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Neural Signaling Chapter 40
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Learning Objective 1 Describe the processes involved in neural signaling: reception, transmission, integration, and action by effectors
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Neural Signaling 1 (1) Reception of information
by a sensory receptor (2) Transmission by an afferent neuron to the central nervous system (CNS) (3) Integration by interneurons in the central nervous system (CNS)
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Neural Signaling 2 (4) Transmission by an efferent neuron
to other neurons or effector (5) Action by effectors the muscles and glands
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Peripheral Nervous System (PNS)
Made up of sensory receptors neurons outside the CNS
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Response to Stimulus
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(e.g., vibration, movement, light, odor) Internal stimulus
External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by external sense organs Detection by internal sense organs Figure 40.1: Response to a stimulus. Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response. TRANSMISSION Sensory (afferent) neurons transmit information Fig. 40-1a, p. 846
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Central Nervous System (brain and spinal cord)
INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses ACTION BY EFFECTORS (muscles and glands) Figure 40.1: Response to a stimulus. Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response. e.g., animal runs away e.g., espiration rate increases; blood pressure rises Fig. 40-1b, p. 846
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(e.g., vibration, movement, light, odor) Internal stimulus
External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by external sense organs Detection by internal TRANSMISSION Sensory (afferent) neurons transmit information Central Nervous System (brain and spinal cord) INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses Figure 40.1: Response to a stimulus. Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response. ACTION BY EFFECTORS (muscles and glands) e.g., animal runs away e.g., espiration rate increases; blood pressure rises Stepped Art Fig. 40-1, p. 846
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KEY CONCEPTS Neural signaling involves reception, transmission, integration, and action by effectors
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Learning Objective 2 What is the structure of a typical neuron?
Give the function of each of its parts
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Neurons Specialized to Cell body receive stimuli
transmit electrical and chemical signals Cell body contains nucleus and organelles
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Dendrites Many branched dendrites extend from cell body of neuron
specialized to receive stimuli and send signals to the cell body
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Axons 1 A single long axon Transmits signals into terminal branches
extends from neuron cell body forms branches (axon collaterals) Transmits signals into terminal branches which end in synaptic terminals
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Axons 2 Myelin sheath Schwann cells surrounds many axons insulates
form the myelin sheath in the PNS
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Axons 3 In the CNS Nodes of Ranvier
sheath is formed by other glial cells Nodes of Ranvier gaps in sheath between successive Schwann cells
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Neuron Structure
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Dendrites covered with dendritic spines Cytoplasm of Schwann cell
Synaptic terminals Axon Axon collateral Cell body Nucleus Myelin sheath Nucleus Figure 40.2: Structure of a multipolar neuron. The cell body contains most of the organelles. Many dendrites and a single axon extend from the cell body. Schwann cells form a myelin sheath around the axon by wrapping their own plasma membranes around the neuron in jelly-roll fashion. Axon Nodes of Ranvier Schwann cell Terminal branches Fig. 40-2, p. 847
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Nerves and Ganglia Nerve Ganglion several hundred axons
wrapped in connective tissue Ganglion mass of neuron cell bodies in the PNS
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Nerve Structure
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Ganglion Cell bodies Myelin sheath Vein Axon Artery (a)
Figure 40.3: Structure of a nerve. Vein Axon Artery (a) Fig. 40-3a, p. 848
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Figure 40.3: Structure of a nerve.
100 µm (b) Fig. 40-3b, p. 848
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Learn more about the structure of neurons and nerves by clicking on the figures in ThomsonNOW.
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Learning Objective 3 Name the main types of glial cells
Describe the functions of each
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Glial Cells Support and nourish neurons
Are important in neural communication
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Glial Cell Types 1 Astrocytes physically support neurons
regulate extracellular fluid in CNS (by taking up excess potassium ions) communicate with one another (and with neurons) induce and stabilize synapses
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Glial Cell Types 2 Oligodendrocytes Schwann cells
form myelin sheaths around axons in CNS Schwann cells form sheaths around axons in PNS
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Glial Cell Types 3 Microglia Ependymal Cells Phagocytic cells
line cavities in the CNS contribute to formation of cerebrospinal fluid serve as neural stem cells
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KEY CONCEPTS Neurons are specialized to receive stimuli and transmit signals; glial cells are supporting cells that protect and nourish neurons and that can modify neural signals
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Learning Objective 4 How does the neuron develop and maintain a resting potential?
