Chapter 6 - Hearing, Taste, Smell

Question 1

What are the ways we describe the physical properties of sound?

Answer 1

Intensity: the loudness of sound, is measured in decibels which describes the amplitude (height) of the sound wave.

Frequency: the pitch of the sound, measured in Hertz which describes the number of waves per second.

Question 2

What is the structure of the external ear and does contribute to the hearing process?

Answer 2

The external ear captures, focuses, and filters sound.

It is made up of the Pinnae: the fleshy external part we refer to as ears. It funnels sound into the ear; mobile pinnae can capture more sound. The shape of the pinnae (ridges and valleys, modify sound enhance and suppress certain frequencies. In humans, we enhance the frequencies associated with speech sounds (2000-5000Hz).

Question 3

What are the structures of the middle ear and how do they contribute to the hearing process?

Answer 3

The Middle ear concentrates sound; it is made up of the ear canal, the tympanic membrane (aka the eardrum), the ossicles, and the oval window.

The Ear Canal: links the outer ear to the tympanic membrane.
The Tympanic Membrane: seals off the end of the ear canal and captures sound vibrations.
Ossicles: three small bones that transmit vibrations from the tympanic membrane to the oval window. They can concentrate and amplify vibrations.
The Oval Window: the location on the cochlea where vibrations are transmitted into the interior of the cochlea.

Question 4

What are the names of the ossicles?

Answer 4

In order from the tympanic membrane to the oval window:

- Maleus, Incus, and Stapes

Question 5

How does the middle ear control Volume?

Answer 5

There are two muscles: Tensor Tympani and Stapedius that attatch to the ossicles and vary the linkage between the bones. When the muscles contract, they reduce the effectiveness of the sound traveling between the ossicles.

Through top down processing, our brain uses these muscles to prevent our own vocalizations from becoming distracting or too loud. When we are about to make a sound, the brain signals for these muscles to contract, effectively dampening the sound before it reaches the cochlea.

Question 6

How does the middle ear control Volume?

Answer 6

There are two muscles: Tensor Tympani and Stapedius that attatch to the ossicles and vary the linkage between the bones. When the muscles contract, they reduce the effectiveness of the sound traveling between the ossicles.

Through top down processing, our brain uses these muscles to prevent our own vocalizations from becoming distracting or too loud. When we are about to make a sound, the brain signals for these muscles to contract, effectively dampening the sound before it reaches the cochlea. `

Question 7

What is the round window in the ear?

Answer 7

The Round window is located on the cochlea, it is flexible and pliable which allows for movement of the cochlear fluid without damaging the hard cochlea.

Question 8

What is the location of transduction in hearing?

Answer 8

The cochlea, specifically, the hair cells in the organ of corti.

Question 9

Describe the structure of the cochlea.

Answer 9

The Cochlea is a coiled, fluid filled, structure with three parallel canals:
- The scala vestibuli (vestibular canal)
- Scala Media (middle canal)
- Scala Tympani (tympanic Canal)
The Scala Media (middle Canal) contains the receptor system called the organ of corti,

Question 10

What is the structure and function of the organ of corti?

Answer 10

Is the part of the cochlea that converts sound waves into neural activity. It has three mains structures:
- The auditory Hair cells (which sit between the two membranes)
- Supporting Cells
- Terminations of the auditory nerve fibers
The organ of corti has two membranes:
- the Tectorial Membrane (at the top)
- The Basal Membrane (at the bottom)

Question 11

How does sound become neural signals once it leaves the ossicles?

Answer 11

When sound vibrations hit the oval window, they cause waves in the fluid of the vestibular canal. This waves cause the basilar membrane to ripple like a shaken rug.

The stereocilia of the hair cells are embedded in tympanic membrane while the base of the hair cells are embedded in the basilar membrane. This causes the stereocilia to move and sway with the movement of the basilar membrane; they pull on the tip links of their neighboring stereocilia which causes (like a cork) the ion channel to open. This allows positively charged ions into the cell, depolarizing it and triggering the release of a neurotransmitter.

Question 12

Describe the structure of an inner hair cell.

Answer 12

The hair cell has small hairs called stereocilia at the top which are connected to one another through tip links.

At the bottom of the hair cell is afferent (away from the hair cell, towards the brain) and efferent (towards the hair cell, away from the brain) nerve endings which connect to the Vestibulocochlear nerve.

Question 13

How is the shape of the basilar membrane significant?

Answer 13

The basilar membrane in smallest and stiffest at the base and wider and looser at the apex.

The difference in shape and stiffness means that it can respond to different frequencies at different locations. The base responds to the highest frequencies whereas the apex responds to the lowest.

Question 14

What are the two groups of hair cells?

Answer 14

Inner Hair cells: fewer IHC than OHC, they are positioned in a single row closer to the central axis of the cochlea
Outer Hair cells: Many more than IHC, organized into three rows and are positioned further from the central axis of the coiled cochlea.

Question 15

What are the 4 different types of neural connections in the hair cells?

Answer 15

IHC Afferents: In the inner hair cells.Action potentials move towards the brain. These AP convey perception of sound and use glutamate

IHC Efferents: In the inner hair cells. Action potentials from the brain to the hair cell. These APs control the responsiveness of the cell and uses ACh (Acetylcholine). This axon actually attatches to the dendrite of the IHC afferent connection rather than the hair cell itself.

OHC Afferents: In the outer hair cell, Action potentials from the hair cell to the brain. Conveys information about the mechanical state of the basilar membrane and uses ACh.

