Chapter 48

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

Biology Eighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 48Chapter 48

Neurons, Synapses, and Signaling


Overview: Lines of Communication

• Cone snail kills prey with venom that disables neurons

• Neurons are nerve cells that transfer information within the body

• Two types of signals to communicate: electrical (long-distance)chemical (short-distance)


• The transmission of information depends on the path of neurons along which a signal travels

• Processing of information

simple clusters of neurons called ganglia

more complex organization of neurons called a brain


Concept 48.1: Neuron organization and structure reflect function in information transfer

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


Introduction to Information Processing

Nervous systems process three stages: sensory input, integration, and motor output

1) Sensors detect external stimuli and internal conditions and transmit information along sensory neurons

2) Sensory information is sent to the brain/ganglia, where interneurons integrate the information

3) Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity


• Many animals have a complex nervous system which consists of:

– A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord

– A peripheral nervous system (PNS), which brings information into and out of the CNS

Fig. 48-3

Sensor

Sensory input

Integration

Effector

Motor output

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)


Neuron Structure and Function• Most of a neuron’s organelles are in the cell

body

• Dendrites - highly branched extensions that receive signals from other neurons

• Axon - a much longer extension that transmits signals to other cells at synapses (connection point between cells where information is passed in the form of chemical messengers called neurotransmitters)

• An axon joins the cell body at the axon hillock


• Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)

• Most neurons are nourished or insulated by cells called glia

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

Form Fits Function - Variation


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

• Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential

• Messages are transmitted as changes in membrane potential

• The resting potential is the membrane potential of a neuron not sending signals


Formation of the Resting Potential

• In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell (Overall Negative Internally)

• Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane

• These concentration gradients represent chemical potential energy


• The opening of ion channels in the plasma membrane converts chemical potential to electrical potential

• A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell

• Anions trapped inside the cell contribute to the negative charge within the neuron

Animation: Resting PotentialAnimation: Resting Potential

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


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

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)


• The equilibrium potential (Eion) is the membrane voltage for a particular ion at equilibrium and can be calculated using the Nernst equation:

Eion = 62 mV (log[ion]outside/[ion]inside)

• The equilibrium potential of K+ (EK) is negative, while the equilibrium potential of Na+ (ENa) is positive


• In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady


Action potentials are the signals conducted by axons

• Neurons contain gated ion channels that open or close in response to stimuli = membrane potential changes.

• When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative

• Hyperpolarization, an increase in magnitude of the membrane potential


• Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential

• For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell

• Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus

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


Production of Action Potentials

• Voltage-gated Na+ and K+ channels respond to a change in membrane potential

• When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell

• The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open

• A strong stimulus results in a massive change in membrane voltage called an action potential

Fig. 48-9c

Strong depolarizing stimulus

+50M

emb

ran

e p

ote

nti

al (

mV

)

–50 Threshold

Restingpotential

–1000 2 3 4

Time (msec)

(c) Action potential

1 5

0

Actionpotential

6

Action Potential

Occurs if a stimulus causes the membrane voltage to cross a particular threshold


• Brief all-or-none depolarization of a neuron’s plasma membrane

• Action potentials are signals that carry information along axons (Not to other cells)

• Hundreds of action potentials per second

• Frequency of action potentials = strength of a stimulus

• An action potential can be broken down into a series of stages

Action Potential

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


• At resting potential

1. Most voltage-gated Na+ and K+ channels are closed, but some K+ channels (not voltage-gated) are open


• When an action potential is generated

2. Voltage-gated Na+ channels open first and Na+ flows into the cell

3. During the rising phase, the threshold is crossed, and the membrane potential increases

4. During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell


5. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close; resting potential is restored

Refractory period after an action potential, a second action potential cannot be initiated

Result of a temporary inactivation of the Na+ channels


Useful Simulations

BioFlix: How Neurons WorkBioFlix: How Neurons Work

Animation: Action PotentialAnimation: Action Potential


Conduction of Action Potentials

• Travels long distances by regenerating itself

• At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

• Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards

• Only travels toward the synaptic terminals

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. 48-12a

Axon Myelin sheath

Schwanncell

Nodes ofRanvier

Schwanncell

Nucleus ofSchwann cell

Node of Ranvier

Layers of myelinAxon

Myelinated axon (cross section)


• Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found

• Jump between the nodes of Ranvier in a process called saltatory conduction

Cell body

Schwann cell

Depolarized region(node of Ranvier)

MyelinsheathAxon


Neurons communicate with other cells at synapses• Electrical synapses, the electrical current flows

from one neuron to another

• Chemical synapses, a chemical neurotransmitter carries information across the gap junction

Most synapses are chemical synapses

Fig. 48-14

Postsynapticneuron

Synapticterminalsof pre-synapticneurons

5 µ

m


• The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal

• The action potential causes the release of the neurotransmitter

• The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

Animation: SynapseAnimation: Synapse

Fig. 48-15

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+


Postsynaptic Potential

• Binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell

• Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential

– 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

End of a neurotransmitter…or is it?


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 can trigger an action potential

• IPSP can counter the effect of an EPSP

• May or may not 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


Modulated Synaptic Transmission

Indirect synaptic transmission - A neurotransmitter binds to a receptor that is not part of an ion channel

•Second messenger in the postsynaptic cell is activated ex) cAMP

•Effects of indirect synaptic transmission have a slower onset but last longer


Neurotransmitters

• Same neurotransmitter can produce different effects in different types of cells

• 5 major classes of neurotransmitters (~100):

1) acetylcholine

2) Biogenic amines

3) Amino acids

4) Neuropeptides

5) Gases

Table 48-1


Acetylcholine

• Acetylcholine is a common neurotransmitter in vertebrates and invertebrates

• In vertebrates it is usually an excitatory transmitter. Ex) muscles (except Heart)

Bacteria produce toxins that causes botulism

Botox


Biogenic Amines – Derived from Animo Acids

Include epinephrine, norepinephrine, dopamine, and serotonin

They are active in the CNS and PNSLSD binds like serotonin to produce

hallucinatory effects.Parkinson’s lack of dopamineTreat depression with drugs to

increase concentration of Bio Amines.


Amino Acids

• Two amino acids are known to function as major neurotransmitters in the CNS:

Gamma-aminobutyric acid (GABA) – Inhibitory by increasing the Cl- movement.

Glutamate – Most common n.t. in brain, excitatory.


Neuropeptides

• Relatively short chains of amino acids, also function as neurotransmitters

• Neuropeptides include substance P (Mediates) and endorphins (Decreases Pain), which both affect our perception of pain

• Opiates bind to the same receptors as endorphins and can be used as painkillers (Euphoria)

Fig. 48-17EXPERIMENT

RESULTS

Radioactivenaloxone

Drug

Proteinmixture

Proteinstrapped on filter

Measure naloxonebound to proteinson each filter


Gases

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

• Produced on demand (as needed), stimulates production of a second messenger)

– Sexual Arousal

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

)

Chapter 48

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