Control Lecture 17: The Auditory System

Question 1

What is the name of the bones that form the ossicles in the middle ear?

Answer 1

Malleus, Staples and Incus (MIS) - These vibrate to transmit sound waves. This covers sound waves into mechanical waves so that they can be transmitted to the inner ear.

Question 2

What is the role of the Eustachian tube?

Answer 2

Connects the middle ear to the nasopharynx. This helps to equilibrate the pressure of the middle ear with that of the external environment. It also allows ventilation and drainage. That in infants is more horizontal rather than vertical in adults. This allows better transmission of infection in infants.

Question 3

Why is the middle ear such as high risk area (5)?

Answer 3

1. Connection to the Nasopharynx allows infection to travel to the middle ear.
2. Mastoid air cells allow transmission of infection in the middle ear to the mastoid fossa.
3. Proximity to the internal carotid artery can cause pulsatile tinnitus.
4. Proximity to the internal jugular (inferiorly) means there is a risk of thrombosis.
5. The area is transferred by the Chorda Tympani (branch of the facial nerve to the tongue). There is an infection risk.

Question 4

What is the role of the:

(a) Round Window,
(b) Oval Window?

Answer 4

(a) Round Window: found at the cochlea, allowing communication between the middle ear and the inner ear.
(b) Oval Window: connective tissue membrane located at the end of the middle ear and the beginning of the inner ear (staples to the cochlea).

Question 5

How is the cochlea organised?

Answer 5

Tonotopically. Low frequency sounds are detected at the apex of the cochlea and progressively higher sounds are detected more at the base.

Question 6

What is the range of frequencies the human ear can hear?

Answer 6

20-20,000 Hz

Question 7

What fluid is found in the:

(a) Bony Labyrinth
(b) Membranous lymph

Answer 7

(a) Perilymph - similar to extracellular fluid

(b) Endolymph - similar to intracellular fluid

Question 8

How is the cochlea organised?

Answer 8

Into the Scala Vestibuli and Scala Tympani. These are both filled with perilymph. They are separated by the cochlear duct (containing endolymph) - SV is found superior to the duct and ST inferior to the duct.

Question 9

How does a sound wave travel from the external ear to the inner ear?

Answer 9

1. Sound waves are funnelled from the external ear into the middle through the external auditory meatus to the tympanic membrane.
2. At the middle ear sound waves travel through the tympanic membrane and cause the ossicles to vibrate. This transmits the sound wave into mechanical energy.
3. The wave travels through the oval window into the cochlea.
4. We transmit waves into the fluid- there is now movement of the perilymph in the Scala vestibuli which is continuous with the Scala tympani.
5. Vibrations are transmitted to the Scala tympani. Movement of the perilymph causes deformation of the fluid in the cochlear duct. In the cochlear duct there is the spiral ganglion and the Organ of Corti.

Question 10

Where is the mechanical sound wave converted to an electrical signal?

Answer 10

In the cochlea duct is the Spiral Organ of Corti. It is a sensory organ that detects and transmits sound. It is made of a Tectorial membrane and a basilar membrane. Hair cells contain cilia which upon movement of the cochlea duct become in contact with the membrane and detect when there is movement. Cellular mechanisms allow transmission of electrical signals via the cochlear nerve.

Question 11

Describe the auditory pathway through the CNS.

Answer 11

1. The primary auditory fibres emerge from the Spiral Organ of Corti are bipolar neurones. These connect to the hair cells and transmit information via the cochlear nerve to the brainstem. The cell body of the 1st order neurones are in the spiral ganglion. They reach the brainstem at the cerebellopontine angle. Primary fibres synapse at the level of the Dorsal and ventral cochlear nuclei.
2. At the cochlear nuclei, the cochlear nerves project bilaterally to the superior olivary nucleus (in the midpons). Cross over occurs here - this is known as the Trapezoid body.
3. From here ascending fibres reach the inferior colliculus (tectum of they midbrain) via the lateral leminiscus bilaterally. There is another opportunity for synapse.
4. From the IC, fibres connect to the Medial Geniculate Nucleus of the Thalamus.
5. The MGN then projects to the primary auditory complex via auditory traditions.

Question 12

Give the terms for the following descriptions:

(a) The Fibres that project from Superior Olivary Nucleus to the Inferior Leminiscus.
(b) The Fibres that cross over at the Superior Olivary Nucleus.
(c) The fibres that travel from the Inferior Colliculus to the Medial Geniculate Nucleus.

Answer 12

(a) Lateral Leminiscus
(b) Trapezoid Body
(c) Brachium of the Inferior Colliculus

Question 13

Where are the ponts of cross over in the auditory pathway?

Answer 13

1. Trapezoid body - At the Level of the Superior Olivary Nucleus.
2. At the Inferior Colliculus - known as the Inferior Colliculus Commissure

Question 14

How is the Auditory Cortex tonotopically organised?

Answer 14

• Lowe frequency sounds are projected to the more anterolateral parts of Heschl’s gyrus
• Higher frequency sounds are projected more posteromedially

Question 15

Describe the pathway for Auditory reflexes.

Answer 15

1. Auditory information reaches the SON via multiple interneurons. Fibres then travel to stapedius muscle via CN VII (via its nuclei) and tensor tympani via CV3 to prevent damage during loud noise. The stapedius muscle tenses the bone to prevent further vibrations. Tensor Tympani prevents further vibration of the tympanic membrane.
2. At the level of the inferior colliculus via interneurons auditory information that has reaches the IC can send interneurons to CN III, IV and VI. There can be connections to the spinal nerves also. This moves the head, eyes, neck and body in response to the sound e.g. when someone calls your name.