1. CSD 3103 anatomy of speech and hearing mechanisms Hearing mechanisms Fall 2008
Central Pathways

2. Cells of the nervous system
The primary cell of the nervous system is the neuron
Non-replaceable
Before we look at the central pathways, we need to discuss the cells of the nervous system in general, and their characteristics
The neuron is the primary cell of the nervous system.
One important characteristics of nerve cells is that that can’t (at this time) be replaced.

3. Typical neuron Important Parts Cell body Dendrites Axon
Fig 7-1 shows a typical neuron--they come in varying shapes and sizes
Important parts
• cell body
• projections or nerve fibers--primary means of communication
- dendrites--short projections that conduct nerve impulses in the direction of the cell body
- axon--a single long process that conducts the nerve impulse to other neurons

4. Neuron specialization
The three major types of neurons, depending on their specialization:
Sensory Neurons
Motor Neurons
Interneurons
There are 3 types of neurons, depending on their specialization:
• sensory neurons
• motor neurons
• interneurons

5. Sensory neurons
Sensory neurons conduct nerve impulses from the ear and deliver sensory information to the brain for processing and interpretation
Afferent refers to this direction of travel and this kind of pathway or system
The type of neurons making up the central auditory pathways are sensory.
These neurons conduct nerve impulses from the ear and deliver sensory info to the brain for processing and interpretation.
In general, sensory impulses travel from the periphery to more central structures of the nervous system
Afferent refers to this direction of travel and this kind of pathway or system.

6. Neuron classification by structure
Neurons are alos classified by their structure:
• monopolar
• bipoloar
• multipolar
Fig 7-2 shows examples
Most of what’s in the CAP are bipolar and multipolar

7. How neurons communicate
Communication between neurons is achieved by the release of small packets of neurotransmitters into the synapse
If the release of neurotransmitters reaches a critical level to the receiving neuron, it will cause an action potential to be generated in the cell body
“All-or-none” behavior
How neurons communicate:
communication between neurons is achieved by the release of small packets of chemicals into the synapse (space between 2 neurons)
The chemicals are called neurotransmitters.
If the release of neurotransmitters reaches a critical level to the receiving neuron, it will cause an action potential to be generated in the cell body.

8. How neurons communicate
The action potential is an electrical event
The action potential travels down the axon to reach another neural cell body
Neurotransmitters are released at the synapse and the process is repeated in a new neuron
This is an electrical event--and the action potential travels down the axon to reach another
neural cell body
Neurotransmitters are released at the synapse and the process starts over in a new neuron.

9. The viii. Cranial nerve (vetibulocochlear)
Sensory neuron
Acoustic Portion
Vestibular Portion
The VIII cranial nerve (vestibulocochlear) is entirely sensory and has 2 parts:
• the acoustic part--transmits auditory impulses from the cochlea
• the vestibular part--transmits vestibular impulses from the sensory cells of the vestibular system (saccule, utricle, ampullae of semicircular canals, etc..).
Fig 2.11 shows the first order neurons leaving the inner ear. You can see the major branch of the vestibular portion of the VIII N. Joining the auditory portion of the nerve. Notice where the facial nerve is as well.
We will only follow the auditory portion of the VIII N.

10. The central auditory pathways
Landmarks:
Auditory nerve
Cochlear nucleus
Superior olivary complex
Lateral lemniscus
Inferior colliculus
Medial geniculate body
Auditory cortex
Fig 14-1 shows a schematic of the whole pathway.
Without exception, every anatomical structure on one side of the brain has an identical structure on the opposite side.

11. The auditory nerve VIII cranial nerve Bipolar neurons
Cell bodies from these neurons lie right outside the cochlea and form the spiral ganglion
One end innervates the individual inner and outer HCs of the cochlea and the other end synapses with the neurons of the cochlear nucleus
These first order neurons are bipolar. The collection of their cell bodies form the spiral ganglion. The second set of axons make their way and synapse on the cochlear nucleus.

