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Slide 1-

Neurons, Synapses, and Signaling Chapter 48
Slide 2-

Overview: Lines of Communication The cone snail kills prey with venom that disables neurons 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)
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Figure 48.1
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Interpreting signals in the nervous system involves sorting a complex set of paths and connections Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain
Slide 5-

Concept 48.1: Neuron organization and structure reflect function in information transfer The squid possesses extremely large nerve cells and has played a crucial role in the discovery of how neurons transmit signals
Slide 6-

Introduction to Information Processing Nervous systems process information in three stages: sensory input, integration, and motor output
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Ganglia Brain Arm Nerve Eye Mantle Nerves with giant axons Figure 48.2
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Figure 48.2a
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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 the information Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity
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Many animals have a complex nervous system that 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 carries information into and out of the CNS The neurons of the PNS, when bundled together, form nerves
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Figure 48.3 Sensor Effector Sensory input Motor output Integration Peripheral nervous system (PNS) Central nervous system (CNS)
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Neuron Structure and Function 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 The cone-shaped base of an axon is called the axon hillock
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Figure 48.4 Nucleus Dendrites Stimulus Axon hillock Cell body Presynaptic cell Signal direction Axon Synapse Neurotransmitter Synaptic terminals Postsynaptic cell Synaptic terminals
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The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters A synapse is a junction between an axon and another cell
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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
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Dendrites Axon Cell body Portion of axon Sensory neuron Interneurons Motor neuron Figure 48.5
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Figure 48.6 Glia 80 m Cell bodies of neurons
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Concept 48.2: Ion pumps and ion channels establish the resting potential of a neuron Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential The resting potential is the membrane potential of a neuron not sending signals Changes in membrane potential act as signals, transmitting and processing information
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Formation of the Resting Potential In a mammalian neuron at resting potential, the concentration of K+ is highest inside the cell, while the concentration of Na+ is highest outside the cell 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
Slide 20-

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 The resulting buildup of negative charge within the neuron is the major source of membrane potential Animation: Resting Potential
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Table 48.1
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Key Na K Sodium- potassium pump Potassium channel Sodium channel OUTSIDE OF CELL INSIDE OF CELL Figure 48.7
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Modeling 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 (Cl–) builds up in the inner chamber At equilibrium, both the electrical and chemical gradients are balanced
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Figure 48.8 Inner chamber  90 mV  62 mV Outer chamber Inner chamber Outer chamber 140 mM KCl 150 mM NaCl 5 mM KCl 15 mM NaCl Potassium channel Sodium channel Artificial membrane K Na Cl Cl (a) Membrane selectively permeable to K (b) Membrane selectively permeable to Na EK  62 mV  90 mV ENa  62 mV  62 mV
Slide 25-

Figure 48.8a Inner chamber 90 mV Outer chamber 140 mM KCl 5 mM KCl Potassium channel Artificial membrane K Cl (a) Membrane selectively permeable to K EK  62 mV  90 mV
Slide 26-

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
Slide 27-

In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady
Slide 28-

Figure 48.8b 62 mV Inner chamber Outer chamber 150 mM NaCl 15 mM NaCl Sodium channel Na Cl (b) Membrane selectively permeable to Na ENa  62 mV  62 mV
Slide 29-

Concept 48.3: Action potentials are the signals conducted by axons Changes in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli
Slide 30-

Figure 48.9 Microelectrode Voltage recorder Reference electrode TECHNIQUE
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When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in magnitude of the membrane potential Hyperpolarization and Depolarization
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Stimulus Threshold Resting potential Hyperpolarizations Time (msec) 50 0 50 100 1 0 2 3 4 5 50 0 50 100 50 0 50 100 Time (msec) 1 0 2 3 4 5 Time (msec) 1 0 2 3 4 5 6 Threshold Resting potential Threshold Resting potential Stimulus Strong depolarizing stimulus Action potential Depolarizations Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K (b) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to Na (c) Action potential triggered by a depolarization that reaches the threshold Figure 48.10
Slide 33-

