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Chapter 17, Part 1 Special Senses!
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Chapter 17, Part 1!The Special Senses!
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SECTION 17-1!Olfaction, the sense of smell, involves olfactory receptors responding to chemical stimuli!
Chapter 17, Part 1 Special Senses!
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Olfaction – A Chemical Sensation!
Olfactory epithelium!1. Olfactory receptors = first-order sensory
neuronsAre bipolar neurons!• Dendrites have cilia (“olfactory hairs”) with
receptors!• Surface area of cilia approximates surface
area of skin!!• Release neurotransmitter - produce action
potentials in olfactory bulb neurons!
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Olfactory Epithelium – 2!
2. Supporting (sustentacular) cells!• Columnar epithelial cells!• Physically support neurons!• Provide nutrients!• Electrically insulate receptors!• Detoxify odorants!
3. Basal (stem) cells!• Produce new receptors (first-order neurons)!
Unusual regeneration of neurons!
Chapter 17, Part 1 Special Senses!
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Olfactory Epithelium – 3!
4. Olfactory (Bowman’s) glands!• Produce mucus!
(Helps dissolve odorant)!5. Innervation of sustentacular cells, olfactory
glands = Facial Nerve (VII)!• Stimulation also stimulates lacrimal glands
and mucous glands!(Smell pepper → eyes water)!
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The Olfactory Organs Figure 17-1!
Where are the first- and second-order neurons?!
Chapter 17, Part 1 Special Senses!
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Olfactory Physiology – 1!
Not well understood!!Low threshold for smell!• Methyl mercaptan in natural gas!• Sensed at a few parts per billion!
Odorant must dissolve in mucus!!May be hundreds of primary scents!!Rapidly adapts (i.e., Phasic)!
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Olfactory Physiology – 2!
For some odorants: Lock and key mechanism!1. Odorant fits into receptor (odorant-binding
protein)!• Each odorant probably binds to a discrete
population of different receptors!2. G-protein activated!3. Adenylate cyclase activated!4. cAMP produced!5. Na+ channels open → generator potential!6. Stimulate second-order neuron in olfactory bulb
via neurotransmitter (glutamate?)!
Chapter 17, Part 1 Special Senses!
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Olfactory Receptors – 2 Spotlight 17-2!
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Olfactory Pathway!
1. Olfactory receptors (first-order neurons)!2. Through olfactory foramina to …!3. Olfactory bulb (second-order neuron cell bodies)!4. Olfactory tract (axons of second-order neurons)!5. Synapse in temporal lobe / limbic structures• Primary olfactory cortex!• Conscious awareness of smell!
6. Relay to frontal lobe!• Doesn’t have to go to thalamus first!
7. Also sent to limbic system!
Chapter 17, Part 1 Special Senses!
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SECTION 17-2!Gustation, the sense of taste, involves taste receptors responding to chemical stimuli!
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Gustation – Taste!
Taste buds contain gustatory receptors!Cell types:!
1. Gustatory receptorsReceptors, not neurons (in contrast to smell)!• Have gustatory hairs (microvilli) !• Develop receptor/generator potentials!• Synapse with first-order neurons!• Release neurotransmitter!• CN VII, IX, X!• Live about 10 days!
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Physiology of Taste – 1!
2. Transitional cells!3. Basal cells!• Probably stem cells!
Physiology of taste• Substance must be dissolved!• Binds receptor!• Receptor potential produced!• Neurotransmitter (?) released
onto first-order neuron (ATP is one neurotransmitter.)!
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Physiology of Taste – 2!
Salty taste!• Cations mostly, but perception is modified by
accompanying anions!• Diffuse through Na+ channels!
e.g. NaCl tastes saltier than MSG!(Glutamate is bigger than Cl-)!
Sour taste!• H+ is stimulus (citric acid → citrate- + H+)!• H+ also diffuses through Na+ channels!
Chapter 17, Part 1 Special Senses!
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Physiology of Taste – 3!
Sweet taste!• Mostly organic molecules!• Sugars, alcohols, amino acids, salts of lead!• G-proteins (gustducins) and cAMP involved!• Binding closes K+ channels → receptor potential!
