Taste

“Taste” is the eighth chapter of the book Sensory Transduction and begins with gustation in insects, describing receptor proteins in insect taste organs and mechanisms of signal production. It proceeds to the anatomy of taste buds and the tongue in mammals and describes the two basic forms of taste transduction: metabotropic and ionotropic. For metabotropic mechanisms, a thorough review is given of the receptor proteins and signal production for bitter, sweet, and umami, concluding with common pathways of transduction for these modalities. The separate ionotropic mechanisms of salty and sour are then reviewed, and the chapter concludes with discussion of our understanding of the neural code for taste.

Mechanoreceptors and touch

“Mechanoreceptors and touch” is the fifth chapter of the book Sensory Transduction and describes general mechanisms of touch sensitivity in animals. It begins with a review of mechanoreception in the single-celled protozoan Paramecium and transduction of touch in the round worm Caenorhabditis elegans. A thorough treatment is next given of the crayfish stretch receptor and insect mechanoreceptors, including a description of NOMPC channels in Drosophila. The chapter then reviews the anatomy and physiology of mechanoreceptors and touch in mammals, both in glabrous and hairy skin. It concludes with recent discoveries of the molecular biology and physiology of Merkel cells, known to be responsible for much of mammalian touch sensation.

Mechanisms of sensation

“Mechanisms of sensation” is the second chapter of the book Sensory Transduction and describes general features of sensory cells, including types of sensory membrane, the specialized organization of membrane and protein within sensory cells, membrane renewal, external specializations of sense cells, mechanisms of stimulus detection, primary and secondary receptor cells, and receptor sensitivity and noise. These general features of sensory cells are illustrated by specific examples taken from a wide variety of organisms, from scallop and crayfish to Drosophila and vertebrates including mammals. The chapter concludes with a description of sex pheromone detection in the male moth, which achieves the physical limit of sensitivity of the receptor to a single molecule of attractant.

Hearing and hair cells

“Hearing and hair cells” is the sixth chapter of the book Sensory Transduction and begins with hearing in insects, describing the anatomy and physiology of tympanal organs and Johnston’s organ. It reviews the literature on vertebrate hair cells, which are the sensory receptors of the inner ear. It begins with the anatomy of hair cells and then describes tip links, hair cell transduction proteins, and our present understanding of the nature and identity of the mechanoreceptive channels, including the role of channel gating in bundle stiffness and adaptation of hair cells. A review is given of the anatomy and physiology of the organs of the lateral line, the vestibular system, and the cochlea, together with a description of endolymph and the endocochlear potential, outer hair cells and tuning in mammals, and the role of electrical resonance in tuning in the turtle basilar papilla.

Channels and electrical signals

“Channels and electrical signals” is the third chapter of the book Sensory Transduction and reviews the structure and function of ion channels, the structure of channel pores, and mechanisms of gating. It introduces ionotropic receptor molecules, which are proteins that function as sensory receptors but are also ion channels, whose gating can produce changes in membrane conductance directly. It then uses the hair cell of the inner ear as an example to introduce the concepts of membrane potentials, the Nernst equation, ion homeostasis, the Goldman voltage equation, and driving force. A description of the technique of voltage clamping follows, together with the application of this technique to the hair cell to explain the method of measuring changes in channel gating and the ion selectivity of channel pores.

Extra sensory receptors

“Extra sensory receptors” is the tenth chapter of the book Sensory Transduction and reviews mechanisms of sensory transduction in three additional sensory modalities: thermoreception, electroreception, and magnetoreception. It describes the physiology and molecular biology of warm and cold receptors in the mammalian skin, including the channels thought to be responsible and mechanisms of channel gating. There follows an extensive description of thermoreceptors in the pit organs of snakes which permit these animals literally to see in the dark. The section on electroreception reviews in detail the mechanism responsible for the astonishing sensitivity of the ampullary receptors of skates, as well as the structure and function of tuberous receptors, electrocytes, and electrolocation. The final section on magnetoreception describes magnetotactic bacteria as well as the evidence for magnetoreception in migrating birds, together with theories—as yet unproved—for the mechanism of animal sensitivity to magnetic fields.

Photoreception

“Photoreception” is the ninth chapter of the book Sensory Transduction and begins with general mechanisms of light detection, photopigment activation, and the variety of pathways of phototransduction using the scallop eye as an example. There is then a thorough treatment of the photoreceptors of arthropods, particularly those of Limulus and Drosophila. Following a description of photoreceptor anatomy, the chapter describes transduction in these arthropods including photoreceptor channels and the role of Ca2+ in the regulation of gain and turnoff. It then proceeds to vertebrate rods and cones, with individual treatment of the topics of transduction in vertebrate photoreceptors, the ion channels of rods and cones, the description and measurement of the photocurrent, pathways responsible for shutting down the light response, light adaptation, pigment renewal, and the recovery of sensitivity after bright light exposure. It concludes with transduction in intrinsically photosensitive retinal ganglion cells.

Chemoreception and the sense of smell

“Chemoreception and the sense of smell” is the seventh chapter of the book Sensory Transduction and begins with a general description of chemoreception, including chemotaxis in bacteria. It then describes olfaction in insects, including new discoveries of the nature of insect receptor proteins and the coding of olfaction in insects. It proceeds to review olfaction in vertebrates, beginning with the primary olfactory epithelium. It describes olfactory receptor proteins, the mechanism of olfactory transduction, and pathways of desensitization and adaptation. The basis of coding in the principal olfactory epithelium is described together with the anatomy and physiology of the olfactory bulb. A final section is provided on the accessory olfactory system and vomeronasal organ, including a description of receptor proteins, transduction cascades, and wiring to the accessory olfactory bulbs.

Metabotropic signal transduction

“Metabotropic signal transduction” is the fourth chapter of the book Sensory Transduction and reviews the structure and function of G-protein cascades, which are essential components of transduction in many sensory receptors. G-protein cascades are found throughout the body and are responsible for mediating the effects of many hormones and synaptic transmitters in the CNS. The chapter describes the components of these cascades, including G-protein-coupled receptors, heterotrimeric G proteins, effector molecules, and second messengers including calcium. It then describes the special properties of channels gated by second messengers, including cyclic-nucleotide-gated channels, which were first discovered in sensory receptors. It concludes with a description of transduction in the lizard parietal eye, where a single cell type can respond to light in two different ways.

The senses

“The senses” is the introductory chapter of the book Sensory Transduction and describes early studies in the anatomy of sense organs and physiology of the senses. It introduces methods in physiology and molecular biology, which were critical tools used to unlock the secrets of sensation, including intracellular and extracellular recording, patch-clamp recording, gene cloning and expression, and structural studies such as cryo-EM. It concludes by describing in detail the discovery of the piezo proteins, which are the stretch-sensitive channels now known to be responsible for much of mammalian touch and proprioception.