neural signaling chapter 40. learning objective 1 describe the processes involved in neural...
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Neural SignalingNeural Signaling
Chapter 40Chapter 40
Learning Objective 1Learning Objective 1
• Describe the processes involved in neural Describe the processes involved in neural signaling: signaling: receptionreception, , transmissiontransmission, , integrationintegration, and , and action by effectorsaction by effectors
Neural Signaling 1Neural Signaling 1
(1) (1) ReceptionReception of information of information• by a sensory receptorby a sensory receptor
(2) (2) TransmissionTransmission by an by an afferent neuronafferent neuron• to the to the central nervous system (CNS)central nervous system (CNS)
(3) (3) IntegrationIntegration by by interneuronsinterneurons• in the in the central nervous system (CNS)central nervous system (CNS)
Neural Signaling 2Neural Signaling 2
(4) Transmission by an (4) Transmission by an efferent neuronefferent neuron• to other neurons or effectorto other neurons or effector
(5) (5) Action by effectorsAction by effectors• the muscles and glandsthe muscles and glands
Peripheral Nervous System (PNS)Peripheral Nervous System (PNS)
• Made up of Made up of • sensory receptors sensory receptors • neurons outside the CNSneurons outside the CNS
Response to Response to StimulusStimulus
Fig. 40-1a, p. 846
External stimulus (e.g., vibration, movement,
light, odor)
Internal stimulus (e.g., change in blood pH or blood pressure)
RECEPTION
Detection by external
sense organs
Detection by internal
sense organs
TRANSMISSION
Sensory (afferent) neurons transmit information
Fig. 40-1b, p. 846
Central Nervous System (brain and spinal cord)
INTEGRATIONInterneurons sort
and interpret information
TRANSMISSION
Motor (efferent) neurons transmit impulses
ACTION BY EFFECTORS (muscles and glands)
e.g., animal runs away
e.g., espiration rate increases; blood pressure rises
Fig. 40-1, p. 846
Stepped Art
External stimulus (e.g., vibration, movement,
light, odor)
Internal stimulus (e.g., change in blood pH or blood pressure)
RECEPTION
Detection by external sense organs
Detection by internal
sense organsTRANSMISSION
Sensory (afferent) neurons transmit information
Central Nervous System (brain and spinal cord)
INTEGRATION
Interneurons sort and interpret information
TRANSMISSIONMotor (efferent) neurons
transmit impulses
ACTION BY EFFECTORS (muscles and
glands)e.g., animal
runs away
e.g., espiration rate increases; blood pressure
rises
KEY CONCEPTSKEY CONCEPTS
• Neural signaling involves reception, Neural signaling involves reception, transmission, integration, and action by transmission, integration, and action by effectorseffectors
Learning Objective 2Learning Objective 2
• What is the structure of a typical What is the structure of a typical neuronneuron??
