Auditory- Olfaction

Question 1

How does hearing work

Answer 1

1. Sound waves come into the pinna of the ear, follow the auditory canal, and hit the tympanic membrane
2. Sound waves are transduced into vibrational energy, transmitting the signal to the malleus -> incus -> stapes which covers the oval window
3. Resulting vibrations from the oval window travel through the perilymph fluid through the scala vestibuli and then through scala tympani. Vibrations leave via the round window

4.Vibrations in the scala vestibuli move the basilar and tectorial membranes within the scala media distorting hair cells leading to transduction at the Organ of Corti

5. Fibers travel as CN VIII (cochlear branch) and meet up with the vestibular branch -> vestibulocochlear nerve

Synapse on cochlear nucleus in medulla -> thalamus -> temporal lobe (auditory cortex) where we can perceive it!

Question 2

Perilymph

Answer 2

Transmits sound waves to endolymph

Found in scala vestibuli and scala tympani

High in sodium and chloride Low in Potassium
(similar to extracellular fluid)

Less positive electric potential

Question 3

Endolymph

Answer 3

Transmits sound waves to hair cells

Found inside scala media

High in potassium
(similar to intracellular fluid)

More positive electric potential

Created by melanocytes in the stria vascularis

Question 4

Signal Transduction

Answer 4

• Hair cells reside in endolymph (high potassium) and base of cell in perilymph (low potassium) creating a gradient.
• When stereocilia move toward rigid kinocilium potassium channels open leading to depolarization and nerve stimulation (also how ampulla and macula densa work!)

Question 5

Two Type of Hair Cells

Answer 5

Inner Hair Cells
Outer Hair Cells

Question 6

Inner Hair Cells

Answer 6

Receptor for the afferent fibers of the cochlear nerve

Question 7

Outer Hair Cells

Answer 7

o Protective function to lead to dampening by limiting movement of tectorial membrane

o “Dampening”

Question 8

Auditory Reflex

Answer 8

• Part of CN VIII synapse on the
  caudal colliculus
• Makes us turn our head to sound!
• May work with other reflexive systems to “hone in” on disturbances or dangers in our environment

Question 9

Clinical Notations

Answer 9

• Pigmented associated deafness
  o White coat color/ Blue eyes (Merle genes in collie/ Shetlands)
  o Melanocytes not present in Stria Vacscularis
  § Leads to degeneration, abnormal endolymph, an death of hair cells
• Many Breed Specific Disease
  o Dalmatians, Catahoula Leopard dogs
• Types of Deafness:
  o Conduction
  § Anything blocking the ear canal
  v Infection, Foreign body, tympanic membrane rupture
  o Sensorineural
  § Nervous system associated
  § Decreases with age

Question 10

Test Hearing

Answer 10

• Basic/ Early Investigative Testing o Breed/ Genetic Predisposition o Behavior at home o Reflexive responses to sound § Cannot drop heavy things on ground § Animal cannot see you o Otoscopic Exam
• Advanced o CT/ MRI o Brainstem Auditory Evoked Response (BAER)

Question 11

Where is the olfactory specific mucosa located

Answer 11

on the
ethmoidal labyrinth and
dorsal nasal septum in
the dog

Odoriferous substances
dissolve in the mucus
overlying the mucosa
and vomeronasal organ

Question 12

Olfactory Epithelium: Cell Types

Answer 12

1.Olfactory Sensory Neurons
* Bipolar neurons
2. Sustentacular Cells
* Provides Structure
* (supporting cell)
3. Basal Cells
* Stem Cells
* Neurogenesis (~2 months)

Question 13

How do axons reach the olfactory bulb

Answer 13

Axons pass through the cribriform plate to the olfactory bulb

Question 14

General Olfactory pathway

Answer 14

1. Axons from the olfactory mucosa and vomeronasal (Jacobson) organ pass through the cribriform plate and synapse in the olfactory bulb neurons
2. From the bulb they travel to the olfactory tract to synapse in

3 potential ways
- Pyriform Cortex
- Limbic Center (behavior/emotion)
- Brainstem (reflexes)

Thalamic relay is not required for cortical perception
- (some axons will go here for secondary cortex perception)

Question 15

Vomeronasal Organ

Answer 15

Important in detecting pheromones

“Flehmen Response”

Vomeronasal system opens directly into the oral cavity and projects axons to the olfactory bulb with differences seen between species

Question 16

Clinical Notations: Olfaction

Answer 16

• Can be tested but may be difficult
• Animal must be blinded to prevent visual input
• Smell must be pleasant
• Cats without the ability to smell have drastic decrease in
  food intake
• Volatile chemicals or odorants (smelling salts, ammonia) result in noxious stimuli to the TRIGEMINAL NERVE
• Can be difficult to test in sick animals. Imaging or EEG are best in sick animals to evaluate
• Can use “slide test” to check for nasal patency
• Anosmia – inability to smell – unilateral difficult to tell