Cochlear nerve
Encyclopedia
The cochlear nerve is a nerve in the head that carries signals from the cochlea
of the inner ear
to the brain. It is part of the vestibulocochlear nerve
, the 8th cranial nerve which is found in higher vertebrates; the other portion of the 8th cranial nerve is the vestibular nerve
which carries spatial orientation information from the semicircular canals. The cochlear nerve is a sensory nerve, one which conducts to the brain
information about the environment, in this case acoustic energy impinging on the tympanic membrane. The cochlear nerve arises from within the cochlea and extends to the brainstem, where its fibers make contact with the cochlear nucleus, the next stage of neural processing in the auditory system
.
s are bipolar
, with the most distal portion being called the peripheral process and the central projection being called the axon
; these two projections are also known as the "peripheral axon" and the "central axon". 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 potential
s, which then "jump" across the cell body (or somata) and continue to propagate along the central axon. In this respect, auditory nerve fibers are somewhat unique bipolar cells in that action potentials pass through the soma. Both the peripheral process and the axon are myelin
ated.
In humans, the number of nerve fibers within the cochlear nerve averages around 30,000. The number of fibers varies significantly across species—for example, the domestic cat has some 50,000 fibers. Auditory nerve fibres provide synaptic connections between the hair cells of the cochlea
and the cochlear nucleus within the brainstem. The cell bodies of the cochlear nerve lie within the central aspect of the cochlea and are collectively known as the spiral ganglion
. This name reflects the fact that the cell bodies, considered as a unit, have a spiral (or perhaps more accurately, a helical) shape, reflecting the shape of the cochlea. The terms "cochlear nerve fiber" and "spiral ganglion cell" are used, to some degree, interchangeably, although the former may be used to more specifically refer to the central axons of the cochlear nerve. These central axons exit the cochlea at its base, where it forms a nerve trunk. In humans, this aspect of the nerve is roughly one inch in length. It projects centrally to the brainstem, where its fibers synapse with the cell bodies of the cochlear nucleus. A good anatomical description of human auditory nerve fibers is provided by Spoendlin and Schrott (1985). Important earlier work was done by Schuknecht.
It was once believed that most of the cochlear nerve fibres were directed to the outer hair cell
s, but it is now understood that at least 90% of the cochlear ganglion cells terminate on inner hair cells, the rest terminating on the outer hair cells.
The transmission between the inner hair cells and the neurons is chemical, using glutamate as a neurotransmitter.
The three major components of the cochlear nuclear complex are: (see figure below)
Each of the three cochlear nuclei are tonotopically organised. The axons from the lower frequency area of the cochlea innervate the ventral portion of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus, while the higher frequency axons project into the dorsal portion of the anteroventral cochlear nucleus and the uppermost dorsal 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.
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....
of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
to the brain. It is part of 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 8th cranial nerve which is found in higher vertebrates; the other portion of the 8th cranial nerve is the vestibular nerve
Vestibular nerve
The vestibular nerve is one of the two branches of the Vestibulocochlear nerve . It goes to the semicircular canals via the vestibular ganglion...
which carries spatial orientation information from the semicircular canals. The cochlear nerve is a sensory nerve, one which conducts to the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
information about the environment, in this case acoustic energy impinging on the tympanic membrane. The cochlear nerve arises from within the cochlea and extends to the brainstem, where its fibers make contact with the cochlear nucleus, the next stage of neural processing 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...
.
Anatomy and connections
In terms of their anatomy, auditory nerve fiberNerve fiber
A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system , myelin...
s are bipolar
Bipolar cell
As a part of the retina, the bipolar cell exists between photoreceptors and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.-Overview:...
, with the most distal portion being called the peripheral process and the central projection being called the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
; these two projections are also known as the "peripheral axon" and the "central axon". The peripheral process is sometimes referred to as a dendrite
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
, although that term is somewhat inaccurate. Unlike the typical dendrite, the peripheral process generates and conducts action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s, which then "jump" across the cell body (or somata) and continue to propagate along the central axon. In this respect, auditory nerve fibers are somewhat unique bipolar cells in that action potentials pass through the soma. Both the peripheral process and the axon are myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
ated.
In humans, the number of nerve fibers within the cochlear nerve averages around 30,000. The number of fibers varies significantly across species—for example, the domestic cat has some 50,000 fibers. Auditory nerve fibres provide synaptic connections between the hair cells of 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....
and the cochlear nucleus within the brainstem. The cell bodies of the cochlear nerve lie within the central aspect of the cochlea and are collectively known as 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...
. This name reflects the fact that the cell bodies, considered as a unit, have a spiral (or perhaps more accurately, a helical) shape, reflecting the shape of the cochlea. The terms "cochlear nerve fiber" and "spiral ganglion cell" are used, to some degree, interchangeably, although the former may be used to more specifically refer to the central axons of the cochlear nerve. These central axons exit the cochlea at its base, where it forms a nerve trunk. In humans, this aspect of the nerve is roughly one inch in length. It projects centrally to the brainstem, where its fibers synapse with the cell bodies of the cochlear nucleus. A good anatomical description of human auditory nerve fibers is provided by Spoendlin and Schrott (1985). Important earlier work was done by Schuknecht.
It was once believed that most of the cochlear nerve fibres were directed to the outer hair cell
Hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear...
s, but it is now understood that at least 90% of the cochlear ganglion cells terminate on inner hair cells, the rest terminating on the outer hair cells.
The transmission between the inner hair cells and the neurons is chemical, using glutamate as a neurotransmitter.
Types of neurons
The cochlear neurons can be divided into two groups: Type I and Type II.- Type I neurons make up 90-95% of the neurons and innervate the inner hair cells. They have a relatively large diameter, and are bipolar and myelinated. Each type I axon innervates only a single hair cell, but each hair cell directs its output to an average of 10 nerve fibres
- Type II cells, which have a relatively small diameter, connect with the outer hair cells, are monopolar and are not myelinated.
Cochlear nuclear complex
The axons from each cochlear nerve terminate in the cochlear nuclear complex which are ipsilaterally located in the medulla of the brainstem. The cochlear nucleus is the first 'relay station' of the auditory nervous system and receives mainly ipsilateral afferent input.The three major components of the cochlear nuclear complex are: (see figure below)
- the dorsal cochlear nucleus (DCN)
- the anteroventral cochlear nucleus (AVCN)
- the posteroventral cochlear nucleus (PVCN)
Each of the three cochlear nuclei are tonotopically organised. The axons from the lower frequency area of the cochlea innervate the ventral portion of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus, while the higher frequency axons project into the dorsal portion of the anteroventral cochlear nucleus and the uppermost dorsal 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.
