Biology Reference
In-Depth Information
homozygosity at these loci will also lead to hearing loss remains to be
determined.
The connexins are a somewhat similar class of genes, which code for the
proteins that line the intercellular pores of gap junctions, where they facil-
itate the movement of small ions or molecules between cells. Defects in two
members of this family,
Cx26
and
Cx31
, have been identified in patients
with deafness (Kelsell et al. 1997; Xia et al. 1998). It is generally believed
that, in the cochlea, the connexins facilitate the recycling of potassium ions
from the hair cells back to the stria vascularis, where they can be actively
transported back into the endolymph. If so, the hearing loss could result
from an interference with this normal homeostatic mechanism.
Lastly, it has long been recognized that patients with
DFN3
have a mixed
hearing loss with a significant sensorineural component in addition to the
conductive loss resulting from their congenitally fixed stapes. It had always
been assumed that the perilymphatic gusher, which is such a characteristic
complication of surgical attempts to mobilize the stapes in this syndrome,
must reflect an abnormal communication between the perilymphatic space
and the epidural space of the CNS. Recent studies of the temporal bone by
CT scans have shown a variety of developmental defects, including enlarge-
ment of the internal auditory canal and persistence of the vestibular aque-
duct. It seems possible that an abnormal mixing of the high-potassium
perilymph with the low-potassium cerebrospinal fluid may exceed the
capacity of the stria vascularis to maintain the normal potassium concen-
tration in the perilymph. If so, this mechanism could provide an explana-
tion for the sensorineural component of the hearing loss.
4.3 Energy
Although the neurosensory structures of the organ of Corti are largely
avascular, the stria vascularis is a highly vascularized structure, as its name
suggests. This component of the cochlea is responsible for maintaining
the endolymphatic potential. The high potassium concentration of the
endolymph has been likened to a battery (Davis 1965), which stores energy
by facilitating the flow of potassium ions across the stereocilia during sound
transduction without requiring the active transport of the ions into the hair
cell. Although the purpose of the battery is not known with certainty, by
limiting the energy requirements of the hair cells it may increase the sen-
sitivity of sound perception by allowing the neurosensory cells in the basilar
membrane to function in a microenvironment that is devoid of turbulent
blood flow.
In view of the energy requirements to sustain this system, perhaps it
should have come as no surprise that genetic defects in the mitochondria
are increasingly being recognized as potential causes for deafness (Fischel-
Ghodsian, Chapter 7). Hearing loss can be a component of several syn-
dromic forms of mitochondrial disease including MERRF and MELAS, but