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Neural Signals Electrical signals transmit information
along axons Plasma membrane of resting neuron (not transmitting an impulse) is polarized Inner surface of plasma membrane is negatively charged relative to extracellular fluid
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Resting Potential Potential difference of about -70 mV
across the membrane Magnitude of resting potential (1) differences in ion concentrations (Na+, K+) inside cell relative to extracellular fluid (2) selective permeability of plasma membrane to these ions
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(a) Measuring the resting potential of a neuron.
Axon 40 20 –20 –40 –60 –70 mV –80 Time Amplifier Figure 40.4: Resting potential. Plasma membrane Electrode placed inside the cell Electrode placed outside the cell + + + – + – + + – – – – – – – + – + – + – + + + (a) Measuring the resting potential of a neuron. Fig. 40-4a, p. 850
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Ions Pass through specific passive ion channels
K+ leak out faster than Na+ leak in Cl- accumulate at inner surface of plasma membrane Large anions (proteins) cannot cross plasma membrane contribute negative charges
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Sodium–Potassium Pumps
Maintain gradients that determine resting potential transport 3 Na+ out for each 2 K+ in Require ATP
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+ + + + + + – – – – – – Extracellular fluid 3 Na+ CI– Na+ Na+ CI– Na+
Diffusion out K+ Na+ K+ K+ Na+ Plasma membrane + + + + + + – – – – – – Na /K pump K+ K+ Na+ Figure 40.4: Resting potential. K+ K+ K+ CI– Diffusion in K+ 2 K+ Na+ A_ A_ A_ CI– CI– CI– Cytoplasm (b) Permeability of the neuron membrane. Fig. 40-4b, p. 850
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KEY CONCEPTS The resting potential of a neuron is maintained by differences in concentrations of specific ions inside the cell relative to the extracellular fluid and by selective permeability of the plasma membrane to these ions
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Learning Objective 5 Compare a graded potential with an action potential Describe the production and transmission of each
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Membrane Potential Membrane is depolarized Membrane is hyperpolarized
if stimulus causes membrane potential to become less negative Membrane is hyperpolarized if membrane potential becomes more negative than resting potential
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Graded Potential A local response Varies in magnitude Fades out
depending on strength of applied stimulus Fades out within a few millimeters of point of origin
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Action Potential 1 Action potential is a wave of depolarization
that moves down the axon Generated when voltage across the membrane declines to a critical point (threshold level) voltage-activated ion channels open Na+ ions flow into the neuron
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Voltage-Activated Ion Channels
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(b) Potassium channels.
Extracellular fluid Activation gate Figure 40.6: Voltage-activated ion channels. Cytoplasm Inactivation gate (a) Sodium channels. (b) Potassium channels. Fig. 40-6, p. 852
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Voltage-Activated Ion Channels During an Action Potential
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Spike (a) Action potential. Depolarization Repolarization
Membrane potential (mV) Threshold level Resting state Figure 40.7: State of voltage-activated ion channels during an action potential. Time (milliseconds) (a) Action potential. Fig. 40-7a, p. 853
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Return to resting state. Repolarization.
