Chapter 48 Neurons, Synapses, and Signaling. Copyright © 2008 Pearson Education, Inc., publishing...
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Transcript of 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).
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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+
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
V. Neurons communicate at synapses
• Electrical synapses = electrical current flows from one neuron to another.
• Chemical synapses = neurotransmitter carries information across the synapse.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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+
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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.