12. The auditory nerve Auditory Portion--30,000 fibers
Vestibular Portion--20,000 fibers
Evidence of tonotopic organization
Spiral Ganglion
Cell bodies of the first order neurons
Bipolar
Cerebello-Pontine angle--where the cerebellum, medulla oblongota and pons meet
The major structures:
• auditory nerve--fibers pass from the modiolus of the cochlear thru the internal auditory canal. This IAC also carries the vestibular portion of the VIII N. There are about 30K separate fibers from the auditory portion and about 20k from the vestibular portion. Talk about the spiraling characteristic of the auditory nerve within the IAC and tonotopic organization (low freqs inside, high freqs outside).
- spiral ganglion--these are the cell bodies of the first order neurons of the auditory portion of the VIII N. These single fibers are bipolar neurons.
The auditory nerve extends mm beyond the IAC, where it attaches to the brainstem where the cerebellum, medulla oblongata and pons (all parts of the CNS) join to form the cerebello-pontine angle. It is here where the auditory and vestibular portions of the VIII N separate. One part of the cochlear bundle descends to the dorsal cochlear nucleus, and the other ascends to the ventral cochlear nucleus.

13. The auditory nerve
Evidence of tonotopic organization

14. The auditory nerve
The individual fibers pass from the modiolus of the cochlea through the internal auditory meatus, which exits at the base of the brain
The IAM also carries fibers from the utricle, saccule, and semicircular canals that form the vestibular portion of the VIII nerve
The vestibular and auditory portions of the VIII N. separate at the cerebellopontine angle
The branch of the facial nerve that courses through the middle ear also exits here
the auditory nerve (eighth cranial nerve)--these fibers, which innervate individual hair cells pass from the modiolus of the cochlea thru the internal auditory canal, which exits at the base of the brain. The IAC also carries the fibers from the utricle, saccule and semicircular canals and form the vestibular portion of the eighth nerve. Talk about how the cochlear fibers are arrayed as they exit the cochlea and define tonotopic organization (low freqs in the center, high freqs on top). Vestibular portion and auditory portion of the eighth nerve separate at the level of the cerebellopontine angle.

15. The cochlear nucleus Two major parts Tonotopic organization
Dorsal and ventral
Tonotopic organization
“must-synapse” station--second order fibers
Preserves, but does not enhance, information received from the auditory nerve
The Cochlear Nucleus--Each division of the cochlear nucleus is organized tonotopically. The nucleus is made up of a variety of different cell types. It’s thought that the cochlear nucleus preserves, but doesn’t enhance info it receives from the auditory nerve. All fibers arising from the cochlea synapse here.

16. Superior Olivary Complex
Most of the fibers from the cochlear nuclei cross and project to the contralateral SOC
Plays a role in the acoustic reflex
Analyzes intensity and time-of-arrival differences between the two ears to help with localization tasks
The Superior Olivary Complex--Most of the fibers from the cochlear nucleus project to the SOC. Most fibers from one ear cross and synapse on the contralateral SOC.
The SOC plays a role in the acoustic reflex (as we have discussed) and analyzing intensity and time of arrival differences between the 2 ears for localization

17. The superior olivery complex
Most (about 80%) of the fibers from the cochlear nuclei cross and project to the contralateral SOC via the trapezoid body
Evidence of tonotopic organization
Plays a major role in the acoustic reflex
Analyzes intensity and time-of-arrival differences between the two ears to help with localization/lateralization tasks
The Superior Olivary Complex--Most of the fibers from the cochlear nucleus project to the SOC. Most fibers from one ear cross and synapse on the contralateral SOC.
The SOC plays a role in the acoustic reflex (as we have discussed) and analyzing intensity and time of arrival differences between the 2 ears for localization

18. The lateral lemniscus
Highway of axons that arise from the SOC and terminate in the midbrain
Lateral lemniscus—major brainstem pathway for the aud system. There are some nuclei, but most the LL is a pathway formed by fibers arising from the contralateral cochlear nucleus that combine with fibers running from the ipsilateral SOC. About 3mm in length, so this is a large tract

