The cochlear nuclear (CN) complex comprises two
cranial nerve nuclei
A cranial nerve nucleus is a collection of neurons (gray matter) in the brain stem that is associated with one or more of the cranial nerves. Axons carrying information to and from the cranial nerves form a synapse first at these nuclei. Lesions ...
in the human
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
, the
ventral cochlear nucleus
In the ventral cochlear nucleus (VCN), auditory nerve fibers enter the brain via the nerve root in the VCN. The ventral cochlear nucleus is divided into the anterior ventral (anteroventral) cochlear nucleus (AVCN) and the posterior ventral (poster ...
(VCN) and the
dorsal cochlear nucleus
The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum"), is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus (VCN), it forms the cochlear nucleus (CN), where all au ...
(DCN).
The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus is layered. Auditory nerve fibers, fibers that travel through the
auditory nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
(also known as the cochlear nerve or eighth cranial nerve) carry information from the inner ear, the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
, on the same side of the head, to the nerve root in the ventral cochlear nucleus.
At the nerve root the fibers branch to innervate the ventral cochlear nucleus and the deep layer of the dorsal cochlear nucleus. All acoustic information thus enters the brain through the cochlear nuclei, where the processing of acoustic information begins. The outputs from the cochlear nuclei are received in higher regions of the
auditory brainstem.
Structure
The cochlear nuclei (CN) are located at the dorso-lateral side of the
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
, spanning the junction of the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
and
medulla
Medulla or Medullary may refer to:
Science
* Medulla oblongata, a part of the brain stem
* Renal medulla, a part of the kidney
* Adrenal medulla, a part of the adrenal gland
* Medulla of ovary, a stroma in the center of the ovary
* Medulla of t ...
.
* The
ventral cochlear nucleus
In the ventral cochlear nucleus (VCN), auditory nerve fibers enter the brain via the nerve root in the VCN. The ventral cochlear nucleus is divided into the anterior ventral (anteroventral) cochlear nucleus (AVCN) and the posterior ventral (poster ...
(VCN) on the ventral aspect of the brain stem, ventrolateral to the
inferior peduncle
The upper part of the posterior district of the medulla oblongata is occupied by the inferior cerebellar peduncle, a thick rope-like strand situated between the lower part of the fourth ventricle and the roots of the glossopharyngeal and vagus nerv ...
.
* The
dorsal cochlear nucleus
The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum"), is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus (VCN), it forms the cochlear nucleus (CN), where all au ...
(DCN), also known as the
tuberculum acusticum or acoustic tubercle, curves over the VCN and wraps around the cerebellar peduncle.
** The VCN is further divided by the nerve root into the posteroventral cochlear nucleus (PVCN) and the anteroventral cochlear nucleus (AVCN).
Projections to the cochlear nuclei
The major input to the cochlear nucleus is from the auditory nerve, a part of cranial nerve VIII (the
vestibulocochlear nerve
The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the bra ...
). The auditory nerve fibers form a highly organized system of connections according to their peripheral innervation of the cochlea. Axons from the
spiral ganglion
The spiral (cochlear) ganglion is a group of neuron cell bodies in the modiolus, the conical central axis of the cochlea. These bipolar neurons innervate the hair cells of the organ of Corti. They project their axons to the ventral and dorsal co ...
cells of the lower frequencies innervate the ventrolateral portions of the ventral cochlear nucleus and lateral-ventral portions of the dorsal cochlear nucleus. The axons from the higher frequency
organ of corti
The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transd ...
hair cells
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment.
...
project to the dorsal portion of the ventral cochlear nucleus and the dorsal-medial portions of the dorsal cochlear nucleus. The mid frequency projections end up in between the two extremes; in this way the tonotopic organization that is established in the cochlea is preserved in the cochlear nuclei. This tonotopic organization is preserved because only a few inner hair cells synapse on the dendrites of a nerve cell in the spiral ganglion, and the axon from that nerve cell synapses on only a very few dendrites in the cochlear nucleus. In contrast with the VCN that receives all acoustic input from the auditory nerve, the DCN receives input not only from the auditory nerve but it also receives acoustic input from neurons in the VCN (T stellate cells). The DCN is therefore in a sense a second order sensory nucleus.
The cochlear nuclei have long been thought to receive input only from the ipsilateral ear. There is evidence, however, for stimulation from the contralateral ear via the contralateral CN, and also the somatosensory parts of the brain.
Projections from the cochlear nuclei
There are three major fiber bundles, axons of cochlear nuclear neurons, that carry information from the cochlear nuclei to targets that are mainly on the opposite side of the brain. Through the
medulla
Medulla or Medullary may refer to:
Science
* Medulla oblongata, a part of the brain stem
* Renal medulla, a part of the kidney
* Adrenal medulla, a part of the adrenal gland
* Medulla of ovary, a stroma in the center of the ovary
* Medulla of t ...
, one projection goes to the contralateral
superior olivary complex
The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system. The SO ...
