Chapter 8 Special Senses

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Chapter 8 Special Senses

  • Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
  • Slides 8.1 – 8.19
  • Seventh Edition
  • Elaine N. Marieb
  • Lecture Slides in PowerPoint by Jerry L. Cook and Melissa Scott
  • This presentation contains copyright protected materials

The Senses

  • The Senses
  • General senses of touch (tactile)
    • Temperature- thermoreceptors (heat)
    • Pressure- mechanoreceptors (movement)
    • Pain- mechanoreceptors
  • Special senses
    • Smell- chemoreceptors (chemicals)
    • Taste- chemoreceptors
    • Sight- photoreceptors (light)
    • Hearing- mechanoreceptors
    • Equilibrium- (balance) mechanoreceptors

The Eye and Vision

  • The Eye and Vision
  • 70 percent of all sensory receptors are in the eyes
  • Each eye has over a million nerve fibers
  • Protection for the eye
    • Most of the eye is enclosed in a bony orbit made up of the lacrimal (medial), ethmoid (posterior), sphenoid (lateral), frontal (superior), and zygomatic and maxilla (inferior)
    • A cushion of fat surrounds most of the eye

Accessory Structures of the Eye

  • Accessory Structures of the Eye
  • Eyelids- brush particles out of eye or cover eye
  • Eyelashes- trap particles and keep them out of the eye

Accessory Structures of the Eye

  • Accessory Structures of the Eye
  • Ciliary glands – modified sweat glands between the eyelashes- secrete acidic sweat to kill bacteria, lubricate eyelashes

Accessory Structures of the Eye

  • Accessory Structures of the Eye
  • Conjunctiva
    • Membrane that lines the eyelids
    • Connects to the surface of the eye- forms a seal
    • Secretes mucus to lubricate the eye
  • Inflammation of the conjunctiva
  • Caused by bacterial or viral infection
  • Highly contagious

Accessory Structures of the Eye

  • Accessory Structures of the Eye
  • Lacrimal apparatus
    • Lacrimal gland – produces lacrimal fluid
    • Lacrimal canals – drains lacrimal fluid from eyes

Accessory Structures of the Eye

  • Accessory Structures of the Eye
    • Lacrimal sac – provides passage of lacrimal fluid towards nasal cavity

Accessory Structures of the Eye

  • Accessory Structures of the Eye
    • Nasolacrimal duct – empties lacrimal fluid into the nasal cavity

Function of the Lacrimal Apparatus

  • Function of the Lacrimal Apparatus
  • Properties of lacrimal fluid
    • Dilute salt solution (tears)
    • Contains antibodies (fight antigens- foreign substance) and lysozyme (enzyme that destroys bacteria)
  • Protects, moistens, and lubricates the eye
  • Empties into the nasal cavity

Extrinsic Eye Muscles

  • Extrinsic Eye Muscles
  • Muscles attach to the outer surface of the eye
  • Produce eye movements
  • When Extrinsic Eye Muscles Contract
    • Superior oblique- eyes look out and down
    • Superior rectus- eyes looks up
    • Lateral rectus- eyes look outward
    • Medial rectus- eyes look inward
    • Inferior rectus- eyes looks down
    • Inferior oblique- eyes look in and up

Structure of the Eye

  • Structure of the Eye

The Fibrous Tunic

  • The Fibrous Tunic
  • Sclera
    • White connective tissue layer
    • Seen anteriorly as the “white of the eye”
    • Semi-transparent
  • The Fibrous Tunic
  • Cornea
    • Transparent, central anterior portion
    • Allows for light to pass through (refracts, or bends, light slightly)
    • Repairs itself easily
    • The only human tissue that can be transplanted without fear of rejection