19. The lateral lemniscus Tonotopic organization is evident
Nuclei within the lateral lemniscus have a large proportion of cells that are sensitive to interaural time differences, binaural input, and interaural intensity differences
Lateral lemniscus—major brainstem pathway for the aud system. There are some nuclei, but most the LL is a pathway formed by fibers arising from the contralateral cochlear nucleus that combine with fibers running from the ipsilateral SOC. About 3mm in length, so this is a large tract
View shows upper pons and lower midbrain. 2 represents the LL tract terminating to the inferior colliculi (1). 6 is the thalamus

20. The inferior colliculus
“must-synapse” station at the level of the midbrain
Highly tonotopic
First evidence of neurons that are sensitive to sound duration
Active in binaural processing
The inferior colliculus is a key auditory region in the midbrain and is an obligatory connection for fibers arising from lower brainstem structures.
It’s also the largest structure in the auditory brainstem pathway

21. Auditory Cortex
Areas of auditory reception are in the temproal lobes on both sides of the cerebral cortex in an area called the superior temporal gyrus or Heschl’s gyrus
The Auditory Cortex--the areas of auditory reception are in the temporal lobes on both sides of the cerebral cortex in an area called the superior temporal gyrus or Heschl’s gyrus.
At one time it was believed that the cortex was the only center of auditory discrim. It’s now known that many discrims may be mediated subcortically. Perceptions of pitch and loudness can be maintained without the cortex.

22. The medial geniculate body
Located in the auditory thalamus
Last subcortical relay in the pathway
Evidence of tonotopic organization
Very active in localization and lateralization
The Medial Geniculate Body--located in the auditory thalamus is the last subcortical relay station of the pathway. At this point, nerve fibers fan out as the auditory radiations and ascend to the cortex
We’re not sure how information is organized and represented at this level.

23. The medial geniculate body
Pathways of neurons projecting from the medial geniculate to cortical areas
Fibers from the MGB fan out into radiations to inervate the cortex in the auditory areas

24. The human brainstem Structures: Cochlear nuclei (1)
Lateral lemnisci (2)
Inferior colliculi (3)
Superior colliculi (4)
Medial geniculates (6)
Auditory thalmi (7)
Fibers from the MGB fan out into radiations to inervate the cortex in the auditory areas

25. The auditory cortex
Areas of auditory reception are in the temproal lobes on both sides of the cerebral cortex
The Sylvian or lateral fissure is the focal point
Primary and secondary auditory areas are above and below this point
At one time it was believed that the cortex was the only center of auditory discrim. It’s now known that many discrims may be mediated subcortically. Perceptions of pitch and loudness can be maintained without the cortex.

26. Central Auditory Pathways
Fig 17-1 is another diagram of the auditory pathways.

27. The corpus callosum
Large fiber tract that connects the two hemispheres of the brain
Allows information (like auditory) to be transferred from one side of the brain to the other
Very important for normal dichotic listening and pitch pattern perception

28. Behavior of the auditory nerve
look at neural responses of the auditory system. How is information coded here??
We know that a certain region of the cochlea is innervated by a single auditory nerve fiber. This fiber makes its way out of the cochlea. What kind of info does it carry??
let’s imagine we were able to put an electrode on just one of these fibers and measure its electrical activity---observe when it fires
One thing we would notice is that even in the absence of any auditory input, the nerve fiber would be active
randomly delivering APs or discharging---key word here is random.
the discharge patterns would not be periodic, nor would they be tied to any auditory event.
If we observed this fiber for a period of time and counted up the number of random firings, we could calculate a spontaneous discharge rate
think of this as the resting activity level of the nerve fiber.
Let’s pretend that for our imaginary fiber, the spontaneous discharge rate is 75 spikes/s
What do you think will happen to the fiber in response to a sine wave it can respond to?
Fig shows how the single neuron responses are tied to the characteristics (temporal) of the signal. Refer to this as “phase locking”.
This figure shows how the single neuron responses are tied to the temporal characteristics of the signal
“phase-locking”

29. Input-output function
discharge rate will increase above 75 spikes/s and will be tied to the presentation of the signal.
as the intensity of the signal increases, the discharge rate will increase.
Fig 6-120

30. Input-output function and histogram
Fig 2-41 shows the same input-output function of a single fiber. Point out dynamic range of fiber and saturation
This figure shows the input-output function of a single fiber. Notice that the firing rate of the neuron increases as the stimulus intensity increases within its dynamic range, and eventually plateaus.