Axon Extracellular fluid Sodium channel Potassium channel Figure 40.7: State of voltage-activated ion channels during an action potential. Cytoplasm 1 Resting state. 2 Depolarization. Return to resting state. 3 Repolarization. 4 (b) The action of the ion channels in the plasma membrane determines the state of the neuron. Fig. 40-7b, p. 853
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Action Potential 2 An all-or-none response
no variation in strength of a single impulse either membrane potential exceeds threshold level or it does not Once begun, an action potential is self-propagating
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Repolarization As an action potential moves down an axon, repolarization occurs behind it
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Transmission of an Action Potential
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Area of depolarization
Stimulus Axon Area of depolarization Potassium channel Sodium channel Figure 40.8: Transmission of an action potential along the axon. When the dendrites (or cell body) of a neuron are stimulated sufficiently to depolarize the membrane to its threshold level, an action potential is generated. Action potential (1) Action potential is transmitted as wave of depolarization that travels down axon. At region of depolarization, Na+ diffuse into cell. Fig. 40-8a, p. 854
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Area of repolarization Area of depolarization
Action potential Figure 40.8: Transmission of an action potential along the axon. When the dendrites (or cell body) of a neuron are stimulated sufficiently to depolarize the membrane to its threshold level, an action potential is generated. (2) As action potential progresses along axon, repolarization occurs quickly behind it. Fig. 40-8b, p. 854
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Refractory Periods During depolarization, the axon enters an absolute refractory period when it can’t transmit another action potential When enough gates controlling Na+ channels have been reset, the neuron enters a relative refractory period when the threshold is higher
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Learn more about ion channels and action potentials by clicking on the figures in ThomsonNOW.
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KEY CONCEPTS Depolarization of the neuron plasma membrane to threshold level generates an action potential, an electrical signal that travels as a wave of depolarization along the axon
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Learning Objective 6 Contrast continuous conduction with saltatory conduction
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Continuous Conduction
Involves entire axon plasma membrane Takes place in unmyelinated neurons
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Saltatory Conduction Depolarization skips along axon from one node of Ranvier to the next more rapid than continuous conduction takes place in myelinated neurons Nodes of Ranvier sites where axon is not covered by myelin Na+ channels are concentrated
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Saltatory Conduction
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Area of action potential Saltatory conduction Nodes of Ranvier Axon
Figure 40.9: Saltatory conduction. This sequence of diagrams (steps ●1 through ●4) illustrates transmission of an action potential in a myelinated neuron. Axon 1 Schwann cell 2 Fig. 40-9a, p. 855
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Direction of depolarization
3 Figure 40.9: Saltatory conduction. This sequence of diagrams (steps ●1 through ●4) illustrates transmission of an action potential in a myelinated neuron. 4 Direction of depolarization Fig. 40-9b, p. 855
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Learning Objective 7 Describe the actions of the neurotransmitters identified in the chapter
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Synapses Junctions between two neurons Most synapses are chemical
or between a neuron and effector Most synapses are chemical some are electrical synapses
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Synaptic Transmission
A presynaptic neuron releases neurotransmitter (chemical messenger) from its synaptic vesicles
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Neurotransmitters 1 Acetylcholine Biogenic amines
triggers contraction of skeletal muscle Biogenic amines norepinephrine, serotonin, dopamine important in regulating mood dopamine is also important in motor function
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Neurotransmitters 2 Some amino acids Neuropeptides (opioids)
glutamate (excitatory neurotransmitter in brain) GABA (widespread inhibitory neurotransmitter) Neuropeptides (opioids) endorphins (e.g. beta-endorphin) enkephalins
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Neurotransmitters 3 Nitric oxide (NO) gaseous neurotransmitter
transmits signals from postsynaptic neuron to presynaptic neuron (opposite direction from other neurotransmitters)
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Learning Objective 8 Trace the events that take place in synaptic transmission Draw diagrams to support your description
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Synaptic Transmission
Calcium ions cause synaptic vesicles to fuse with presynaptic membrane releases neurotransmitter into synaptic cleft Neurotransmitter diffuses across the synaptic cleft combines with specific receptors on a postsynaptic neuron
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Synaptic Transmission
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Plasma membrane of postsynaptic neuron
Synaptic vesicles Plasma membrane of postsynaptic neuron Figure 40.10: Synaptic transmission. 0.25 µm (a) The TEM shows synaptic terminals filled with synaptic vesicles. Fig a, p. 858
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Figure 40.10: Synaptic transmission.
