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Stimulationof sensory receptors results in the opening or closing of ionchannels and generation of a receptor or generator potential(a membrane potential change in the receptor). When threshold is reached, the stimulus has been transduced into an electrical signal, and information is transmitted via afferentpathways to the central nervous system (CNS), which integrates such information and transmits signals back to effectorsystems. The somatosensory system includes mechanoreceptors, thermal receptors, and nociceptors (pain receptors) thatrespond to stimuli in the skin or at the body surface.
The termsomatovisceral sensory system is also used, because suchreceptors are also located in visceral organs, muscle, andjoints.Several types of sensory receptors are found in the skin (andother tissues); these can be differentiated by the stimulito which they respond, as well as by the degree of adaptationthey exhibit to intensity, duration, and change in stimulus(Fig.
5.1):■Meissner’s corpuscles are mechanoreceptors located inthe dermal papillae, especially in fingertips, palms, soles,lips, face, tongue, and genital skin (nonhairy skin). Theseare rapidly adapting receptors with small receptive fieldsthat allow point discrimination and detection of lowfrequency stimuli such as flutter.■■■■■■■Pacinian corpuscles are rapidly adapting mechanoreceptors that detect pressure and vibration. The lamellated capsule of these receptors allows them to respondspecifically to rapid changes in pressure as well as vibration, as opposed to continual pressure or slow changesin pressure (Fig.
5.2).Merkel’s disks are slowly adapting mechanoreceptorsthat respond through small receptive fields to pressureand touch, particularly to indentation of the skin.Hair follicle receptors consist of nerve endings wrappedaround the base of hair follicles. These rapidly adaptingmechanoreceptors detect movement across the surfaceof the skin.Ruffini’s corpuscles are mechanoreceptors found in thedermis and in joints; they respond to stretch and areslowly adapting.Thermal receptors are sensory nerves with endings inthe skin that respond to temperature.
They are subdivided into warm and cold receptors. Both are slowlyadapting.Nociceptors are sensory nerves with endings in skin,cornea, muscle, joints, and visceral endings. Varioustypes of nociceptors respond to temperature andmechanical and chemical stimuli.Muscle and joint receptors include muscle spindles andGolgi tendon organs. These receptors are important inproprioception and coordination of motor activity andare considered in more detail later (see “The MotorSystem”).The receptive field of a sensory neuron is the area overwhich a stimulus will trigger firing of the neuron. Arapidly adapting receptor is one that responds quickly to astimulus but adapts rapidly to a constant stimulus, returning toits normal firing rate.Transduction of Somatosensory SignalsSomatosensory (pain, touch, pressure, temperature) signalsoriginating below the head and in the head are ultimately conveyed to the primary somatosensory area, located in the postcentral gyrus of the parietal lobe of the cerebral cortex.
The60The Nervous System and MuscleMeissner’s corpusclePore of sweat glandStratum corneumStratumlucidumStratumgranulosumStratumspinosumStratumbasaleInternal sheathHair follicleEpidermisFree nerve endingsHair shaftMelanocyteArrector muscle of hairSebaceous glandCuticleExternal sheathGlassymembraneConnectivetissue layerDermisDermalpapilla(of papillarylayer)ReticularlayerHair cuticleSweat glandSubcutaneous tissueHair matrixPapilla of hair folliclePaciniancorpuscleArterySubcutaneousartery and veinVeinSensory nervesElastic fibersDetail of Merkel’s diskBasalepithelialcellsCytoplasmicprotrusionMitochondriaExpandedaxon terminalSkin ligaments(retinacula cutis)DesmosomesBasement membraneAxon terminalMitochondrionSchwann cellCutaneousnerveMotor(autonomic)nerveCross sectionMerkelcellLobulatednucleusGranulatedvesiclesAxonSchwann cellsSchwanncellDetail of free nerve endingFigure 5.1 Skin and Cutaneous Receptors Mechanoreceptors, thermoreceptors, and nociceptorstransduce the effects of touch, temperature, and painful stimuli into neural signals.
Within the skin, mechanoreceptors known as Meissner’s corpuscles, pacinian corpuscles, and Merkel’s disks respond to varioustypes of mechanical stimulation. Temperature and painful stimuli are detected by free nerve endings.Sensory PhysiologyCLINICAL CORRELATEDermatomesDermatomes are areas of the skin associated with a specific pairof dorsal nerve roots. In other words, sensory signals originatingin the skin within a dermatome are largely transmitted to thespinal cord through one of a specific pair of afferent sensory nerveroots, although there is significant overlap between adjacent dermatomes. Clinically, dermatomes are useful in identifying the site61of spinal nerve damage, based on the region of skin in which sensation is affected.