• Give the function of each of its partsGive the function of each of its parts
NeuronsNeurons
• Specialized to Specialized to • receive stimuli receive stimuli • transmit electrical and chemical signalstransmit electrical and chemical signals
• Cell bodyCell body • contains nucleus and organelles contains nucleus and organelles
DendritesDendrites
• Many branched Many branched dendritesdendrites• extend from cell body of neuronextend from cell body of neuron• specialized to receive stimuli and send signals specialized to receive stimuli and send signals
to the cell bodyto the cell body
Axons 1Axons 1
• A single long A single long axonaxon • extends from neuron cell body extends from neuron cell body • forms branches (forms branches (axon collaterals)axon collaterals)
• Transmits signals into Transmits signals into terminal branchesterminal branches• which end in which end in synaptic terminalssynaptic terminals
Axons 2Axons 2
• Myelin sheathMyelin sheath • surrounds many axonssurrounds many axons• insulatesinsulates
• Schwann cellsSchwann cells • form the myelin sheath in the PNS form the myelin sheath in the PNS
Axons 3Axons 3
• In the CNSIn the CNS• sheath is formed by other glial cells sheath is formed by other glial cells
• Nodes of RanvierNodes of Ranvier • gaps in sheath between successive Schwann gaps in sheath between successive Schwann
cells cells
Neuron StructureNeuron Structure
Fig. 40-2, p. 847
Dendrites covered with dendritic spines
Cytoplasm of Schwann cell
Synaptic terminals
AxonAxon collateralCell body
Nucleus
Myelin sheath
NucleusAxon Nodes of
RanvierSchwann cell
Terminal branches
Nerves and GangliaNerves and Ganglia
• NerveNerve • several hundred axons several hundred axons • wrapped in connective tissuewrapped in connective tissue
• GanglionGanglion • mass of neuron cell bodies in the PNSmass of neuron cell bodies in the PNS
Nerve StructureNerve Structure
Fig. 40-3a, p. 848
Ganglion
Cell bodies
Myelin sheath
ArteryVein Axon
(a)
Fig. 40-3b, p. 848
100 µm(b)
Learn more about the structure Learn more about the structure of neurons and nerves by of neurons and nerves by clicking on the figures in clicking on the figures in
ThomsonNOW.ThomsonNOW.
Learning Objective 3Learning Objective 3
• Name the main types of Name the main types of glial cellsglial cells
• Describe the functions of eachDescribe the functions of each
Glial CellsGlial Cells
• Support and nourish neuronsSupport and nourish neurons
• Are important in neural communication Are important in neural communication
Glial Cell Types 1Glial Cell Types 1
• AstrocytesAstrocytes • physically support neurons physically support neurons • regulate extracellular fluid in CNS (by taking regulate extracellular fluid in CNS (by taking
up excess potassium ions) up excess potassium ions) • communicate with one another (and with communicate with one another (and with
neurons)neurons)• induce and stabilize synapsesinduce and stabilize synapses
Glial Cell Types 2Glial Cell Types 2
• OligodendrocytesOligodendrocytes • form myelin sheaths around axons in CNSform myelin sheaths around axons in CNS
• Schwann cellsSchwann cells • form sheaths around axons in PNSform sheaths around axons in PNS
Glial Cell Types 3Glial Cell Types 3
• Microglia Microglia • Phagocytic cellsPhagocytic cells
• Ependymal Cells Ependymal Cells • line cavities in the CNS line cavities in the CNS • contribute to formation of cerebrospinal fluid contribute to formation of cerebrospinal fluid • serve as neural stem cellsserve as neural stem cells
KEY CONCEPTSKEY CONCEPTS
• Neurons are specialized to receive stimuli Neurons are specialized to receive stimuli and transmit signals; glial cells are and transmit signals; glial cells are supporting cells that protect and nourish supporting cells that protect and nourish neurons and that can modify neural neurons and that can modify neural signalssignals
Learning Objective 4Learning Objective 4
• How does the neuron develop and How does the neuron develop and maintain a maintain a resting potentialresting potential??
Neural SignalsNeural Signals
• Electrical signals transmit informationElectrical signals transmit information• along axons along axons
• Plasma membrane of resting neuron (not Plasma membrane of resting neuron (not transmitting an impulse) is transmitting an impulse) is polarizedpolarized
• Inner surface of plasma membrane is Inner surface of plasma membrane is negatively chargednegatively charged
• relative to extracellular fluidrelative to extracellular fluid
Resting PotentialResting Potential
• Potential difference of about -70 mV Potential difference of about -70 mV • across the membraneacross the membrane
• Magnitude of resting potentialMagnitude of resting potential(1) differences in ion concentrations (Na(1) differences in ion concentrations (Na++, K, K++) )
inside cell relative to extracellular fluid inside cell relative to extracellular fluid
(2) selective permeability of plasma membrane (2) selective permeability of plasma membrane to these ionsto these ions
Fig. 40-4a, p. 850
Axon
40
20
0
–20
–40
–60 –70 mV
–80Time
Amplifier
Plasma membrane
Electrode placed inside the cell Electrode placed
outside the cell+
–– –
– – – –
– –– – +
++
+ +–
+ +
+ +
+ +
(a) Measuring the resting potential of a neuron.
