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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Command and Control Center
• The human brain contains about 100 billion nerve cells, or neurons
• Each neuron may communicate with thousands of other neurons
• Functional magnetic resonance imaging is a technology that can reconstruct a three-dimensional map of brain activity
• Brain imaging and other methods reveal that groups of neurons function in specialized circuits dedicated to different tasks
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Brainbow mouse section of brainhttp://www.conncad.com/gallery/brainbow.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 48.1: Nervous systems consist of circuits of neurons and supporting cells
• All animals except sponges have a nervous system
• What distinguishes nervous systems of different animal groups is how neurons are organized into circuits
Nerve net
Hydra (cnidarian)
Radialnerve
Nervering
Sea star (echinoderm)
The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets
Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring
Brain
Ganglia
Squid (mollusc)
Brain
Salamander (chordate)
Spinalcord(dorsalnervecord)
Sensoryganglion
•Nervous systems in molluscs correlate with lifestyles
•Sessile molluscs have simple systems, whereas more complex molluscs have more sophisticated systems
LE 48-3
SensorSensory input
Motor output
Integration
Effector Peripheral nervoussystem (PNS)
Central nervoussystem (CNS)
In vertebrates, the central nervous system consists of a brain and dorsal spinal cord
The PNS connects to the CNSNervous systems process information in three stages: sensory input, integration, and motor output
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Sensory neurons transmit information from sensors that detect external stimuli and internal conditions
• Sensory information is sent to the CNS, where interneurons integrate the information
• Motor output leaves the CNS via motor neurons, which communicate with effector cells
• The three stages of information processing are illustrated in the knee-jerk reflex
LE 48-4
Quadricepsmuscle
Cell body ofsensory neuron in dorsal rootganglion
Sensory neuron
Spinal cord(cross section)
Whitematter
Hamstringmuscle
Graymatter
Motor neuronInterneuron
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Neuron Structure
• Most of a neuron’s organelles are in the cell body
• Most neurons have dendrites, highly branched extensions that receive signals from other neurons
• The axon is typically a much longer extension that transmits signals to other cells at synapses
• Many axons are covered with a myelin sheath
LE 48-5
Dendrites
Cell body
Nucleus
Axon hillock Axon
Signaldirection
Presynaptic cellMyelin sheath
Synapticterminals
Synapse
Postsynaptic cell
Dendrites
Cell body
Axon
InterneuronsSensory neuron Motor neuron
Neurons have a wide variety of shapes that reflect input and output interactions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Supporting Cells (Glia)
• Glia are essential for structural integrity of the nervous system and for functioning of neurons
• Types of glia: astrocytes, radial glia, oligodendrocytes, and Schwann cells
50 µ
m
In the CNS, astrocytes provide structural support for neurons and regulate extracellular concentrations of ions and neurotransmitters
LE 48-8
Axon Nodes ofRanvier
Schwanncell
Myelin sheathNucleus ofSchwann cell
Schwanncell
Nodes of Ranvier
Layers of myelinAxon
0.1 µm
Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) form the myelin sheaths around axons of many vertebrate neurons
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Myelination in the central and peripheral nervous systems
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dorsal view of the human brain showing the progression of myelination (“white matter”) over the cortical surface during adolescence
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron
• Across its plasma membrane, every cell has a voltage called a membrane potential
• The cell’s inside is negative relative to the outside
• Membrane potential of a cell can be measured
• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) form the myelin sheaths around axons of many vertebrate neurons
Microelectrode
Referenceelectrode
Voltagerecorder
–70 mV
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Resting Potential
• Resting potential is the membrane potential of a neuron that is not transmitting signals
• Resting potential depends on ionic gradients across the plasma membrane
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concentration of Na+ is higher in the extracellular fluid than in the cytosol
• The opposite is true for K+
• By modeling a neuron with an artificial membrane, we can better understand resting potential
LE 48-10
CYTOSOL EXTRACELLULARFLUID
[Na+]15 mM
[K+]150 mM
[A–]100 mM
[Na+]150 mM
[K+]5 mM
[Cl–]120 mM[Cl–]
10 mM
Plasmamembrane
LE 48-11
150 mMKCl
Innerchamber
Outerchamber
–92 mV
Potassiumchannel
Membrane selectively permeable to K+ Membrane selectively permeable to Na+
5 mMKCl
Artificialmembrane
K+
Cl–
150 mMNaCl
Innerchamber
Outerchamber
+62 mV
Sodiumchannel
15 mMNaCl
Na+
Cl–
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A neuron that is not transmitting signals contains many open K+ channels and fewer open Na+ channels in its plasma membrane
• Diffusion of K+ and Na+ leads to a separation of charges across the membrane, producing the resting potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gated Ion Channels
• Gated ion channels open or close in response to one of three stimuli:
– Stretch-gated ion channels open when the membrane is mechanically deformed
– Ligand-gated ion channels open or close when a specific chemical binds to the channel
– Voltage-gated ion channels respond to a change in membrane potential