Stimulus Threshold Resting potential Hyperpolarizations Time (msec) 50 0 50 100 1 0 2 3 4 5 Membrane potential (mV) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K Figure 48.10a
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Opening other types of ion channels triggers 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
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Stimulus Threshold Resting potential Depolarizations Time (msec) 50 0 50 100 1 0 2 3 4 5 Membrane potential (mV) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na Figure 48.10b
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Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus These are not the nerve signals that travel along axons, but they do have an effect on the generation of nerve signals Graded Potentials and Action Potentials
Slide 37-

If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage called an action potential Action potentials have a constant magnitude, are all-or-none, and transmit signals over long distances They arise because some ion channels are voltage-gated, opening or closing when the membrane potential passes a certain level
Slide 38-

Strong depolarizing stimulus Threshold Resting potential Time (msec) 50 0 50 100 1 0 2 3 4 5 Membrane potential (mV) 6 (c) Action potential triggered by a depolarization that reaches the threshold Action potential Figure 48.10c
Slide 39-

Generation of Action Potentials: A Closer Look An action potential can be considered as a series of stages At resting potential Most voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed
Slide 40-

OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Threshold Resting potential Time Membrane potential (mV) 50 100 50 0 Na K Key 1 1 Resting state Figure 48.11-1
Slide 41-

When an action potential is generated Voltage-gated Na+ channels open first and Na+ flows into the cell During the rising phase, the threshold is crossed, and the membrane potential increases During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell
Slide 42-

OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Threshold Resting potential Time Membrane potential (mV) 50 100 50 0 Na K Key 2 1 1 2 Resting state Depolarization Figure 48.11-2
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OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Action potential Threshold Resting potential Time Membrane potential (mV) 50 100 50 0 Na K Key 2 1 3 1 2 3 Resting state Depolarization Rising phase of the action potential Figure 48.11-3
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OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Action potential Threshold Resting potential Time Membrane potential (mV) 50 100 50 0 Na K Key 2 1 3 4 1 2 3 4 Resting state Depolarization Rising phase of the action potential Falling phase of the action potential Figure 48.11-4
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5. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close and resting potential is restored
Slide 46-

OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Action potential Threshold Resting potential Time Membrane potential (mV) 50 100 50 0 Na K Key Resting state Undershoot Depolarization Rising phase of the action potential Falling phase of the action potential Figure 48.11-5
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Figure 48.11a Action potential Threshold Resting potential Time Membrane potential (mV) 50 100 50 0
Slide 48-

During the refractory period after an action potential, a second action potential cannot be initiated The refractory period is a result of a temporary inactivation of the Na+ channels BioFlix: How Neurons Work Animation: Action Potential
Slide 49-

Conduction of Action Potentials At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane Action potentials travel in only one direction: toward the synaptic terminals
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Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards
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Figure 48.12-1 Na Action potential Axon Plasma membrane Cytosol
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Figure 48.12-2 K K Na Na Action potential Axon Plasma membrane Cytosol Action potential
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Figure 48.12-3 K K K K Na Na Na Action potential Axon Plasma membrane Cytosol Action potential Action potential
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Evolutionary Adaptation of Axon Structure 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
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Axon Myelin sheath Schwann cell Nodes of Ranvier Node of Ranvier Layers of myelin Axon Schwann cell Nucleus of Schwann cell 0.1 m Figure 48.13
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Figure 48.13a 0.1 m
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Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction
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Cell body Schwann cell Depolarized region (node of Ranvier) Myelin sheath Axon Figure 48.14
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Concept 48.4: Neurons communicate with other cells at synapses At electrical synapses, the electrical current flows from one neuron to another At chemical synapses, a chemical neurotransmitter carries information across the gap junction Most synapses are chemical synapses
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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: Synapse
Slide 61-

