Y khoa, y dược - The peripheral nervous system and reflex activity: Part A

13 The Peripheral Nervous System and Reflex Activity: Part APeripheral Nervous System (PNS)All neural structures outside the brainSensory receptorsPeripheral nerves and associated gangliaMotor endingsFigure 13.1Central nervous system (CNS)Peripheral nervous system (PNS)Motor (efferent) divisionSensory (afferent)divisionSomatic nervoussystemAutonomic nervoussystem (ANS)SympatheticdivisionParasympatheticdivisionSensory ReceptorsSpecialized to respond to changes in their environment (stimuli)Activation results in graded potentials that trigger nerve impulsesSensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brainClassification of ReceptorsBased on:Stimulus typeLocationStructural complexityClassification by Stimulus TypeMechanoreceptors—respond to touch, pressure, vibration, stretch, and itchThermoreceptors—sensitive to changes in temperaturePhotoreceptors—respond to light energy (e.g., retina)Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)Classification by LocationExteroceptorsRespond to stimuli arising outside the bodyReceptors in the skin for touch, pressure, pain, and temperatureMost special sense organsClassification by LocationInteroceptors (visceroceptors)Respond to stimuli arising in internal viscera and blood vesselsSensitive to chemical changes, tissue stretch, and temperature changesClassification by LocationProprioceptorsRespond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and musclesInform the brain of one’s movementsClassification by Structural ComplexityComplex receptors (special sense organs)Vision, hearing, equilibrium, smell, and taste (Chapter 15)Simple receptors for general senses:Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle senseUnencapsulated (free) or encapsulated dendritic endings Unencapsulated Dendritic EndingsThermoreceptorsCold receptors (10–40ºC); in superficial dermis Heat receptors (32–48ºC); in deeper dermisUnencapsulated Dendritic EndingsNociceptorsRespond to:PinchingChemicals from damaged tissueTemperatures outside the range of thermoreceptorsCapsaicinUnencapsulated Dendritic EndingsLight touch receptorsTactile (Merkel) discsHair follicle receptorsTable 13.1Encapsulated Dendritic EndingsAll are mechanoreceptorsMeissner’s (tactile) corpuscles—discriminative touchPacinian (lamellated) corpuscles—deep pressure and vibrationRuffini endings—deep continuous pressureMuscle spindles—muscle stretchGolgi tendon organs—stretch in tendonsJoint kinesthetic receptors—stretch in articular capsulesTable 13.1From Sensation to PerceptionSurvival depends upon sensation and perceptionSensation: the awareness of changes in the internal and external environmentPerception: the conscious interpretation of those stimuliSensory IntegrationInput comes from exteroceptors, proprioceptors, and interoceptorsInput is relayed toward the head, but is processed along the waySensory IntegrationLevels of neural integration in sensory systems:Receptor level—the sensor receptorsCircuit level—ascending pathwaysPerceptual level—neuronal circuits in the cerebral cortexFigure 13.2123 Receptor level(sensory receptionand transmissionto CNS) Circuit level(processing inascending pathways) SpinalcordCerebellumReticularformationPonsMusclespindleJointkinestheticreceptorFree nerveendings (pain,cold, warmth) Medulla Perceptual level (processing incortical sensory centers)MotorcortexSomatosensorycortexThalamusProcessing at the Receptor LevelReceptors have specificity for stimulus energy Stimulus must be applied in a receptive fieldTransduction occursStimulus energy is converted into a graded potential called a receptor potential Processing at the Receptor LevelIn general sense receptors, the receptor potential and generator potential are the same thingstimulusreceptor/generator potential in afferent neuronaction potential at first node of RanvierProcessing at the Receptor LevelIn special sense organs:stimulusreceptor potential in receptor cell release of neurotransmittergenerator potential in first-order sensory neuronaction potentials (if threshold is reached)Adaptation of Sensory ReceptorsAdaptation is a change in sensitivity in the presence of a constant stimulusReceptor membranes become less responsiveReceptor potentials decline in frequency or stopAdaptation of Sensory ReceptorsPhasic (fast-adapting) receptors signal the beginning or end of a stimulusExamples: receptors for pressure, touch, and smell Tonic receptors adapt slowly or not at allExamples: nociceptors and most proprioceptorsProcessing at the Circuit LevelPathways of three neurons conduct sensory impulses upward to the appropriate brain regionsFirst-order neuronsConduct impulses from the receptor level to the second-order neurons in the CNSSecond-order neuronsTransmit impulses to the thalamus or cerebellumThird-order neuronsConduct impulses from the thalamus to the somatosensory cortex (perceptual level)Processing at the Perceptual LevelIdentification of the sensation depends on the specific location of the target neurons in the sensory cortexAspects of sensory perception:Perceptual detection—ability to detect a stimulus (requires summation of impulses)Magnitude estimation—intensity is coded in the frequency of impulsesSpatial discrimination—identifying the site or pattern of the stimulus (studied by the two-point discrimination test)Main Aspects of Sensory PerceptionFeature abstraction—identification of more complex aspects and several stimulus propertiesQuality discrimination—the ability to identify submodalities of a sensation (e.g., sweet or sour tastes)Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music)Figure 13.2123 Receptor level(sensory receptionand transmissionto CNS) Circuit level(processing inascending pathways) SpinalcordCerebellumReticularformationPonsMusclespindleJointkinestheticreceptorFree nerveendings (pain,cold, warmth) Medulla Perceptual level (processing incortical sensory centers)MotorcortexSomatosensorycortexThalamusPerception of PainWarns of actual or impending tissue damageStimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykininImpulses travel on fibers that release neurotransmitters glutamate and substance PSome pain impulses are blocked by inhibitory endogenous opioids Structure of a NerveCordlike organ of the PNSBundle of myelinated and unmyelinated peripheral axons enclosed by connective tissueStructure of a NerveConnective tissue coverings include:Endoneurium—loose connective tissue that encloses axons and their myelin sheathsPerineurium—coarse connective tissue that bundles fibers into fasciclesEpineurium—tough fibrous sheath around a nerveFigure 13.3bBloodvesselsFascicleEpineuriumPerineuriumEndoneuriumAxonMyelin sheath(b)Classification of NervesMost nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibersPure sensory (afferent) or motor (efferent) nerves are rareTypes of fibers in mixed nerves:Somatic afferent and somatic efferentVisceral afferent and visceral efferentPeripheral nerves classified as cranial or spinal nervesGangliaContain neuron cell bodies associated with nervesDorsal root ganglia (sensory, somatic) (Chapter 12)Autonomic ganglia (motor, visceral) (Chapter 14)Regeneration of Nerve FibersMature neurons are amitoticIf the soma of a damaged nerve is intact, axon will regenerateInvolves coordinated activity among:Macrophages—remove debrisSchwann cells—form regeneration tube and secrete growth factorsAxons—regenerate damaged partCNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regenerationFigure 13.4 (1 of 4)EndoneuriumDropletsof myelin FragmentedaxonSchwann cells Site of nerve damage The axonbecomesfragmented atthe injury site. 1Figure 13.4 (2 of 4)Schwann cellMacrophage Macrophagesclean out thedead axon distalto the injury. 2Figure 13.4 (3 of 4)Fine axon sproutsor filaments Aligning Schwann cellsform regeneration tube 3 Axon sprouts,or filaments,grow through aregeneration tubeformed bySchwann cells. Figure 13.4 (4 of 4)Schwann cellSite of newmyelin sheathformation4 The axonregenerates anda new myelinsheath forms. Single enlargingaxon filament