Transcript Neural pathways

Lecture 10
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◦ By far largest division
◦ Involved in thought, perception, higher functions ...
◦ Entirely covered by sheets of gray matter – cerebral
cortex
◦ Gyri (bulges) & sulci (grooves) (deep sulci – fissures)
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Cerebellum
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Brainstem
◦ Co-ordinates muscle activity, processes information
from vestibular system, …
◦ Continuous with top of spinal cord
◦ Divided into midbrain, pons, medulla
◦ Contains tegmentum (part of the mid brain)– central
core (continuous within all three divisions)
◦ ... and non-tegmental portions – various different
structures attached to tegmentum
◦ Central attachment of most cranial nerves
 Nuclei involved in initial stages of processing sensory input,
many reflex arcs
 NVIII attaches in area where cerebellum, pons & medulla
meet – cerebello-pountine angle
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NVIII passes from cranial cavity to cochlea
through canal in temporal bone – internal
auditory meatus
The internal auditory meatus is a canal in
the petrous part of the temporal bone of
the skull, on each side, and serves as the
passageway for the cranial nerves,
namely cranial nerve VII and cranial nerve VIII,
and for the labyrinthine artery, between
the middle and inner ear.
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Fibres form cochlear division of NVIII
Cell bodies form spiral ganglion within
modiolus (Rosenthal’s canal)
Terminate centrally in cochlear nucleus in
brainstem (ipsilaterally only)
Approximately 30,000 in man (in each ear)
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90-95% innervate IHCs (‘true receptor cells’) –
‘Type I’ auditory neurons
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Relatively large diameter
Bipolar sensory neuron
Myelinated (both processes as well as cell body)
‘Inner radial fibres’ within organ of Corti
Each IHC connects with ~ 20 fibres
But each fibre innervates only one IHC
Innervates closest to point of entry
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A bipolar cell is a type of neuron which has
two extensions. Bipolar cells are specialized
sensory neurons for the transmission of
special senses. As such, they are part of the
sensory pathways for smell, sight, taste,
hearing and vestibular functions.
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Remaining 5-10% innervate OHCs– ‘Type II’
neurons
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Relatively small diameter
Monopolar (‘pseudomonopolar’) (Fig 5)
Not myelinated
‘Outer spiral fibres’ within organ of Corti
Each OHC connects with ~ 6 fibres
Each fibre innervates ~ 10 OHCs
Crosses tunnel of Corti, runs basally for ~ 0.6 mm
before innervating OHCs
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All reported data are from Type I cells
Individual neurons exhibit spontaneous firing
(action potentials) – spontaneous rate
Presentation of stimulus – increases firing
rate (See Fig 7)
◦ Primary afferents always excitatory (on the whole)
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The afferent
innervations to
the organ of corti.
Notice that the
type I auditory
neurons in the
spiral ganglion
continue in the
organ of Corti as
inner radial fibers
to inner hair cells.
Type II auditory
neurons continue
as outer spiral
fibers to outer
hair cells .
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A) afferent and
efferent
innervation of the
organ of Corti.
Efferent fibers are
shown in black.
B) arrangement of
type I and type II
afferent auditory
nerve fibers to
inner and outer
hair cells.
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Brainstem nuclei
◦ Cochlear nucleus
◦ Superior olivary complex
 Binaural processing
◦ Inferior colliculus
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Medial geniculate body
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Located in _______________
◦ Spans junction of pons and medulla
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First ‘relay station’ in auditory nervous system
Receives ipsilateral afferent input only
Three distinct anatomical divisions
◦ Antero-Ventral Cochlear Nucleus (AVCN)
◦ Postero-Ventral CN (PVCN)
◦ Dorsal CN (DCN)
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Each primary (‘first order’) afferent bifurcates
twice at CN
◦ One branch from first bifurcation to AVCN
◦ Other branch bifurcates again to innervate both
PVCN and DCN
◦ Thus each primary fibre innervates all three
divisions of CN
◦ Each branch may synapse with several CN (‘second
order’) neurons
◦ Each CN neuron may receive information from one
or more primary neurons
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Each CN division exhibits cochleotopic
organisation
Various anatomical cell types described – e.g.
