Chapter 15: The Special Senses

1
Chapter 15 The Special Senses

• J.F. Thompson, Ph.D. J.R. Schiller, Ph.D. G.
  Pitts, Ph.D.

2
The Five Special Senses

• Smell and taste chemical senses (chemical
  transduction)
• Sight light sensation (light transduction)
• Hearing sound perception (mechanical
  transduction)
• Equilibrium static and dynamic balance
  (mechanical transduction)

3
Special Sensory Receptors

• Distinct types of receptor cells are confined to
  the head region
• Located within complex and discrete sensory
  organs (eyes and ears) or in distinct epithelial
  structures (taste buds and the olfactory
  epithelium)

4
The Chemical Senses Taste and Smell

• The receptors for taste (gustation) and smell
  (olfaction) are chemoreceptors (respond to
  chemicals in an aqueous solution)
• Chemoreception involves chemically gated ion
  channels that bind to odorant or food molecules

5
Taste
6
Location of Taste Buds

• Located mostly on papillae of tongue
• Two of the types of papillae
• fungiform
• circumvallate

7
Taste Buds

• Each papilla contains numerous taste buds
• Each taste bud contains many gustatory cells
• The microvilli of gustatory cells have
  chemoreceptors for tastes

8
The Five Basic Tastes

• Sweet sugars, alcohols, some amino acids, lead
  salts
• Sour H ions in acids
• Salty Na and other metal ions
• Bitter many substances including quinine,
  nicotine, caffeine, morphine, strychnine, aspirin
• Umami the amino acid glutamate (beef taste)

9
Taste Transduction

• Incompletely understood
• A direct influx of various ions (Na, H) or the
  binding of other molecules which leads to
  depolarization of the receptor cell
• Depolarization of the receptor cell causes it to
  release neurotransmitter that stimulates nerve
  impulses in the sensory neurons of gustatory
  nerves

10
Sensory Pathways for Taste

• Afferent impulses of taste stimulate many
  reflexes which promote digestion (increased
  salivation, and gastrointestinal motility and
  secretion)
• Bad taste sensations can elicit gagging or
  vomiting reflexes

11
Smell
12
Location of Olfactory (Odor) Receptors
13
Odor Receptors

• Bipolar neurons
• Collectively constitute cranial nerve I
• Unusual in that they regenerate (on a 60 day
  replacement cycle)

14
Odors

• Very complicated
• Humans can distinguish thousands
• More than a thousand different odorant-binding
  receptor molecules have been identified
• Different combinations of specific
  molecule-receptor interactions produce different
  odor perceptions

15
Transduction of Smell

• Binding of an odorant molecule to a specific
  receptor activates a G-protein and then a second
  messenger (cAMP)
• cAMP causes gated Na and Ca2 channels to open,
  leading to depolarization

16
Olfactory Pathway

• One path leads from the olfactory bulbs via the
  olfactory tracts to the olfactory cortex where
  smells are consciously interpreted and identified
• Another path leads from the olfactory bulbs via
  the olfactory tracts to the thalamus and limbic
  system where smells elicit emotional responses
• Smells can also trigger sympathetic nervous
  system activation or stimulate digestive processes

17
Vision
18
Surface Anatomy of the Eye

• Eyebrows divert sweat from the eyes and
  contribute to facial expressions
• Eyelids (palpebrae) blink to protect the eye from
  foreign objects and lubricate their surface
• Eyelashes detect and deter foreign objects

19
Conjunctiva

• A mucous membrane lining the inside of the
  eyelids and the anterior surface of the eyes
• forms the conjunctival sac between the eye and
  eyelid
• Forms a closed space when the eyelids are closed
• Conjunctivitis (pinkeye) inflammation of the
  conjunctival sac

20
The Lacrimal Apparatus

• Lacrimal Apparatus
• lacrimal gland
• lacrimal sac
• nasolacrimal duct
• Rinses and lubricates the conjunctival sac
• Drains to the nasal cavity where excess moisture
  is evaporated

21
Extrinsic Eye Muscles

• Lateral, medial, superior, and inferior rectus
  muscles (recall, rectus straight) superior and
  inferior oblique muscles

22
Internal Anatomy of the Eye--Tunics

• Fibrous tunic sclera cornea
• Vascular tunic choroid layer
• Sensory tunic retina

23
Internal Anatomy of the Eye

• Anterior Segment contains the Aqueous Humor
• Iris
• Ciliary Body
• Suspensory Ligament
• Lens
• Posterior Segment contains the Vitreous Humor

24
Autonomic Regulation of the Iris
Pupil Constricts
Pupil Dilates
25
The Two Layers of the Retina

• Outer pigmented layer has a single layer of
  pigmented cells, attached to the choroid tunic,
  which absorbs light to prevent light scattering
  inside
• Inner neural layer has the photosensory cells and
  various kinds of interneurons in three layers

26
Neural Organization in the Retina

• Photoreceptors rods (for dim light) and cones
  (3 colors blue, green and red, for bright light)
• Bipolar cells are connecting interneurons
• Ganglion cells axons become the Optic Nerve

