Special Senses Concepts

Question 1

Describe the structure of the olfactory receptors and other cells involved in olfaction.

Answer 1

Olfactory receptor cells= Bipolar, first order, ciliated neurons that act as chemoreceptors to bind odorant molecules at the apical surface of the olfactory epithelium and transduce stimulation (binding) into receptor potentials - and then into action potentials sent along the axons of the neurons, through the olfactory foramina of the cribriform plate of the ethmoid bone to synapse on second order neurons of the olfactory bulb

Supporting cells= pseudostratified, ciliated columnar cells of the olfactory epithelium that provide structural and metabolic support to the olfactory receptor cells. Also, mimic myelin sheaths by “insulating” the nerve olfactory receptor cells

Basal cells= mitotically active (stem cells) at the basal end of the olfactory epithelium. They divide to replace worn-out olfactory receptors every month or so.

Question 2

Describe the sensation of olfaction in terms of the type of sensory receptor and steps needed to transduce odorants. Describe the location of the olfactory epithelium and some nearby structures.

Answer 2

1. Air inhaled through the nostrils
2. Air humidified, cleansed, and connected among the nasal conchae
3. Odorant molecules in air bind to the olfactory receptors
4. Stimulation (chemoreception) transduced to receptor potential
5. Action potential sent along the bipolar olfactory receptor
6. Axons of olfactory receptor synapse (EPSP) on the neurons of the olfactory bulb
7. CN I runs to the primary olfactory area, then on to the olfactory association area (oribitalfronal area) with lots of collaterals to the limbic system and hypothalamus

Question 3

Outline the neural pathway for olfaction.

Answer 3

First-order olfactory receptor synapses on the dendrites or cell bodies of second-order olfactory neurons in the olfactory bulb. Olfactory bulbs sit superior to the cribriform plate of the ethmoid bone and lateral to the crista galli. Then to the primary olfactory area, then on to the olfactory association area. Lots of collaterals along the way are sent to the limbic system and hypothalamus.

Question 4

Explain how only a couple hundred odorants can account for tens of thousands of smells.

Answer 4

You get tens of thousands of smells through combinations of odorant molecules

Question 5

Discuss the rate of adaptation of olfactory receptors and compare it to that of other fast and slow-adapting receptors.

Answer 5

The rate of adaptation of olfactory receptors is fast adapting. About 50% adaptation in the first second after stimulation and eventually enough of a continued slow adaptation become completely insensitive and you become nose blind to certain strong odors within a minute or so.

Question 6

Name the 5 primary tastes and be able to link gustation to olfaction, conceptually.

Answer 6

Sweet, sour, bitter, salty, umami
- 80% of taste is actually olfactory
- Taste is completely wrapped up in olfaction with 1000s of “flavors” steeming from combinations of the 5 tastes plus olfaction

Question 7

Contrast the structure, function, and location of the 4 different papillae of the tongue.

Answer 7

Vallate papilla= about 12 very big, taste, posterior 1/3 of tongue

Fungiform papilla= spread all over the tongue (mushroom-shaped), taste, anterior ⅔ of tongue

Filiform papilla= no taste, increased friction, good for licking, bumpy, center of the tongue

Foliate papilla= very lateral aspects of the tongue, but degenerate early in childhood

Question 8

Describe the structure and function of gustatory receptors, focusing on which type of cell they are.

Answer 8

Epithelial cells with hairs push through the taste pore and out into the oral cavity. Chemoreceptors bind sweet, bitter, and umami tastants to trigger secondary messenger paths to depolarize the cell with Ca influx to trigger NT release from synaptic vesicles and the synaptic cleft

Question 9

Compare the structure and function of the other types of cells in the gustatory epithelium to those of the olfactory epithelium.

Answer 9

Taste buds are oval-shaped bodies found within the papillae and connected to the outside world via a taste pore. These taste buds hold the gustatory receptor cells. Inside the taste bud is a “mini-epithelium” with 3 cell types: gustatory receptor, supporting cells, and basal cells. So, same concept as olfactory cells, mitotically active and replacing.

Question 10

Describe the basics of the neural pathway for gustation.

Answer 10

• two-step process
• Gustatory receptor cells transduce binding of tastant molecules, but the real 1st order neuron is a “sensory” neuron found on the basal side of the taste bud
• The receptor creates a “receptor” potential and needs to synapse on the 1st-order neuron to generate an AP

Question 11

Explain how Na, as a tastant, can be both a ligand and a transported cation.

Answer 11

The molecule that binds to the metabotropic receptor is sodium as salty food is high in sodium, also causing sodium to rush in when the receptor is activated.

Question 12

Compare gustatory adaptation to that of other sensory modalities.