bushy (spherical and globular), octopus,
multipolar, stellate cells
Distribution of each varies in different
divisions
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Predominant cell type – bushy cells (1 or 2
profusely branching dendrites)
◦ Primary afferent terminates in large nerve ending that
envelopes cell body – endbulb of Held
 Allows for ‘one-to-one’ transmission of action potentials
◦ Neural responses (firing patterns) of these cells are
very much like Type I primary afferents
◦ Cells used to be regarded as simple ‘relays’, no real
processing of afferent input
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AVCN also contains multipolar cells
◦ Several profusely branching dendrites, irregularly
shaped cell bodies
◦ More complex firing patterns than bushy cells
◦ Sensitive to changes in acoustic stimuli
◦ In particular, onset and offset of sounds, as well as
changes in intensity, frequency
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Axons of both cell types leave AVCN as large
tract – ventral acoustic stria (becomes
trapezoid body further along)
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Predominant cell type – octopus cells (C in Fig
1)
◦ Large cell bodies with thick dendrites extending
from one side of cell body only
◦ Cells receive input from several adjacent Type I
primary afferents
◦ Cells therefore sensitive to bands of frequencies
◦ Axons leave PVCN as intermediate acoustic stria
(also called stria of Held)
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Also contains multipolar cells, similar to those
in AVCN described above
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Contains stellate cells exclusively (in human)
◦ Many dendrites, in star-shaped arrangement
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Function unknown
Axons leave DCN as dorsal acoustic stria or
stria of Monakow
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Group of nuclei in pons
Receives input from cochlear nuclear
complexes (primarily AVCN) on both sides
◦ First stage in auditory pathway to receive binaural
input
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Three major nuclei, surrounded by smaller,
more diffuse nuclei
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Largest component of SOC (in humans)
Each neuron receives input from left and right
AVCN (from low-frequency fibres)
Axons project to higher centres via ipsilateral
lateral lemniscus tract
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MSO neurons are sensitive to difference in
arrival time of sound at each ear (ITD)
Mechanism is thought to be along lines of on
‘coincidence detection’ model first proposed
by Jeffress
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Jeffress’ ‘coincidence detection’ model of lowfrequency sound localisation
Delay lines
Coincidence
detectors
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ITD
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Response
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ITD
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Response
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Smallest component of SOC
Neurons receive input from contralateral
AVCN only (high-frequency fibres)
Axons terminate in ipsilateral LSO
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Each neuron receives input from ipsilateral
AVCN (high-frequency fibres) and ipsilateral
MNTB
◦ Thus – binaural, high-frequency input
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Axons contribute to lateral lemnisci on both
sides
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LSO neurons are sensitive to interaural
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(Neurons also code for horizontal sound
localisation, but for high frequencies)
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Smaller groups of nuclei surrounding the
three main nuclei
Include cell bodies of olivocochlear bundle
(OCB) – provide efferent innervation of
cochlea
Medial and lateral groups (in vicinity of MSO
and LSO respectively)
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Axons from both MSO and LSO also project
bilaterally to nuclei of N VII (________ nerve),
also within pons
Synapse with neurons that innervate
(ipsilateral) stapedius muscle
SOC therefore important stage in stapedius
reflex loop
◦ Note existence of an additional pathway between
CN and (ipsilateral) facial nucleus
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LL is the major tract of the auditory brainstem
The lateral lemniscus is a tract of axons in
the brainstem that carries information about
sound from the cochlear nucleus to various
brainstem nuclei and ultimately the
contralateral inferior colliculus of
the midbrain. Three distinct, primarily
inhibitory, cellular groups are located
interspersed within these fibers, and are thus
named the nuclei of the lateral lemniscus
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Group of nuclei located in midbrain
Major nucleus is central nucleus of inferior
colliculus (CNIC)
◦ Largest brainstem auditory nucleus
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Also contains pericentral nucleus and
external nucleus
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Vast majority of axons forming lateral
lemniscus terminate in ipsilateral CNIC
◦ Some via additional synapses at the interstitial
nuclei within tract
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A few lateral lemniscus axons terminate in
contralateral CNIC (via commissure of Probst)
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Most axons of CNIC cells form brachium of IC,
which leaves brainstem to travel to ipsilateral
thalamus
A few CNIC axons cross midline (commissure
of IC), then either
◦ Synapse on cells within contralateral CNIC; or
◦ Pass through contralateral CNIC to join
(contralateral) brachium of IC
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All the major ascending pathways (crossed
and uncrossed) converge here (see Fig 4)
◦ (Pathways then diverge again – somewhat unique
organisation amongst sensory systems)
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Lower centres extract different features of
acoustic signal
◦ e.g. frequencies, frequency bands, onsets, offsets,
changes in intensity, localisation ...
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Information needs to be integrated as
different aspects of same acoustic signal
Integration (‘synthesis’) thought to
commence in CNIC
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CNIC also processes binaural information
independently of lower stages
◦ Appears to
◦ Enhance sensitivity to ITD, IID demonstrated by SOC
neurons
◦ Extract more sophisticated information from
binaural input
 e.g. change in ITD, possibly encoding motion (rather
than location) of a sound source
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Auditory portion of diencephalon
Complex group of nuclei within thalamus
Receives input from IC, processes and relays to
cerebral cortex (via internal capsule)
Three divisions – ventral, dorsal, medial
◦ Ventral largely specific to auditory information
◦ Dorsal and medial divisions less so
 Receive information from non-auditory pathways (as well
as CNIC)
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Most fibres of brachium of IC terminate here
Receives & further processes detailed
auditory information, e.g. on location,
onset/offset, frequency, intensity, …
Axons project to primary auditory cortex
Ventral MGB also receives considerable input
from primary auditory cortex
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Axons project mainly to association auditory
cortices
Proposed function in directing and
maintaining attention
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Also receives input from vestibular,
somesthetic, visual systems
◦ Regarded as a ‘multimodal’ nucleus, rather than
purely auditory
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Axons project to both auditory & nonauditory cortices
Also receives axons from widespread areas of
cerebral cortex
Proposed function – multi-sensory
arousal/attention system
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