Choroid Layer

  • Choroid Layer
  • Blood-rich nutritive tunic
  • Pigment prevents light from scattering (opaque- blocks light from getting in, has melanin)
  • Choroid Layer
  • Modified interiorly into two structures
    • Cilliary body – smooth muscle (contracts to adjust the shape of the lens)
    • Iris- pigmented layer that gives eye color (contracts to adjust the size of the pupil- regulates entry of light into the eye)

Sensory Tunic (Retina)

  • Sensory Tunic (Retina)
  • Contains receptor cells (photoreceptors)
    • Rods
    • Cones
    • Signals leave the retina toward the brain through the optic nerve
  • Sensory Tunic (Retina)
  • Signals pass from photoreceptors via a two-neuron chain
    • Bipolar neurons and Ganglion cells
  • Rhodopsin- visual purple, in high concentration in RODS
  • Composed of opsin and retinal (a derivative of vitamin A) proteins
  • When light hits the protein it “bleaches”- turns yellow and then colorless. It straightens out and breaks down into opsin and retinal.
  • There are three different other opsins beside rhodopsin, with absorption for yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light.

Neurons of the Retina and Vision

  • Neurons of the Retina and Vision
  • Rods
    • Most are found towards the edges of the retina
    • Allow dim light vision and peripheral vision (more sensitive to light, do not respond in bright light)
    • Perception is all in gray tones

Neurons of the Retina and Vision

  • Neurons of the Retina and Vision
  • Cones
    • Allow for detailed color vision
    • Densest in the center of the retina
    • Fovea centralis – area of the retina with only cones
    • Respond best in bright light
  • No photoreceptor cells are at the optic disk, or blind spot

Cone Sensitivity

  • Cone Sensitivity
  • There are three types of cones
  • Different cones are sensitive to different wavelengths - red- long - green- medium - blue- short
  • Color blindness is the result of lack of one or more cone type
  • How do we see colors?
  • To see any color, the brain must compare the input from different kinds of cone cells—and then make many other comparisons as well.
  • The lightning-fast work of judging a color begins in the retina, which has three layers of cells. Signals from the red and green cones in the first layer are compared by specialized red-green "opponent" cells in the second layer. These opponent cells compute the balance between red and green light coming from a particular part of the visual field. Other opponent cells then compare signals from blue cones with the combined signals from red and green cones.
  • An inherited trait that is transferred on the sex chromosomes (23rd pair)- sex-linked trait
  • Occurs more often in males
  • Can not be cured or corrected
  • Comes from a lack of one or more types of color receptors.
  • Most are green or red or both and that is due to a lack of red receptors.
  • Another possibility is to have the color receptors missing entirely, which would result in black and white vision.


  • Lens
  • Biconvex crystal-like structure
  • Held in place by a suspensory ligament attached to the ciliary body
  • Refracts light greatly

Internal Eye Chamber Fluids

  • Internal Eye Chamber Fluids
  • Aqueous humor
    • Watery fluid found in chamber between the lens and cornea
    • Similar to blood plasma
    • Helps maintain intraocular pressure
    • Provides nutrients for the lens and cornea
    • Reabsorbed into venous blood through the canal of Schlemm
  • Refracts light slightly

Internal Eye Chamber Fluids

  • Internal Eye Chamber Fluids
  • Vitreous humor
    • Gel-like substance behind the lens
    • Keeps the eye from collapsing
    • Lasts a lifetime and is not replaced
  • Refracts light slightly
  • Holds lens and retina in place

Lens Accommodation

  • Lens Accommodation
  • Light must be focused to a point on the retina for optimal vision
  • The eye is set for distance vision (over 20 ft away)
  • 20/20 vision- at 20 feet, you see what a normal eye would see at 20 feet (20/100- at 20, normal person would see at 100)
  • The lens must change shape to focus for closer objects
  • Nearsightedness, or myopia is the difficulty of seeing objects at a distance.
  • Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface.
  • Concave lenses are used to correct the problem.
  • Hyperopia, or farsightedness, is when light entering the eye focuses behind the retina.
  • Hyperoptic eyes are shorter than normal.
  • Hyperopia is treated using a convex lens.