Answer 12

The threshold is lowest for bitter tastants…poisons
Sweet and salty have a much higher threshold
Adaptation is complete within 1-5 minutes of continuous stimulation (take small bites and savor)

Question 13

Describe the location and function of accessory structures of the eye.

Answer 13

Eyebrows: above eyes, protection

Eyelashes: protect eyes from foreign substances

Eyelids: upper and lower structures anterior to the eyeball that open and close to expose, or protect the eyeball

7 extrinsic eye muscles:
Lateral, medial, superior, and inferior rectus= mostly moving the eye linearly. Up, down, over, and out
Inferior and superior oblique= aid in “rolling” the eyes
Levator palpebrae= opens the eye by lifting the upper palpebra (eyelid)

Lacrimal apparatus= lubricate eyes

Question 14

Review the 7 extrinsic muscles of the eye, including insertions, roles, and CN innervation

Answer 14

Lateral, medial, superior, and inferior rectus= Cranial nerve 6 (abducens), moving the eye linearly. Up, down, over, and out
Inferior and superior oblique= Cranial nerve 4 (trochlear), aid in “rolling” the eyes
Levator palpebrae= Cranial nerve 3 (oculomotor), opens the eye by lifting the upper palpebra (eyelid)

originate on the temporal or lacrimal bone and inserts on the sclera of the eys

Question 15

Trace the path of lacrimation, including the relevant structures.

Answer 15

The lacrimal gland secretes tears into excretory lacrimal ducts, which distribute tears over the surface of the eyeball. The superior or inferior lacrimal canaliculi drain tears into the lacrimal sac, which drains tears into the nasolacrimal duct, which drains tears into the nasal cavity.

Question 16

Review 2 small ANS adjustments to vision

Answer 16

Accommodation: viewing near and distant objects differently
Pupi diameter to match the amount of available light

Question 17

Discuss the fluids of the eye, using an anterior/posterior cavity analysis

Answer 17

anterior cavity= aqueous humor, liquid-like substance
posterior cavity= vitreous humor, gel-like substance

Question 18

Discuss the importance of the fovea centralis in image formation.

Answer 18

The fovea centralis is a small indentation in the center of the macula lutea, which is the dead center of the posterior retina receiving the projected image. This area only has cones, so it’s the area of highest visual activity and why we always orient to and look directly at things we want to see the clearest, having the image landing on the fovea centralis.

Question 19

Discuss the roles of the two primary layers of the retina and the associated cells and structures found in each.

Answer 19

Pigmented layer= melanin granules, pigment epithelial cell

Neural layer (inner segment and outer segment) = rod and cones, mitochondrion, Golgi complex, nucleus, synaptic terminal, synaptic vesicles, discs

Question 20

Describe phototransduction in detail, starting with light passing through the neural layer and ending with depolarization of bipolar sensory neurons.

Answer 20

• When light hits the opsin and trans-retinal is briefly set free, trans-retinal sets off a molecular chain reaction that shuts down the influx of Na into the photoreceptor
• This hyperpolarizes the photoreceptor and stops the flow of glutamate, an inhibitory NT of the eye, onto the bipolar cells
• This stops the inhibition of bipolar cells
• This means in the light, the bipolar cells are actively depolarizing and signaling the CNa (transduction is occurring)

Question 21

Contrast rods and cones, structurally and functionally

Answer 21

Rods: rod-shaped
- rhodopsin
- Night vision
- Fuzzy vision
- Peripheral vision
- Active only in low light

Cones: cone-shaped
- 3 different Opsins
- Color vision
- Edge detection
- Central vision
- Only in bright light

Question 22

Discuss, briefly, how amacrine and horizontal cells can modify phototransduction.

Answer 22

If you’re in bright light, they’re quieting now the stimulation and if you’re in a dark room they amplify what little signal is there so you can create more signal

Question 23

Describe the neural pathway for vision.

Answer 23

• Ganglion cells synapse onto CN II axons that coalesce at the optic disc to pass through the retina and onto the CNS
• Via the optic chasm, immediately to the lateral geniculate nuclei of the thalamus then onto the primary visual area with collaterals to:
  - Superior colliculi of the midbrain
  - Other nuclei of the midbrain coordinate pupil size, accommodation, and head movement
  - The suprachiasmatic nucleus of the hypothalamus (sleep & circadian rhythms)

Question 24

Discuss the role of tensor tympani and stapedius in modifying sound transduction.

Answer 24

The tensor tympani is a small skeletal muscle under ANS control that contracts to stiffen the malleus against the tympani membrane to dampen excess vibration from loud noises

The stapedius is a small muscle in the body under ANS control that contracts to dampen the vibration of stapes on the oval window in “loud” environments

Question 25

Contrast the 2 major parts of the inner ear, relative to sound transduction and equilibrium.

