Cochlear nuclei
Encyclopedia
The cochlear nuclei are two heterogeneous collections of neurons in the mammalian brainstem that receive input from the cochlear nerve
Cochlear nerve
The cochlear nerve is a nerve in the head that carries signals from the cochlea of the inner ear to the brain...

, which carry sound information from the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....

e. The outputs from the CN are to higher regions of the auditory brainstem.

Anatomy

The CN is located at the dorso-lateral side of the brainstem, spanning the junction of the pons and medulla.

Each CN can be anatomically divided into 2 regions:
  • (a) the dorsal cochlear nucleus
    Dorsal cochlear nucleus
    The dorsal cochlear nucleus , is a cortex-like structure on the dorso-lateral surface of the brainstem...

    (DCN), corresponding to the tuberculum acusticum on the dorso-lateral surface of the inferior peduncle; and
  • (b) the ventral or accessory cochlear nucleus, placed between the two divisions of the nerve, on the ventral aspect of the inferior peduncle.


The ventral cochlear nucleus is further divided 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
Vestibulocochlear nerve
The vestibulocochlear nerve is the eighth of twelve cranial nerves, and is responsible for transmitting sound and equilibrium information from the inner ear to the brain...

). The auditory nerve fibers form a highly organized system of connections according to their peripheral innervation of the cochlea. Axons from the spiral ganglion
Spiral ganglion
The spiral ganglion is the group of nerve cells that serve the sense of hearing by sending a representation of sound from the cochlea to the brain...

 cells of the lower frequencies innervate the lateral-ventral portions of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus. In contrast, the axons from the higher frequency organ of corti
Organ of Corti
The organ of Corti is the organ in the inner ear of mammals that contains auditory sensory cells, or "hair cells."The organ was named after the Italian anatomist Marquis Alfonso Giacomo Gaspare Corti , who conducted microscopic research of the mammaliean auditory system.-Structure and function:The...

 hair cells project to the dorsal portion of the anteroventral 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 frequency spectrum is preserved. In this way, the cochlear nuclei inherit the tone based organization of the cochleae. This so-called 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 synapes on only a very few dendrites in the cochlear 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 projections from the cochlear nuclei. Through the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

, one projection goes to the contralateral superior olivary complex (SOC) via the trapezoid body
Trapezoid body
The trapezoid body is part of the acoustic pathway. It is a bundle of fibers and cells in the pontine tegmentum. It consists of fibers arising from the ventral cochlear nucleus. A collection of nerve cells inside forms a trapezoid nucleus. The superior olivary nucleus is situated on the dorsal...

, whilst the other half shoots to the ipsilateral SOC. This projection is called the ventral acoustic stria (or, more commonly, the trapezoid body). Another projection, called the dorsal acoustic stria (DAS, also known as the stria of von Monakow), rises above the medulla into the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...

 where it hits the nucleus of the lateral lemniscus
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...

 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
Inferior colliculus
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex...

. 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.

All 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 (MSO) via Trapezoid Body (TB) – Ipsilateral and contralateral stimulation for low frequency sounds.

  • Lateral superior olive (LSO) directly and via TB – Ipsilateral stimulation for high frequency sounds.

  • Medial Nucleus of Trapezoid body
    Trapezoid body
    The trapezoid body is part of the acoustic pathway. It is a bundle of fibers and cells in the pontine tegmentum. It consists of fibers arising from the ventral cochlear nucleus. A collection of nerve cells inside forms a trapezoid nucleus. The superior olivary nucleus is situated on the dorsal...

     (MNTB) – Contralateral stimulation.

  • Inferior colliculus
    Inferior colliculus
    The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex...

     – Contralateral stimulation.

  • Periolivary nuclei (PON) – Ipsilateral and Contralateral stimulation.

  • Lateral lemniscus
    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...

     (LL) and Lemniscal Nuclei (LN) – Ipsilateral and Contralateral Stimulation.

