chapter 48 neurons, synapses, and signaling. copyright © 2008 pearson education, inc., publishing...

Chapter 48Chapter 48

Neurons, Synapses, and Signaling

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

Overview: Lines of Communication

• Neurons are nerve cells that transfer information within the body.

• Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance).


• Sensors detect external stimuli and internal conditions and transmit information along sensory neurons.

• Sensory information is sent to the brain or ganglia, where interneurons integrate / process the information.

• Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity = response.

Information Processing

Sensor: Detects stimulus

Sensory input

IntegrationProcessing

Effector:Does response

Motor output

Peripheral nervous

system (PNS)

Central nervous

system (CNS)


I. Neurons

• Dendrites = receive signals from other neurons.

• Axon = longer extension that transmits signals from its terminal branches to other cells at synapses.

NeuronsDendrites

Stimulus

Nucleus

Cellbody

Axonhillock

Presynaptic cell

Axon

Synaptic terminalsSynapse

Postsynaptic cellNeurotransmitters


• Synaptic terminal of one axon passes information across the synapse in the form of chemical messengers (neurotransmitters)

• Presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell).

II. Synapse


III. Resting Potential

• At resting potential, the [K] is greater inside the cell, while the [Na] is greater outside the cell.

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

• The concentration gradients represent chemical potential energy.


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

• Resting neuron contains open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell, leading to negative charge inside cell

The Basis of the Membrane Potential

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

OUTSIDECELL

Na+Key

K+

Sodium-potassiumpump

Potassiumchannel

Sodiumchannel

INSIDECELL


IV. Production of Action Potentials

• Na+ and K+ channels respond to a change in membrane potential.

• Stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell.

• This increases the depolarization and causes even more Na+ channels to open.

• Strong stimulus = a massive change in membrane voltage = action potential (signal)


• AP occurs if stimulus causes voltage to cross a threshold.

• AP is a all-or-none depolarization of the plasma membrane.

Strong depolarizing stimulus

+50

Mem

bra

ne

po

ten

tia

l (m

V)

–50 Threshold

Restingpotential

–1000 2 3 4

Time (msec)

(c) Action potential = change in membrane voltage

1 5

0

Actionpotential

6

The role of voltage-gated ion channels in the generation of an action potential

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


V. Conduction of Action Potentials

• APs can travel long distances by regenerating along the axon.

• Action potentials travel toward the synaptic terminals.

Conduction of an Action Potential

SignalTransmission

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

ActionpotentialK+

K+

Na+


A. Conduction Speed

• Speed of AP increases with the axon’s diameter.

• Axons are insulated by a myelin sheath (speed increases)

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

Schwann cells and the myelin sheath

Axon Myelin sheath

Schwanncell

Nodes ofRanvier

Schwanncell

Nucleus of

Schwann cell

Node of Ranvier

Layers of myelin

Axon


• APs are formed at nodes of Ranvier (gaps in the myelin)

• APs in myelinated axons jump between the nodes of Ranvier = saltatory conduction

Saltatory conduction

Cell body

Schwann cell

Depolarized region(node of Ranvier)

Myelinsheath

Axon


V. Neurons communicate at synapses

• Electrical synapses = electrical current flows from one neuron to another.

• Chemical synapses = neurotransmitter carries information across the synapse.


• Presynaptic neuron puts the neurotransmitter in synaptic vesicles

• The action potential causes the release of the neurotransmitter.

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

Chemical synapse

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontaining

neurotransmitter

5

6

K+Na+


VI. Generation of Postsynaptic Potentials

• Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell.

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


• 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.

Summation of postsynaptic potentials

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


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; 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.

chapter 48 neurons, synapses, and signaling. copyright © 2008 pearson education, inc., publishing...

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