Cochlear nerve

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Cochlear nerve
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Diagrammatic longitudinal section of the cochlea. (Cochlear nerve is in center, shown as striped.)
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Part of the cochlear division of the acoustic nerve, highly magnified.
Details
From Vestibulocochlear nerve
Identifiers
Latin nervus cochlearis
MeSH D003056
TA98 A14.2.01.133
TA2 6318
FMA 53431
Anatomical terms of neuroanatomy

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 from the cochlea of the inner ear directly to the brain. The other portion of the vestibulocochlear nerve is the vestibular nerve, which carries spatial orientation information to the brain from the semicircular canals, also known as semicircular ducts. [1]

Contents

Anatomy and connections

In terms of anatomy, an auditory nerve fiber is either bipolar or unipolar, with its distal projection being called the peripheral process, and its proximal projection being called the axon; these two projections are also known as the "peripheral axon" and the "central axon", respectively. The peripheral process is sometimes referred to as a dendrite, although that term is somewhat inaccurate. Unlike the typical dendrite, the peripheral process generates and conducts action potentials, which then "jump" across the cell body (or soma) and continue to propagate along the central axon. In this respect, auditory nerve fibers are somewhat unusual in that action potentials pass through the soma. Both the peripheral process and the axon are myelinated.

In humans, there are on average 30,000 nerve fibers within the cochlear nerve. [2] The number of fibers varies significantly across species; the domestic cat, for example, has an average of 50,000 fibers. The peripheral axons of auditory nerve fibers form synaptic connections with the hair cells of the cochlea via ribbon synapses using the neurotransmitter glutamate. The central axons form synaptic connections with cells in the cochlear nucleus of the brainstem.

The cell bodies of the cochlear nerve lie within the cochlea and collectively form the spiral ganglion, named for the spiral shape it shares with the cochlea. These central axons exit the cochlea at its base and form a nerve trunk, which, in humans, is approximately one inch long. This travels in parallel with the vestibular nerves through the internal auditory canal, through which it connects to the brainstem. There, its fibers synapse with the cell bodies of the cochlear nucleus.

Types of neurons

In mammals, cochlear nerve fibers are classified as either type I or type II.

Cochlear nuclear complex

In mammals, the axons from each cochlear nerve terminate in the cochlear nuclear complex that is ipsilaterally located in the medulla of the brainstem. The cochlear nucleus is the first 'relay station' of the central auditory system and receives mainly ipsilateral afferent input.

The three major components of the cochlear nuclear complex are (see figure below):

Each of the three cochlear nuclei are organized to sort sound according to a specific spacial arrangement. As such, sound frequencies detected by the cochlea are transmitted electrically to specific positions in the cochlear nuclei. The axons from the low-frequency region of the cochlea project to the ventral portion of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus. The axons from the high-frequency region project to the dorsal portion of the anteroventral cochlear nucleus and the uppermost dorsal portions of the dorsal cochlear nucleus. The axons from the intermediate frequency region project to intermediate targets. Through this process, a spatial representation of sound is created by electrical nerve impulses through the cochlear complex.

See also

Related Research Articles

Articles related to anatomy include:

<span class="mw-page-title-main">Brainstem</span> Posterior part of the brain, adjoining and structurally continuous

The brainstem is the stalk-like part of the brain that connects the forebrain with the spinal cord. In the human brain, the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch.

<span class="mw-page-title-main">Vestibulocochlear nerve</span> Cranial nerve VIII, for hearing and balance

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 brain. Through olivocochlear fibers, it also transmits motor and modulatory information from the superior olivary complex in the brainstem to the cochlea.

<span class="mw-page-title-main">Saccule</span> Bed of sensory cells in the inner ear

The saccule is a bed of sensory cells in the inner ear. The saccule is from Latin saccus 'sack'. The saccule translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves vertically, the sensory cells of the saccule are disturbed and the neurons connected to them begin transmitting impulses to the brain. These impulses travel along the vestibular portion of the eighth cranial nerve to the vestibular nuclei in the brainstem.

<span class="mw-page-title-main">Auditory system</span> Sensory system used for hearing

The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs and the auditory parts of the sensory system.

<span class="mw-page-title-main">Lateral lemniscus</span> Brain structure

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.

<span class="mw-page-title-main">Hair cell</span> Auditory sensory receptor nerve 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.

<span class="mw-page-title-main">Inferior colliculus</span> Midbrain structure involved in the auditory pathway

The inferior colliculus (IC) 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 inferior colliculus has three subdivisions: the central nucleus, a dorsal cortex by which it is surrounded, and an external cortex which is located laterally. Its bimodal neurons are implicated in auditory-somatosensory interaction, receiving projections from somatosensory nuclei. This multisensory integration may underlie a filtering of self-effected sounds from vocalization, chewing, or respiration activities.

