الفهرس 
Embryology and anatomy of the cochlea and spiralOnly 14 pages are availabe for public view
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Abstract
spiral ganglion cells are derived from the ectoderm adjoining the auditory vesicle, by 14 weeks, the spiral ganglion cells show a gradual decrease in the nucleus-to-cell area ratio that may reflect a morphological adaptation to function (Wright et al., 2006).
More information about the fine structure and organization of spiral ganglion provides a better understanding of the likely site of electrical stimulation of the neural elements in patients with cochlear implants (Tylstedt and Rask-Andersen, 2001).
The mammalian spiral ganglion consists mainly of the bipolar cell bodies of primary afferent auditory neurons. The cell bodies are enclosed in Rosenthal’s canal (spiral canal) in the attached margin of the osseous spiral lamina, which spirals around the modiolus of the cochlea. The central processes of the spiral ganglion neurons pass through the internal auditory meatus forming the auditory nerve, and the peripheral processes project to the organ of Corti where they connect with the hair cells (Glueckert et al., 2005b; Cartee et al., 2006).
Two types of cells are distinguished in spiral ganglion: large (type I) and small (type II) ganglion cells.
(A) large type I ganglion cells: make up 90-95% of the total population, myelinated with a round nucleus, and are thought to innervate inner hair cells (Richard and Mark, 2003).
(B) Small type II ganglion cells: make up only 5–10% of the total population and unmyelinated with an irregular nucleus which is thought to innervate outer hair cells(Thiers et al., 2000).
SPIRAL GANGLION NEURON scored as monopolar, bipolar, multipolar, pseudomonopolar or no processes (Whitlon et al., 2007).
Glueckert et al (2005b) investigated some immediately fixed human spiral ganglions using scanning electron microscopy (SEM) of cochleae from normal hearing volunteers, taken out at surgery due to life-threatening petroclival meningioma .
(1)The human spiral ganglion of the basal turn was located in a more or less well-defined bony channel (RC or spiral canal).
(2) The human spiral ganglion of the middle turn the concentration of ganglion cells increased and cells frequently impinged on each other.
In the human specimen axosomatic synapses on type I human SGCs are extremely rare. In contrast, axosomatic synapses were present on approximately 50% of type II SGCs (Thiers et al., 2000).
The high number of presumed axodendritic synapses suggests that type II’s are profusely innervated by fibers of the olivocochlear bundle and that the olivocochlear efferent system performs its peripheral function not only via its axosomatic synapses on the outer hair cell (OHC)
The number of neurons in the normal human spiral ganglion is important for evaluating changes associated with aging or pathological conditions. There have been a few studies on the number of Spiral ganglion neurons, but the results are conflicting (Whitlon et al., 2007).
Spiral ganglion neurons are the primary afferent neurons that transmit auditory information from the organ of Corti to the cochlear nuclei.
DA receptors can be divided into two groups which their effects on the intracellular signaling pathway are different: (1) D1-like (D1 and D5) receptors that activate adenyl cyclase and (2) D2-like (D2 and D4) receptors that inhibit adenyl cyclase activity (Inoue et al., 2006).
Immunoreactivity for all DA receptor subtypes (D1-D5) was found in most, but not all, spiral ganglion neurons regardless of cell size.
Serotonin immunoreactivity observed in the cytoplasm of spiral ganglion neurons strongly suggests that the source of serotonin-containing fibers is from the afferent system. These results suggest that serotonin in the spiral ganglion cells may be involved in the mechanism of acoustic processing however the source of serotonin in the cochlea is unknown (Vicente-Torres et al., 2003).
Last studies showed that endothelin 1 is widely distributed in cardiovascular and noncardiovascular system (Naidoo et al., 2004), and also distributed in endolymphatic sac, vestibule and stria vascularis of inner ear (Xu et al., 2007). Endothelin 1 in spiral ganglion cells may have a direct influence on transmission of nerve impulses.
Degeneration of the SGNs and its processes in many types of SNHL may occur as a primary or a secondary event. Alam et al. (2007) demonstrated that there are also two phases in the degeneration of SGNs after deafness induced by ototoxic drug in rats: (1) an early phase (2) later phase.
It is known that an excessive release of glutamate into the synaptic cleft leads to a swelling and destruction of the afferent nerve terminals and reduces the number of SGNs of type I in the developing cochlea (Steinbach and Lutz, 2007). This may lead to hearing loss.