Hyperpolarizations
Graded potential hyperpolarizations Graded potential depolarizations
5Time (msec)
Restingpotential
43210
Threshold
–100
–50
0
Mem
bran
e po
tent
ial (
mV)
Stimuli+50
Depolarizations
5Time (msec)
Restingpotential
43210
Threshold
–100
–50
0
Mem
bran
e po
tent
ial (
mV)
Stimuli+50
Action potential
5Time (msec)
Restingpotential
43210
Threshold
–100
–50
0
Mem
bran
e po
tent
ial (
mV)
Stronger depolarizing stimulus
+50Actionpotential
6
If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those channels
Some stimuli trigger a hyperpolarization, an increase in magnitude of the membrane potentialOther stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Hyperpolarization and depolarization are called graded potentials
• The magnitude of the change in membrane potential varies with the strength of the stimulus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Production of Action Potentials
• Depolarizations are usually graded only up to a certain membrane voltage, called the threshold
• A stimulus strong enough to produce depolarization that reaches the threshold triggers a response called an action potential
• An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane
• It carries information along axons
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Voltage-gated Na+ and K+ channels are involved in producing an action potential
• When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell
• As the action potential subsides, K+ channels open, and K+ flows out of the cell
• During the refractory period after an action potential, a second action potential cannot be initiated
Action Potential
Resting potential
Threshold
Mem
bran
e po
tent
ial
(mV)
Actionpotential
Time–100
–50
+50
0
Potassiumchannel
Extracellular fluid
Plasma membrane
Na+
Resting state
Inactivationgate
Activationgates
Sodiumchannel K+
Cytosol
Na+
Depolarization
K+
Na+
Na+
Rising phase of the action potential
K+
Na+
Na+
Falling phase of the action potential
K+
Na+
Na+
Undershoot
K+
Na+
An action potential is generated as Na+ flows inward across the membrane at one location.
Na+
Action potential
Axon
Na+
Action potentialK+
The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.
K+
Na+
Action potentialK+
The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.
K+
Conduction of Action Potentials
•An action potential can travel long distances by regenerating itself along the axon
•At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Conduction Speed
• The speed of an action potential increases with the axon’s diameter
• In vertebrates, axons are myelinated, also causing an action potential’s speed to increase
• Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction
LE 48-15
Cell body
Schwann cell
Depolarized region(node of Ranvier)
Myelinsheath
Axon
Na+ and K+ channels are concentrated at the nodes to initiate a series of action potentials
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 48.4: Neurons communicate with other cells at synapses
• In an electrical synapse, current flows directly from one cell to another via a gap junction
• The vast majority of synapses are chemical synapses
• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic terminal
LE 48-16
Postsynapticneuron
Synapticterminalsof pre-synapticneurons
5 µm
LE 48-17
Postsynaptic cellPresynaptic cell
Synaptic vesiclescontainingneurotransmitter
Presynaptic membrane
Voltage-gatedCa2+ channel
Ca2+Postsynaptic membrane
Postsynaptic membrane
Neuro-transmitter
Ligand-gatedion channel
Na+
K+
Ligand-gatedion channels
Synaptic cleft
When an action potential reaches a terminal, the final result is release of neurotransmitters into the synaptic cleft
•Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Postsynaptic potentials fall into two categories:
– Excitatory postsynaptic potentials (EPSPs)
– Inhibitory postsynaptic potentials (IPSPs)
• After release, the neurotransmitter diffuses out of the synaptic cleft
• It may be taken up by surrounding cells and degraded by enzymes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Postsynaptic neuron
Terminal branchof presynaptic neuron
E1 E1
Axonhillock
E1E2
E1
I
Actionpotential
E1E1 + E2
Spatial summationof EPSP and IPSP
Spatial summation
I E1 + I
Actionpotential
E1
Temporal summation
E1
Threshold of axon ofpostsynaptic neuron
E1
Subthreshold, nosummation
E1
Restingpotential
Mem
bran
e po
tent
ial (
mV)
–70
0
•If two EPSPs are produced in rapid succession, an effect called temporal summation occurs
•In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together
•Through summation, an IPSP can counter the effect of an EPSP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Indirect Synaptic Transmission
• In indirect synaptic transmission, a neurotransmitter binds to a receptor that is not part of an ion channel
• This binding activates a signal transduction pathway involving a second messenger in the postsynaptic cell
• Effects of indirect synaptic transmission have a slower onset but last longer
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Neurotransmitters• The same neurotransmitter can produce different effects in
different types of cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Acetylcholine
• Acetylcholine is a common neurotransmitter in vertebrates and invertebrates
• It can be inhibitory or excitatory
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Biogenic Amines
• Biogenic amines include epinephrine, norepinephrine, dopamine, and serotonin
• They are active in the CNS and PNS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Amino Acids and Peptides
• Four amino acids are known to function as neurotransmitters in the CNS
• Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gases
• Gases such as nitric oxide and carbon monoxide are local regulators in the PNS