Barry W. Connors
Sensory receptors convert environmental energy into neural signals
Sensation is a cognitive process that requires the full powers of the central nervous system (CNS). Sensation begins with the sensory receptors that actually interface with the world, and these receptors use energy from the environment to trigger electrochemical signals that can be transmitted to the brain—a process called sensory transduction. An understanding of transduction processes is crucial for several reasons. Without these processes, sensation fails. Moreover, a variety of diseases that specifically affect sensory receptors can impair or abolish sensation without damaging the brain. Transduction also sets the basic limits of perception. It determines the sensitivity, range, speed, versatility, and vigor of a sensory system.
We have a variety of senses, each tuned to particular types of environmental energy. These sensory modalities include the familiar ones of seeing, hearing, touching, smelling, and tasting, as well as our senses of pain, balance, body position, and movement. In addition, other intricate sensory systems of which we are not conscious monitor the internal milieu and report on the body's chemical and metabolic state. Early in the 19th century, the physiologist Johannes Müller recognized that neurons that are specialized to evaluate a particular type of stimulus energy will produce the appropriate sensation regardless of how they are activated. For example, banging your eye can produce perceptions of light even in the dark, and seizure activity in a region of the cortex devoted to olfaction can evoke repulsive smells even in a rose garden. This property has been called univariance; in other words, the sensory receptor and its subsequent neural circuits do not know what stimulated them—they give the same type of response regardless. Specificity for each modality is ensured by the structure and position of the sensory receptor.
Sensory transduction uses adaptations of common molecular signaling mechanisms
Evolution is a conservative enterprise. Good ideas are retained, and with slight modification they are adapted to new purposes. Sensory transduction is a prime example of this principle. The sensory processes that are now understood at the molecular level use systems that are closely related to the ubiquitous signaling molecules in eukaryotic cells. Some modalities (vision, olfaction, some types of taste, and other chemoreception) begin with integral membrane proteins that belong to the superfamily of G protein–coupled receptors (GPCRs; see pp. 51–52). The second-messenger pathways use the same substances that are used for so many nonsensory tasks in cells, such as cyclic nucleotides, inositol phosphates, and kinases. Other sensory systems (mechanoreceptors, including the hair cells of audition and the vestibular organs, as well as some taste cells) use modified membrane ion channels in the primary transduction process. Although the structures of most of these channels have not yet been determined, their biophysical properties are generally unremarkable, and they are likely to be related to other, nonsensory ion channels. Indeed, the gating of many ion channels from “nonsensory” cells is sensitive to the physical distortion of the membrane that they lie in, which implies that mechanosensitivity is a widespread (although perhaps epiphenomenal) feature of integral membrane proteins.
To achieve a specificity for certain stimulus energies, many sensory receptors must use specialized cellular structures. These, too, are usually adapted from familiar components. Various receptors are slightly modified epithelial cells. Some situate their transduction sites on modified cilia, whereas others use muscle cells or collagen fibers to channel the appropriate forces to the sensory axon. Many are neurons alone, often just bare axons with no specialization visible by microscopy. Most sensory transduction cells (e.g., oxygen and taste sensors, but not olfactory receptors) lack their own axon to communicate with the CNS. For these cells, the communication system of choice is a relatively standard, Ca2+-dependent system of synaptic transmission onto a primary sensory neuron.
Sensory transduction requires detection and amplification, usually followed by a local receptor potential
Functionally, sensory transducers follow certain general steps. Obviously, they must detect stimulus energy, but they must do so with enough selectivity and speed that stimuli of different types, from different locations, or with different timing are not confused. In most cases, transduction also involves one or more steps of signal amplification so that the sensory cell can reliably communicate the detection of small stimuli (e.g., a few stray photons or a smattering of drifting molecules) to a large brain in an environment with much sensory noise. The sensory cell must then convert the amplified signal into an electrical change by altering the gating of some ion channel. This channel gating leads to alterations of the membrane potential (Vm) in the receptor cell—otherwise known as a receptor potential. The receptor potential is not an action potential but a graded electrotonic event that can either modulate the activity of other channels (e.g., voltage-gated Na+ or Ca2+ channels) or trigger action potentials in a different portion of the same cell. Very often, the receptor potential regulates the flux of Ca2+ into the cell and thus controls the release of some synaptic transmitter molecule onto the sensory afferent neuron.
Ultimately, receptor potentials determine the rate and pattern of action potential firing in a sensory neuron. This firing pattern is the signal that is actually communicated to the CNS. Useful information may be encoded in many features of the firing, including its rate, its temporal patterns, its periodicity, its consistency, and its patterns when compared with the signals of other sensory neurons of the same or even different modalities.