1. Nervous system works because information flows from neuron to neuron
The Synapse
Nervous system works because information flows from neuron to neuron
Neurons functionally connected by synapses
Junctions that mediate information transfer
From one neuron to another neuron
Or from one neuron to an effector cell
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2. Synapse Classification
Axodendritic—between axon terminals of one neuron and dendrites of others
Axosomatic—between axon terminals of one neuron and soma of others
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3. Important Terminology
Presynaptic neuron
Neuron conducting impulses toward synapse
Sends the information
Postsynaptic neuron (in Pns may be a neuron, muscle cell, or gland cell)
Neuron transmitting electrical signal away from synapse
Receives the information
Most function as both
PLAY
Animation: Synapses
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4. Figure 11.16 Synapses. Axodendritic synapses Dendrites Axosomatic
Cell body
Axoaxonal
synapses
Axon
Axon
Axosomatic
synapses
Cell body (soma)
of postsynaptic
neuron
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5. Chemical Synapses
Specialized for release and reception of chemical neurotransmitters
Typically composed of two parts
Axon terminal of presynaptic neuron
Contains synaptic vesicles filled with neurotransmitter
Neurotransmitter receptor region on postsynaptic neuron's membrane
Usually on dendrite or cell body
Two parts separated by synaptic cleft
Fluid-filled space
Electrical impulse changed to chemical across synapse, then back into electrical
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6. 30 – 50 nm wide (~1/1,000,000th of an inch)
Synaptic Cleft
30 – 50 nm wide (~1/1,000,000th of an inch)
Prevents nerve impulses from directly passing from one neuron to next
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7. Transmission across synaptic cleft
Chemical event (as opposed to an electrical one)
Depends on release, diffusion, and receptor binding of neurotransmitters
Ensures unidirectional communication between neurons
PLAY
Animation: Neurotransmitters
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8. Information Transfer Across Chemical Synapses
AP arrives at axon terminal of presynaptic neuron
Causes voltage-gated Ca2+ channels to open
Ca2+ floods into cell
protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane
Exocytosis of neurotransmitter into synaptic cleft occurs
Higher impulse frequency  more released
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9. Information Transfer Across Chemical Synapses
Neurotransmitter diffuses across synapse
Binds to receptors on postsynaptic neuron
Often chemically-gated ion channels
Ion channels are opened
Causes an excitatory or inhibitory event (graded potential)
Neurotransmitter effects terminated
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10. Termination of Neurotransmitter Effects
Within a few milliseconds neurotransmitter effect terminated in one of three ways
Reuptake
By astrocytes or axon terminal
Degradation
By enzymes
Diffusion
Away from synaptic cleft
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11. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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12. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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13. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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14. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane. 4
Postsynaptic
neuron
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15. ion channels, resulting in graded potentials.
Figure Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Ion movement
Graded potential 5
Binding of neurotransmitter opens
ion channels, resulting in graded
potentials.