Postischemic administration of Ginsenoside Rb1 (gRb1) has protective effects against ischemic injury to SGCs. So the administration of gRb1 prevented apoptotic cell death and minimized injury resulting from cochlear ischemia, so gRb1 may be effective in the treatment of sensorineural hearing loss (Fujita et al., 2007).
Cochlear implantation is a well established method of rehabilitating severe to profound deafness (Geers et al., 2002). One possible common link between the preoperative factors and the outcome of the cochlear implant procedure is the survival of eighth nerve structures, especially the spiral ganglion neurons (Rejali et al., 2007), which are directly stimulated by the cochlear implant electrode.
A regener¬ation of inner ear neurons induced by growth factors (Richardson et al., 2007), stem cell (Martinez – Monedero et al., 2007), Growth cones (Anderson et al., 2006) And chronic electrical stimulation (Coco et al., 2007) is a challenging future.
(1) The role of endogenous stem cells in regeneration of the spiral ganglion:
The regeneration capacity of tissues is determined in part by whether they contain endogenous stem cells. These tissues have stem cells that constantly replace lost cells in the adult, and their designation as adult tissue stem cells is based on their continued ability to selfrenew and differentiates (Martinez-Monedero et al., 2007).
# Progenitor stem cells in spiral ganglion
SGNs have been reported to regrow fibers to varying extent after damage in different animal models. Several studies have suggested that the endings regrow after damage by glutamate toxicity in guinea pigs. Experimental sectioning of the auditory nerve in mice leads to extensive regrowth of fibers into the cochlea (Sugawara et al., 2005).
(2) Role 0f growth factors in regeneration of the spiral ganglion:
Spiral ganglion cell survival and development has been shown to be influenced by several growth factors (Richardson et al., 2007) such as:
(1) Neurotrophin 3 (Ntf3),
(2) Brain-derived neurotrophic factor (BDNF),
(3) Glial cell-derived neurotrophic factor (GDNF),
(4) Leukemia inhibitory factor (LIF) -type cytokines,
(5) Bone morphogenetic protein 4 (BMP4) and
(6) Ciliary-derived neurotrophic factor (CNTF).
*Maruyama et al. (2008) demonstrates that GDNF, when administered locally to the cochlea directly after deafening, enhances the electrical responsiveness, compared to untreated animals. And recently demonstrated that, combination therapy including both neurotrophic factors and antioxidants would be of interest for inner ear treatment.
(3) Role of growth cones in regeneration of the spiral ganglion
Development and regeneration of the nervous system is dependent on the locomotive and navigational capacity of nerve growth cones (GC) (Ming et al., 2001)
The GC forms a sensori-motor tip that guides a growing neurite along a precisely defined pathway and establishes synapses with their target cells. These trafficking motor heads guide axon growth in response to (a) chemical and (b) electrical cues (Ming et al., 2001).
When dissociated SG cells were cultured on coated substrates together with neurotrophins (BDNF and NT-3) and nerve growth factor (GDNF), from day two and forward, spherocytes attached and developed into elongating neurons, with regenerating neurites (peripheral axons). Two to four days after seeding, spherocytes started to attach to the substrate, grew in size and began to sprout, with neurites regenerating during a period of 7–14 days.
(4) Role of Chronic electrical stimulation (ES) in regeneration of the spiral ganglion
The effect of Intracochlear electrical stimulation (ICES) on SGNs survival in ototoxically deafened animals is controversy (Miller, 2001).
*Shepherd et al. (2005) have shown no evidence of (ICES) -induced trophic support of SGNs.
*Coco et al. (2007) reported a significant regional increase in SG survival in partially deafened cats and a consistent increase in the size of stimulated SG cells
Factors influencing neurotrophic effects of ES on SGNs:
(1) Animals stimulated using a monopolar electrode at the round window has a little effect of ICES on SG density (Leake et al., 2008).
(2) Animals stimulated using higher frequency modulated signals show greater trophic effects of ICES.
(3) Long periods of ICES, one interesting finding was that chronic ICES reduced SG degeneration throughout most of the cochlea (Leake and Rebscher, 2004).
(4) Another important factor for eliciting trophic effects is efficacy or distribution of electrical activation across the auditory nerve.
(5) Specific features of these electrodes such as geometry, size; orientation of the stimulating sites and proximity of the device to the SG neurons (Rebscher et al., 2007).
(6) The status of the SGNs following deafening might play a critical role in determining the efficacy of ES on SGN rescue (Coco et al., 2007).
(7) Cochlear pathology and CI insertion trauma results in regional SG loss beyond that caused by the deafening procedure (Kiefer et al., 2005).