(SOC) via the
trapezoid body
The trapezoid body (the ventral acoustic stria) is part of the auditory pathway where some of the axons coming from the cochlear nucleus (specifically, the anterior cochlear nucleus) decussate (cross over) to the other side before traveling on to ...
, whilst the other half shoots to the ipsilateral SOC. This pathway is called the ventral acoustic stria (VAS or, more commonly, the trapezoid body). Another pathway, called the dorsal acoustic stria (DAS, also known as the stria of von Monakow), rises above the medulla into the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
where it hits the nuclei of the
lateral lemniscus
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 in ...
along with its kin, the intermediate acoustic stria (IAS, also known as the stria of Held). The IAS decussates across the medulla, before joining the ascending fibers in the contralateral lateral lemniscus. The lateral lemniscus contains cells of the nuclei of the lateral lemniscus, and in turn projects to the
inferior colliculus
The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The ...
. The inferior colliculus receives direct, monosynaptic projections from the superior olivary complex, the contralateral dorsal acoustic stria, some classes of stellate neurons of the VCN, as well as from the different nuclei of the lateral lemniscus.
Most of these inputs terminate in the inferior colliculus, although there are a few small projections that bypass the inferior colliculus and project to the medial geniculate, or other forebrain structures.
*
Medial superior olive
The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system. The S ...
(MSO) via trapezoid body (TB) – Ipsilateral and contralateral stimulation for low frequency sounds.
*
Lateral superior olive
The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system. The S ...
(LSO) directly and via TB – Ipsilateral stimulation for high frequency sounds.
*Medial nucleus of
trapezoid body
The trapezoid body (the ventral acoustic stria) is part of the auditory pathway where some of the axons coming from the cochlear nucleus (specifically, the anterior cochlear nucleus) decussate (cross over) to the other side before traveling on to ...
(MNTB) – Contralateral stimulation.
*
Inferior colliculus
The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The ...
– Contralateral stimulation.
*
Periolivary nuclei (PON) – Ipsilateral and contralateral stimulation.
*
Lateral lemniscus
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 in ...
(LL) and lemniscal nuclei (LN) – Ipsilateral and contralateral stimulation.
Histology
Three types of principal cells convey information out of the ventral cochlear nucleus: Bushy cells, stellate cells, and octopus cells.
* ''Bushy cells'' are found mainly in the anterior ventral cochlear nucleus (AVCN). These can be further divided into large spherical, small spherical and globular bushy cells, depending on their appearance, and also their location. Within the AVCN there is an area of large spherical cells; caudal to this are smaller spherical cells, and globular cells occupy the region around the nerve root. An important difference between these subtypes is that they project to differing targets in the superior olivary complex. Large spherical bushy cells project to the ipsilateral and contralateral medial superior olive. Globular bushy cells project to the contralateral medial nucleus of the trapezoid body, and small spherical bushy cells likely project to the lateral superior olive. They have a few (1-4) very short dendrites with numerous small branching, which cause it to resemble a “bush”. The bushy cells have specialized electrical properties that allow them to transmit timing information from the auditory nerve to more central areas of the auditory system. Because bushy cells receive input from multiple auditory nerve fibers that are tuned to similar frequencies, bushy cells can improve the precision of the timing information by in essence averaging out jitter in timing of the inputs. Bushy cells can also be inhibited by sounds adjacent to the frequency to which they are tuned, leading to even sharper tuning than seen in auditory nerve fibers. These cells are usually innervated only by a few auditory nerve fibres, which dominate its firing pattern. These afferent nerve fibres wrap their terminal branches around the entire soma, creating a large synapse onto the bushy cells, called an "
endbulb of Held". Therefore, a single unit recording of an electrically stimulated bushy neuron characteristically produces exactly one action potential and constitutes the primary response.
* ''Stellate cells'' (aka multipolar cells), have longer dendrites that lie parallel to fascicles of auditory nerve fibers. They are also called chopper cells, in reference to their ability to fire a regularly spaced train of action potentials for the duration of a tonal or noise stimulus. The chopping pattern is intrinsic to the
electrical excitability
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
of the stellate cell, and the firing rate depends on the strength of the auditory input more than on the frequency. Each stellate cell is narrowly tuned and has inhibitory sidebands, enabling the population of stellate cells to encode the spectrum of sounds, enhancing spectral peaks and valleys. These neurons provide acoustic input to the DCN.
* ''Octopus cells'' are found in a small region of the posterior ventral cochlear nucleus (PVCN). The distinguishing features of these cells are their long, thick and tentacle-shaped dendrites that typically emanate from one side of the cell body. Octopus cells produce an "Onset Response" to simple tonal stimuli. That is, they respond only at the onset of a broad-band stimulus. The octopus cells can fire with some of the highest temporal precision of any neuron in the brain. Electrical stimuli to the auditory nerve evoke a graded excitatory postsynaptic potential in the octopus cells. These EPSPs are very brief. The octopus cells are thought to be important for extracting timing information. It has been reported that these cells can respond to click trains at a rate of 800 Hz.
Two types of principal cells convey information out of the
dorsal cochlear nucleus
The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum"), is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus (VCN), it forms the cochlear nucleus (CN), where all au ...
