • is focused on the back surface of the eye

  • triggers impulses that are sent to the brain and interpreted

the eye

• eye: a hollow, fluid-filled sphere about 2.5 cm in diameter that responds to light stimuli

• about 1/16 of the surface is visible

• enclosed in a bony orbit & cushioned by fat

external structures

• eyebrows

  • provide shape and prevent perspiration from reaching the eyes

  • important in nonverbal communication and facial expression

• eyelids

  • protect anterior surface

• eyelashes

  • trigger reflex blinking

lacrimal apparatus

• the lacrimal gland release dilute salt solution (tears)

• lacrimal canals drain tears into the lacrimal sac

• tears eventually drain into the nasal cavity

tears

• cleanse, protect, moisten, and lubricate the eye

• contain antibodies and lysozyme, an enzyme that destroys bacteria

• emotional tears contain a natural painkiller

external eye muscles

• six muscles originate from the bony orbit and insert onto the outer surface of the eye

• allow eye movements including following a moving object

• maintain the shape of the eyeball and hold it in the orbit

conjuctiva

• conjuctiva: a transparent muscuous membrane

• lines the eyelids and covers the anterior surface of the eyeball

• ends at the edge of the cornea

• conjunctivitis (pink eye): inflammation of the conjuctiva

internal structures

• retina: innermost layer of the eyeball

• choroid: pigmented layer

• sclera: tough outer layer

• cornea: transparent covering

• pupil: opening

• iris: controls the size of the pupil

anterior view of the eye

• pupil: opening through which light enters

• sclera: protective white layer

• iris: colored part of the eye that controls the size of the pupil

lens

• flexible biconcave structure that focuses light

• held in place by suspensory ligaments that extend from smooth muscle structures called ciliary bodies

two chambers of the eye

• the anterior chamber (in front of the lens) is filled with a clear watery fluid called aqueous humor 

• the posterior chamber is filled with a gel-like substance called vitreous humor

path of light

• cornea

• aqueous humor

• pupil

• lens

• vitreous humor

• light is bent or refracted as it passes through the structures and fluids listed above. this refraction focuses light on the surface of the retina

the retina

• the sensory layer of the eye where light is focused

• contains millions of photoreceptors cells: rods and cones

• the fovea centralis is the area of sharpest vision

photoreceptors

• rods allow us to see in gray tones in dim light and provide peripheral vision; they are most dense at the edge of the retina.

• cones allow us to see color in bright light and are most denses at the center of the retina

vision

• images are formed on the retina as real images, that is they are reversed left to ride, upside down, and smaller than the objec

• vision occurs when the brain receives images from the retinas of both eyes and interprets them

eye reflexes

• eye blink reflex: eyelids close in response to foreign objects touching the cornea

• pupillary reflex: pupil constricts in bright light and dilates when light is removed

• accommodation response: compensates for changing focus from a distant object to a closer one

human outer ear

• human ear is broken into 3 parts:

  • external ear: auricle or pinna

    • gathers and directs sound waves

  • auditory canal or external acoustic meatus or external eardrum

    • inside are ceruminous fluid that are produced by ceruminous glands (a type of sweat gland)

    • makes earwax

      • pick up foreign particles, debris

      • impedes hearing

  • tympanic membrane or eardrum

eardrum

• aka. tympanic membrane: thin translucent connective tissue membrane covered by skin on its external surface and mucosa internally

• shapes like a flattened cone

• sound waves will make the eardrum vibrate

• eardrum can heal if damaged, but damaged eardrum can impede hearing

middle ear

• middle ear or tympanic cavity: a small air filled mucosal lined cavity found in the petrous part of the temporal bone

• made of ossicles (smallest bone in the human body)

• ends at oval and round window

• job is to transfer sound wave vibrations of the eardrum to fluid waves in inner ear structures

auditory ossicles

• ossicles: the 3 tiny bones that transfer sound waves to the oval window

  • malleus (hammer)

  • incus (anvil)

  • stapes (stirrup)

• ligaments help to suspend them place and synovial joints that links them together

• their vibration transfers and amplifies sound waves as it travels through the middle ear

the eusachian tube

• also called the pharyngotympanic or auditory tube: connects the middle ear with the nasopharynx

• helps equalize pressure in the middle ear, keeping it equal with air pressure outside the body

• sometimes the tube is too horizontal and does not allow for it to drain properly

• caused muffle hearing, pain, tinnitus, reduced hearing, problem with balance

two openings to inner ear

1. vibrations of the spades are transmitted into the vestibule through the membrane-covered oval window

2. the membrane of the round window vibrates in opposite phase to oval window vibrations

• these 2 windows vibrate independently of one another

inner ear

• inner ear (aka. bony labyrinth): maze of bony chambers

• 3 areas:

  • cochlea

    • helps with hearing

    • composed to membrane and bone (soft and hard)