Images Formed on the Retina

  • Images Formed on the Retina
  • If the image is focused at the spot where the optic disk is located, nothing will be seen. This is known as the blind spot. There are no photoreceptors there, as nerves and blood vessels pass through this point.

Visual Pathway

  • Visual Pathway
  • Photoreceptors of the retina
  • Optic nerve
  • Optic nerve crosses at the optic chiasma

Visual Pathway

  • Visual Pathway
  • Optic tracts
  • Thalamus (axons form optic radiation)
  • Visual cortex of the occipital lobe

Eye Reflexes

  • Eye Reflexes
  • Internal muscles are controlled by the autonomic nervous system
    • Bright light causes pupils to constrict through action of radial (iris) and ciliary muscles
    • Viewing close objects causes accommodation
  • External muscles control eye movement to follow objects- voluntary, controlled at the frontal eye field
  • Viewing close objects causes convergence (eyes moving medially)

The Ear

  • The Ear
  • Houses two senses
    • Hearing (interpreted in the auditory cortex of the temporal lobe)
    • Equilibrium (balance) (interpreted in the cerebellum)
  • Receptors are mechanoreceptors
  • Different organs house receptors for each sense

Anatomy of the Ear

  • Anatomy of the Ear
  • The ear is divided into three areas
    • Outer (external) ear
    • Middle ear
    • Inner ear
    • (Add C. “INNER EAR” to notes)

The External Ear

  • The External Ear
  • Involved in hearing only
  • Structures of the external ear
    • Pinna (auricle)- collects sound
    • External auditory canal- channels sound inward

The External Auditory Canal

  • The External Auditory Canal
  • Narrow chamber in the temporal bone- through the external auditory meatus
  • Lined with skin
  • Ceruminous (wax) glands are present
  • Ends at the tympanic membrane (eardrum)

The Middle Ear or Tympanic Cavity

  • The Middle Ear or Tympanic Cavity
  • Air-filled cavity within the temporal bone
  • Only involved in the sense of hearing

The Middle Ear or Tympanic Cavity

  • The Middle Ear or Tympanic Cavity
  • Two tubes are associated with the inner ear
    • The opening from the auditory canal is covered by the tympanic membrane (eardrum)
    • The auditory tube connecting the middle ear with the throat (also know as the eustacian tube)
      • Allows for equalizing pressure during yawning or swallowing
      • This tube is otherwise collapsed

Bones of the Tympanic Cavity

  • Bones of the Tympanic Cavity
  • Three bones span the cavity
    • Malleus (hammer)
    • Incus (anvil)
    • Stapes (stirrip)

Bones of the Tympanic Cavity

  • Bones of the Tympanic Cavity
  • Vibrations from eardrum move the malleus
  • These bones transfer sound to the inner ear

Inner Ear or Bony Labyrinth

  • Inner Ear or Bony Labyrinth
  • Also known as osseous labyrinth- twisted bony tubes
  • Includes sense organs for hearing and balance
  • Filled with perilymph
  • Inner Ear or Bony Labyrinth
  • Vibrations of the stapes push and pull on the membranous oval window, moving the perilymph through the cochlea. The round window is a membrane at the opposite end to relieve pressure.

Inner Ear or Bony Labryinth

  • Inner Ear or Bony Labryinth
  • A maze of bony chambers within the temporal bone

Organ of Corti

  • Organ of Corti
    • Located within the cochlea
    • Receptors = hair cells on the basilar membrane
  • Scala tympani
  • Scala vestibuli
    • Gel-like tectorial membrane is capable of bending hair cells (endolymph in the membranous labyrinth of the cochlear duct flows over it and pushes on the membrane)
  • Organ of Corti
  • Scala tympani
  • Scala vestibuli
  • Organs of Hearing
  • Organ of Corti
    • Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe
  • Scala tympani
  • Scala vestibuli

