1、Chapter 43The Cells of the Nervous SystemNervous System CellsNervous system=neurons+glial cells神經膠質細胞Neurons carry informationGlial cells sustain neuronsIndividual nerve cells are hard to see because they are so intertwined.Golgi developed a method for staining the nerve cells.Ramon y Cajal demonstr
2、ated that the nervous system consists of cellsMost nerve cells have the same kind of organelles as other cells,sometimes more of themThe Nerve Cell(Neuron)Neuron:a cell body and cell extensions(dendrites+axons)Cell body:region of the neuron around the nucleusAxon:a single,long cell extension that of
3、ten makes distant connections;carry signals away from the cell body and to other cells;usually have myelin sheath for insulation;can transmit impulses over long distancesDendrite:short extension;relay signals from other cells to the cell body;no myelin sheath;specialized to receives impulses from se
4、nsory cells or axons or other neurons;cell may have a few or manyNeuronsGlial CellsGlial cells:Do not carry information;protect and nurture the neurons.Astrocytes:the most numerous glial cells in the brain;provide appropriate ions for nerve function as well as support.3-times more numerous than neur
5、ons Astrocyte functions:Help regulate the ion levels and chemical signals in the extracellular fluid so that neurons can transmit their signals Provide structural support and serve as guides and scaffolding during development of the nervous system Play a role in the blood-brain barrier,strictly cont
6、rols the flow of substances from the blood into the brain Myelin sheath increases the speed of nerve signals.The Insulation of AxonsMost neurons do not exist alone,but have non-neuronal companion cells nearby,called neuroglia cells(神經膠質細胞),which nourish and support neurons and form myelin sheath(髓鞘)
7、around the axon of neurons.Schwann cells:one type of neuroglial cells that associate with sensory and motor neurons of peripheral nervous system by wrapping around their long axons and forming myelin sheath.Myelin sheath,a fatty wrap,created by multiple layers of many Schwann cell membranes;insulate
8、 the axonThe uncovered spots are called nodes of RanvierThe nodes and the myelin sheath create conditions that speed the nerve impulse as it is conducted along the surface of the axonOuter membrane of Schwann cellThe Insulation of AxonsAn Overview of How Neurons WorkHow do nerve cells conduct nerve
9、impulses?Similar to transmit the voice via the telephone wire(sound energy to electrical energy then back to sound energy)Nerves respond to electrical impulses:Specialized receptor cells in the sensory organs respond to stimuli by generating electrical signals.The generated electrical signals then t
10、ravel in the nervous system.Nerve cells transmit information through nervous system from environment to brain and spinal cord,between cells within the brain,and from the brain and spinal cord to other parts of body.Nerve ImpulsesHow nerve signaling differs from electric current in a wire:Electric cu
11、rrent involves electrons jumping from atom to atom,nerve signals involve ions moving across cell membranes.Electric current moves much faster than nerve impulses Electric current loses energy as it flows,nerve signals renew their strength as they move along the nerve.Electric current varies widely i
12、n size(millions of volts to millivolts),nerve signals carry the same voltage Neurons have threshold,lowest stimulus that causes reactionDifferences in intensity of stimulus indicated by:Number of impulses per second Number of cells respondingNeuron Electrical Potential Cell membranes in neurons have
13、 channels,gated channels and pumps to control the flow of ions Maintain concentration difference across membrane Electrical charge difference across the membrane is voltage Current is the flow of ionsMeasuring Action PotentialCells are Electrically NeutralComparison of ion concentration inside and o
14、utside a typical mammalian cellComponentIntracellular concentration(mM)Extracellular concentration(mM)CationsNa+5-15145K+1405Mg2+0.51-2Ca2+10-41-2H+7 x 10-5(pH 7.2)4 x 10-5(pH 7.4)AnionsCl-5-15110*Normally,cells must contain equal quantities of+and charges(i.e.electrically neutral).In fact,most cell
15、ular components are negatively charged(HCO3-,PO43-,proteins,and nucleic acids).Sodium-Potassium PumpsThe ions that play the principal role in the development of the electrical potential are sodium ions(Na+)and potassium ions(K+)The sodium-potassium pump(Na+-K+pump)is an ATPases embedded in the membr
16、ane of neurons and many other cells,which pump Na+out of cell and K+back into cell by active transport using ATP as energy source.The Na+-K+pump works as a long-term mechanism to lower the interior sodium ion concentration.Potassium Ions Diffuse out of the CellK+can simply diffuse back out of the ce
17、ll through either voltage-gated channel during membrane depolarization or a leakage channel open to them.i.e.K+actively enter the cell through Na+-K+channel,but exit by voltage-gated channelBut Na+can only move back into the cell very slowly,Na+channel is closed at-70 mV.Moreover,this small amount o