IonsIons
• Pass through specific Pass through specific passive ion channelspassive ion channels• KK++ leak out faster than Na leak out faster than Na++ leak in leak in • ClCl-- accumulate at inner surface of plasma accumulate at inner surface of plasma
membrane membrane
• Large anions (proteins)Large anions (proteins)• cannot cross plasma membranecannot cross plasma membrane• contribute negative chargescontribute negative charges
Sodium–Potassium PumpsSodium–Potassium Pumps
• Maintain gradients that determine resting Maintain gradients that determine resting potential potential
• transport 3 Natransport 3 Na++ out for each 2 K out for each 2 K++ in in
• Require ATPRequire ATP
Fig. 40-4b, p. 850
Diffusion out
Extracellular fluid
Plasma membrane
Cytoplasm
2 K+Diffusion in
Na /K pump
3 Na+
K+ Na+
CI–
K+
K+
K+
K+
K+
K+ K+
K+
Na+ Na+
Na+
Na+
Na+
Na+
A_ A
_A_
CI–
CI–
CI–
CI–CI–
+ + + + + +
– – – – – –
(b) Permeability of the neuron membrane.
KEY CONCEPTSKEY CONCEPTS
• The resting potential of a neuron is The resting potential of a neuron is maintained by differences in maintained by differences in concentrations of specific ions inside the concentrations of specific ions inside the cell relative to the extracellular fluid and by cell relative to the extracellular fluid and by selective permeability of the plasma selective permeability of the plasma membrane to these ionsmembrane to these ions
Learning Objective 5Learning Objective 5
• Compare a Compare a graded potentialgraded potential with an with an action action potentialpotential
• Describe the production and transmission Describe the production and transmission of eachof each
Membrane PotentialMembrane Potential
• Membrane is Membrane is depolarizeddepolarized • if stimulus causes membrane potential to if stimulus causes membrane potential to
become less negativebecome less negative
• Membrane is Membrane is hyperpolarizedhyperpolarized• if membrane potential becomes more if membrane potential becomes more
negative than resting potentialnegative than resting potential
Graded PotentialGraded Potential
• A local response A local response
• Varies in magnitudeVaries in magnitude• depending on strength of applied stimulus depending on strength of applied stimulus
• Fades out Fades out • within a few millimeters of point of originwithin a few millimeters of point of origin
Action Potential 1Action Potential 1
• Action potentialAction potential is a is a wave of depolarizationwave of depolarization • that moves down the axonthat moves down the axon
• Generated whenGenerated when• voltage across the membrane declines to a voltage across the membrane declines to a
critical point critical point (threshold level)(threshold level)• voltage-activated ion channelsvoltage-activated ion channels open open• NaNa++ ions flow into the neuron ions flow into the neuron
Voltage-Activated Ion ChannelsVoltage-Activated Ion Channels
Fig. 40-6, p. 852
Extracellular fluid
Activation gate
Cytoplasm Inactivation gate
(b) Potassium channels.(a) Sodium channels.
Voltage-Activated Ion ChannelsVoltage-Activated Ion ChannelsDuring an Action PotentialDuring an Action Potential
Fig. 40-7a, p. 853
Spike
Depolarization Repolarization
Threshold level
Resting state
Mem
bra
ne
po
ten
tial
(m
V)
Time (milliseconds)
(a) Action potential.
Fig. 40-7b, p. 853
Axon
Extracellular fluid
Sodium channel
Potassium channel
Cytoplasm
Resting state. Depolarization. Repolarization. Return to resting state.
1 2 3 4
(b) The action of the ion channels in the plasma membrane determines the state of the neuron.
