Download 48 nervous system essential knowledge

Chapter 48
Neurons, Synapses, and
Signaling
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Why do multicellular organism have nervous
systems?
• Must have one centralized organ that can detect
and integrate external and internal environment
and coordinate function
• Relying on typical cell signaling (diffusion) of
molecules over a long distance is too slow to
respond to immediate danger.
• Electrical impulses allow signals to be transmitted
rapidly from one part of the body to another.
• Electrical impulses on a can travel 120 m/s.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The neuron is the basic structure of the nervous
system that reflects function
1. Dendrites: Detection
2. Cell body: Signal integration
3. Axon: Signal transmission
4. Myelin sheath: Electrical insulator
–
Formed by Schwann cells
–
Signal jumps from node to node.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Nervous system requires a large amount of
energy to generate electrical activity
• The brain is ~3% of the body by mass but requires
approximately 30% of the body’s glucose.
• Why? It must maintain a “resting potential” or
difference in charge across the cell membrane of
the axon.
• The Na+/ K+ ATP Pump keeps the neuron
Polarized: inside is more negative than the outside
by
• An electrical impulse is a rapid and transient.
reversal of charge across the membrane (inside
becomes more positive than the outside)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neurons transmit electrical impulses along the axon
called an Action Potential
Action potential: a
rapid and temporary
change in membrane
potential.
Caused by opening
and closing of voltage
gated ion channels.
Membrane potential (mV)
Strong depolarizing stimulus
+50
Action
potential
0
–50 Threshold
Resting
potential
–100
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0
1 2 3 4 5
Time (msec)
6
Neuron Cell Membrane
Opens at ~ +35 mV
Closes at ~ -75mV
Opens at ~ -45 mV
Closes at ~ +35 mV
Closed
Na+/ K+ Pump
Na+
Channel
K+
Channel
Voltage gated
After closing they have a refractory
phase where they cannot open. This
prevents the action potential from
traveling in the wrong direction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neuron membranes are polarized by Na+/K+
pump.
•
Membrane Polarization: The Na+/K+ pump
establishes an electrical potential across the
membranes called the “resting potential”
•
Na+/K+ pump is powered by ATP
3 Na+ out
+ + + + +
-
Na+/ K+ Pump
2 K+ in
- - - Na+
K+
Channel Channel
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Outside of cell is
more positive than
inside
“Resting Potential”
inside is~ -70mV
Depolarization:
Stimulus triggers Na+ channels to open
•
Ligand-gated ion channels allow Ca2+ to enter the
cell slowly (slightly depolarizing the membrane)
•
When membrane depolarizes to -45mV, Na+
channels open
•
Na+ rushes into the cell, causing local
depolarization of the membrane
•
Na+ channels close at +35 mV
Na+
3 Na+ out
Na+/ K+ Pump
+ -
+ -
+ -
+ - +
K+
2 K+ in Na+
Channel
Channel
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Repolarization occurs when Na+ channels close
and K+ Channels open
•
At +35 mV K+ gated channels open.
•
K+ exits the cell.
•
The cell membrane becomes hyperpolarized
•
K+ channel closes at -75mV
•
Na+/K+ pump reestablishes the resting
K+
potential at -70 mV
3 Na+ out
+ + + + +
Na+/ K+ Pump
-
- - - Na+
K+
2 K+ in Channel Channel
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Na+ Channels
close
K+ Channels
open
Ligand gated
channels open,
allowing some
Ca2+ to enter
+50
Action
potential
Membrane potential (mV)
Na+ Channels
open
Strong depolarizing stimulus
0
–50 Threshold
Resting
potential
–100
0
(c)
1 2 3 4 5
Time (msec)
6
K+ Channels
Close
Action potential = change in membrane voltage
Conduction of an
Action Potential
Axon
Signal
Transmission
Plasma
membrane
Action
potential
Cytosol
Na+
K+
Action
potential
Na+
K+
K+
Action
potential
Na+
K+
Saltatory conduction: the action potential
“jumps” along the membrane, traveling faster
than if myelin sheath were absent.
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
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Transmission of information between neurons
occurs across synapses using neurotransmitters.
5
Synaptic vesicles
containing
neurotransmitter
Voltage-gated
Ca2+ channel
1
Postsynaptic
membrane
Ca2+
4
2
Synaptic
cleft
Presynaptic
membrane
3
Ligand-gated
ion channels
6
K+
Na+
Generation of Postsynaptic Potentials
• Neurotransmitter & Receptor binding can be
excitatory or inhibitory
• The same neurotransmitter can be excitatory or
inhibitory in different parts of the brain
– Depends on the properties of the target
receptor
• Hyperpolarization (more negative) = Inhibitory
• Depolarization (more positive)= excititory
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• After release, the neurotransmitter
– May diffuse out of the synaptic cleft
– May be taken up by surrounding cells
– May be degraded by enzymes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Summation of postsynaptic potentials
All inputs (excitatory and inhibitory) are summed or
added together in the cell body.
If the combined input is enough to reach threshold
at the axon hillock an action potential will occur.
Action potentials is all or nothing.
Terminal branch
of presynaptic
neuron
E2
E1
E2
Membrane potential (mV)
Postsynaptic
neuron
E1
I
I
Axon
hillock
0
Action
potential
Threshold of axon of
postsynaptic neuron
Resting
potential
–70
E1
E1
E1
(a) Subthreshold,
summation
no
E1
(b) Temporal
summation
Nervous System Review
1.
Explain the roles of dendrites, cell body, axon, and synapse in neuron function.
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 how the action potential travels along the axon.
6.
Describe the events that lead to the release of neurotransmitters into the synaptic cleft.
7.
How do neurotransmitters affect the postsynaptic neuron?
8.
Why is it imprecise to define a neurotransmitter as excitatory or inhibitory?
9.
Independent reading challenge: see chapter 48-49. The brain can be divided up into different regions responsible for
different functions. Choose one of the functions or regions below and describe how the brain processes that function
or where the function occurs in the brain.
–
Vision
–
Hearing
–
Muscle movement
–
Abstract thought and emotions
–
Neuro-hormone production
–
Forebrain (cerebrum), midbrain (brainstem) and hindbrain (cerebellum)
–
Right and left cerebral hemispheres in humans
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