18、f Na+leakage is usually offset by the Na+-K+pump.The Na+-K+channel works as long term mechanism that lowers the concentration of positively charged Na+within its interior.A chemical gradient of Na+w/higher conc.of Na+outside the cell than inside the cell is establishedAn electrical gradient is also
19、established,the inside of a nerve cell along its membrane is more“-”charged than the outside of the cell along the membraneThe membrane of the neuron is said to be polarized when a charge difference across the membrane is generated.Voltage-gated channelThe Neuron at Rest:The Resting PotentialHow do
20、neurons conduct electricity?At resting state,the membrane of neuron is electrically charged which inside of cell is negative relative to extracellular fluid and the charge difference measured across the membrane as the resting potential=-70 mVElectrical(membrane)potential refers to the amount of pot
21、ential energy stored in membrane by separation of positively and negatively charged ions.The net potential is due to the imbalance of charge on both side of membrane,which is due to the leakage of K+ions from the inside of cell to the outsideVoltage-gated channelConducting an Impulse:The Action Pote
22、ntialA neuron transmits a nerve impulse when it is excited by an internal or external environmental change called a stimulus(pl.stimuli).Stimuli:Sound,light,pressure,temperature,and chemical activitiesInitiation of nerve impulse transmission:by changing the electrical potential difference of the rec
23、eptor cell or nerve cell membrane at that spot.Depolarization:The change in electrical potential difference is called depolarization.Stimuli result in opening the ion channels and changing the electrical potential difference across the receptor cell or nerve cell membrane.The depolarization results
24、in opens and closes of the transmembrane Na+channel and allow Na+ions to enter the cell;i.e.,Na+channels are operated by electricityA generator potential is called when a local membrane depolarization occurs.The movement of Na+ions across the membrane induces a further depolarization of the nerve ce
25、ll membrane.If depolarization reaches threshold level,(from 70 mV to+30 mV)it triggers an action potential(“all or nothing”)in the sensory neurons.Action potential(or wave of depolarization)is a rapid reversal in the membranes electrical potential across a portion of the membrane.Action potential mo
26、ves down the membrane of nerve cells and induces a rapid local Na+influx and membrane depolarization.The depolarization of the cell membrane lasts for only a few milliseconds(ms)because the sodium channels close quickly.After sodium channel inactivation,K+ions are released from the voltage-sensitive
27、 potassium channels to the extracellular environment.The released K+ions in extracellular environment help to re-polarize the membrane(or regenerating resting potential).The inactivated Na+channel cannot be reopened until the reestablishing of membrane resting potential(called refractory).The refrac
28、tory period for sodium channel is about milliseconds long.During this period,a nerve impulse cannot be conducted through this portion of membrane and,hence,blocks the retrograde transmission of the nerve impulse.Transmission of Nerve ImpulseThe nerve impulse is an all-or-none(all-or-nothing)response
29、.The amount of stimulation applied to the receptor or neuron must be sufficient to open enough sodium channels to generate an action potential.The wave of action potential has a constant amplitude(the height or strength of the wave,from-70 mV to+30 mV).An action potential at one point on the nerve c
30、ell membrane is a stimulus to neighboring regions of the cell membrane.Myelin SheathMany nerves are covered with myelin sheaths.Insulates axon so action potentials only at gaps in sheath called“nodes of Ranvier.”This allows signal to move more quickly saltatory(jumping)conduction.Up to 100 times fas
31、ter than in unmyelinated nerves.Action potential only at nodes,saltatory conduction Axons less leakyMultiple Sclerosis;an autoimmune disease in which myelin is destroyed.Speedy Neurons:Saltatory ConductionThe axons of many sensory and motor neurons are covered by stacks of lipid-rich layers of Schwa
32、nn cell(neuroglial)membrane called myelin sheath.Myelin sheath functions as an insulator to prevent the membrane depolarization beneath it.Nodes of Ranvier are(gaps in myelin sheath):Locations of where the pumps and ion channels are found and concentrated.An action potential can only be found at the
33、 gaps and the ion movement at these spots are enhanced.Saltatory ConductionThe nerve impulse“jumps”from one node to the next and results in a faster propagation of action potential known as saltatory conductionImpulses conducted by myelinated neurons travel much faster than impulses conducted by ner
