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

13 The Peripheral Nervous System and Reflex Activity: Part DMotor EndingsPNS elements that activate effectors by releasing neurotransmitters Review of Innervation of Skeletal MuscleTakes place at a neuromusclular junctionAcetylcholine (ACh) is the neurotransmitter ACh binds to receptors, resulting in:Movement of Na+ and K+ across the membraneDepolarization of the muscle cellAn end plate potential, which triggers an action potentialFigure 9.8NucleusActionpotential (AP)Myelinated axonof motor neuronAxon terminal of neuromuscular junctionSarcolemma ofthe muscle fiberCa2+Ca2+Axon terminalof motor neuronSynaptic vesiclecontaining ACh MitochondrionSynaptic cleftJunctionalfolds of sarcolemmaFusing synaptic vesiclesAChSarcoplasm ofmuscle fiberPostsynaptic membraneion channel opens;ions pass.Na+K+AChNa+K+Degraded AChAcetylcholinesterasePostsynaptic membraneion channel closed;ions cannot pass. Action potential arrives at axon terminal of motor neuron. Voltage-gated Ca2+channels open and Ca2+enters the axon terminal. Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis. Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.123456Review of Innervation of Visceral Muscle and GlandsAutonomic motor endings and visceral effectors are simpler than somatic junctionsBranches form synapses en passant via varicositiesAcetylcholine and norepinephrine act indirectly via second messengersVisceral motor responses are slower than somatic responsesFigure 9.27SmoothmusclecellVaricosities releasetheir neurotransmittersinto a wide synaptic cleft (a diffuse junction).SynapticvesiclesMitochondrionAutonomicnerve fibersinnervatemost smoothmuscle fibers.VaricositiesLevels of Motor ControlSegmental levelProjection levelPrecommand levelFigure 13.13aFeedbackReflex activityMotoroutputSensoryinput(a) Levels of motor control and their interactionsPrecommand Level(highest)• Cerebellum and basal nuclei• Programs and instructions (modified by feedback)Projection Level (middle) • Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.)• Convey instructions to spinal cord motor neurons and send a copy of that information to higher levelsSegmental Level (lowest)• Spinal cord• Contains central pattern generators (CPGs) InternalfeedbackSegmental LevelThe lowest level of the motor hierarchyCentral pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of musclesControls locomotion and specific, oft-repeated motor activityProjection LevelConsists of:Upper motor neurons that direct the direct (pyramidal) system to produce voluntary skeletal muscle movementsBrain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG-controlled motor actionsProjection motor pathways keep higher command levels informed of what is happeningPrecommand LevelNeurons in the cerebellum and basal nucleiRegulate motor activityPrecisely start or stop movementsCoordinate movements with postureBlock unwanted movementsMonitor muscle tonePerform unconscious planning and discharge in advance of willed movementsPrecommand LevelCerebellumActs on motor pathways through projection areas of the brain stemActs on the motor cortex via the thalamusBasal nucleiInhibit various motor centers under resting conditionsFigure 13.13aFeedbackReflex activityMotoroutputSensoryinput(a) Levels of motor control and their interactionsPrecommand Level(highest)• Cerebellum and basal nuclei• Programs and instructions (modified by feedback)Projection Level (middle) • Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.)• Convey instructions to spinal cord motor neurons and send a copy of that information to higher levelsSegmental Level (lowest)• Spinal cord• Contains central pattern generators (CPGs) InternalfeedbackFigure 13.13b(b) Structures involvedPrecommand level • Cerebellum• Basal nucleiProjection level • Primary motor cortex• Brain stem nucleiSegmental level • Spinal cordReflexesInborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulusLearned (acquired) reflexes result from practice or repetition, Example: driving skillsReflex ArcComponents of a reflex arc (neural path)Receptor—site of stimulus actionSensory neuron—transmits afferent impulses to the CNSIntegration center—either monosynaptic or polysynaptic region within the CNSMotor neuron—conducts efferent impulses from the integration center to an effector organEffector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secretingFigure 13.14Receptor Sensory neuronIntegration centerMotor neuronEffectorSpinal cord(in cross section)InterneuronStimulusSkin12345Spinal ReflexesSpinal somatic reflexesIntegration center is in the spinal cordEffectors are skeletal muscleTesting of somatic reflexes is important clinically to assess the condition of the nervous systemStretch and Golgi Tendon ReflexesFor skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary Muscle spindles inform the nervous system of the length of the muscleGolgi tendon organs