Download Synaptic Integration (SUMMATION)

Questions
Q: What happens to a nerve impulse once
it reaches the end of an axon?
Q: How does one neuron communicate
with another?
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Synapse structure and function
17-2
Transmission of a Signal Across Synapse
17-3
Synapses
• Nerve impulses reach synaptic ending
making the axomembrane permeable to
calcium ions (Ca2+ ).
• Ca2+ causes microfilaments to pull
synaptic vesicles to the inner
membrane of the presynaptic
membrane.
17-4
Synaptic Cleft
• Neurotransmitter molecules are
released into synaptic cleft, where they
bind with receptors on the postsynaptic
membrane.
• Depending on the kind of
neurotransmitter and/or type of
receptor, the response can be either:
• Excitation or Inhibition
17-5
Synaptic Integration (SUMMATION)
Excitatory Signals
• Occurs when membrane potential of
postsynaptic membrane increase
• Postsynaptic membrane depolarization
• Na+ channels open, positive ions enter
cell
• Increases likelihood of nerve impulse
to happen
17-6
Synaptic Integration (SUMMATION)
Inhibitory Signals
• Membrane potential of postsynaptic
membrane decreases(making the
inside become more negative)
• Decreases likelihood of nerve impulse
17-7
Synaptic Integration (SUMMATION)
• Transmission across a synapse is oneway because only the ends of axons
have synaptic vesicles that are able to
release neurotransmitters to affect the
potential of the next neurons.
• A neuron is on the receiving end of
many synapses -- some may be giving
inhibitory and some may give
stimulatory impulses.
17-8
Synaptic Integration (SUMMATION)
• Whether or not the neuron they are
attached to fires depends on the
SUMMARY EFFECT of all the excitatory
neurotransmitters received.
• Threshold voltage must be reached for
an action potential to occur.
17-9
Synaptic Integration (SUMMATION)
• If amount of excitatory
neurotransmitters received is sufficient
to overcome the amount of inhibitory
neurotransmitters received, the neuron
fires. If not, only local excitation
occurs
17-10
Summation
17-11
17-12
All or None
• A signal in an individual neuron cannot
be strong or weak; it is “all-or-none”
• If the neuron carries too many signals
in quick succession, it may end up with
all the Na+ inside and all the K+ outside.
17-13
All or None
• The Na+/K+ pump will be unable to work
quickly enough to restore the resting
potential
• This weakened stimulation is called
Neural Fatigue
17-14
Fate of Neurotransmitters
• Neurotransmiters are small molecules
• can be single amino acids, short chains
of amino acids, or derivatives of protein
• Neurotransmitters are quickly
deactivated or broken down to prevent
them from continually acting on
postsynaptic membrane.
• This can occur by:
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Fate of Neurotransmitters
a) neurotransmitter is degraded by
enzymes
e.g. acetylcholinesterase (AChE)
breaks down acetylcholine (Ach)
b)synaptic vesicles
• Reabsorbs/repackages the
neurotransmitter
17-16
Types of Neurotransmitters
• Proper brain and nervous system
function depends on the proper
balance of excitatory and inhibitory
synaptic transmitters.
17-17
Types of Neurotransmitters
Excitatory transmitters
e.g. ACETYLCHOLINE (ACh)
norepinephrine (NE)
Inhibitory transmitters
e.g. GABA (gamma aminobutyric acid - a
type of amino acid)
17-18
Drug Action
DRUGS can:
• Stimulate release of neurotransmitter
• Block release of neurotransmitter
• Bind with neurotransmitter
• Mimic neurotransmitter
• Block receptors
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Drug Action
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2
1
3
5
The 5 ways that drugs can act at synapes!
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Drug Action
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Drugs
Drugs either promote or decrease the action of
neurotransmitters, either stimulating or
inhibiting the action of excitatory
transmitters or inhibitory transmitters.
Stimulants either enhance excitatory
transmitters or block the action of inhibitory
transmitters
Depressants either enhance the action of an
inhibitory transmitter or block the action of
an excitatory transmitter.
17-22
Caffeine & Theophylline
• Both block the action of adenosine
• Adenosine inhitbits the release of
neurotransmitters
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Nicotine
• Enhances the action of acetylcholine
Alcohol
• Enhances action of inhibitory
transmitter GABA
17-24
Think-Pair-Share Review
1. What is the difference in action
between an inhibitory and excitatory
neurotransmitter?
2. What is ‘summation’?
17-25