Action Potential 2Action Potential 2
• An An all-or-noneall-or-none response response• no variation in strength of a single impulseno variation in strength of a single impulse• either membrane potential exceeds either membrane potential exceeds threshold threshold
levellevel or it does not or it does not
• Once begun, an Once begun, an action potentialaction potential is self- is self-propagatingpropagating
RepolarizationRepolarization
• As an action potential moves down an As an action potential moves down an axon, axon, repolarizationrepolarization occurs behind it occurs behind it
Transmission of Transmission of an Action an Action PotentialPotential
Fig. 40-8a, p. 854
Stimulus
Axon
Area of depolarization
Potassium channel
Sodium channelAction potential
(1) Action potential is transmitted as wave of depolarization that travels down axon. At region of depolarization, Na+ diffuse into cell.
Fig. 40-8b, p. 854
Area of repolarization Area of depolarization
Action potential
(2) As action potential progresses along axon, repolarization occurs quickly behind it.
Refractory PeriodsRefractory Periods
• During During depolarizationdepolarization, the axon enters an , the axon enters an absolute refractory periodabsolute refractory period
• when it can’t transmit another action potential when it can’t transmit another action potential
• When enough gates controlling NaWhen enough gates controlling Na++ channels have been reset, the neuron channels have been reset, the neuron enters a enters a relative refractory periodrelative refractory period
• when the threshold is higherwhen the threshold is higher
Learn more about ion channels Learn more about ion channels and action potentials by clicking and action potentials by clicking on the figures in ThomsonNOW.on the figures in ThomsonNOW.
KEY CONCEPTSKEY CONCEPTS
• Depolarization of the neuron plasma Depolarization of the neuron plasma membrane to threshold level generates an membrane to threshold level generates an action potential, an electrical signal that action potential, an electrical signal that travels as a wave of depolarization along travels as a wave of depolarization along the axonthe axon
Learning Objective 6Learning Objective 6
• Contrast Contrast continuous conductioncontinuous conduction with with saltatory conductionsaltatory conduction
Continuous ConductionContinuous Conduction
• Involves entire axon plasma membraneInvolves entire axon plasma membrane
• Takes place in unmyelinated neuronsTakes place in unmyelinated neurons
Saltatory ConductionSaltatory Conduction
• Depolarization skips along axon from one Depolarization skips along axon from one node of Ranviernode of Ranvier to the next to the next
• more rapid than continuous conductionmore rapid than continuous conduction• takes place in myelinated neuronstakes place in myelinated neurons
• Nodes of RanvierNodes of Ranvier • sites where axon is not covered by myelinsites where axon is not covered by myelin• NaNa++ channels are concentrated channels are concentrated
Saltatory Saltatory ConductionConduction
Fig. 40-9a, p. 855
Area of action potential
Saltatory conduction
Nodes of Ranvier
AxonSchwann cell
2
1
Fig. 40-9b, p. 855
Direction of depolarization
3
4
Learning Objective 7Learning Objective 7
• Describe the actions of the Describe the actions of the neurotransmittersneurotransmitters identified in the chapter identified in the chapter
SynapsesSynapses