For example, sensations in the feet are conveyedto the spinal cord through nerve roots at L4, L5, or S1, dependingon the area of the foot, with some overlap in regions. In shingles(herpes zoster), the herpes virus usually resides dormant in nervecell bodies of the dorsal root ganglia, but when active, will specifically affect the skin in the corresponding dermatome on one orboth sides (depending which ganglia are infected).Dermatome patternC2C3C4C3 C5C6C4C5T1T1T2T2T3T3T4T5T4T6T7T5T8T6T9T7T10T8T11T12T9L1T10L2L3L4T11L5S1L1 T12S2L2C2C6T1C5C8C6C7C8C7C8C6C8C7C6S3 C8S4S5S2–3L3C7L1S1L5S2L4L2L3L5S2 S1L4S1S1L5L5L4Levels of principal dermatomesC5ClaviclesC5, 6, 7 Lateral parts of upper limbsC8; T1 Medial sides of upper limbsC6ThumbC6, 7, 8 HandC8Ring and little fingersT4Level of nipplesT10Level of umbilicusafferent somatosensory signals originating below the head areconveyed to the dorsal root ganglia and subsequently throughthe spinothalamic and spinoreticular tracts of the anterolateralsystem, ultimately reaching the primary somatosensory cortex(Fig.
5.3). Signals involved in proprioception, as well as signalsgenerated by vibratory and tactile stimuli, are carried throughthe fasciculus gracilis and fasciculus cuneatus to the ventralposterolateral nucleus (VPN) of the thalamus. The lateral cervical system carries some proprioceptive, vibratory, and tactilesignals as well. These pathways reach synapses in the thalamusand before extending to the cerebral cortex.In the head, touch, pressure, pain, and temperature result inafferent nerve traffic to nerve cell bodies in the trigeminalT12Inguinal or groin regionsL1, 2, 3, 4 Anterior and inner surfaces of lower limbsL4, 5; S1 FootL4Medial side of great toeL5; S1, 2 Outer and posterior sides of lower limbsS1Lateral margin of foot and little toeS2, 3, 4 Perineum(semilunar) ganglia of the trigeminal nerve.
Signals involvedin proprioception are carried to neuronal cell bodies in themesencephalic nucleus of the trigeminal nerve (CN V). Projections in this system are mainly to the contralateral ventralposterolateral nucleus (VPN) of the thalamus, subsequentlyreaching the primary somatosensory cortex (postcentralgyrus) (Fig. 5.4).THE VISUAL SYSTEMThe visual system performs the remarkable task of detectinglight stimuli between 400 and 700 nanometers in wavelength,transducing the stimuli into electrical signals, and transmitting62The Nervous System and MusclePacinian corpuscle as pressure transducerA. Sharp “on and off” changes in pressureat start and end of pulse applied to lamellated capsule are transmitted to centralaxon and provoke generator potentials,which in turn may trigger action potentials; there is no response to a slowchange in pressure gradient.
Pressure atcentral core and, accordingly, generatorpotentials are rapidly dissipated byviscoelastic properties of capsule. (Actionpotentials may be blocked by pressure ata node or by drugs.)PressureTo amplifierGenerator potentialAction potentialFirst nodeMyelin sheathPressureTo amplifierLamellated capsuleCentral coreGenerator potentialUnmyelinated axon terminalB. In absence of capsule,axon responds to slow aswell as to rapid changesin pressure.
Generatorpotential dissipatesslowly, and there is no“off” response.Action potentialPressureNa+ⴙⴙⴙⴙⴙⴙⴙⴙⴚⴚⴚⴚⴚⴚⴚⴚⴙⴙⴙⴙⴙⴙⴚⴚⴚⴚⴚⴙIf resultant depolarization at first node is greatenough to reach threshold, an action potentialappears and is propagated along nerve fiber.Pressure applied to axon terminal directlyor via capsule causes increased permeabilityof membrane to Na+, thus setting up ionicgenerator current through first node.ⴙⴙⴙⴙⴙⴙⴙⴙⴙⴚⴙⴙⴙ ⴙⴙⴚⴚⴚⴚⴚⴚⴚⴚⴚⴙⴚⴚⴚ ⴚⴚFigure 5.2 Pacinian Corpuscle Pressure and vibration are transduced into electrical potentialchanges by pacinian corpuscles, which are rapidly adapting mechanoreceptors. The lamellated capsulesurrounding the axon terminal dissipates slow pressure changes, and thus, the axon responds only to rapidchanges in pressure (A), in contrast to an axon lacking a capsule (B). The action potentials produced arecarried through afferent fibers to the dorsal root ganglia (see Fig.
4.9).these signals to the CNS, where they are used to construct athree-dimensional representation of objects in the visual fieldthat includes color and brightness and detects motion.Light passes through the cornea of the eye, through the pupil(the circular opening of the iris), and is then focused by thelens onto the retina (Fig. 5.5). Within the retina, photoreceptors known as rods and cones contain the pigment rhodopsin,which absorbs photons. Color perception is mainly mediatedby cones, whereas rods are sensitive photoreceptors adaptedto perceive light at low intensity. The visual fields of the twoeyes overlap, with the center being projected, upside-down,on the sensitive macular zone of the retina (Fig. 5.6).
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