27
Neural Organization in the Retina

• Horizontal Cells enhance contrast (light versus
  dark boundaries) and help differentiate colors
• Amacrine cells detect changes in the level of
  illumination

28
The Optic Disc

• Axons of ganglion cells exit to form the optic
  nerve
• Blood vessels enter to serve the retina by
  running on top of the neural layer
• The location of the blind spot in our vision

29
Micrograph of the Retina

• Light must cross through the capillaries and the
  two layers of interneurons to reach the
  photoreceptors, the rods and cones

Light
30
Opthalmoscope Image of the Retina

• The Macula Lutea (yellow spot) is the center of
  the visual image
• The Fovea Centralis is a central depression where
  light falls more directly on cones providing for
  the sharpest image discrimination
• Light bouncing off RBCs hemoglobin causes red
  eye in flash photos

31
Circulation of the Aqueous Humor

• Ciliary process at the base of the iris produces
  aqueous humor
• Scleral venous sinus returns aqueous humor to the
  blood stream
• Glaucoma any disturbance that increases aqueous
  humor volume and pressure which causes pain
  ultimately the vitreous humor crushes the retina
  causing blindness

32
Hearing
33
External Ear

• Pinna (auricle) focuses sound waves on the
  tympanic membrane
• Ceruminous glands guard the external auditory
  canal

34
Middle Ear Auditory Tube

• Three auditory ossicles (bones) serve as a lever
  system to transmit sound to the inner ear
• Pharyngotympanic (auditory tube) connects to
  pharynx, allowing air pressure to equalize on
  both side of the tympanic membrane

35
Middle Ear Ossicles (median view)

• Malleus (hammer), incus (anvil) and stapes
  (stirrup) act to increase the vibratory force on
  the oval window
• Tensor tympani and stapedius muscles control the
  tension of this lever system to prevent damage to
  the delicate tympanic and round window membranes

36
The Membranous Labyrinth

• A series of tiny fluid-filled chambers in the
  temporal bone
• Cochlea tranduces sound waves
• Semicircular canals and their ampullae transduce
  balance and equilibrium
• The vestibule connects the two portions

37
The Cochlea Two Coiled Tubes

• Larger outer tube is folded but continuous (like
  a coiled letter U) the scala vestibuli and
  scala tympani contains perilymph fluid
• Smaller inner tube is the scala media (cochlear
  duct) contains endolymph fluid

38
The Spiral Organ of Corti

• Between the scala tympani and the scala
  media/cochlear duct is the complex receptor
  system the spiral organ of Corti
• Sensory Hair Cells stand on the basilar membrane
  and their processes are attached to the Tectorial
  Membrane

39
Wave Pulses in the Cochlea

• Stapes moving at the oval window creates pulses
  of vibration in the perilymph of the scala
  vestibuli and scala tympani
• Harmonic vibrations are created at right angles
  in the endolymph of the scala media which move
  the basilar membrane

40
Transduction of Sound Waves

• Movement against the tectorial membrane
  stimulates the hair cells to send impulses to the
  auditory cortex
• Round window moves to accommodate the vibrations
  initiated by the stapes

41
Wave Pulses in the Cochlea

• Stapes moving at the oval window creates pulses
  of vibration in the perilymph of the scala
  vestibuli and scala tympani
• Harmonic vibrations are created at right angles
  in the endolymph of the scala media which move
  the basilar membrane

42
Transduction of Sound Waves
43
Resonance of Basilar Membrane

• High notes are detected at the base of the
  cochlea
• Low notes are detected at the apex
• Due to differences in the width and flexibility
  of the basilar membrane

44
Auditory Pathway

• Afferent impulses for sounds are routed
• Vestibulocochlear Nerve VIII (cochlear branch)
• Nuclei in the medulla oblongata where motor
  responses can turn the head to focus on sound
  sources
• Primary Auditory Cortex in the temporal lobe for
  conscious interpretation

45
Balance and Coordination
46
Macula in the Saccule Utricle

• Chambers near the oval window filled with
  perilymph
• CaCO3 otoliths (ear stones) slide over the
  surface lining cells in response to gravity
• Static equilibrium tells the CNS which way is
  up

47
Macular Transduction

• Hair cells stereocilia move in response to the
  sliding otoliths
• To send impulses to the CNS for interpretation

48
Semicircular Canals

• Three endolymph-filled tubes in the bony
  labyrinth
• Each C-shaped loop is in a plane at right angles
  to the other two
• Each has an expanded ampulla containing a sensory
  structure, the cupula

49
Ampullar Transduction

• Movement in the plane of one of the canals causes
  endolymph to flow and bends the cupola
• Hair cells stereocilia move in response to the
  movement

• Dynamic equilibrium tells the CNS which way is
  the head or body is moving

50
Pathways of Balance and Orientation

• Integration of sensory modalities
• Sight
• Proprioception
• Static equilibrium
• Dynamic equilibrium
• Output to skeletal muscles to position
• Eyes
• Head and neck
• Trunk

51
Take a Tour of the Virtual Ear at
http//www.augie.edu/perry/ear/hearmech.htm
52
End Chapter 15