<span class="mw-page-title-main">Pseudounipolar neuron</span>

A pseudounipolar neuron is a type of neuron which has one extension from its cell body. This type of neuron contains an axon that has split into two branches. They develop embryologically as bipolar in shape, and are thus termed pseudounipolar instead of unipolar.

<span class="mw-page-title-main">Spinocerebellar tract</span> Nerve tract in humans

The spinocerebellar tract is a nerve tract originating in the spinal cord and terminating in the same side (ipsilateral) of the cerebellum.

<span class="mw-page-title-main">Vestibulospinal tract</span> Neural tract in the central nervous system

The vestibulospinal tract is a neural tract in the central nervous system. Specifically, it is a component of the extrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information from nuclei to motor neurons. The vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract. The function of these motor commands is to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head.

<span class="mw-page-title-main">Cochlear nucleus</span> Two cranial nerve nuclei of the human brainstem

The cochlear nucleus (CN) or cochlear nuclear complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN). The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus is layered. Auditory nerve fibers, fibers that travel through the auditory nerve carry information from the inner ear, the cochlea, 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.

<span class="mw-page-title-main">Superior olivary complex</span> Collection of brainstem nuclei related to hearing

The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that is located in pons, functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system. The SOC is intimately related to the trapezoid body: most of the cell groups of the SOC are dorsal to this axon bundle while a number of cell groups are embedded in the trapezoid body. Overall, the SOC displays a significant interspecies variation, being largest in bats and rodents and smaller in primates.

<span class="mw-page-title-main">Interaural time difference</span> Difference in time that it takes a sound to travel between two ears

The interaural time difference when concerning humans or animals, is the difference in arrival time of a sound between two ears. It is important in the localization of sounds, as it provides a cue to the direction or angle of the sound source from the head. If a signal arrives at the head from one side, the signal has further to travel to reach the far ear than the near ear. This pathlength difference results in a time difference between the sound's arrivals at the ears, which is detected and aids the process of identifying the direction of sound source.

<span class="mw-page-title-main">Spiral ganglion</span> Anatomical structure

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 cochlear nuclei as the cochlear nerve, a branch of the vestibulocochlear nerve.

<span class="mw-page-title-main">Alpha motor neuron</span> Large lower motor neurons of the brainstem and spinal cord

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

Binaural fusion or binaural integration is a cognitive process that involves the combination of different auditory information presented binaurally, or to each ear. In humans, this process is essential in understanding speech as one ear may pick up more information about the speech stimuli than the other.

<span class="mw-page-title-main">Ventral cochlear nucleus</span>

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 (posteroventral) cochlear nucleus (PVCN). In the VCN, auditory nerve fibers bifurcate, the ascending branch innervates the AVCN and the descending branch innervates the PVCN and then continue to the dorsal cochlear nucleus. The orderly innervation by auditory nerve fibers gives the AVCN a tonotopic organization along the dorsoventral axis. Fibers that carry information from the apex of the cochlea that are tuned to low frequencies contact neurons in the ventral part of the AVCN; those that carry information from the base of the cochlea that are tuned to high frequencies contact neurons in the dorsal part of the AVCN. Several populations of neurons populate the AVCN. Bushy cells receive input from auditory nerve fibers through particularly large endings called end bulbs of Held. They contact stellate cells through more conventional boutons.

The olivocochlear system is a component of the auditory system involved with the descending control of the cochlea. Its nerve fibres, the olivocochlear bundle (OCB), form part of the vestibulocochlear nerve, and project from the superior olivary complex in the brainstem (pons) to the cochlea.

<span class="mw-page-title-main">Anatomy of the cerebellum</span> Structures in the cerebellum, a part of the brain

The anatomy of the cerebellum can be viewed at three levels. At the level of gross anatomy, the cerebellum consists of a tightly folded and crumpled layer of cortex, with white matter underneath, several deep nuclei embedded in the white matter, and a fluid-filled ventricle in the middle. At the intermediate level, the cerebellum and its auxiliary structures can be broken down into several hundred or thousand independently functioning modules or compartments known as microzones. At the microscopic level, each module consists of the same small set of neuronal elements, laid out with a highly stereotyped geometry.

References

  1. Palmer, A R (1987). "Physiology of the cochlear nerve and cochlear nucleus". British Medical Bulletin. 43 (4): 838–855. doi:10.1093/oxfordjournals.bmb.a072221. ISSN   1471-8391. PMID   3329928.
  2. Spoendlin H, Schrott A (1989). "Analysis of the human auditory nerve". Hear Res. 43 (1): 25–38. doi:10.1016/0378-5955(89)90056-7. PMID   2613564. S2CID   37038957.

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