neural signaling

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.