Chapter 48Chapter 48Neurons, Synapses, and Signaling

Fig. 48-2

Nerveswith giant axonsGanglia

MantleEye

Brain

Arm

Nerve

The squid possesses extremely large nerve cells and is a good model for studying neuron function

Fig. 48-8

TECHNIQUE

Microelectrode

Voltagerecorder

Referenceelectrode

Intracellular recording

Fig. 48-3

Sensor

Sensory input

Integration

Effector

Motor output

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Fig. 48-4

Dendrites

Stimulus

Nucleus

Cellbody

Axonhillock

Presynapticcell

Axon

Synaptic terminalsSynapse

Postsynaptic cellNeurotransmitter

Fig. 48-5

Dendrites

Axon

Cellbody

Sensory neuron Interneurons

Portion of axon Cell bodies of

overlapping neurons

80 µm

Motor neuron

Fig. 48-6

OUTSIDECELL

[K+]5 mM

[Na+]150 mM

[Cl–]120 mM

INSIDECELL

[K+]140 mM

[Na+]15 mM

[Cl–]10 mM

[A–]100 mM

(a) (b)

OUTSIDECELL

Na+Key

K+

Sodium-potassiumpump

Potassiumchannel

Sodiumchannel

INSIDECELL

Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron

Modeling of the Resting Potential

• Resting potential can be modeled by an artificial membrane that separates two chambers

– The concentration of KCl is higher in the inner chamber and lower in the outer chamber

– K+ diffuses down its gradient to the outer chamber

– Negative charge builds up in the inner chamber

• At equilibrium, both the electrical and chemical gradients are balanced

The plasma membrane of a resting neuron contains many open potassium channels but only a few open sodium channels)

Fig. 48-7

Innerchamber

Outerchamber

–90 mV

140 mM 5 mM

KCIKCI

K+

Cl–

Potassiumchannel

(a) Membrane selectively permeable to K+ (b) Membrane selectively permeable to Na+

+62 mV

15 mMNaCI

Cl–

150 mMNaCI

Na+

Sodiumchannel

EK = 62 mV (log5 mM

140 mM ) = –90 mV ENa = 62 mV (log 150 mM15 mM

= +62 mV)

1. Chemical concentrations

2. Ion channels— selective permeability or gated ion channels

Fig. 48-9

Stimuli

+50 +50

Stimuli

00

Mem

bra

ne

po

ten

tial

(m

V)

Mem

bra

ne

po

ten

tial

(m

V)

–50 –50Threshold Threshold

Restingpotential

Restingpotential

Hyperpolarizations–100 –100

0 1 2 3 4 5

Time (msec)

(a) Graded hyperpolarizations

Time (msec)

(b) Graded depolarizations

Depolarizations

0 1 2 3 4 5

Strong depolarizing stimulus

+50

0

Mem

bra

ne

po

ten

tial

(m

V)

–50 Threshold

Restingpotential

–100

Time (msec)0 1 2 3 4 5 6

(c) Action potential

Actionpotential

Graded potentials:

the magnitude of the change in membrane potential varies with the the strength of the stimulus.

Fig. 48-9a

Stimuli

+50

Mem

bra

ne

po

ten

tia

l (m

V)

–50 Threshold

Restingpotential

Hyperpolarizations–100

0 2 3 4

Time (msec)

(a) Graded hyperpolarizations

0

1 5

Hyperpolarization:

Stimulusincrease the outflow of positive ions or the inflow of negative ionsthe separation of charge or polarity increase.

(-60~-80-90 mV)

K+

Fig. 48-9b

Stimuli

+50

Mem

bra

ne

po

ten

tia

l (m

V)

–50 Threshold

Restingpotential

Depolarizations

–1000 2 3 4

Time (msec)

(b) Graded depolarizations

1 5

0

Depolarizatiom:

The reduction in the magnitude of the membrane potentials

often involves gated sodium channels

Na+

Fig. 48-9c

Strong depolarizing stimulus

+50

Mem

bra

ne

po

ten

tia

l (m

V)

–50 Threshold

Restingpotential

–1000 2 3 4

Time (msec)

Action potential

1.Threshold2. All or none

1 5

0

Actionpotential

6

A voltage-gated ion channels that open or close in response to a change in the membrane potential

-55 mV

Na+

Fig. 48-10-1

KeyNa+

K+

+50

Actionpotential

Threshold

0

1

4

51

–50

Resting potential

Mem

bra

ne

po

ten

tial

(mV

)

–100Time

Extracellular fluid

Plasmamembrane

Cytosol

Inactivation loop

Resting state

Sodiumchannel

Potassiumchannel

Depolarization

Undershoot

2

3

1

Fig. 48-10-2

KeyNa+

K+

+50

Actionpotential

Threshold

0

1

4

51

–50

Resting potential

Mem

bra

ne

po

ten

tial

(mV

)

–100Time

Extracellular fluid

Plasmamembrane

Cytosol

Inactivation loop

Resting state

Sodiumchannel

Potassiumchannel

Depolarization

Undershoot

2

3

2

1

Fig. 48-10-3

KeyNa+

K+

+50

Actionpotential

Threshold

0

1

4

51

–50

Resting potential

Mem

bra

ne

po

ten

tial

(mV

)

–100Time

Extracellular fluid

Plasmamembrane

Cytosol

Inactivation loop

Resting state

Sodiumchannel

Potassiumchannel

Depolarization

Rising phase of the action potential

Undershoot

2

3

2

1

3

Fig. 48-10-4

KeyNa+

K+

+50

Actionpotential

Threshold

0

1

4

51

–50

Resting potential

Mem

bra

ne

po

ten

tial

(mV

)

