Download Central Auditory Pathways

CSD 3103
anatomy of speech and hearing mechanisms
Hearing mechanisms
Fall 2008
Central Pathways
Cells of the nervous
system

The primary cell of the nervous system is the
neuron

Non-replaceable
Typical neuron
 Important Parts
Cell body
Dendrites
Axon
Neuron specialization
The three major types of neurons, depending on
their specialization:
Sensory Neurons
Motor Neurons
Interneurons
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
Neuron classification
by structure
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
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
The viii. Cranial nerve
(vetibulocochlear)
Sensory neuron
Acoustic Portion
Vestibular Portion
The central auditory
pathways
Landmarks:
Auditory nerve
Cochlear nucleus
Superior olivary complex
Lateral lemniscus
Inferior colliculus
Medial geniculate body
Auditory cortex
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
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 auditory nerve
Evidence of
tonotopic
organization
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 cochlear nucleus
Two major parts
Dorsal and ventral
Tonotopic organization
“must-synapse” station--second
order fibers
Preserves, but does not enhance,
information received from the
auditory nerve
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 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 lateral lemniscus
Highway of axons
that arise from
the SOC and
terminate in
the midbrain
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
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
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 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
Pathways of neurons projecting
from the medial geniculate
to cortical areas
The human brainstem
Structures:
Cochlear nuclei (1)
Lateral lemnisci (2)
Inferior colliculi (3)
Superior colliculi (4)
Medial geniculates (6)
Auditory thalmi (7)
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
Central Auditory Pathways
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
Behavior of the
auditory nerve
This figure shows how the single neuron responses are tied to
the temporal characteristics of the signal
“phase-locking”
Input-output
function
Input-output function and
histogram
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.
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
Single fiber tuning
curve
Portions of response areas for each of three cochlear
neurons with CFs of 100, 1000, and 10,000 Hz
Single fiber tuning
curves of many
frequencies
Tuning curves of auditory neurons with a variety of
characteristic frequencies
Post-stimulus time
histogram
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.