inform the brain as to the amount of tension in the muscle and tendonsMuscle SpindlesComposed of 3–10 short intrafusal muscle fibers in a connective tissue capsuleIntrafusal fibersNoncontractile in their central regions (lack myofilaments) Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibersMuscle SpindlesContractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivityNote: extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent fibersFigure 13.15Secondary sensoryendings (type II fiber) Efferent (motor)fiber to muscle spindlePrimary sensoryendings (type Iafiber)Connectivetissue capsuleMuscle spindleTendonSensory fiberGolgi tendonorgan Efferent (motor)fiber to extrafusalmuscle fibersExtrafusal musclefiberIntrafusal musclefibersMuscle SpindlesExcited in two ways:External stretch of muscle and muscle spindleInternal stretch of muscle spindle:Activating the  motor neurons stimulates the ends to contract, thereby stretching the spindleStretch causes an increased rate of impulses in Ia fibersFigure 13.16a, b(a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber.MusclespindleIntrafusalmuscle fiberPrimarysensory (la)nerve fiberExtrafusalmuscle fiberTime(b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. TimeMuscle SpindlesContracting the muscle reduces tension on the muscle spindleSensitivity would be lost unless the muscle spindle is shortened by impulses in the  motor neurons– coactivation maintains the tension and sensitivity of the spindle during muscle contractionFigure 13.16c, d(d) - Coactivation. Both extrafusal and intrafusal muscle fibers contract. Muscle spindle tension is main- tained and it can still signal changes in length.Time(c) Only motor neurons activated. Only the extrafusal muscle fibers contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes. TimeStretch ReflexesMaintain muscle tone in large postural musclesCause muscle contraction in response to increased muscle length (stretch)Stretch ReflexesHow a stretch reflex works:Stretch activates the muscle spindleIIa sensory neurons synapse directly with  motor neurons in the spinal cord motor neurons cause the stretched muscle to contractAll stretch reflexes are monosynaptic and ipsilateral Stretch ReflexesReciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit the  motor neurons of antagonistic musclesExample: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relaxFigure 13.17 (1 of 2)Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist. When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord. The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.The events by which muscle stretch is damped Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch. Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).Initial stimulus(muscle stretch)Cell body ofsensory neuronSensoryneuronMuscle spindleAntagonist muscle Spinal cord123a3bFigure 13.17 (1 of 2), step1Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist. When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.The events by which muscle stretch is dampedInitial stimulus(muscle stretch)Cell body ofsensory neuronSensoryneuronMuscle spindleAntagonist muscle Spinal cord1Figure 13.17 (1 of 2), step 2Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist. When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord. The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.The events by which muscle stretch is dampedInitial stimulus(muscle stretch)Cell body ofsensory neuronSensoryneuronMuscle spindleAntagonist muscle Spinal cord12Figure 13.17 (1 of 2), step 3aStretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist. When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord. The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.The events by which muscle stretch is damped Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.Initial stimulus(muscle stretch)Cell body ofsensory neuronSensoryneuronMuscle spindleAntagonist muscle Spinal cord123aFigure 13.17 (1 of 2), step 3bStretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist. When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord. The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.The events by which muscle stretch is damped Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch. Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).Initial stimulus(muscle stretch)Cell body ofsensory neuronSensoryneuronMuscle spindleAntagonist muscle Spinal cord123a3bFigure 13.17 (2 of 2)The patellar (knee-jerk) reflex—a specific example of a stretch reflexMusclespindleQuadriceps(extensors)Hamstrings(flexors)PatellaPatellarligamentSpinal cord(L2–L4) Tapping the patellar ligament excitesmuscle spindles in the quadriceps. The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee. Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons. The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.Excitatory synapseInhibitory synapse+–123a3b123a3b3bFigure 13.17 (2 of 2), step 1The patellar (knee-jerk) reflex—a specific example of a stretch reflexMusclespindleQuadriceps(extensors)Hamstrings(flexors)PatellaPatellarligamentSpinal cord(L2–L4) Tapping the patellar ligament excitesmuscle spindles in the quadriceps.Excitatory synapseInhibitory synapse+–11Figure 13.17 (2 of 2), step 2The patellar (knee-jerk) reflex—a specific example of a stretch reflexMusclespindleQuadriceps(extensors)Hamstrings(flexors)PatellaPatellarligamentSpinal cord(L2–L4) Tapping the patellar ligament excitesmuscle spindles in the quadriceps. Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.Excitatory synapseInhibitory synapse+–1212Figure 13.17 (2 of 2), step 3aThe patellar (knee-jerk) reflex—a specific example of a stretch reflexMusclespindleQuadriceps(extensors)Hamstrings(flexors)PatellaPatellarligamentSpinal cord(L2–L4) Tapping the patellar ligament excitesmuscle spindles in the quadriceps. The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee. Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.Excitatory synapseInhibitory synapse+–123a123aFigure 13.17 (2 of 2), step 3bThe patellar (knee-jerk) reflex—a specific example of a stretch reflexMusclespindleQuadriceps(extensors)Hamstrings(flexors)PatellaPatellarligamentSpinal cord(L2–L4) Tapping the patellar ligament excitesmuscle spindles in the quadriceps. The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee. Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons. The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.Excitatory synapseInhibitory synapse+–123a3b123a3b3bGolgi Tendon ReflexesPolysynaptic reflexesHelp to prevent damage due to excessive stretch Important for smooth onset and termination of muscle contractionGolgi Tendon ReflexesProduce muscle relaxation (lengthening) in response to tensionContraction or passive stretch activates Golgi tendon organs Afferent impulses are transmitted to spinal cord Contracting muscle relaxes and the antagonist contracts (reciprocal activation)Information transmitted simultaneously to the cerebellum is used to adjust muscle tensionFigure 13.18+Excitatory synapse–Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract.InterneuronsSpinal cordQuadriceps(extensors)GolgitendonorganHamstrings(flexors)123a3bFigure 13.18, step 1+Excitatory synapse–Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. InterneuronsSpinal cordQuadriceps(extensors)GolgitendonorganHamstrings(flexors)1Figure 13.18, step 2+Excitatory synapse–Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. InterneuronsSpinal cordQuadriceps(extensors)GolgitendonorganHamstrings(flexors)12Figure 13.18, step 3a+Excitatory synapse–Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.InterneuronsSpinal cordQuadriceps(extensors)GolgitendonorganHamstrings(flexors)123aFigure 13.18, step 3b+Excitatory synapse–Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract.InterneuronsSpinal cordQuadriceps(extensors)GolgitendonorganHamstrings(flexors)123a3bFlexor and Crossed-Extensor ReflexesFlexor (withdrawal) reflexInitiated by a painful stimulusCauses automatic withdrawal of the threatened body partIpsilateral and polysynapticFlexor and Crossed-Extensor ReflexesCrossed extensor reflexOccurs with flexor reflexes in weight-bearing limbs to maintain balanceConsists of an ipsilateral flexor reflex and a contralateral extensor reflexThe stimulated side is withdrawn (flexed)The contralateral side is extendedFigure 13.19AfferentfiberEfferentfibersExtensorinhibitedFlexorstimulatedSite of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.ArmmovementsInterneuronsEfferentfibersFlexorinhibitedExtensorstimulated+ Excitatory synapse– Inhibitory synapseSuperficial ReflexesElicited by gentle cutaneous stimulationDepend on upper motor pathways and cord-level reflex arcsSuperficial ReflexesPlantar reflexStimulus: stroking lateral aspect of the sole of the footResponse: downward flexion of the toesTests for function of corticospinal tracts Superficial ReflexesBabinski’s sign Stimulus: as aboveResponse: dorsiflexion of hallux and fanning of toesPresent in infants due to incomplete myelinationIn adults, indicates corticospinal or motor cortex damage Superficial ReflexesAbdominal reflexesCause contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skinVary in intensity from one person to anotherAbsent when corticospinal tract lesions are presentDevelopmental Aspects of the PNSSpinal nerves branch from the developing spinal cord and neural crest cellsSupply both motor and sensory fibers to developing muscles to help direct their maturationCranial nerves innervate muscles of the headDevelopmental Aspects of the PNSDistribution and growth of spinal nerves correlate with the segmented body planSensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased numbers of synapses per neuron, and slower central processingPeripheral nerves remain viable throughout life unless subjected to trauma