Cell types & Physiology

There are four types of principal cells found in the cochlear nuclei: Bushy cells, stellate cells, octopus cells, and fusiform cells.
  • Bushy cells are found in the anterior ventral cochlear nucleus (AVCN). These can be further divided into 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. They have a few (1-4) very short dendrites with numerous small branching, which cause it to resemble a “bush”. The bushy cells are only found in the ventral portion of the AVCN itself. 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. Some bushy cells can even improve the precision of the timing information. Bushy cells have responses very similar to those in the auditory nerve. The primary difference is that spontaneous activity is decreased by stimulation by adjacent frequencies, therefore leading to an even sharper tuning curve than seen in auditory nerve cells. 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), morphologically, have a radial, star-like dendritic tree, which is where they get their name. 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 of the stellate cell, and the firing rate depends on the strength of the auditory input more than on the frequency.

  • 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 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 specific frequency or frequency range at higher amplitudes. The octopus cells can fire with some of the highest temporal precision of any neuron in the brain. Electrical stimuli to the auditory nerve has been shown to evoke a graded post synaptic potential in the octopus cells. These EPSP’s are very brief. The octopus cells are thought to be important extracting timing information. It has been reported that these cells can respond to click trains at a rate of 800 Hz.

  • Fusiform cells (also known as pyramidal cells) are found in the Dorsal Cochlear Nucleus (DCN). See the separate page concerning the DCN
    Dorsal cochlear nucleus
    The dorsal cochlear nucleus , is a cortex-like structure on the dorso-lateral surface of the brainstem...

    .


Neurotransmitters: There are four neurotransmitters responsible for transmission of neural impulses, namely, GABA
Gabâ
Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...

, Norepinephrine
Norepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

, Glutamate, and Acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

.

Function

The CN is the first relay station in the auditory system
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...

 and can be characterized as a point of divergence in the representation of auditory information. Information is brought to the CN from the ipsilateral cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....

 via the cochlear nerve
Cochlear nerve
The cochlear nerve is a nerve in the head that carries signals from the cochlea of the inner ear to the brain...

. In general, the cells of the cochlear nuclei tend to preserve or even enhance the timing information that is provided by the each fiber of the cochlear nerve. The information is used by higher brainstem regions to achieve computational objectives (such as sound source location or improvement in signal to noise ratio). The cochlear nucleus receives input from each spiral ganglion, but also receives input from other parts of the brain, such as auditory cortex, pontine nuclei
Pontine nuclei
The pontine nuclei are a part of the pons involved in motor activity. Corticopontine fibres carry information from the primary motor cortex to the ipsilateral pontine nucleus in the ventral pons, and the pontocerebellar projection then carries that information to the contralateral cerebellum via...

, trigeminal ganglion
Trigeminal ganglion
The trigeminal ganglion is a sensory ganglion of the trigeminal nerve that occupies a cavity in the dura mater, covering the trigeminal impression near the apex of the petrous part of the temporal bone.-Relations:It is somewhat crescentic in shape, with its convexity...

 and nucleus, dorsal column nuclei
Dorsal column nuclei
In neuroanatomy, the dorsal column nuclei are a pair of nuclei in the brainstem. The name refers collectively to the cuneate nucleus and gracile nucleus, which are present at the junction between the spinal cord and the medulla oblongata...

 and the second dorsal root ganglion
Dorsal root ganglion
In anatomy and neuroscience, a dorsal root ganglion is a nodule on a dorsal root that contains cell bodies of neurons in afferent spinal nerves.-Unique unipolar structure:...

. 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 nucleus it is first necessary to understand the way sound information is represented by the fibers of the auditory nerve. 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
Spiral ganglion
The spiral ganglion is the group of nerve cells that serve the sense of hearing by sending a representation of sound from the cochlea to the brain...

 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 as well as the particular timing of individual action potentials. The particular physiology and morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

of each cochlear nuclei cell type enhances different aspects of sound information.

External links

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