Bitter taste!• Long-chain organic substances !
e.g. alkaloids from some plants!• Quinine, caffeine, strychnine, nicotine!
• Involves G-proteins, second messengers!
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Physiology of Taste – 4!
Umami!• Pleasant, rich taste!• Characteristic of chicken or beef broth!• Stimulus = Amino acids (especially glutamate),
peptides, nucleotides!• G-proteins involved!
Water!• Information processed in hypothalamus!• Water in mouth for 20 min →!
↓ ADH secretion!↑ Water loss by kidneys!
Chapter 17, Part 1 Special Senses!
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Taste Receptors Spotlight 17 bottom!
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Gustatory Reception Figure 17-3!
Chapter 17, Part 1 Special Senses!
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SECTION 17-3!Internal eye structures contribute to vision, while accessory eye structures provide protection!!You are already very familiar with eye anatomy from your lab work.!
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Eye Sectional Anatomy Figure 17-5!
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Tunics of the Eye – 1!
1. Fibrous tunic/layer = sclera and cornea!Sclera!• Whites of the eye!• Irregularly arranged dense CT!
Cornea!• Clear!• Regularly arranged CT!• No blood vessels!
Canal of Schlemm (scleral venous sinus)!• Returns aqueous humor to circulation!
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Tunics of the Eye – 2!
2. Vascular tunic/layer (uvea)!A. Choroid!• Route for blood vessels and lymphatics!• Pigmented!• Supplies nutrients, etc. to eye!
B. Ciliary body has two major parts:!1. Ciliary muscle!
Attached to suspensory ligaments!Changes shape of lens (accommodation)!
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Accommodation Figure 17-11!
Ciliaris muscle contracts,!Lens relaxes (thickens)!
Lens! Avascular! Composed of crystallins!! Cataracts = deterioration! of lens proteins!
Near vision!
Far vision!Ciliaris muscle relaxes,!Lens stretched (flattens)!
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Tunics of the Eye – 3!
B. Ciliary body (cont.)!2. Ciliary processes!• Epithelial folds!• Secrete aqueous humor!
Na+ actively transported!Water, Cl- and HCO3
- follow!• Fluid returns to circulation through canal of
Schlemm (scleral venous sinus)!
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The Circulation of Aqueous Humor Figure 17-9!
of retina
Note pigmented epithelium of retina!
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Tunics of the Eye – 4!
C. Iris - regulates amount of light entering eye!a. Body:!• Loose CT with melanocytes!• Two groups of smooth muscle!
Circular muscle - parasympathetic!o Contraction makes pupil smaller!
Radial muscle - sympathetic!o Contraction makes pupil larger!
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The Pupillary Muscles Figure 17-6!
Sympathetic! Parasympathetic!
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Tunics of the Eye – 5!
C. Iris (cont.)!Posterior surface!• Epithelium with melanin granules!• a.k.a. non-optical retina!
Anterior surface!• No epithelium; melanocytes and fibroblasts
present!
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Eye Color!
Influenced by:!• Melanocyte density and distribution in body
and anterior surface (with fibroblasts there)!• Melanin granule density on posterior
(epithelial) surface!If:!• Few melanocytes in body, light is refracted,
bounces back, eye looks blue!• Lots of melanocytes in body, light is
absorbed, eye looks brown, black, green!
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Tunics of the Eye – 6 – The Retina!
Retina = nervous tunic/layer!1. Pigmented layer (becomes non-optical retina)!2. Optical portion (neural retina)!Major layers:!
A. Photoreceptor layer!• 6 million cones, 120 million rods!• Rods - black/white, shades of gray!• Cones - color vision!
B. Bipolar cell layer!C. Ganglion cell layer (axons form optic nerve)!
Course evals!2/27 – 3/19!
Canvas!
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Retina – continued!
3. Visual information pathway!• Photoreceptors → bipolar cells → ganglion
cells → lateral geniculate of thalamus → occipital cortex!
4. Optic disc (“Blind spot”)!• Ganglion cell axons exit here!• Blood vessels enter/exit here!• No photoreceptors!
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The Organization of the Retina Figure 17-7b, c!
Chapter 17, Part 1 Special Senses!
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Fovea Centralis (“Central Pit”)!Found within macula lutea (“yellow spot”)!• Eye movements focus light here!• Area of highest visual acuity!