  • vestibule

    • tells position of head

    • or equilibrium position

  • semicircular canals

    • helps with balance

static equilibrium

• receptors in the vestibule report on the position of the head with respect to gravity when the body is not moving

• tiny stones called otoliths stimulate hair cells which inform the brain of head position

  • ex. when you turn to the side, they float to the side

  • ex. when you are upside down, they circulate

  • otoliths tells you where you are when youre not moving

dynamic equilibrium

• refers to maintaining proper head positioning when the body is moving

• arranged in 3 planes, semicircular canals are filled with thick fluid that moves in response to body movement

  • anterior canal

  • lateral canal

  • posterior canal

• fluid stimulates hair cells which send impulses to along the vestibular nerve to the cerebellum

  • these canals have ducts which communicates with utricles

  • each duct has an enlarged swelling called the amupulla which houses the equilibrium receptors (or crista ampullaris)

  • they respond to the angular movement of the head and body

how sound travels through the ear

1. sounds enter the oracle, into the acoustic meatus, hit the tmpanic membrane, goes to the cochlea, the cochlear nerve will send the message to the brain

2. in short, impulses from hair cells (these are specific receptors in the cochlea) travel along the cochlear nerve or auditory nerve; these then merge with the vestibular nerve to form the 8th cranial nerve

hearing

• endolymph: fluid contained in the cochlea

• when sound waves reach the oval and round windows, cochlear fluid is set in motion

• hair cells, located in the organ of corti, are stimulated to transmit impulse along the cochlear nerve

• different hair cells will detect different sound pitches

  • high pitched sound stimulates short hair cells

  • low pitched sound stimulates long hair cells

• your range of hearing can be affected if hair cells are damaged

disorders and conditions of the ear

• ear infections

  • inflammation of the middle ear is relatively common in children

  • if large amounts of fluid accumulate, tubes can be inserted to allow drainage and relieve pressure

• otitis media: a group of infections in the middle ear (symptom is sore throat)

  • common cause of hearing loss is children because their eustachian tube is shorter and can run horizontally

  • an acute infection (meaning that it can be fixed)

two main types of deafness

• sensorineural 

  • hearing loss caused by damage to receptors in the organ of corti, nerves, or other nervous system structures

• conduction (acute: fixable)

  • occurs when something interferes with the transmission of sound vibrations

  • ex. build-up of earwax, fusion of ossicles, ruptured eardrum, etc.

american sign language

• american sign language (asl): a visual language that allows the brain to process linguistic information through the eyes

• involves shape, placement, and movement of the hands as well as facial expressions and body movements

• if you are fluent in ASL, you are considered bilingual

noise induced hearing loss

• sounds can be harmful to hearing if they are too loud even for brief period of time or if they are both loud and long-lasting

• sound intensity is measured in decibels

• sound over 120 db can be painful and damaging to hair cells

• sound is a pressure disturbance

• it is defined in terms of frequency

  • 160: shotgun

  • 140: jetplane

  • 90: hair dryer

  • 80: vacuum cleaner

  • 60: conversation

  • 30: whisper

  • 10: breathing

• sound is a pressure disturbance defined in frequencies

  • frequencies comes in wavelength

• the higher the frequency the higher the pitch

• amplitude is the height of the wave or crest

  • measures the intensity of the sound

• loudness os our subjective interpretation of the sound

  • it is measured in decibels

  • the higher the decibel the louder the sound

the sense of smell

• thousands of olfactory (smell) receptors are located in the rood of each nasal cavity

• olfactory receptor cells are neurons with olfactory hairs

olfactory bulb

• hair cells are stimulated by chemicals dissolved in mucus

• impulses are transmitted along olfactory nerve fiber that lead to the olfactory bulb in the brain

the sense of taste

• taste buds are the receptors for the sense of taste

• they are found throughout the oral cavity, but most are located on the tongue

• usually found on projections called papillae

3 types of papillae

• filiform: provide texture to the tongue

• fungiform: contain taste buds, large and darker

• circumvallate: rare, located at the back of the tongue, contain taste buds in the side walls

gustatory cells

• gustatory cells: taste receptor cells

• have long microvilli called gustatory hairs that protrude through a taste pore

• impulses travel to the brain through various cranial nerves

• each taste bud contains approximately 50-75 gustatory cells

5 basic taste sensations

• sweet receptors detect sugar

• sour -> acids

• bitter -> alkaloids

• salty -> metal ions

• umami -> amino acid glutamate which is responsible for “beef taste”

developmental aspects

special senses a birth

• eyes are not fully functional for several years

• responses to sounds are mostly reflexive

• babies have a highly developed sense of smell and more taste buds than adults

effect of aging on eyesight

• decreasing lens elasticity makes it difficult to focus on close objects

• lacrimal glands become less active leading to dry eyes

• lens loses clarity

effect of aging on other senses

• organ of corti and other ear structures being to deteriorate leading to hearing loss

• about half of people over age 80 cannot smell at all and taste is also diminished