Answer 25

Cochlear= for hearing

Semicircular canals and vestibules= for equilibrium

Question 26

Discuss the role of the outer hair cells in modifying sound transduction.

Answer 26

In loud environments, motor neurons cause contractile proteins in the outer hair cells to relax, which pulls the basilar membrane down and dampens its vibrations (dampening noise). The opposite occurs in quiet places to increase basilar membrane vibration and acoustic sensitivity.

Question 27

Trace a path of light from outside the eye to the fovea centralis, naming all the structures light passes through and where it can be refracted.

Answer 27

Cornea (light bends)
Lens (light bends)
Fovea centralis

Three different media: air, liquid, and gel
every time light moves from one density to the next, light refracts

Question 28

Compare and contrast, in detail, the structures and functions of the 3 tunics of the eye.

Answer 28

Fibrous tunic:
Cornea= curved to focus light onto the retina
Sclera= white of the eye

Vascular tunic:
choroid= absorbs light and prevents backscatter of light
Ciliary body=secrete aqueous humor
irs=dilates or constricts to determine the amount of light entering the eyes

Sensory tunic:
Pigmented layer= pigmented structure/ prevents backscatter of light
Neural layer= contains 3 layers: photoreceptors, bipolar, ganglion

Question 29

Working from a sagittal view of the eye and starting anteriorly, discuss the structures of the palpebra and their functions, moving posteriorly

Answer 29

Orbicularis oculi= close the eye
Tarsal plate= gives support and strength to the eyelid
Tarsal gland= secretes fluid to keep eyelids from sticking to each other
conjunctiva= mucous production
Palpebral conjunctiva= lines the inner aspect of the eyes and covers the eyes when closed
The bulbar conjunctiva= extends from the eyelid to contact the sclera of the eye at all times

Question 30

Trace the path of a sound wave, identifying the key structures vibrated by sound waves, and the correct sequence of vibration through these structures.

Answer 30

• The auricle collects and focuses sound waves
• The external auditory meatus conducts sound waves to the medial wall of the outer ear
• The malleus transmits the vibration of sound waves entering the ear and striking the tympanic membrane into the vibration of bones
• Malleus articulates, via a tiny synovial joint, with the incus so vibration in the malleus from external sound waves set the incus vibrating which also sets the stapes vibrating
• Stapes is connected to the oval window, which is much smaller than the tympanic membrane, so the vibration of the tympanic membrane is concentrated and more forceful at the oval window which travels to the cochlear, where hearing is produced

Question 31

Describe the mechanical transduction of sound waves by inner hair cells, starting with sound waves traveling through the perilymph of the scala vestibuli and ending with depolarization of inner hair cells.

Answer 31

• Sound is transmitted to the fluid of the inner ear, and perilymph of the scala vestibule, through vibrations of the tympanic membrane, malleus, incus, and stapes. The cochlear duct (scala media) is filled with rich K+ endolymph
• The inner hair cells are embedded in the basilar membrane and stereocilia in the tectorial membrane. When the basilar membrane vibrates, the endolymph flows across the cilia and moves them, and the movement of cilia by flowing endolymph opens channels in the cilia.

Question 32

Compare and contrast perilymph and endolymph (bony & membranous labyrinths) in all inner ear structures.

Answer 32

Perilymph: Fills scala vestibule and scala tympani
- The outer bony labyrinth is lined with periosteum which is continuous with the petrous portion of the temporal bone and is “osseous” tissue filled with perilymph
- Perilymph is an osseous version of lymph

Endolymph: Fills scala media
- Endolymph is a very special fluid high in [K+] and surrounded by the bony labyrinth

Question 33

Describe the equilibrium pathways.

Answer 33

Equilibrium: purely gravity
From the vestibular ganglion, vestibular and SCC sensory afferent info travels along the vestibular branch of the CN VIII to the CNS

Directly to the vestibular nuclei of the pons and medulla and from there to:
- Nuclei of CN III, IV, & VI
- Cerebellum (along with proprioceptive somatic info & visual stimuli for a “state” check)
- The skeletal muscles via the vestibulospinal pathway (coordination/balance/posture)
- To primary somatosensory area and frontal eye field area (via the thalamus)

Question 34

Contrast the exact details of transduction in the spiral organ of Corti, maculae of the vestibule, and ampullae of the semicircular canals.

Answer 34

Transduction in the spiral organ of corti for hearing is through vibrations. The vibrations move the hair cells in the spiral organ of corti to produce hearing.

Transduction for equilibrium in the vesticle apparatus is through gravity. Moving your head moves endolympths which move hair cells, controlling equilibrium