Auditory System

1. Diagram the major ascending pathways of the auditory system.

See diagram A on page 75 of Dr. Godfrey’s notes.

Signals from the inner ear are conducted from the cochlea to the cochlear nucleus on the ipsilateral side by the auditory nerve. The cochlear nucleus on both sides then travels to the superior olivary complex and inferior colliculus through the trapezoid body and lateral lemniscus at the level of the pons-medullary junction. The superior auditory complex goes to the nuclei of lateral lemniscus at the level of the pons and inferior colliculus at the level of the midbrain by going through the lateral lemniscus, bilaterally. Inferior colliculus then projects to ispsilateral medial geniculate at the level of the thalamus through the brachium of inferior colliculus. Medial geniculate projects to ispilateral auditory cortex (area 41 and 42 of superior temporal gyrus) by transversing the temporal gyri of Heschl through auditory radiations. There are descending pathways all along the system.

2. Describe the structural features of the middle ear that facilitate transmission of sound to the cochlea.

The middle ear contains the tympanic membrane and the middle ear bones to help convert air vibrations into cochlear fluid vibrations.

Tympanic Membrane
The tympanic membrane has 20x the area as the oval window membrane and acts to help convert air vibrations into cochlear fluid vibrations.

Middle Ear Bones
Malleus, Incus, Stapes

Middle Ear Muscles
The tensor tympani connects to the malleus and the stapedius connects to the stapes; contraction of these muscles decreases transmission of vibrations through the middle ear at specific frequencies.

Eustachian Tube
This tube helps equalize the pressure across the tympanic membrane, providing better conditions for hearing.

3. Compare the ionic compositions of the endolymph and perilymph.

Endolymph is similar to intracellular fluid with high K+ and low Na+ concentrations. Perilymph is similar to extracellular fluid with high Na+ and low K+ concentrations.

4. Describe the process whereby vibrations entering the ear are converted to electrical activity of the auditory nerve.

(1) Entry of air vibrations into external acoustic meatus
(2) Vibration of tympanic membrane
(3) Vibration of middle ear bones
(4) Vibration of perilymph in cochlea
(5) Vibration of endolymph in cochlea
(6) Vibration of organ of corti
(7) Vibration of hair cell cilia
(8) K+ entry from endolymph into hair cells causes depolarization
(9) Depolarization opens Ca2+ channels in hair cell membranes, and facilitates release of excitatory neurotransmitters from hair cell basal parts.
(10) Excitatory neurotransmitter interacts with receptors on auditory nerve fiber peripheral terminals, leading to EPSP of auditory nerve fibers
(11) EPSP leads to opening of voltage gated Na2+ channels and action potential of auditory nerve fibers

5. State the approximate numbers of inner and outer hair cells.

There are approximately 4,000 inner hair cells arranged in 1 row within the organ of corti. There are approximately 20,000 outer hair cells arranged in rows of 3 or 4 within the organ of corti.

6. Describe the relationships of type I and type II auditory nerve fibers to the inner and outer hair cells.

90% of auditory nerve fibers are myelinated type I fibers that innervate only a few (one or two) inner hair cells. The remaining 10% are unmyelinated type II fibers that innervate many (20+) outer hair cells. Both type I and II fibers terminate in the cochlear nucleus.

7. Describe how sound frequency and intensity are coded in the cochlea, auditory nerve, and cochlear nucleus.

Frequency coding in the cochlea is determined by which part of the corti spiral is set into vibration (and, for low frequencies, the frequency of the vibration). Amplitude is coded in the organ of corti by the amplitude of corti vibrations and the length of the organ of corti that is set in vibration.

In the auditory nerve, the frequency is coded by which specific fibers have increased firing (and, for low frequencies, the pattern of action potentials). Amplitude is coded by the auditory nerve by the firing rates of individual fibers and the number of active fibers.

In the cochlear nucleus, frequency is coded by location of active cells in the cochlear nucleus: higher frequencies are represented more dorsally while lower frequencies are represented more ventrally in the 3 major subdivisions of the cochlear nucleus. Amplitude is still coded by the firing rates of individual cells and the number of active cells.

8. Name 3 types of neurons in the cochlear nucleus and describe how their structural specializations are correlated with their physical properties.