Characteristics:!• No rods, cones only!• Cones tightly packed together!• Axons and blood vessels directed away from
fovea!• Ganglion cell bodies smaller!
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The Organization of the Retina Figure 17-7a!
Chapter 17, Part 1 Special Senses!
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Photoreceptors!
Rods and cones contain photopigments!
Outer segment contains pigments!• Visual pigments = opsins!• Melanin in epithelium!• Old photoreceptor discs
recycled by epithelium!Inner segment!• Cytoplasm, organelles !• Na+ pumps• Synapses with bipolar cell!
Inner!segment!
Bipolar!cell!
LIGHT!
CHOROID COAT!
Outer!segment!
Pigmented epithelium!
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Photopigments!
All contain:!1. Opsin = glycoprotein (different types exist)!2. Retinal = Vitamin A derivative!
In cones:!• One of three slightly different pigments!
In rods:!• Pigment = rhodopsin (“visual purple”)!• Rhodopsin = scotopsin + 11-cis-retinal!
ü Cis = bent form, trans = straight form!ü Only cis form can bind with opsin
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Rods and Cones Figure 17-14!
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Excitation of Rods – The Big Picture!
1. Light strikes rod!2. Cis-retinal changes shape → trans-retinal!3. Trans-retinal pulls away from opsin!4. Membrane permeability to Na+ changes!5. Receptor potential is produced!6. Receptor potential influences neurotransmitter
release by rod onto bipolar cell!
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Photoreception – 1 see the great Spotlight 17-16!
In darkness:!• Na+ continually pumped out of inner segment!• Na+ leaks back in at outer segment through
chemically-gated Na+ channels!Na+ movement called “the dark current”!!
Na+ entry into outer segment:!a. Cyclic GMP (cGMP) produced continuously!b. cGMP binds chemically-gated Na+ channels!c. cGMP keeps Na+ channels open in dark!
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Photoreception – 2!
Darkness (cont.)!“Dark current”!
Continuous depolarization = receptor potential!Leads to continuous release of glutamate!Glutamate inhibits bipolar cells!
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Photoreception Figure 17-16!
Darkness !
Opsin!inactive!
Opsin!active!
OUTER!SEGMENT!
INNER!SEGMENT!Na+!
Pump!
Light!
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Photoreception – 3!
Light striking rod causes:!1. Cis-retinal changes shape to trans-retinal!2. This activates opsin!3. Opsin activates transducin (a G-protein)!4. Transducin activates phosphodiesterase (PDE)!5. PDE breaks down cGMP → GMP!6. cGMP no longer holds open Na+ channels!7. Less Na+ leaks into cell (but it’s still being pumped out)!8. Cell hyperpolarizes!9. Less glutamate is released to inhibit bipolar cells!10. Bipolar cells excited, excite ganglion cells!
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Photoreception Figure 17-17c, d!
Light !
Pump!
Hyperpolarized!
Na+!pump!
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Bleaching and Regeneration Figure 17-17!
It takes time to reassemble rhodopsin, for [cGMP] to return to normal and open the gates, and to inhibit bipolar cells.!
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Convergences of Input to Bipolar Cells!
1. Rods: up to 600 inputs to one bipolar cell!• High sensitivity to light!• Low acuity (image sharpness)!
2. Cones: usually 1:1 convergence on bipolar cells!• Lower sensitivity to light!• High acuity!
3. Overall:!• About 130 million photoreceptors → 1 million
ganglion cells!• i.e. a convergent pathway !
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Other Retinal Cells!
Horizontal cells!• Inhibit bipolar cells to increase contrast!• Assist in color vision!
Amacrine cells!• Excited by bipolar cells!• Signal changes in illumination!
!(Yes, its really much more complicated than the
overview we are examining!)!
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Color Vision!
Three types of cones:!Respond maximally to different wavelengths!
1. Blue-sensitive cones!2. Red-sensitive cones (actually orange)!3. Green-sensitive cones!
!Young-Helmholtz Theory!• Nervous system interprets color based on the
degree of stimulation of the various cone types!• Interpretation is partly by retina, partly by brain
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Cones and Color Vision Figure 17-16!