Presynaptic cell Postsynaptic cell Axon Presynaptic membrane Synaptic vesicle containing neurotransmitter Postsynaptic membrane Synaptic cleft Voltage-gated Ca2 channel Ligand-gated ion channels Ca2 Na K Figure 48.15
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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, generating a postsynaptic potential
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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
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After release, the neurotransmitter May diffuse out of the synaptic cleft May be taken up by surrounding cells May be degraded by enzymes
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Summation of Postsynaptic Potentials 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
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Figure 48.16 Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 m
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Figure 48.17 Terminal branch of presynaptic neuron Postsynaptic neuron Axon hillock E1 E2 E1 E2 E1 E2 E1 E2 I I I I 0 70 Membrane potential (mV) Threshold of axon of postsynaptic neuron Resting potential Action potential Action potential I E1 E1 E1 E1 E1  E2 E1  I Subthreshold, no summation (a) (b) Temporal summation (c) Spatial summation Spatial summation of EPSP and IPSP (d) E1
Slide 68-

If two EPSPs are produced in rapid succession, an effect called temporal summation occurs
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Terminal branch of presynaptic neuron Postsynaptic neuron Axon hillock E1 E2 E1 E2 I I 0 70 Membrane potential (mV) Threshold of axon of postsynaptic neuron Resting potential Action potential E1 E1 E1 E1 Subthreshold, no summation (a) (b) Temporal summation Figure 48.17a
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In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together The combination of EPSPs through spatial and temporal summation can trigger an action potential
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E1 E2 E1 E2 I I Action potential I E1  E2 E1  I (c) Spatial summation Spatial summation of EPSP and IPSP (d) E1 Figure 48.17b
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Through summation, an IPSP can counter the effect of an EPSP The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential
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Modulated Signaling at Synapses In some synapses, a neurotransmitter binds to a receptor that is metabotropic In this case, movement of ions through a channel depends on one or more metabolic steps
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Binding of a neurotransmitter to a metabotropic receptor activates a signal transduction pathway in the postsynaptic cell involving a second messenger Compared to ligand-gated channels, the effects of second-messenger systems have a slower onset but last longer
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Neurotransmitters There are more than 100 neurotransmitters, belonging to five groups: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases A single neurotransmitter may have more than a dozen different receptors
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Table 48.2
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Acetylcholine Acetylcholine is a common neurotransmitter in vertebrates and invertebrates It is involved in muscle stimulation, memory formation, and learning Vertebrates have two major classes of acetylcholine receptor, one that is ligand gated and one that is metabotropic
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Amino Acids Amino acid neurotransmitters are active in the CNS and PNS Known to function in the CNS are Glutamate Gamma-aminobutyric acid (GABA) Glycine
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Biogenic Amines Biogenic amines include Epinephrine Norepinephrine Dopamine Serotonin They are active in the CNS and PNS
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Neuropeptides Several neuropeptides, relatively short chains of amino acids, also function as neurotransmitters Neuropeptides include substance P and endorphins, which both affect our perception of pain Opiates bind to the same receptors as endorphins and can be used as painkillers
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Figure 48.18 Radioactive naloxone Drug Protein mixture Radioactive naloxone and a test drug are incubated with a protein mixture. Proteins are trapped on a filter. Bound naloxone is detected by measuring radioactivity. EXPERIMENT RESULTS 2 1 Drug Opiate Concentration That Blocked Naloxone Binding Morphine Methadone Levorphanol Phenobarbital Atropine Serotonin Yes Yes Yes No No No 6  109 M 2  108 M 2  109 M No effect at 104 M No effect at 104 M No effect at 104 M
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Radioactive naloxone Drug Protein mixture Radioactive naloxone and a test drug are incubated with a protein mixture. Proteins are trapped on a filter. Bound naloxone is detected by measuring radioactivity. EXPERIMENT 2 1 Figure 48.18a
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RESULTS Drug Opiate Concentration That Blocked Naloxone Binding Morphine Methadone Levorphanol Phenobarbital Atropine Serotonin Yes Yes Yes No No No 6  109 M 2  108 M 2  109 M No effect at 104 M No effect at 104 M No effect at 104 M Figure 48.18b
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Gases Gases such as nitric oxide and carbon monoxide are local regulators in the PNS
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Action potential Membrane potential (mV) Falling phase Rising phase Threshold (55) Depolarization Undershoot Resting potential Time (msec) 50 0 50 100 70 0 1 2 3 4 5 6 Figure 48.UN01
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Figure 48.UN02
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Figure 48.UN03
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Figure 48.UN04