Sensory transduction
Have you wondered how our senses work? Well, the body requires specific sensors to perceive
the environment. There are sensory cells for all five senses. In this Chalk Talk episode,
we’ll be introducing you to sensory transduction. Let’s start by going through the common
features shared by all five senses.
The process of perception can be divided into two phases. The first phase is transduction,
the conversion of a physical or chemical stimulus into an impulse, the signal. The second phase
is transformation, which conducts the impulse to the central nervous system. In this episode,
we’ll be focusing on the transduction of stimuli into sensory cells.
To start this episode, you can select the sense you’d like to watch either by clicking
on the slide or by selecting it from the drop-down list in the control bar. You’ll be returned
to the overview at the end of each section. In other words, you can watch the sections
in random order.
The physical signal perceived by sight is electromagnetic radiation with a visible wavelength
range of approximately 380 to 780 nanometres.
Visual stimuli are transformed by photoreceptors cells, namely rod cells and cone cells in
the retina. These photoreceptor cells are able to detect light, as they comprise special
proteins called opsins. As cofactors, opsins use light-sensitive molecules, such as 11-cis-retinal
that is present in all rod cells. When light strikes the rod cells, 11-cis retinal is converted
to all-trans-retinal. The protein complex of opsin and 11-cis-retinal is called rhodopsin,
whereas the opsin and all-trans-retinal complex is termed metarhodopsin.
The activated metarhodopsin now initiates a signal transduction pathway in the photoreceptors
termed phototransduction. Metarhodopsin initially binds to the G protein transducin, resulting
in its activation. The released alpha subunit of transducin interacts with the inhibitory
subunit of cGMP-dependent phosphodiesterase, resulting in its activation. cGMP phosphodiesterase
breaks down cGMP to GMP, reducing intracellular cGMP concentrations. As a result, the cGMP-dependent
cation channels of the rod cells close and the cytosolic concentration of cations decrease,
leading to an overall negative intracellular charge. This hyperpolarization causes a decrease
in glutamate release. It represents an electrochemical light signal and is transmitted to downstream
neurons.
During hearing, the signals perceived are sound waves with a frequency ranging between
20 Hertz and 20 Kilohertz. However, this range isn’t strictly defined. A frequency below
the hearing range could be perceived as pressure or felt as vibrations. In contrast, a frequency
higher than the upper auditory threshold can’t be heard by most people due to middle ear
geometry.
The airborne sound reaching the inner ear is transmitted to the fluid-filled cochlea.
Without any adjusting processes, this would cause ninety-eight percent of sound to be
reflected, as the acoustic impedance of air and water differ greatly. Therefore, the ear
employs a process called impedance matching, in which auditory ossicles, which are located
in the middle ear, transmit sound as vibrations to the oval window of the cochlea. This process
ensures the transfer of 60% of the incoming sound and results in our ability to hear.
The sound wave transmitted to the cochlear generates a travelling wave in the basilar
membrane. This wave causes the membrane to be set in motion. The basilar membrane is
the narrowest and most stiff directly behind the oval window, but widens and becomes more
flexible towards the apex of the cochlear, known as the helicotrema. Incoming sound waves
initially cause the anterior part of the basilar membrane to vibrate. These vibrations travel
along the cochlear until they decay. A key factor in hearing is that the position of
the maximal vibration of the basilar membrane depends on the sound wave frequency. Therefore,
the travelling wave leads to a spatial resolution of sound frequencies. Sound waves with a high
frequency cause the basilar membrane to move most strongly near the oval window, in contrast
to low frequency sound waves that occur close to the helicotrema. The detection through
secondary sensory cells in the Organ of Corti occurs at the site of maximum basilar membrane
motion.
Because of their appearance, auditory sensory cells are called hair cells. There are two
distinct types: inner hair cells and outer hair cells. The main task of outer hair cells
is to enhance mechanical stimulation in order to decrease the threshold of hearing. Their
stereocilia are in contact with the tectorial membrane and transfer basilar membrane motion
to the tectorial membrane. As a result, this causes a drag of the potassium-rich endolymph.
In turn, this deflects the stereocilia of the inner hair cells that are not in contact
with the tectorial membrane. The bending causes tension of the hair cell plasma membrane,
which opens mechanosensitive ion channels and enables potassium ions from the endolymph
to enter the hair cell. The result is an overall positively charged intracellular state. This
depolarization causes the release of glutamate, which stimulates afferent nerve fibers.
The sense of smell is responsive to chemical stimuli. Odors are a mixture of small volatile
molecules that diffuse through the air and enter the nose.
Olfactory sensory cells are located in the olfactory epithelium that lines the nasal
cavity.
To detect odorants, olfactory sensory cells have specific receptors expressed on the ciliary
membrane. These receptors contain polypeptide chains that form a binding pocket for odorant
molecules. Although the receptors aren’t very selective and can bind to different members
of a family of molecules, humans require approximately 350 different receptors that work together
to enable the sense of smell. However, each olfactory sensory cell only expresses one
receptor type at any given time.
When a fragrance attaches to its matching receptor, it subsequently activates a G protein.
