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It is used todescribe the savory taste of protein-rich dishes such as thosecontaining meat. The recent discovery of a specific taste receptor that binds glutamates, which are prevalent in protein-richfood, has led to the recognition of the “umami taste receptor,”along with the traditional sweet, bitter, salty, and sour receptorson taste buds. The flavor-enhancing quality of monosodiumglutamate (MSG) can be ascribed to stimulation of the umamireceptor.72The Nervous System and MuscleA.
TongueB. Section through vallate papillaFoliatepapillaeTaste budsFungiform papillaeDuct of gustatory(Ebner’s)glandC. Taste budEpitheliumBasement membraneVallate papillaeNerve plexusMicrovilliNerve fibers emerging from taste budsTaste poreTaste cellsFibroblastCollagenMicrovilliDesmosomesSchwann cellEpitheliumD. Detail of taste poreBasement membraneLarge nerve fiberSmall nerve fiberGranulesLarge nerve fiberIntercellular spaceE. Detail of base of receptor cellsFigure 5.12 Taste Receptors Taste receptors are found mainly in the taste buds of the three typesof papillae of the tongue (A and B), but also in taste buds on the palate, larynx, and pharynx. Goblet-shapedtaste buds (C) comprise taste cells with microvilli protruding through the taste pore (D).
Nerve fibers arefound in close association with hair cells (D and E). Taste cells, through receptors, respond to chemicalsignals (taste), ultimately producing depolarization and action potentials in afferent nerve fibers. At least fivetypes of taste receptors exist: sweet, salty, sour, bitter, and umami (savory).transmission of signals to the brainstem (see Fig. 5.11).By virtue of the orientation of the maculae of the twootolithic organs, linear acceleration in any direction can bedetected.CHEMICAL SENSESTaste CellsGustation (taste) and olfaction (smell) are two chemicalsenses involved in detecting chemical stimuli in the environ-ment and the substances we ingest.
Traditionally, tastes havebeen classified as sweet, bitter, salty, and sour. More recently,a fifth classification, umami (savory) has become widelyaccepted (see preceding box). Taste cells (also called tastereceptor cells or gustatory receptor cells) are found mainly inthe taste buds of the papillae of the tongue, but also in tastebuds of the palate, larynx, and pharynx. Three types of papillae are recognized, based on anatomical structure: the fungiform, foliate, and circumvallate papillae (Fig.
5.12). Tastebuds within the papillae consist of groups of taste cells, alongwith supporting cells and basal cells; the latter differentiate tocontinuously replace taste cells. Individual taste cells respondSensory Physiology73Ventral posteromedial (VPM) nucleus of thalamusSensory cortex (just below face area)Lateral hypothalamic areaAmygdalaMesencephalicnucleusandMotor nucleusof trigeminal n.Pontine taste areaTrigeminal (V) n.Maxillary n.Mandibular n.PonsPterygopalatine ganglionGreater petrosal n.Geniculate ganglionFacial (VII) n.andNervus intermediusRostral part of nucleusof solitary tractOticganglionLingual n.ChordatympaniGlossopharyngeal (IX) n.Fungiform papillaeFoliate papillaeValiate papillaeLower part of medulla oblongataEpiglottisLarynxPetrosal (inferior) ganglion of glossopharyngeal n.Nodose (inferior) ganglion of vagus n.Vagus (X) n.Superior laryngeal n.Figure 5.13 Taste Pathways Regions of the tongue are innervated by cranial nerves VII, IX, and X.These pathways transmit afferent signals to the medulla (specifically, the nucleus tractus solitarius), fromwhich they are conveyed to other brain regions, eventually reaching the sensory cortex.through receptors to all of the fundamental taste types,although they may respond best to one type.
Microvilli of tastecells recognize chemical signals in saliva, resulting in activation of G protein–coupled signal transduction mechanisms orchanges in ion conductance, ultimately resulting in depolarization and action potentials in primary afferent nerve fibers.Various regions of the tongue are innervated by cranial nervesVII, IX, and X, which carry the signal to the medulla, fromwhich it is transmitted to other regions of the brain, ultimatelyto the sensory cortex (Fig. 5.13).Many people will recall seeing a diagram of the tongue,depicting localization of perception for sweet, salty,bitter, and sour tastes to various regions on its surface. Althoughit was thought for many years that different areas of the tongueare responsible for detection of various tastes, it is now believedthat taste perception is not exclusively differentiated in thismanner.
In other words, the “taste map” seen in many texts isnot well defined.74The Nervous System and MuscleA. Distribution of olfactory epithelium (blue area)Cribriform plateof ethmoid boneOlfactorybulbLateralnasal wallSeptumB. Schema of section through olfactory mucosaCribriform plateSchwann cellOlfactory glandUnmyelinated olfactory axonsBasement membraneSustentacular cellsEndoplasmic reticulumNucleusOlfactory cellsDendritesTerminal bars (desmosomes)Olfactory rod (vesicle)VilliCiliaMucusFigure 5.14 Olfactory Receptors Chemical stimuli in the form of odorants result in depolarization ofsensory olfactory cells of the olfactory epithelium, located in the upper nasal cavity (A).