Mechanisms of Hearing

  • Mechanisms of Hearing
  • Vibrations from sound waves move tectorial membrane (pass through the endolymph fluid filling the membranous labyrinth in the cochlear duct)
  • Hair cells are bent by the membrane
  • Mechanisms of Hearing
  • An action potential starts in the cochlear nerve
  • The signal is transmitted to the midbrain (for auditory reflexes and then directed to the auditory cortex of the temporal lobe)
  • Continued stimulation can lead to adaptation (over stimulation to the brain makes it stop interpreting the sounds)
  • Mechanisms of Hearing

Organs of Equilibrium

  • Organs of Equilibrium
  • Receptor cells are in two structures
    • Vestibule
    • Semicircular canals

Organs of Equilibrium

  • Organs of Equilibrium
  • Equilibrium has two functional parts

Static Equilibrium

  • Static Equilibrium
  • Maculae – receptors in the vestibule
    • Report on the position of the head
    • Send information via the vestibular nerve
  • Static Equilibrium
  • Anatomy of the maculae
    • Hair cells are embedded in the otolithic membrane
    • Otoliths (tiny stones) float in a gel around the hair cells

Function of Maculae

  • Function of Maculae
    • Movements cause otoliths to bend the hair cells (gravity moves the “rocks” over and pulls the hairs)

Dynamic Equilibrium

  • Dynamic Equilibrium
  • Whole structure is the ampulla
  • Crista ampullaris – receptors in the semicircular canals
    • Tuft of hair cells
    • Cupula (gelatinous cap) covers the hair cells

Dynamic Equilibrium

  • Dynamic Equilibrium
  • Action of angular head movements
    • The cupula stimulates the hair cells
    • Movement of endolymph pushes the cupula over and pulls the hairs
    • An impulse is sent via the vestibular nerve to the cerebellum

Chemical Senses – Taste and Smell

  • Chemical Senses – Taste and Smell
  • Both senses use chemoreceptors
  • Both senses complement each other and respond to many of the same stimuli

Olfaction – The Sense of Smell

  • Olfaction – The Sense of Smell
  • Olfactory receptors are in the roof of the nasal cavity
    • Neurons with long cilia
    • Chemicals must be dissolved in mucus for detection
  • Olfaction – The Sense of Smell
  • Impulses are transmitted via the olfactory nerve
  • Interpretation of smells is made in the cortex (olfactory area of temporal lobe)

The Sense of Taste

  • The Sense of Taste
  • Taste buds house the receptor organs
  • Location of taste buds

The Tongue and Taste

  • The Tongue and Taste
  • The tongue is covered with projections called papillae
    • Filiform papillae – sharp with no taste buds
    • Fungifiorm papillae – rounded with taste buds
    • Circumvallate papillae – large papillae with taste buds
  • Taste buds are found on the sides of papillae

Structure of Taste Buds

  • Structure of Taste Buds
  • Gustatory cells are the receptors
    • Have gustatory hairs (long microvilli)
    • Hairs are stimulated by chemicals dissolved in saliva

Structure of Taste Buds

  • Structure of Taste Buds
  • Impulses are carried to the gustatory complex (pareital lobe) by several cranial nerves because taste buds are found in different areas
    • Facial nerve
    • Glossopharyngeal nerve
    • Vagus nerve

Taste Sensations

  • Taste Sensations
  • Sweet receptors
    • Sugars
    • Saccharine
    • Some amino acids
  • Sour receptors
    • Acids
  • Bitter receptors
    • Alkaloids
  • Salty receptors
    • Metal ions
  • Umami
    • Glutamate, aspartate (MSG, meats)

Developmental Aspects of the Special Senses

  • Developmental Aspects of the Special Senses
  • Formed early in embryonic development
  • Eyes are outgrowths of the brain
  • All special senses are functional at birth

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