The G protein stimulates adenylyl cyclase, which converts ATP to cAMP. cAMP is an effector
of the cAMP-mediated ion channel, more commonly termed CNG channel. It’s intracellular binding
to CNG channels causes the channels to open, enabling the influx of sodium and calcium
ions from the extracellular space into the sensory cell. As a result, the cellular interior
is positively charged and depolarizes. This depolarization is enhanced by the intracellular
binding of calcium to chloride ion channels, causing them to open. The efflux of negatively
charged chloride ions from the cell increases the surplus of positive charge in the cell.
The produced receptor potential generates an action potential that is transmitted to
the downstream neurons. The sense of taste is based on the detection
of molecules as a chemical stimulus. There are five basic tastes, which can be perceived
and differentiated by secondary sensory cells. These tastes are sour, salty, sweet, bitter,
and umami, also known as savory.
The taste receptors of the five basic tastes can be subdivided into ionotropic and metabotropic
receptors. The taste receptors for sour and salty tastes are ionotropic and directly depolarize
the cell.
The sour taste is mediated by potassium channels. The binding of extracellular protons to these
potassium channels causes them to alter their conformation and close, preventing the efflux
of potassium ions. The resulting increased intracellular potassium concentration leads
to positive polarization of the cell, causing it to depolarize.
The salty taste is detected using epithelial sodium channels, which directly absorb the
sodium ions present in salty food. This also leads to an increased positive charge in the
cytosol of sensory cells, causing them to depolarize.
In contrast, the taste receptors for bitter, sweet, and umami are metabotropic, meaning
they initiate a signaling cascade and can only indirectly depolarize the cell. The taste
molecules are extracellularly bound to matching receptors and are bound intracellularly to
a G protein.
The G protein stimulates phospholipase C. This enzyme cleaves the membrane phospholipid
PIP2 to the second messenger IP3 and DAG. IP3 subsequently binds to the calcium channels
in the endoplasmic reticulum, which leads to the release of calcium from the endoplasmic
reticulum into the cytosol. As a result, various downstream processes can be initiated in the
cell, such as the opening of non-selective cation channels or an ATP-dependent signaling
cascade. All of these processes eventually cause the cell to depolarize. In addition,
depolarization is enhanced by the G protein stimulating the production of cAMP and cGMP.
This stimulation causes the potassium channels in the cell membrane to close, limiting potassium
efflux.
Depolarization of the gustatory cells finally results in the release of glutamate, which
stimulates afferent nerve fibers. The sensation of touch detects mechanical
forces impacting the skin. Pressure is perceived by Merkel cells and bulbous corpuscles, vibrations
through lamellar corpuscles, and touch by tactile corpuscles and free nerve endings.
These sensory cells are distributed unevenly throughout the body and can be found especially
on the fingertips, the tongue, and in the mouth.
Signal transduction of the individual tactile cells still remains a research focus. Therefore,
we’ll focus on the fundamentals to provide a general overview. Tactile cells have mechanosensitive
ion channels for perceiving stimuli. These are channel proteins located in the plasma
membrane, with some of the channels connected to the cytoskeleton or the extracellular matrix.
As a result of mechanical stimuli, the channel proteins deform and open, enabling the influx
of cations into the cell. The positive polarization of the cell now leads to the opening of voltage-gated
channels in the cell membrane. The result is an even increased influx of cations, mainly
sodium and calcium, ultimately causing the cell to depolarize. An action potential is
formed and transmitted to downstream neurons. Let’s compare the similarities and differences
between the various signal transduction pathways.
During vision, electromagnetic radiation is detected through chemical changes in the cells.
The recorded signal is converted via a G protein-dependent signaling cascade, which leads to hyperpolarization
of the secondary sensory cell. This is the result of a decrease in intracellular sodium
and calcium levels. The signal is transmitted to afferent nerve fibers using the transmitter
glutamate. Sight is the only sense in which the cell hyperpolarizes in response to a stimulus.
During hearing, a sound wave is converted to an electrical signal through the mechanical
motion of the secondary sensory cells. The opening of mechanosensitive ion channels causes
the cells to depolarize, mainly through the influx of potassium ions into the cell. The
signal is then transmitted as an action potential to downstream neurons via the transmitter
glutamate.
Smell is mediated by odor molecules, which represent a chemical signal. These fragrances
bind to specific receptors on primary sensory cells and convert the stimuli into cellular
depolarization via a G protein-dependent signaling cascade. Depolarization is generated by the
influx of sodium and calcium ions and the efflux of chloride ions. The signal is transmitted
as an action potential to downstream neurons.
When it comes to taste, flavors act as chemical signals on secondary sensory cells. There
are two different signal transduction pathways. Activation of ionotropic receptors leads to
cation channel opening and increases ion levels in the cell, causing the cell to depolarize.
Activation of metabotropic receptors is caused by the binding of taste molecules and activates
associated G proteins. These initiate a signaling cascade resulting in increased calcium levels
in the cell, causing it to depolarize. Taste receptors transmit their signal to afferent
nerve fibers using the transmitter glutamate.
The sense of touch detects mechanical stimuli by primary sensory cells, which have mechanosensitive
cation channels. Their opening positively polarizes the cell interior, leading to the
opening of voltage-gated ion channels. This results in the influx of mainly sodium and
calcium ions in the cell, causing the cell to depolarize. An action potential is formed
that is transmitted to downstream neurons.