OHC Efferents: In the outer hair cell. Action potentials from the brain to the hair cell. Controls the sensitivity to sound by controlling the stiffness of the basilar membrane. The stiffness if controlled by modifying the length of the OHC, longer means if is more firmly embedded between the two membranes, restricting movement of the basilar membrane. Shorter means the basilar membrane can move more freely. Uses GABA.

Question 16

What is a tuning curve?

Answer 16

A graph of a single auditory nerve fibers responses to sound based on their frequency and intensity.

All IHC have maximum sensitivity to particular frequencies but can respond to other frequencies if the intensity is high enough.

Question 17

Describe the movement of auditory signals starting in the cochlea and ending in the auditory cortex.

Answer 17

Auditory signals move from the Cochlea, to the cochlear nucleus, to the superior olivary nucleus, to the inferior colliculus, to the medial geniculate nucleus, to the auditory cortex.

The superior olivary nuclei are the first areas to receive bilateral input in that they get information from both ears. This is important for the localization of sound.
Th medial geniculate nucleus is located in the the thalamus which controls the direction of transmission of sensory input.

Speech sounds activated other more specialized auditory areas as well.

Question 18

What does it mean to say that the auditory pathway has tonotopical organization?

Answer 18

It means that the auditory pathway is organized in terms of sound frequency. The pathway is arranged in a map low to high frequencies. Auditory nerves are excited by some frequencies but inhibited by similar ones for more precise feedback.

Question 19

Describe to ways we encode pitch.

Answer 19

Place coding: pitch is determined by the location of the activated hair cells.
Temporal Coding: pitch is encoded based on the frequency of auditory stimuli encoded in the rate of firing of auditory neurons. Essentially, pitch is determined by the number of AP in a given time compared to the number of cycles/second.

Question 20

Describe two ways we localize sound.

Answer 20

Interaural Intensity Differences: comparing the intensity (loudness) of a sound between the two ears. Sound appears less intense to the ear pointing away from the origin point because head casts a sound shadow. This effect works best on localizing high frequency sounds.

Interaural temporal Differences: the difference between two ears for the amount of time it takes for a sound to arrive. There are two kinds of time differences:
- Onset disparity: the time difference between two ears in hearing the start of the sound
- Ongoing phase disparity: continued mismatch between the two ears in the time of arrival of all the peaks and troughs that make up a sound wave.
This effect works best on localizing low frequency sounds.

Question 21

What is Amusia?

Answer 21

A disorder characterized by the inability to discern or sing tunes accurately. Associated with abnormal functioning in the right frontal lobe and impoverished connectivity between the frontal and temporal cortex.

Question 22

What are the three types of deafness?

Answer 22

Conduction Deafness: the ear fails to convert sound vibrations into waves of fluid in the cochlea. Caused by defects in the external and middle ear

Sensorineural deafness: permanent damage to the hair cells or to the vestibularcochlear nerve.

Central Deafness: the auditory areas of the brain are unable to process or interpret action potentials from sound stimuli in a meaningful way. Caused by damage to the brain.

Question 23

What are the five basic known tastes detected by the tongue and how are they processed?

Answer 23

Sweet: Is a metobotropic process. proteins T1R2 and T1R3 bind to protein receptors in combination which triggers a second messenger system.
Salty: Sodium ion channels pick up the Na+ ions in the food. Cl- may play are part too.
Sour: Caused by the acidity of the food. Hydrogen ion channels pick up the H+ in all acids.
Bitter: Metobotropic process. Protein T2R receptors activate a second messenger system. This is a highly sensitive taste process which evolved out of survival needs (bitter is a indicator of toxicity.)
Umami: detected by two types of processes
- A metobotropic glutamate receptor responds to glutamate in the food and activates a secondary messenger system.
- T1R1 and T1R3 combination receptors respond to proteins in the food.

Question 24

What is the difference between taste and flavour?

Answer 24

Taste is the perception of chemicals in the food via the tongue.

Flavour is a combination of taste and smell.

Question 25

Describe the structure of our taste receptors.

Answer 25

The tongue is covered in papillae which are small ridges that increase the surface area of the tongue.
Taste buds are located on the sides of the papilla and they use fine projections called microvilli to contact tastants in the saliva

Question 26

What is the Gustatory Pathway?

Answer 26

It moves from the tongue, to the brain-stem nuclei, to the thalamus, and then to the somatosensory cortex.
In the Somatosensory cortex, other factors of flavour are integrated with taste including temperature, and texture.

Question 27

What is the olfactory epithelium?

Answer 27

It is the receptor neurons in the nose. It is made up of three types of cells: supporting cells, basal cells, and receptor neurons.
- Each receptor has a long dendrite extending into the mucous surface of the nasal passage. Odorants are inhaled and interact with the olfactory receptor proteins on these dendrites. An unmyelinated axon travels to the olfactory bulb in the brain and then projects in to multiple other areas.

Question 28

What is the olfactory pathway starting from the olfactory receptor neurons?

Answer 28

Olfactory receptor neurons connect to the olfactory bulb. The olfactory bulb connects to the amygdala, primary olfactory cortex, and the hypothalamus. The amygdala and periform cortex connect to the hypothalamus and the medial dorsal thalamus. The hypothalamus connects to the lateral posterior bitofrontal cortex. the medial dorsal thalamus connects to the secondary olfactory cortex.

Question 29

What is the vomeronasal system?

Answer 29

This system detects pheromones. Receptors are found in the vemeronasal organ of the epithlial cells near the olfactory epithelium.

These receptors are involved in reproductive behaviour and in signals of genetic relatedness.