34、ve fibers without insulation.Large diameter myelinated neurons can transmit neuronal signals in a speed up to 120 meters per second.(A signal can transmit from your toes to your brain in less than one second.)Myelin Degeneration and Neuronal DiseasesMultiple sclerosis(MS硬化),one of leading causes of
35、serious neurological diseases in adults(1 in 800 in US)Results from the destruction of large patches of myelin sheath around neurons of CNS and cause the formation of hardened scars(sclerosis)in multiple places that interfere with the transmission of nerve impulse.The slowed transmission of signals
36、in the nervous system results in a gradual loss of motor activity and an alteration in the way in which impulses are interpreted in the brainInfection with certain virus,such as herpes virus that cause roseola(玫瑰疹)in children,may be a trigger which stimulating the immune system later in life to atta
37、ck not only the virus,but also the nerve-insulating myelin as well.Acute disseminated encephalomyelitis(ADE腦脊髓炎)is caused by viral infections in childhood such as measles(痲疹),German measles,smallpox(天花),chickenpox(水痘)and mumps(腮腺炎).Transmitting Information between Neurons and Other Cellssynapse(突觸):
38、The place where a neuron communicates with another neuron or an effector cell(muscles or glands)synaptic cleft:The minute space(gap)separating axon and effectors,is a place that electrical signal is changed into a chemical signalpresynaptic membrane:The membrane on the axon side of the synaptic clef
39、t Nerve impulse causes release of neurotransmitter from presynaptic membrane into synaptic cleft and then reach the receptors in the postsynaptic membraneSynapseChemical SynapseChemical SynapseMay be excitatory or inhibitory(reduce possibility of action potential),can amplify signalAt least 20 diffe
40、rent neurotransmittersAmino acids,glutamate and glycineDerived from amino acids,GABA(from Glu),serotonin(from Trp),dopamine,norepinephrine,pinephrine(from Tyr)Neurotransmitter diffuses across synaptic cleft,binds to receptors on postsynaptic cellNeuromuscular junction,neurotransmitter is acetylcholi
41、neTrigger for release is influx of Ca2+from voltage gated channelsCa2+diffuses across synaptic cleft,binds to receptorReceptor is ligand-gated channelNeurotransmitters are stored in vesicles at the end of axons.When an action potential arrives at pre-synaptic terminal,calcium flows into the cell.Tri
42、ggers the release of neurotransmitter into synapse.Neurotransmitter moves to post-synaptic cell and binds to receptors.Neuron-to-Muscle ConnectionsNeuron-to-Neuron ConnectionsAn action potential moves down axon to the pre-synapse.This depolarization stimulates a flow of Ca2+into the cell(Ca2+),occur
43、s at the pre-synaptic terminal.In response to nerve impulse-induced calcium influx,the membrane of secretory vesicles fuses with plasma membrane of the pre-synapse and triggers the release of neurotransmitters from synaptic vesicles into synaptic cleft.Neurotransmitters are small organic molecules s
44、tored in thousands of small,membrane-bound sacs(secretory vesicles).Each sac contains from 10,000 to 100,000 molecules of neurotransmitters.Neurotransmitters diffuse to the other side of the gap and combine(stimulate)w/specific receptors in postsynaptic membrane.Receptor stimulation opens ion channe
45、ls and propagates next action potential;Synapse may be excitatory or inhibitoryDifferent neurotransmitters bind to different receptors:An advantage for using chemical junctions over direct electrical connectionsChemicals such as drugs can compete w/neurotransmitters and modify the activity of neurot
46、ransmitters How Neurotransmitters WorkReceptorsNeurotransmitters can have different effects on different cellsAcetylcholine(Ac)receptors come in two varieties:Nicotinic receptors:in skeletal muscles,cause contractions Muscarinic毒菌鹼receptors:in smooth muscle and heart muscle,Ac inhibits production of
47、 action potential,i.e.cause inhibition Results:Ac reduces the heart rate and slows the peristaltic contractions of the smooth muscles in the intestineEnding Signal Neurotransmitters must be cleared from the synaptic cleft after impulse by three strategies Diffusion:diffuse away from the synaptic cle
48、ft Reuptake:back into the presynaptic cleft or into surrounding glial cells Enzymatic breakdown:destroy in the synaptic cleft using extracellular enzymesPsychoactive DrugsNot all the molecules that bind to neurotransmitter receptors are neurotransmittersPsychoactive drugs can bind to neurotransmitte
49、r receptors Ex.Nicotine(from tobacco),curare(a frog toxin used in poison darts),a-bungarotoxin(from snake venom)-all bind to nicotinic acetylcholine receptor Nicotine activates acetylcholine receptor Curare箭毒(frog toxin)blocks receptor,prevents muscle contractionAtropine 阿托品;顛茄鹼from belladonna莨菪plan
50、t blocks muscarine receptors,relaxes intestines,inhibits diarrheaPesticides,malathion馬拉松and parathion巴拉松inhibit breakdown of acetylcholine overstimulate muscle contractionsBenzodiazepines(anti-anxiety)increase effectiveness of GABAProzac 百憂解(SSRI)blocks reuptake of serotonin,elevates moodPainkillers
