Lecture 6: Audition

Question 1

Describe the auditory pathway

Answer 1

Primary afferents arise from hair cells in the organ of corti and project to the ipsilateral dorsal and ventral cochlear nuclei in the medulla via the cochlear nerve (cell bodies are contained in the spiral ganglion).

Neurons that synapse on the ventral cochlear nucleus project to:

• contralateral lateral and medial superior olive (via the trapezoid body)
• ipsilateral lateral and medial superior olive

Neurons arise from the ipsilateral lateral and medial superior olive and the ipsilateral dorsal cochlear nucleus and project to the inferior colliculus via the lateral lemniscus.

There are 3 nuclei in the inferior colliculus (central and dorsal/external)

• The central nucleus of the inferior colliculus projects to the ventral nucleus of the medial geniculate nucleus of the thalamus. Neurons that arise from the ventral nucleus of the MGN project to the primary auditory cortex (transverse temporal gyrus of heschel)
• The dorsal/external nuclei of the inferior colliculus project to the medial/dorsal nuclei of the MGN. Neurons that arise from the medial/dorsal nuclei of the MGN project to the secondary auditory cortex (planum temporale)

Question 2

What is the function of the superior olive?

Answer 2

The superior olive has two nuclei and is involved in sound localisation:

• the lateral superior olive which is involved in the intensity of sound
• the medial superior olive which is involved in the time difference of sound

It receives projections from the ipsilateral and contralateral cochlear nuclei (bilateral input for sound localisation) and projects to the inferior colliculus which has an auditory map of visual space.

Question 3

What is the function of the cochlear nuclei?

Answer 3

Enhance and preserve timing and intenisty information

Question 4

What is the function of the inferior colliculus?

Answer 4

The inferior colliculus is a nexus

• It integrates auditory information with other information (eg. emotion, somatosensation)
• Filters/discriminates auditory information (eg. dampening down of internal sounds like breathing)
• Involved in the auditory motor (startle) reflex
• Gives rise to two distinct pathways to the primary and secondary auditory cortices

Question 5

How many nuclei are there in the medial geniculate nucleus? What are their projections?

Answer 5

There are 3 nuclei (ventral and medial/dorsal)

• The ventral nucleus of the MGN recieves projections from the central nucleus of the inferior colliculus and projects to the primary auditory cortex (transverse temporal gyrus of heschel)
• The medial/dorsal nuclei recieve projections from the dorsal/external nuclei of the inferior colliculus and projects to the secondary auditory cortex

Question 6

How many nuclei are there in the superior olive? What are their projections and functions?

Answer 6

There are 2 nuclei involved in sound localisation - both nuclei compare inputs from both ears:

1. The lateral superior olive (involved in the intensity of sound)
   * recieves projections from ipsilateral and contralateral ventral cochlear nuclei
2. The medial superior olive (involved in the time difference of sound)
   * recieves projections from ipsilateral and contralateral ventral cochlear nuclei

Question 7

What is the function of the secondary auditory cortex?

Answer 7

The secondary auditory cortex or planum temporale is involved in understanding and relating sound

Question 8

what is the range of frequencies that we can hear?

Answer 8

We can hear frequencies between 20 Hz - 20,000 Hz

Question 9

What is tonotopy? Where are low and high frequencies represented? Use cochlear nuclei as an example of how this works.

Answer 9

Every part of the auditory system is mapped from low frequency to high frequency and different information from the same frequency project to different regions.

• Cochlear
  
  • Higher frequencies are conveyed from base of cochlear
  • Lower frequencies are represented at the tip of the cochlear
• Transverse temporal gyrus of Heschel
  
  • Higher frequencies are more caudal
  • Lower frequencies are more rostral

Cochlear Nuclei example
A particular cochlear nerve fibre has a frequency of 20,000 Hz, it synapses with a particular sheet of cells called the isofrequency strip (consisting of bushy cells, octopus and fusiform cells.

Each one of the cells extracts key information of that sound and project to different regions.

• Fusiform and octopus cells project to the inferior colliculus
• Bushy cells prokect to the superior olive

Question 10

What type of cells project from cochlear nuclei to the inferior colliculus?

Answer 10

Bushy cells and octopus cells

Question 11

What type of cells project from the ventral cochlear nucleus to the superior olive

Answer 11

Bushy cells

Question 12

How are sounds localised?

Answer 12

Sounds are localised in two ways

1. Intenisty (is it louder in one ear) - processed by the lateral superior olive
2. Time difference (which ear does it reach first?) - processed by the medial superior olive

Question 13

What are the three main types of deafness?

Answer 13

• Conductive deafness
  
  • problem with the mechanical workings of external/middle ear
• Senorineural deafness
  
  • problem with the neural circuitry of inner ear (organ of corti or cochlear nerve
  • often caused by age or excessive loud noise
• Pure word deafness (also known as Wernicke’s aphasia)
  
  • Caused by lesion in wernicke’s area (nothing wrong with auditory pathway)

Question 14

What causes unilateral deafness?

Answer 14

A lesion or damage to the inner, middle and external ear or cochlear nuclei.

Question 15

What causes bilateral deafness?

Answer 15

A lesion beyond cochlear nuclei, from superior olive upwards