Questions

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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

Transmission of a Signal Across Synapse

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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.

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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

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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

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Synaptic Integration (SUMMATION)

Inhibitory Signals• Membrane potential of postsynaptic

membrane decreases(making the inside become more negative)

• Decreases likelihood of nerve impulse

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Synaptic Integration (SUMMATION)

• Transmission across a synapse is one-way 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.

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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.

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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

Summation

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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.

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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

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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

 

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Types of Neurotransmitters

• Proper brain and nervous system function depends on the proper balance of excitatory and inhibitory synaptic transmitters.

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Types of Neurotransmitters

Excitatory transmitters

e.g. ACETYLCHOLINE (ACh)

norepinephrine (NE)

 Inhibitory transmitters

e.g. GABA (gamma aminobutyric acid - a type of amino acid)

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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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The 5 ways that drugs can act at synapes!

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5

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.

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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

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Think-Pair-Share Review

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1. What is the difference in action between an inhibitory and excitatory neurotransmitter?

2. What is ‘summation’?