–100Time

Extracellular fluid

Plasmamembrane

Cytosol

Inactivation loop

Resting state

Sodiumchannel

Potassiumchannel

Depolarization

Rising phase of the action potential Falling phase of the action potential

Undershoot

2

3

2

1

3 4

Refracory periods:

1. The inactivation of sodium channels

In one direction

Fig. 48-10-5

KeyNa+

K+

+50

Actionpotential

Threshold

0

1

4

51

–50

Resting potential

Mem

bra

ne

po

ten

tial

(mV

)

–100Time

Extracellular fluid

Plasmamembrane

Cytosol

Inactivation loop

Resting state

Sodiumchannel

Potassiumchannel

Depolarization

Rising phase of the action potential Falling phase of the action potential

5 Undershoot

2

3

2

1

3 4

The membrane’s permeability to K+ than it is at the resting potentials

Fig. 48-11-1

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Fig. 48-11-2

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

Fig. 48-11-3

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

ActionpotentialK+

K+

Na+

Conduction Speed

• The speed of an action potential increases with the axon’s diameter

• In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase

• Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

Fig. 48-12

Axon

Schwanncell

Myelin sheathNodes ofRanvier

Node of Ranvier

Schwanncell

Nucleus ofSchwann cell

Layers of myelinAxon

0.1 µm

A. myenlin sheath

1.Oligodendrocyte (CNS)and Schwann cells (PNS)

2. As increasing the axon’s diameter: Its space efficency

B. Those myelinated axons can be packed into the space

Fig. 48-13

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

Axon

Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

Fig. 48-14

Postsynapticneuron

Synapticterminalsof pre-synapticneurons

5 µ

m

Electrical synapseChemical synapse

Fig. 48-15

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+

• Postsynaptic potentials fall into two categories:

– Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold

– Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold

• After release, the neurotransmitter– May diffuse out of the synaptic cleft– May be taken up by surrounding cells – May be degraded by enzymes

Generation of Postsynaptic Potentials

Fig. 48-16

Terminal branchof presynapticneuron

E1

E2

I

Postsynapticneuron

Threshold of axon ofpostsynaptic neuron

Restingpotential

E1 E1

0

–70

Mem

bra

ne

po

ten

tial

(m

V)

(a) Subthreshold, no summation

(b) Temporal summation

E1 E1

Actionpotential

I

Axonhillock

E1

E2 E2

E1

I

Actionpotential

E1 + E2

(c) Spatial summation

I

E1 E1 + I (d) Spatial summation

of EPSP and IPSP

E2

E1

I

Generation of Postsynaptic Potentials

Summation of Postsynaptic Potentials

• Unlike action potentials, postsynaptic potentials are graded and do not regenerate

• Most neurons have many synapses on their dendrites and cell body

• A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron

• If two EPSPs are produced in rapid succession, an effect called temporal summation occurs

Fig. 48-16ab

E1

E2

Postsynapticneuron

I

Terminal branchof presynapticneuron

E2

0

–70

Threshold of axon ofpostsynaptic neuron

Restingpotential

Mem

bra

ne

po

ten

tia

l (m

V)

E1E1

(a) Subthreshold, no summation

(b) Temporal summation

E1 E1

Actionpotential

I

Axonhillock

E1

• In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

• The combination of EPSPs through spatial and temporal summation can trigger an action potential– Through summation, an IPSP can counter the effect of an

EPSP

– The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential

Fig. 48-16cd

E1

E2

Mem

bra

ne

po

ten

tia

l (m

V)

Actionpotential

I

E1 + E2

(c) Spatial summation (d) Spatial summation of EPSP and IPSP

E1

E1 E1 + II

I

E2

0

–70

Table 48-1

Neurotransmitters

Table 48-1a

Acetylcholine ~is a common neurotransmitter in vertebrates and invertebrates~ In vertebrates it is usually an excitatory transmitter

Biogenic amines include epinephrine, norepinephrine, dopamine, and serotonin

---They are active in the CNS and PNS

Table 48-1b

Two amino acids are known to function as major neurotransmitters in the CNS: gamma-aminobutyric acid (GABA) and glutamate

substance P and endorphins, which both affect our perception of painOpiates bind to the same receptors as endorphins and can be used as painkillers

Gases such as nitric oxide and carbon monoxide are local regulators in the PNS

Heme oxygenase CO regulates the release of hypothalamic hormones in CNSIn the PNS, it acts as an inhibitory neurotransmitter that hyperpolarizes intestinal smooth muscle cells.

Fig. 48-17EXPERIMENT

RESULTS

Radioactivenaloxone

Drug

Proteinmixture

Proteinstrapped on filter

Measure naloxonebound to proteinson each filter

Fig. 48-UN1

Action potential

Fallingphase

Risingphase

Threshold (–55)

Restingpotential

Undershoot

Time (msec)

Depolarization–70–100

–50

0

+50

Me

mb

ran

e p

ote

nti

al (

mV

)

You should now be able to:

1. Distinguish among the following sets of terms: sensory neurons, interneurons, and motor neurons; membrane potential and resting potential; ungated and gated ion channels; electrical synapse and chemical synapse; EPSP and IPSP; temporal and spatial summation

2. Explain the role of the sodium-potassium pump in maintaining the resting potential

3. Describe the stages of an action potential; explain the role of voltage-gated ion channels in this process

4. Explain why the action potential cannot travel back toward the cell body

5. Describe saltatory conduction

6. Describe the events that lead to the release of neurotransmitters into the synaptic cleft

7. Explain the statement: “Unlike action potentials, which are all-or-none events, postsynaptic potentials are graded”

8. Name and describe five categories of neurotransmitters