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16. terminated by reuptake through transport proteins, enzymatic
Figure Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Enzymatic
degradation
Reuptake
Diffusion away
from synapse 6
Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
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17. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane. 4
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
Binding of neurotransmitter opens
ion channels, resulting in graded
potentials. 5
Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse. 6
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18. Synaptic delay is rate-limiting step of neural transmission
Time needed for neurotransmitter to be released, diffuse across synapse, and bind to receptors
0.3–5.0 ms
Synaptic delay is rate-limiting step of neural transmission
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19. Postsynaptic Potentials
Neurotransmitter receptors cause graded potentials that vary in strength with
Amount of neurotransmitter released and
Time neurotransmitter stays in area
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20. Postsynaptic Potentials
Types of postsynaptic potentials
EPSP—excitatory postsynaptic potentials
IPSP—inhibitory postsynaptic potentials
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21. Excitatory Synapses and EPSPs
Neurotransmitter binding opens chemically gated channels
Allows simultaneous flow of Na+ and K+ in opposite directions
Na+ influx greater than K+ efflux  net depolarization called EPSP (not AP)
EPSP help trigger AP if EPSP is of threshold strength
Can spread to axon hillock, trigger opening of voltage-gated channels, and cause AP to be generated
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22. Inhibitory Synapses and IPSPs
Reduces postsynaptic neuron's ability to produce an action potential
Makes membrane more permeable to K+ or Cl–
If K+ channels open, it moves out of cell
If Cl- channels open, it moves into cell
Therefore neurotransmitter hyperpolarizes cell
Inner surface of membrane becomes more negative
AP less likely to be generated
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23. Synaptic Integration: Summation
A single EPSP cannot induce an AP
EPSPs can summate to influence postsynaptic neuron
IPSPs can also summate
Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons
Only if EPSP's predominate and bring to threshold  AP
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24. Language of nervous system
Neurotransmitters
Language of nervous system
50 or more neurotransmitters have been identified
Most neurons make two or more neurotransmitters
Neurons can exert several influences
Usually released at different stimulation frequencies
Classified by chemical structure and by function
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25. Classification of Neurotransmitters: Chemical Structure
Acetylcholine (ACh)
First identified; best understood
Released at neuromuscular junctions ,by some ANS neurons, by some CNS neurons
Synthesized from acetic and choline by enzyme choline acetyltransferase
Degraded by enzyme acetylcholinesterase (AChE)
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26. Classification of Neurotransmitters: Chemical Structure
Biogenic amines
Catecholamines
Dopamine, norepinephrine (NE), and epinephrine
Synthesized from amino acid tyrosine
Indolamines
Serotonin and histamine
Serotonin synthesized from amino acid tryptophan; histamine synthesized from amino acid histidine
Broadly distributed in brain
Play roles in emotional behaviors and biological clock
Some ANS motor neurons (especially NE)
Imbalances associated with mental illness
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27. Classification of Neurotransmitters: Chemical Structure
Peptides (neuropeptides)
Substance P
Mediator of pain signals
Endorphins
Beta endorphin, dynorphin and enkephalins
Act as natural opiates; reduce pain perception
Gut-brain peptides
Somatostatin and cholecystokinin
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28. Classification of Neurotransmitters: Function
Great diversity of functions
Can classify by
Effects – excitatory versus inhibitory
Actions – direct versus indirect
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29. Classification of Neurotransmitters: Function
Effects - excitatory versus inhibitory
Neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)
Effect determined by receptor to which it binds
GABA and glycine usually inhibitory
Glutamate usually excitatory
Acetylcholine and NE bind to at least two receptor types with opposite effects
ACh excitatory at neuromuscular junctions in skeletal muscle
ACh inhibitory in cardiac muscle
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30. Basic Concepts of Neural Integration
Neurons function in groups
Groups contribute to broader neural functions
There are billions of neurons in CNS
Must be integration so the individual parts fuse to make a smoothly operating whole
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31. Figure 11.22 Simple neuronal pool.
Presynaptic
(input) fiber
Facilitated zone
Discharge zone
Facilitated zone
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32. Patterns of Neural Processing: Serial Processing
Input travels along one pathway to a specific destination
System works in all-or-none manner to produce specific, anticipated response
Example – spinal reflexes
Rapid, automatic responses to stimuli
Particular stimulus always causes same response
Occur over pathways called reflex arcs
Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector
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33. Figure 11.24 A simple reflex arc.
Stimulus
Interneuron 1
Receptor 2
Sensory neuron 3
Integration center 4
Motor neuron 5
Effector
Spinal cord (CNS)
Response
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34. Patterns of Neural Processing: Parallel Processing
Input travels along several pathways
Different parts of circuitry deal simultaneously with the information
One stimulus promotes numerous responses
Important for higher-level mental functioning
Example: a sensed smell may remind one of an odor and any associated experiences
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