Sensory Transduction
Sensory transduction involves the conversion of a stimulus from
the external or internal environment into
an electrical signal for transmission through the nervous system. This process
is performed by all sensory systems and in general involves either:
•
a chemical
process in the retina, tongue or olfactory epithelium; or
•
a mechanical
process in the cochlea and somatosensory systems.
These contrasting modes of
transduction are best characterized in some of the special senses.
Phototransduction
Phototransduction is the process by which light energy in the
form of photons is translated into electrical energy in the form of potential
changes in the photoreceptors (rods and cones) in the retina. The following
sequence of events defines it:
•
Photons
are captured in pigments in the photoreceptor outer segment.
•
This
results in an amplification process using the G-protein, transducin and cyclic
guanosine monophosphate (cGMP) as the secondary messenger.
•
This
causes a reduction in cGMP concentrations which leads to channel closure.
•
The
closure of these channels, which allows Na+ and Ca2+ to
enter the photoreceptor in the dark, leads to a hyperpolarization response, the
degree of which is graded according to the number of photons captured by the
photoreceptor pigment.
The hyperpolarization response
leads to reduced glutamate release by the photoreceptor on to bipolar and
horizontal cells (see Chapter 24). The termination of the photoreceptor
response to a continuous unvarying light stimulus is multifactorial, but
changes in intracellular Ca2+ concentration are important. The light
insensitive Ca2+ pump in the outer segment coupled to the closure of
the cation channel leads to a significant reduction in intracellular Ca2+
concentrations which is important in terminating the photoreceptor response as
well as mediating light (or background) adaptation.
A number of rare congenital forms
of night blindness have now been associated with specific deficits
within the photo transduction
pathway.
Olfactory transduction
Olfactory transduction is similarly a chemically mediated process. The
olfactory receptor cells are bipolar neurones consisting of a dendrite with a
knob on which are found the cilia, and an axonal part that projects as the
olfactory nerve to the olfactory bulb on the underside of the frontal lobe. The
presence of cilia, which contain the olfactory receptors, greatly increases the
surface area of the olfactory neuroepithelium and so increases the probability
of trapping odorant molecules. The following sequence of events defines it:
1. The binding of the odorant molecule to the
receptor leads to the activation of Golf.
2. This activates adenylate cyclase type III which
hydrolyses adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP).
3. cAMP then binds to and activates specific
cation channels, thus allowing Na+ and Ca2+ to influx
down their concentration gradients.
4. This not only partly depolarizes the receptor,
but also leads to the activation
of a Ca2+-dependent Cl− channel and the subsequent Cl−
efflux then further depolarizes the olfactory receptor.
5. There are probably additional transduction
processes present in the olfactory receptor using inositol triphosphate as the
secondary messenger.
6. This can lead to the generation of action
potentials at the cell body, which are then conducted down the olfactory nerve
axons to the olfactory bulb.
The Ca2+ influx is also
important in adaptation by resetting the transduction response.
Auditory transduction
In contrast to both phototransduction
and olfactory transduction, the process of auditory transduction in the
inner ear involves the mechanical displacement of stereocilia on the
hair cells of the cochlea (see Chapter 27). The following sequence of events
defines it:
1. The sensory stimulus, a sound wave, causes
displacement of the stapedial foot process in the oval window which generates
waves in the perilymphatic filled scala vestibuli and tympani of the cochlea.
2. This leads to displacement of the basilar
membrane on which the hair cells are to be found in the organ of Corti. These
cells transduce the sound waves into an electrical response by a process of
mechanotransduction. The stereocilia at the apical end of the hair cell are
linked by tip links, which are attached to ion channels.
3. The sound causes the stereocilia to be
displaced in the direction of the largest stereocilia (or kinocilium) which
creates tension within the tip links which then pull open an ion channel.
4. This ion channel then allows K+ (not
Na+, as the endolymph within the scala media is rich in K+
and low in Na+) and Ca2+ to flow into the hair cell and
by so doing depolarizes it.
5. This depolarization leads to the release of
neurotransmitter at the base of the hair cell which activates the afferent
fibres of the cochlear nerve.
The continued displacement of the
stereocilia in response to a sound is countered by a process of adaptation with
a repositioning of the ion channel such that it is now shut in response to that
degree of tip link tension. This is achieved by the influx of Ca2+
through the ion transduction channels which leads via actin– myosin in the
stereocilia to a new repositioning of the ion channel. A number of syndromes
with congenital deafness have now been identified as being caused by
abnormalities in the myosin found in hair cells.
Other transduction processes
Transduction in the somatosensory
receptors, nociceptors, thermoreceptors, taste receptors and muscle spindle are
discussed in Chapters 30, 31, 32
and 36 respectively.