Fig bc, p. 858
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(b) How a neural impulse is transmitted across a synapse.
Axon of presynaptic neuron Synaptic terminal Voltage-gated Ca2+ channel 1 Synaptic vesicle Ca2+ 2 3 Neuro- transmitter molecule Figure 40.10: Synaptic transmission. 4 Ligand-gated channels 5 Postsynaptic membrane (b) How a neural impulse is transmitted across a synapse. Receptor for neurotransmitter Postsynaptic neuron Fig b, p. 858
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Synaptic terminal Synaptic cleft
Ca2+ Synaptic terminal Presynaptic membrane Synaptic cleft Na+ Postsynaptic membrane Figure 40.10: Synaptic transmission. (c) Neurotransmitter binds with receptor. Ligand-gated channel opens, resulting in depolarization. Fig c, p. 858
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Neurotransmitter Receptors
Many are proteins that form ligand-gated ion channels Others work through a second messenger such as cAMP
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Learn more about synaptic transmission by clicking on the figure in ThomsonNOW.
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KEY CONCEPTS Neurons signal other cells by releasing neurotransmitters at synapses
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Learning Objective 9 Compare excitatory and inhibitory signals and their effects
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Binding of Neurotransmitter to a Receptor
Binding causes either excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP) Depending on the type of receptor
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EPSPs and IPSPs EPSPs IPSPs bring neuron closer to firing
move neuron farther away from its firing level
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Learning Objective 10 Define neural integration
Describe how a postsynaptic neuron integrates incoming stimuli and “decides” whether or not to fire
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Neural Integration Process of summing (integrating) incoming signals
Summation process of adding and subtracting incoming signals
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Summation Each EPSP or IPSP is a graded potential
vary in magnitude depending on strength of stimulus applied Summation of several EPSPs brings neuron to critical firing level
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Temporal Summation Occurs when repeated stimuli cause new EPSPs to develop before previous EPSPs have decayed
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Spatial Summation Occurs when several closely spaced synaptic terminals release neurotransmitter simultaneously stimulating postsynaptic neuron at several different places
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Neural Integration
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Threshold level Resting potential
Postsynaptic membrane potential (mV) Resting potential Figure 40.11: Neural integration. E1 and E2 are excitatory stimuli. I represents an inhibitory stimulus. Time (msec) (a) Subthreshold (no summation). (b) Temporal summation. (c) Spatial summation. (d) Spatial summation of EPSPs and IPSPs. Fig , p. 860
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KEY CONCEPTS During integration, incoming neural signals are summed; temporal and spatial summation can bring a neuron to threshold level
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Learning Objective 11 Distinguish among convergence, divergence, and reverberation Explain why each is important
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Neural Circuits Complex neural circuits are possible because of associations such as convergence and divergence
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Convergence A single neuron is affected by converging signals from two or more presynaptic neurons Allows CNS to integrate incoming information from various sources
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Divergence A single presynaptic neuron stimulates many postsynaptic neurons allowing widespread effect
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Neural Circuits
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Figure 40.12: Neural circuits.
(a) Convergence of neural input. Several presynaptic neurons synapse with one postsynaptic neuron. (b) Divergence of neural output. A single presynaptic neuron synapses with many postsynaptic neurons. Fig , p. 861
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Reverberating Circuits
Important in rhythmic breathing mental alertness short-term memory Depend on positive feedback new impulses generated again and again until synapses fatigue
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Reverberating Circuits
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1 2 (a) Simple reverberating circuit. An axon collateral of the second neuron turns back on its own dendrites, so the neuron continues to stimulate itself. Figure 40.13: Reverberating circuits. Fig a, p. 861
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Interneuron Axon collateral 1 2 3
(b) Reverberating circuit with interneuron. An axon collateral of the second neuron synapses with an interneuron. The interneuron synapses with the first neuron in the sequence. New impulses are triggered again and again in the first neuron, causing reverberation. Figure 40.13: Reverberating circuits. Fig b, p. 861
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