• Junctions between two neuronsJunctions between two neurons• or between a neuron and effectoror between a neuron and effector
• Most synapses are chemicalMost synapses are chemical• some are electrical synapses some are electrical synapses
Synaptic TransmissionSynaptic Transmission
• A A presynaptic neuronpresynaptic neuron releases releases neurotransmitterneurotransmitter (chemical messenger) (chemical messenger) from its from its synaptic vesiclessynaptic vesicles
Neurotransmitters 1Neurotransmitters 1
• AcetylcholineAcetylcholine • triggers contraction of skeletal muscletriggers contraction of skeletal muscle
• Biogenic aminesBiogenic amines• norepinephrine, serotonin, dopaminenorepinephrine, serotonin, dopamine• important in regulating mood important in regulating mood • dopaminedopamine is also important in motor function is also important in motor function
Neurotransmitters 2Neurotransmitters 2
• Some Some amino acidsamino acids• glutamateglutamate (excitatory neurotransmitter in brain) (excitatory neurotransmitter in brain)• GABAGABA (widespread inhibitory neurotransmitter) (widespread inhibitory neurotransmitter)
• NeuropeptidesNeuropeptides (opioids) (opioids)• endorphinsendorphins (e.g. (e.g. beta-endorphinbeta-endorphin) ) • enkephalinsenkephalins
Neurotransmitters 3Neurotransmitters 3
• Nitric oxide (NO)Nitric oxide (NO) • gaseous neurotransmittergaseous neurotransmitter• transmits signals from postsynaptic neuron to transmits signals from postsynaptic neuron to
presynaptic neuron (opposite direction from presynaptic neuron (opposite direction from other neurotransmitters)other neurotransmitters)
Learning Objective 8Learning Objective 8
• Trace the events that take place in Trace the events that take place in synaptic transmissionsynaptic transmission
• Draw diagrams to support your descriptionDraw diagrams to support your description
Synaptic TransmissionSynaptic Transmission
• Calcium ions cause synaptic vesicles to Calcium ions cause synaptic vesicles to fuse with presynaptic membranefuse with presynaptic membrane
• releases neurotransmitter into releases neurotransmitter into synaptic cleft synaptic cleft
• Neurotransmitter diffuses across the Neurotransmitter diffuses across the synaptic cleft synaptic cleft
• combines with specific receptors on a combines with specific receptors on a postsynaptic neuronpostsynaptic neuron
Synaptic TransmissionSynaptic Transmission
Fig. 40-10a, p. 858
Synaptic vesicles
Plasma membrane of postsynaptic
neuron
0.25 µm
(a) The TEM shows synaptic terminals filled with synaptic vesicles.
Fig. 40-10bc, p. 858
Fig. 40-10b, p. 858
Axon of presynaptic neuron
Synaptic terminal
Voltage-gated Ca2+ channel 1
Ca2+Synaptic vesicle2
Neuro- transmitter molecule
3
4
Ligand-gated channels
Postsynaptic membrane
5
Postsynaptic neuron
Receptor for neurotransmitter
(b) How a neural impulse is transmitted across a synapse.
Fig. 40-10c, p. 858
Synaptic terminal
Ca2+
Presynaptic membrane
Synaptic cleft
Na+
Postsynaptic membrane
(c) Neurotransmitter binds with receptor. Ligand-gated channel opens, resulting in depolarization.
Neurotransmitter ReceptorsNeurotransmitter Receptors
• Many are proteins that form Many are proteins that form ligand-gated ligand-gated ion channels ion channels
• Others work through a Others work through a second messengersecond messenger such as such as cAMPcAMP
Learn more about synaptic Learn more about synaptic transmission by clicking on the transmission by clicking on the
figure in ThomsonNOW.figure in ThomsonNOW.
