Summary: Info on spiral ganglion sought

["Dr L S Smith (Staff)" <l.s.smith@xxxxxxxxxxxxx>]

Some little time ago, I put out a request on the AUDITORY mailing list:

> I am looking for information on the neural structure of the spiral
ganglion:
> specifically
>

> (i) What is the pattern of innervation from the cochlear nerve to the
spiral
> ganglion? So far as I can work out (mostly from Pickles, "Intro to the
> Physiology of Hearing", p34-7), spiral ganglion cells are bipolar, with
one
> process (dendritic?) innervating the inner hair cells (mostly), and the
> other process becoming the auditory nerve. Is there any spread in the
> innervation of the spiral ganglion cells by the IHC's?
> (ii) Do the neurons of the spiral ganglion interact with (excite/inhibit
> through interneurons) each other?

and I received useful replies from Peter Cariani, Ben Clopton, Steve
Greenberg, Glenis Long, and Kevin Baker.  What follows is my attempt to
summarise what they said, and what I have gleaned from the references. Any
responsibility for errors, misunderstandings etc. is, of course, mine!

(i) The spiral ganglion has two types of cells, type 1, consituting 90-95%
of the cells in cat [1], and type 2. Type 1 cells are bipolar, with one
projection innervating the inner hair cells (IHCs), and the the other
projection becoming the auditory nerve. The projection to the IHCs
innervates 1 or a small number of IHC's: the experimental evidence is
summarised in [2][3]. It appears that where the fibers that innervate the
IHC's are small, they innervate just 1 IHC, but where the fibers are
larger, they may branch, and innervate 2 or 3 adjacent IHC's.


Type 2 cells innervate the OHC's, are larger, and monopolar. The synapses
onto the OHC's are believed to be reciprocal [1][2][3].

There are far fewer IHC's than spiral ganglion type 1 cells. In cat there
are about 3000 IHC's, and 50000 cochlear nerve fibers: in man between 2800
and 4400 IHC's and 25000-30000 cells in the spiral ganglion [1, pp23-4].
Each spiral ganglion type 1 cell projection to an IHC makes a number of
synapses on the IHC: 10-30 in cat [1], about 10 in man [2]. At the IHC each
SG type 1 afferent makes 1 synapse in cat[1][2], but many in man [2]. The
synaptic structure is described in [3]. The different SG type 1 cells
afferent to a single IHC can have very different spontaneous rates and
thresholds.

There are also efferent fibers near the IHC's. These originate in the
lateral olivocochlear system, and make axodendritic synapses with the
afferent fibers [3]. There are approximately 30 such synapses per fiber
[1].


There are also efferent fibers at the OHC's: these come from the medial
olivocochlear nucleus [1][2].

Type 1 SG neurons are largely myelinated in cat (95%, in [2]), but not in
man (5% in [2]). The projection to the IHC ceases to be myelinated after it
passes through the habenular opening.

So: there is virtually no spread in the activation from the IHC's to the SG
type 1 cells.

(ii) There do not appear to be any inhibitory synapses in the SG. There
appear to be some synapses on spiral ganglion cells: synapses on the cell
body of small spiral ganglion cells [2]. The origin of the synapsing fibers
is believed to be the olivocochlear body [2]. There are dendrodendritic
synapses believed to be on type 2 peripheral processes.


So: since the type 1 SG cell bodies are unmyelinated, there may be ephaptic
synapses, but there is little direct evidence for them. There is little
evidence for synaptic interaction.

[1] Webster D.B., Popper A.N., Fay R.R. (eds), The mammalian auditory
pathway: neuroanatomy, Springer-Verlag, 1992.
[2] Nadol J.B., Jr., Comparative anatomy of the cochlea and auditory nerve
in mammals, Hearing research 34, , 253-265, 1988.
[3] Nadol J.B., Jr., Synaptic morphology of inner and outer hair cells of
the human organ of Corti, Journal of electron microscopy technique, 15,
187-196, 1990.

In conclusion, may I thank the respondees for their help: I have certainly
learned a lot about the IHC/OHC/SG system!

--Leslie Smith
Dr Leslie S. Smith
Dept of Computing and Mathematics, Univ of Stirling
Stirling FK9 4LA Scotland

l.s.smith@cs.stir.ac.uk   (NeXTmail welcome)
Tel (44) 1786 467435 Fax (44) 1786 464551
www http://www.cs.stir.ac.uk/~lss/