Genes for photoreceptor proteins in cones

The gene for the blue receptor is autosomal; the
genes for the red and green receptors are X
chromosomal. The absorption spectra of the
three receptors show maxima of 426 nm for
blue, about 530 for green, and about 550 for red.
The red receptorwas discovered to be polymorphic,
with two somewhat different absorption
maxima at 552 and 557 nm.

Hearing and Deafness

Acoustic signals are essential for an animal’s
ability to respond appropriately to its environment.
Hearing is orchestrated by a large ensemble
of proteins acting in concert. Specialized
sensory cells in the cochlea of the inner ear
process the incoming sound waves, converting
them into cellular information that is relayed to
the brain via the acoustic nerve. A missing or
defective protein involved in the hearing
process results in hearing loss. Hearing loss is
common in humans. One out of 1000 newborns
lacks the ability to hear or has severely impaired
hearing. Two categories of genetic hearing loss
can be distinguished: nonsyndromic and syndromic.
In the former category the genetic defect
is limited to the ear; in the latter the ear is
one of several organ systems affected.
The types of genes implicated when defective as
the cause of nonsyndromic hearing loss include
those encoding proteins involved in cytoskeletal
structure, transcription factors, ion
channels (potassium channel), and intercellular
gap channels composed of junction connexins.

The main components of the ear

The auditory system consists of the outer ear,
the middle ear, and the inner ear. Sound waves
are funneled through the outer ear (auricle) and
transmitted through the external ear canal to
the tympanic membrane, which they cause to
vibrate. These vibrations are transmitted
through the tympanic cavity of the middle ear
by a chain of three movable bones, the malleus,
the incus, and the stapes. Three major cavities
form the inner ear: the vestibule, the cochlea,
and the semicircular canals. The chochlea is the
site where auditory signals are processed. The
cochlea contains amembranous labyrinth filled
with a fluid, the endolymph. The vestibular apparatus
includes three semicircular canals
oriented at 90! degree angles to each other.
They respond to rotatory and linear acceleration.
Signals received here are transmitted via
the vestibular nerve, which fuses with the
cochlear nerve to form the acoustic nerve. The
latter transmits the information to the brain.

The cochlea

The cochlea contains the cochlear duct, which
forms the organ of Corti. The organ of Corti converts
sound waves in the endolymph of the
cochlea into intracellular signals. These are
transmitted to auditory regions of the brain.
The organ of Corti contains two types of sensory
cells: one row of inner hair cells and three rows
of outer hair cells. The inner hair cells are pure
receptor cells. Vibrations induced by sound lead
to slight deflections of the stereocilia and open
potassium channels at the tips of the stereocilia.
The influx of potassium ions at the tips of the
cilia of the hair cells (see C) causes a change in
membrane potential that results in a nerve impulse,
which is transmitted as an auditory signal
to the auditory cortex of the brain.
Potassium ions are recycled to the supporting
cells and the spiral ligament into the endolymph
of the scala media. The tectorial membrane
amplifies the sound waves as a resonator.

The outer hair cell

The outer hair cells combine sensory function
with the ability to elongate and contract when
acoustically stimulated. The apical pole of a hair
cell carries an array of about 100 cylindrical
stereocilia in a V-shaped arrangement. Each
stereocilium contains an actin molecule, which
enables it to elongate or to contract. The tips of
the stereocilia are connected by tip links. The
potassium channels are formed by the KCNQ4
protein (yellow) and by connexins (red). Important
for the structural integrity and dynamics of
the hair cells is a cytoskeleton involving actin,
myosin 7A, myosin 15, and the protein diaphanous.

Chromosomal locations of human deafness genes

Almost every human chromosome harbors at
least one gene involved in nonsyndromic
monogenic hearing loss. The diagrammatic
presentation shown here is limited to nonsyndromic
hearing loss.

Odorant Receptor Gene Family

Vertebrates can differentiate thousands of individual
odors. Although their ability to distinguish
differences in color is based on only three
classes of photoreceptors, their sense of smell is
regulated by a large multigene family of receptors
that are highly specific for individual
odorants. In fish, about 100 and in mammals
about 1000 genes code for specific olfactory receptors.
These genes are expressed exclusively
in the olfactory epithelium of the nasal mucous
membrane.