Transcript Nervous system power point # 3

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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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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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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Figure 11.16 Synapses.
Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonal
synapses
Axon
Axon
Axosomatic
synapses
Cell body (soma)
of postsynaptic
neuron
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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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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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Synaptic Cleft
• 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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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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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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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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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon
terminal.
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
© 2013 Pearson Education, Inc.
Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon
terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
© 2013 Pearson Education, Inc.
Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon
terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
3 Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
© 2013 Pearson Education, Inc.
Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon
terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
3 Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis
4 Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane.
© 2013 Pearson Education, Inc.
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
Figure 11.17 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.
© 2013 Pearson Education, Inc.
Figure 11.17 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.
© 2013 Pearson Education, Inc.
Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon
terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
3 Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
4 Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter opens
ion channels, resulting in graded
potentials.
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
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Synaptic Delay
• 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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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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Postsynaptic Potentials
• Types of postsynaptic potentials
– EPSP—excitatory postsynaptic potentials
– IPSP—inhibitory postsynaptic potentials
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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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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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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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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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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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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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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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Classification of Neurotransmitters:
Function
• Great diversity of functions
• Can classify by
– Effects – excitatory versus inhibitory
– Actions – direct versus indirect
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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
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– ACh excitatory at neuromuscular junctions in skeletal
muscle
– ACh inhibitory in cardiac muscle
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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Figure 11.22 Simple neuronal pool.
Presynaptic
(input) fiber
Facilitated zone
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Discharge zone
Facilitated zone
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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Figure 11.24 A simple reflex arc.
Stimulus
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord (CNS)
Response
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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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