USMLE Step 1 Physiology Review 53 07 Neuronal Synapses (2 of 3)

USMLE Step 1 Physiology Review 53 07 Neuronal Synapses (2 of 3)

 

 

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Begin 53 07 Neuronal Synapses (2 of 3) Transcription

Okay, let’s talk about presynaptic inhibition.

Where does presynaptic inhibition occur?

  • At axo-axonic synapses.

What is the term for a junction between the synaptic axonic terminus of one neuron with the axon of another neuron?

  • An axo-axonic synapse.

This is a little complicated, so let me try to clarify things before we start talking about this.  In a simple case of presynaptic inhibition, there are three neurons involved.  For the purpose of this explanation, let’s call them the inhibitor neuron, the middle neuron and the target neuron.  The middle neuron is a postsynaptic neuron relative to the inhibitor neuron, but a presynaptic relative to the target neuron.  Signals in the middle neuron originate from a point or points before the axo-axonic synapse.  Signals travel in the middle neuron past the axo-axonic synapse and on to the target neuron.

What effect does the axo-axonic synapse between the inhibitor neuron and the middle neuron have on the release of neurotransmitter from the middle neuron?

  • It reduces the amount of neurotransmitter released from the middle neuron.

Recall from earlier in this tape, what ion induces the fusion of synaptic vesicles with the membrane wall, resulting in the exocytosis of the neurotransmitter into the synaptic cleft?

  • Calcium.

What is the ionic cause of the inhibition of neurotransmitter release from the middle neuron?

  • There is a reduction of the influx of calcium into the middle neuron.

Student Doctor, please pause the tape and summarize the information discussed this far on presynaptic inhibition.

  • Presynaptic inhibition occurs at axo-axonic synapses.  An axo-axonic synapse is a junction between the synaptic axonic terminus of one neuron with the axon of another neuron.  In a simple case of presynaptic inhibition, there are three neurons involved.  Let’s call them the inhibitor neuron, the middle neuron and the target neuron.  The middle neuron is a postsynaptic neuron relative to the inhibitor neuron, but a presynaptic relative to the target neuron.  The axo-axonic synapse between the inhibitor neuron and the middle neuron reduces the amount of neurotransmitter released from the middle neuron.  Calcium induces the fusion of synaptic vesicles with the membrane wall.  Reduction in the influx of calcium into the middle neuron causes the inhibition of neurotransmitter release from the middle neuron.

If the inhibitor neuron is not active, how does this affect the action potential in the middle neuron?

  • There is a normal action potential in the middle neuron if the inhibitor neuron is not active.

If the inhibitor neuron is active, how does this affect the action potential of the middle neuron?

  • The action potential in the middle neuron is reduced if the inhibitor neuron is active.

Consequently, if the inhibitor neuron is active, how does this affect the EPSP, or excitatory postsynaptic potential, in the target neuron?

  • The EPSP has a lower amplitude.

Is the synapse between the inhibitory neuron and the middle neuron an excitatory synapse or an inhibitory synapse?

  •  It is an excitatory synapse.

Is the synapse between the middle neuron and the target neuron excitatory or inhibitory?

  • Excitatory.

If the synapses are excitatory and they produce and EPSP, or excitatory postsynaptic potential, rather than an inhibitory postsynaptic potential, why is this mechanism considered inhibitory?

  • Because the EPSP that is generated is of a reduced amplitude.

Does most nerve to nerve communication occur via chemical communication or electrical communication?

  • Chemical communication.

When nerve to nerve communication does occur via electrical transmission, it is accomplished through what kind of junctions?

  • Gap junctions.

What are gap junctions?

  • They are cylindrical tubes of protein that span the membranes of adjacent cells.

What can pass directly from cell to cell through gap junctions?

  • Ions and small molecules.

Student Doctor, please pause the tape and summarize presynaptic inhibition discussed since the last summary, as well as the information on electrical synapses.

If the inhibitor neuron is not active, there is a normal action potential.  If the inhibitor neuron is active, the action potential in the middle neuron is reduced.  Consequently, if the inhibitor neuron is active an EPSP with a decreased amplitude is generated in the target neuron.  The synapses between the inhibitory neuron and the middle neuron and between the middle neuron and the target neuron are both excitatory synapses.  Even though the synapses are excitatory and they produce an EPSP rather than an IPSP, this mechanism is considered inhibitory because the EPSP that is generated is of a reduced amplitude relative to the normal.  Most neuronal communication occurs via chemical communication.  When neuronal communication does occur via electrical transmission, it is accomplished through gap junctions.  Gap junctions are cylindrical tubes of protein that span the membranes of adjacent cells and permit the direct passage ions and small molecules.

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