The spiral (cochlear) ganglion is a group of neuron cell bodies in the
modiolus, the conical central axis of 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 (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...
. 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 (CN VIII).
Structure
Neurons
A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
whose
cell bodies lie in the spiral ganglion are strung along the bony core of the cochlea, and send fibers (
axon
An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...
s) into the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
(CNS). These
bipolar neurons are the first neurons in the
auditory system
The auditory system is the sensory system for the sense of hearing. It includes both the ear, sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, incre ...
to fire an action potential, and supply all of the brain's auditory input. Their
dendrites make
synaptic contact with the base of
hair cells, and their axons are bundled together to form the auditory portion of
eighth cranial nerve. The number of neurons in the spiral ganglion is estimated to be about 35,000–50,000.
Two apparent subtypes of spiral ganglion cells exist. Type I spiral ganglion cells comprise the vast majority of spiral ganglion cells (90–95% in cats and 88% in humans), and exclusively innervate the
inner hair cells. They are myelinated, bipolar neurons. Type II spiral ganglion cells make up the remainder. In contrast to Type I cells, they are unipolar and unmyelinated in most mammals. They innervate the
outer hair cells, with each Type II neuron sampling many (15–20) outer hair cells. In addition, outer hair cells form reciprocal synapses onto Type II spiral ganglion cells, suggesting that the Type II cells have both afferent and efferent roles.
Development
The
rudiment of the
cochlear nerve appears about the end of the third week as a group of
ganglion cells closely applied to the cephalic edge of the
auditory vesicle. The ganglion gradually splits into two parts, the
vestibular ganglion and the spiral ganglion. The axons of neurons in the spiral ganglion travel to 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 conti ...
, forming the
cochlear nerve.
Gallery
file:Gray928.png, Diagrammatic longitudinal section of the cochlea
file:Organ of corti.svg, Organ of Corti
References
External links
Slide and overview at anatomy.dal.caSlide at cytochemistry.netImage at
University of New England, Maine
{{Authority control
Auditory system
Sensory ganglia
Vestibulocochlear nerve
Otology
Audiology