KEY CONCEPTSKEY CONCEPTS
• Neurons signal other cells by releasing Neurons signal other cells by releasing neurotransmitters at synapsesneurotransmitters at synapses
Learning Objective 9Learning Objective 9
• Compare Compare excitatoryexcitatory and and inhibitoryinhibitory signals signals and their effectsand their effects
Binding of Neurotransmitter Binding of Neurotransmitter to a Receptorto a Receptor
• Binding causes eitherBinding causes either• excitatory postsynaptic potential (EPSP)excitatory postsynaptic potential (EPSP)• oror inhibitory postsynaptic potential (IPSP) inhibitory postsynaptic potential (IPSP)
• Depending on the type of receptorDepending on the type of receptor
EPSPs and IPSPsEPSPs and IPSPs
• EPSPsEPSPs • bring neuron closer to firing bring neuron closer to firing
• IPSPsIPSPs • move neuron farther away from its firing levelmove neuron farther away from its firing level
Learning Objective 10Learning Objective 10
• Define Define neural integrationneural integration
• Describe how a postsynaptic neuron Describe how a postsynaptic neuron integrates incoming stimuli and “decides” integrates incoming stimuli and “decides” whether or not to firewhether or not to fire
Neural IntegrationNeural Integration
• Process of summing (integrating) incoming Process of summing (integrating) incoming signalssignals
• SummationSummation • process of adding and subtracting incoming process of adding and subtracting incoming
signalssignals
SummationSummation
• Each EPSP or IPSP is a graded potentialEach EPSP or IPSP is a graded potential• vary in magnitudevary in magnitude• depending on strength of stimulus applied depending on strength of stimulus applied
• SummationSummation of several EPSPs of several EPSPs• brings neuron to critical firing levelbrings neuron to critical firing level
Temporal SummationTemporal Summation
• Occurs when repeated stimuli cause new Occurs when repeated stimuli cause new EPSPs to develop before previous EPSPs EPSPs to develop before previous EPSPs have decayedhave decayed
Spatial SummationSpatial Summation
• Occurs when several closely spaced Occurs when several closely spaced synaptic terminals release synaptic terminals release neurotransmitter simultaneouslyneurotransmitter simultaneously
• stimulating postsynaptic neuron at several stimulating postsynaptic neuron at several different placesdifferent places
Neural IntegrationNeural Integration
Fig. 40-11, p. 860
Threshold level
Resting potential
Po
stsy
nap
tic
mem
bra
ne
po
ten
tial
(m
V)
Time (msec)
(a) Subthreshold (no summation).
(b) Temporal summation.
(c) Spatial summation.
(d) Spatial summation of EPSPs and IPSPs.
KEY CONCEPTSKEY CONCEPTS
• During integration, incoming neural signals During integration, incoming neural signals are summed; temporal and spatial are summed; temporal and spatial summation can bring a neuron to summation can bring a neuron to threshold levelthreshold level
Learning Objective 11Learning Objective 11
• Distinguish among Distinguish among convergenceconvergence, , divergencedivergence, and , and reverberationreverberation
• Explain why each is importantExplain why each is important
Neural CircuitsNeural Circuits
• Complex Complex neural circuitsneural circuits are possible are possible because of associations such as because of associations such as convergenceconvergence and and divergencedivergence
ConvergenceConvergence
• A single neuron is affected by A single neuron is affected by converging converging signals from two or more presynapticsignals from two or more presynaptic neurons neurons
• Allows CNS to integrate incoming Allows CNS to integrate incoming information from various sourcesinformation from various sources
DivergenceDivergence
• A single presynaptic neuron stimulates A single presynaptic neuron stimulates many postsynaptic neuronsmany postsynaptic neurons
• allowing widespread effectallowing widespread effect
Neural CircuitsNeural Circuits
Fig. 40-12, p. 861
(a) Convergence of neural input. Several presynaptic neurons synapse with one postsynaptic neuron.
(b) Divergence of neural output. A single presynaptic neuron synapses with many postsynaptic neurons.
Reverberating CircuitsReverberating Circuits
• Important in Important in • rhythmic breathingrhythmic breathing• mental alertness mental alertness • short-term memory short-term memory
• Depend on positive feedbackDepend on positive feedback• new impulses generated again and again until new impulses generated again and again until
synapses fatigue synapses fatigue
Reverberating CircuitsReverberating Circuits
Fig. 40-13a, p. 861
1 2
(a) Simple reverberating circuit. An axon collateral of the second neuron turns back on its own dendrites, so the neuron continues to stimulate itself.
Fig. 40-13b, p. 861
Interneuron
Axon collateral
1 2 3
(b) Reverberating circuit with interneuron. An axon collateral of the second neuron synapses with an interneuron. The interneuron synapses with the first neuron in the sequence. New impulses are triggered again and again in the first neuron, causing reverberation.