31. Single fiber tuning curve
Characteristic frequency (tip)
Tail
Difference in slope between the low frequency and high frequency sides
Shaded area is the response area
After observing the change in discharge rate from our one fiber in response to increasing the intensity of our pure tone or sine wave, we are going to arbitrarily decide that 100 spikes/s represents the threshold level of responding for this fiber.
in other words, when the discharge rate is 100 spike/s or higher, the fiber has been “captured” by a pure tone
if the discharge rate is less than 100 spikes/s, then the fiber is not responding to the tone
Now we’re going to do a little experiment.
We’re going to present a variety of different pure tones to the ear of this system.
with each pure tone we present, we will keep an eye on the discharge rate of the fiber.
for each pure tone presented, we will find the intensity of the pure tone that will give us a discharge rate of 100 spikes/s---criterion discharge rate
We’ll do this running through a range of frequencies from Hz
Plot the response---single fiber tuning curve
dB SPL for criterion discharge rate as a function of frequency
point out tip (characteristic frequency of fiber)
point out low frequency tail
make high frequency slope very steep
fig 8.5
tuning curve of auditory fiber with cf=10,000 Hz
important points:
one fiber has the capability of responding and providing info about a wide range of frequencies---if the intensity is high enough
each fiber responds “best” to a very narrow range of frequencies.
shaded area:
if frequency and intensity of pure tone falls within the response region, the fiber will respond at or above criterion discharge rate
if frequency and intensity of pure tone falls outside of the response region, the fiber will not respond.

32. Single fiber tuning curve
Fig 8.6: tuning curves of fibers with 3 center frequencies
Portions of response areas for each of three cochlear neurons with CFs of 100, 1000, and 10,000 Hz

33. Single fiber tuning curves of many frequencies
Fig 2-42 tuning curves of neurons with a variety of cfs.
Discuss issue of tonotopic organization
certain regions of the BM vibrate with greatest displacement depending on frequency
cfs of fibers represent an orderly display along length of
BM--fibers with high cf are near base, fibers with low cf are near apex
as fibers exit cochlea, high frequencies are on top, low frequencies are inside.
This kind of organization continues all the way up.
How else is information about a pure tone coded at the peripheral neural levels?
Tuning curves of auditory neurons with a variety of characteristic frequencies

34. Post-stimulus time histogram
Here’s the situation:
we’ve isolated a single fiber auditory neuron with cf of 1600 Hz.
we present this fiber with a 1000 Hz pure tone at an intensity high enough to be within the fiber’s response area.
the tone will be on for 1 s.
during the presentation of the tone, we are going to keep track of when, after the onset of the tone the fiber responds, each time it responds
keep track of the length of time between one AP to the next (intervals)
We’re going to repeat this many times and develop a histogram of how many times the fiber responds to the tone and what the interval duration is from the last firing.
number of times as a function of duration of intervals
PST (post stimulus time) histograms
what we find is that some durational units are more likely to
have events in them than other durational units
there seem to be some “prime times” when the fiber will jump in and start firing.
the time between these high probability times is equal to the period of the stimulating pure tone.
go through fig 8.15 to illustrate---phase locking behavior of neural fibers. This will come up again.

35. Pst histograms
Selected histograms showing that neural firing during a pure tone is timed to the period of the tone. The dots along each x-axis correspond to the multiples of the period of the tone.
Fig 2-44
single fiber with cf=1600 Hz
pure tones of 412 Hz, 600 Hz, 900 Hz, 1000 Hz
presented all at 80 dB SPL for 1 s
Notice that the overall number of spikes increases as you get closer to the fiber’s CF
Implications: single fiber with cf not equal to stimulating tone can provide frequency (period) info about stimulating tone
system looks at which fiber is firing maximally as well as temporal characteristics of discharge of other fibers.