Spherical Bushy Cells
Spherical bushy cells found in the anterior part of the ventral cochlear nucleus (AVCN) and have small dendritic fields with few large terminals from auditory nerve fibers. As a result, they respond with high fidelity to the original signal from the auditory nerve. Its axons mainly project bilaterally to the superior olivary complex.

Octopus Cells
Octopus cells are found in the posterior part of the ventral cochlear nucleus (PVCN) and have large dendritic fields oriented perpendicular to the trajectory of auditory nerve fibers such that it receives terminals from many auditory nerve fibers with a wide range of best frequencies. Responses are characterized by precise firing at stimulus onset with broad tuning curves (i.e., capable of receiving a broad frequency range and sensitive to changes only with quick adaptation). Its axons project mainly contralaterally to the superior olivary complex and nuclei of lateral lemniscus.

Fusiform Cells
Fusiform cells are found in the dorsal cochlear nucleus (DCN) and have large dendritic fields oriented in parallel to the trajectory of auditory nerve fibers. They have small bouton terminals from auditory nerve fibers and from many terminals of interneurons and descending pathways. As a result, its response patter is very different from the auditory nerve because it includes much lateral inhibition and other descending pathway modulation. Its axons mainly project contralaterally to the inferior colliculus.

9. Describe the connections of the superior olivary complex that are involved in spatial localization of sound source and explain how sounds can be localized by the functions of these connections.

There are three parts to the superior olivary complex: the lateral superior olivary nucleus (LSO), medial superior olivary nucleus (MSO), and the medial nucleus of the trapezoid body (MNTB). The convergence of pathways from both cochlear nuclei is involved in the coding of sound location in space.

Low Frequency
Excitatory fibers from spherical bushy cells of the AVCN project bilaterally to the dendrites of the MSO neurons such that the left and right MSO neurons each receive excitatory signals from the left and right cochlear nuclei.

Excitation of the MSO neuron depends on special summation of inputs from both cochlear nuclei. Depending on the timing of arrival of sound at each ear, the time of arrival of action potentials to terminals of spherical bushy cells on the MSO neurons will differ. Largest response of MSO neurons occur when inputs from the two sides arrive simultaneously. Because of the difference in spatial input arrangements, different MSO neurons will be most active for different interaural time differences.

High Frequency
Excitatory fibers from spherical bushy cells of ipsilateral AVCN project to neurons of LSO. Excitatory projections from globular bushy cells of contralateral caudal AVCN project to neurons of the MNTB. Neurons of the MNTB send inhibitory fibers to ipsilateral LSO neurons, using glycine neurotransmitter. Excitation of LSO neuron depends on the balance of input from AVCN and MNTB. The more AVCN relative to MNTB input, the more active the LSO neuron. Sound directly lateral to one ear gives off a large response of neurons on the ispilateral side, but no response on the contralateral side.

10. Describe the neural components of the centrifugal pathway from the superior olivary complex to the cochlea.

The centrifugal pathway from the superior olivary complex to the cochlea is called the olivocochlear bundle (OCB) and functions as a negative feedback pathway. These fibers originate from the superior olivary complex and terminate on auditory nerve fibers under inner hair cells and on outer hair cells. Activation of the OCB leads to acetylcholine release and decreased response in auditory nerve fibers.

11. State one key feature of the auditory function of each of the higher auditory centers: inferior colliculus, medial geniculate nucleus, and auditory cortex.

Inferior Colliculus
Located in the midbrain, the inferior colliculus receives converging input from cochlear nuclei, superior olivary complex, and nuclei of lateral lemniscus. It contains a detailed map of auditory space and tonotopic organization.

Medial Geniculate Nucleus
The medial geniculate is located in the thalamus and receives all information to be sent to the auditory cortex. It has more complex responses such as giving more attention to behaviorally significant sounds.

Auditory Cortex
Located in the superior part of the temporal lobe, the auditory cortex contains the primary auditory cortex and peripheral belt areas. Its major ascending inputs come from the medial geniculate. The primary auditory cortex contains orthogonal maps of frequency, and binaural interaction (alteration of EE stripes which are neurons excited by sound to either ear, and EI stripes which are neurons excited by sound to one ear and inhibited by sound to the other ear). The auditory cortex is also involved in speech processing.