(DCN) to the contralateral inferior colliculus. The principal cells receive two systems of inputs. Acoustic input comes to the deep layer through several paths. Excitatory acoustic input comes from auditory nerve fibers and also from stellate cells of the VCN. Acoustic input is also conveyed through inhibitory interneurons (tuberculoventral cells of the DCN and "wide band inhibitors" in the VCN). Through the outermost molecular layer, the DCN receives other types of sensory information, most importantly information about the location of the head and ears, through parallel fibers. This information is distributed through a cerebellar like circuit that also includes inhibitory interneurons.
* ''Fusiform cells'' (also known as ''pyramidal cells''). Fusiform cells integrate information through two tufts of dendrites, the apical dendrites receiving multisensory, excitatory and inhibitory input through the outermost molecular layer and the basal dendrites receiving excitatory and inhibitory acoustic input from the basal dendrites that extend into the deep layer. These neurons are thought to enable mammals to analyze the spectral cues that enable us to localize sounds in elevation and when we lose hearing in one ear.
* ''Giant cells'' also integrate inputs from the molecular and deep layers but input from the deep layer is predominant. It is unclear what their role is in hearing.
Function
The cochlear nuclear complex is the first integrative, or processing, stage in the
auditory system
The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, increasin ...
. Information is brought to the nuclei from the
ipsilateral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
via the
cochlear nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
. Several tasks are performed in the cochlear nuclei. By distributing acoustic input to multiple types of principal cells, the auditory pathway is subdivided into parallel ascending pathways, which can simultaneously extract different types of information. The cells of the ventral cochlear nucleus extract information that is carried by the auditory nerve in the timing of firing and in the pattern of activation of the population of auditory nerve fibers. The cells of the dorsal cochlear nucleus perform a non-linear spectral analysis and place that spectral analysis into the context of the location of the head, ears and shoulders and that separate expected, self-generated spectral cues from more interesting, unexpected spectral cues using input from the
auditory cortex
The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to ...
,
pontine nuclei
Pontine may refer to:
* Having to do with the pons, a structure located in the brain stem (from ''pons'', "bridge")
* Pontine Marshes, a region of Italy near Rome
* Pontine Islands
The Pontine Islands (, also ; it, Isole Ponziane ) are an ar ...
,
trigeminal ganglion
A trigeminal ganglion (or Gasserian ganglion, or semilunar ganglion, or Gasser's ganglion) is the sensory ganglion at the base of each of the two trigeminal nerves (CN V), occupying a cavity (Meckel's cave) in the dura mater, covering the trigemi ...
and nucleus,
dorsal column nuclei
In neuroanatomy, the dorsal column nuclei are a pair of nuclei in the dorsal columns in the brainstem. The name refers collectively to the cuneate nucleus and gracile nucleus, which are present at the bottom of the medulla oblongata. Both nucl ...
and the second
dorsal root ganglion
A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsa ...
. It is likely that these neurons help mammals to use spectral cues for orienting toward those sounds. The information is used by higher brainstem regions to achieve further computational objectives (such as sound source location or improvement in
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
). The inputs from these other areas of the brain probably play a role in sound localization.
In order to understand in more detail the specific functions of the cochlear nuclei it is first necessary to understand the way sound information is represented by the fibers of the
auditory nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
. Briefly, there are around 30,000 auditory nerve fibres in each of the two auditory nerves. Each fiber is an axon of a
spiral ganglion
The spiral (cochlear) ganglion is a group of neuron cell bodies in the modiolus, the conical central axis of the cochlea. These bipolar neurons innervate the hair cells of the organ of Corti. They project their axons to the ventral and dorsal co ...
cell that represents a particular frequency of sound, and a particular range of loudness. Information in each nerve fibre is represented by the rate of
action potentials
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...
as well as the particular timing of individual action potentials. The particular physiology and
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
* Morphology (archaeology), study of the shapes or forms of artifacts
* Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
of each cochlear nucleus cell type enhances different aspects of sound information.
See also
*
Auditory system
The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, increasin ...
*
Cochlear nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
*
Unipolar brush cell
Unipolar brush cells (UBCs) are a class of excitatory glutamatergic interneuron found in the granular layer of the cerebellar cortex and also in the granule cell domain of the cochlear nucleus.
Structure
The UBC has a round or oval cell body w ...
Additional images
File:Gray683.png, Dissection of brain-stem. Lateral view.
File:Gray696.png, The cranial nerve nuclei schematically represented; dorsal view. Motor nuclei in red; sensory in blue.
File:Gray698.png, Primary terminal nuclei of the afferent (sensory) cranial nerves schematically represented; lateral view.
File:Gray713.png, Scheme showing the course of the fibers of the lemniscus; medial lemniscus in blue, lateral in red.
File:Lower pons horizontal KB.svg, Cross section of lower pons showing the cochlear nucleus (#1) labeled at the top right.
References
External links
*
*
Medical research councilShore lab
{{Authority control
Cranial nerve nuclei
Auditory system
Medulla oblongata
Pons