An olfactory cell,through G protein–coupled receptors, is capable of responding to more than one odor type. Odorants aretrapped and also removed by secretions of olfactory glands (B).Olfactory CellsThe sense of smell has an important role in appetite inhumans, but this sense is poorly developed compared withthat of other mammals, in which olfaction can play a majorrole in reproductive and territorial behavior. The six basictypes of odors we sense are camphor, floral, ethereal, musky,putrid, and pungent.
The sensory olfactory cells are foundin the olfactory epithelium of the nasal cavity, along withsupporting (sustentacular) and basal cells. As is the case withbasal cells in taste buds, these basal cells differentiate to continuously replace sensory cells. The cilia of olfactory cellsextend into the nasal mucosa, where they bind odorantmolecules on specific G protein–coupled receptors (Fig.5.14).
Binding of an odorant activates signal transductionmechanisms within the olfactory cells, resulting in openingof ion channels and depolarization. Olfactory cells are primaryafferent neurons, and action potentials are carried by theiraxons to the olfactory bulb, where they synapse with mitralcells (Fig. 5.15). Projections from the mitral cells form theolfactory tract, which carries signals to higher centers, suchas the piriform and orbitofrontal cortices, as well as theamygdala.Sensory Physiology75Efferent fibersAfferent fibersGranule cell (excited byand inhibiting to mitraland tufted cells)Fibers from contralateral olfactory bulbFibers to contralateral olfactory bulbMitral cellRecurrent processAnterior commissureTufted cellMedial olfactory striaPeriglomerular cellOlfactory trigone andolfactory tubercleGlomerulusOlfactorynerve fibersAnterior perforatedsubstanceLateral olfactorystriaLateral olfactorytract nucleusPiriform lobeUncusAmygdala(in phantom)Entorhinal areaOlfactory epitheliumOlfactory nervesOlfactory bulbOlfactory tractAnterior olfactory nucleusCribriform plate of ethmoid boneFigure 5.15 Olfactory Pathway Axons of bipolar olfactory nerves synapse with tufted and mitral celldendrites in glomeruli of the olfactory bulb (detail, including modulatory interneurons, in inset diagram).
Mitralcell axons form the olfactory tract, which extends to the anterior commissure (where fibers project back tothe contralateral olfactory bulb), and the olfactory trigone, ultimately projecting to the primary olfactorycortex, lateral entorhinal cortex, and amygdala.This page intentionally left blank77CHAPTER6The Somatic Motor SystemThe motor system consists of two subdivisions, the somaticmotor system and the autonomic motor system. The somaticnervous system controls voluntary muscle activity throughmotor neurons and skeletal muscle. The autonomic nervoussystem is responsible for the involuntary muscle activityinvolved in visceral organ function and maintenance of theinternal environment.tendons.
They are innervated by group Ib afferent nerves.Excessive stretch or contraction of muscle will cause activation of Golgi tendon organs and their afferent nerves, producing a reflexive relaxation of the muscle. Golgi tendon organsare also important in proprioception; their role in this process,along with the role of muscle spindles and other receptors, isillustrated in Figures 6.2 and 6.3.MUSCLE SPINDLESSPINAL REFLEXESThe somatic motor system, through the actions of skeletalmuscle, motor neurons, the spinal cord, and the brain, isresponsible for the complex task of controlling movement andposture. This task is accomplished through involuntary spinalreflexes and voluntary muscle activity, and is dependent oncoordinated contraction and relaxation of muscle.
Most skeletal muscle fibers are the extrafusal fibers that are innervatedby α-motor neurons and contract to generate force necessaryfor movement and postural adjustment.Stereotypic motor responses to sensory signals integratedcompletely within the spinal cord are called spinal reflexes.These are the simplest motor responses, but they may bemodified by the central nervous system (CNS) during morecomplex motor activity. The reflex arc of a spinal reflex beginswith stimulation of sensory receptors; the signal is carried tothe spinal cord through sensory afferent nerves, is integratedwithin the spinal cord, is conducted to the muscle throughmotor neurons, and finally produces contraction or relaxation.There are three basic types of spinal cord reflexes (Fig.
6.4):Intrafusal fibers are a second category of muscle fibers foundspecifically within muscle spindles; these spindles are distributed in parallel with extrafusal fibers and function as sensoryorgans that detect length changes in muscle (Fig. 6.1). Thesensitivity of intrafusal fibers is regulated by the γ-motorneurons that innervate them. Muscle spindles are abundantin muscles that control fine movements, for example in theeye. They have important roles in coordinating fine movements, in the return of muscles to their normal resting length,and in proprioception. Intrafusal fibers are further classifiedas nuclear bag fibers and nuclear chain fibers.
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