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 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 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.
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.