Chapter 1. Neurotransmission: Bridging the Divide

1.0.1 Neurotransmission: Bridging the Divide

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Neurotransmission: Bridging the Divide
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neuron
Specialized nerve cell engaged in information processing.
neurotransmissionneurotransmission (synaptic transmission)
Process that occurs when a neurotransmitter is released from a presynaptic neuron and binds to a receptor on the postsynaptic neuron.
synaptic cleft
Gap separating the neuronal presynaptic membrane from the postsynaptic membrane.
synapse
Spatial junction between one neuron and another; forms the information transfer site between neurons.
presynaptic membrane
Axon terminal membrane on the transmitter, or output, side of a synapse.
neurotransmitter
Chemical with an excitatory or inhibitory effect when released by a neuron onto a target.
activated (binding) proteins
Calcium molecule with a positive charge; protein that binds two or more molecules together.
microtubules
Transport structures that carry substances to the axon terminal.
storage granule
Membranous compartment that holds several vesicles containing a neurotransmitter.
transporter
Protein molecule that pumps substances across a membrane.
voltage-gated Ca2+ channelvoltage-gated Ca2+ channel
A gated protein channel on the presynaptic cell that opens when an action potential arrives at the axon terminal, allowing Ca2+ ions to flow into the cell.
postsynaptic membrane
Membrane on the transmitter, or input, side of a synapse.
dendrite
Branching extension of a neuron’s cell membrane; greatly increases the cell’s surface area; collects information from other cells.
dendritic spine
Protrusion that greatly increases the dendrite’s surface area; typical point of dendritic contact with the axons of other cells.
soma
Core region of the cell containing the nucleus and other organelles for making proteins.
synaptic vesicles
Membranous compartment that encloses a fixed number (called a quantum) of neurotransmitter molecules.
action potential
Large, brief reversal in the polarity of an axon membrane.
voltage-activated calcium channels
A gated protein channel on the presynaptic cell that opens when an action potential arrives at the axon terminal, allowing Ca2+ ions to flow into the cell.
transmitter-activated receptor
Protein that has a binding site for a specific neurotransmitter and is embedded in the membrane of a cell.
autoreceptor
Self-receptor in a neuronal membrane; that is, it responds to the same transmitter released by the neuron; part of a negative feedback loop allowing the neuron to adjust its output.
enzymes
Complex proteins made by living cells to aid in specific biochemical reactions, such as degradation of neurotransmitters.

Neurotransmission: Bridging the Divide

By: Dr. Diana Lim, Concordia University

1.1 Introduction

Introduction

The primary form of communication between neurons in the mammalian nervous system is neurotransmission (also known as synaptic transmission). During this process, chemical signals are sent from one neuron to the other across a small space known as the synaptic cleft. In this activity, you will set up a synapse and identify its basic components.

After completing this activity, you should be able to:

  • Describe the structure of a synapse.
  • Identify and describe the five basic steps of anterograde neurotransmission.
  • Explain how neurons communicate.

This activity relates to the following principles of nervous system function:

  • Principle 10: The Nervous System Works by Juxtaposing Excitation and Inhibition

1.2 Setting Up the Synapse: The Presynaptic Membrane

Setting Up the Synapse: The Presynaptic Membrane
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The presynaptic membrane is found on the transmitter (output) side of the synapse. The presynaptic cell sends the signal across the membrane. The neurotransmitters are stored in and released from the axon terminal of the presynaptic cell.

Let’s explore the primary components of the presynaptic terminal. Match the structure to the correct description in the presynaptic terminal. You have three attempts to match the descriptions correctly.

1.3 Setting Up the Synapse: The Postsynaptic Membrane

Setting Up the Synapse: The Postsynaptic Membrane
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The postsynaptic membrane is on the receptor (input) side of the synapse. This is the membrane that will receive the chemical signal. A close-up diagram of the synapse may show a dendrite (or a dendritic spine) of the postsynaptic cell.

Now let’s organize the components of the postsynaptic terminal. Match each structure to its description.

Review the diagram below showing the structures of the pre- and post-synaptic membranes.

Image

Now let's see how these two cells communicate.

1.4 Steps 1 and 2: Neurotransmitter Synthesis, Packaging, and Storage

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Steps 1 and 2: Neurotransmitter Synthesis, Packaging, and Storage

The process of neurotransmission can be broken into five steps. The first two steps involve synthesis of neurotransmitters, followed by their packaging and storage.

Some neurotransmitters are made in the soma and then transported to the axon terminal. Other neurotransmitters, such as acetylcholine (ACh), are made directly in the axon terminal (watch the Neurotransmitter Synthesis animation). At the axon terminal, neurotransmitters are packaged and stored in synaptic vesicles (watch the Neurotransmitter Packaging and Storage animation).

Video Box

Step 1: Neurotransmitter Synthesis

Video Box

Step 2: Neurotransmitter Packaging and Storage

1.5 Check Your Understanding

Check Your Understanding

Before we move on to the other steps in neurotransmission, let's do a quick check of your understanding. Complete the following sentence:

Question

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Well done! Synaptic vesicles are round granules in the presynaptic terminal that hold the neurotransmitters. Each vesicle holds a fixed number of neurotransmitter molecules.
Synaptic vesicles are round granules in the presynaptic terminal that hold the neurotransmitters. Each vesicle holds a fixed number of neurotransmitter molecules.

1.6 Step 3: Neurotransmitter Release

Step 3: Neurotransmitter Release

The third step in neurotransmission occurs in response to an action potential arriving at the axon terminal, which causes voltage-activated calcium channels at the terminal to open and calcium ions (Ca2+) to flow into the cell. These Ca2+ ions bind to proteins to form a complex that binds to and moves the vesicles to the presynaptic membrane. The neurotransmitter is released into the synaptic cleft by the process of exocytosis.

Step 3: Triggering Neurotransmitter Release

1.7 Check Your Understanding

Check Your Understanding

Let's do another quick check of your understanding.

Question 1.1

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Calcium ions enter the presynaptic cell and form complexes with binding proteins in the axon terminal. These complexes stimulate the vesicles to move toward and bind to the membrane in order to release their contents.

1.8 Step 4: Receptor-Site Activation

Step 4: Receptor-Site Activation

In the fourth step in neurotransmission, the transmitter crosses the synaptic cleft and binds to a transmitter-activated receptor. The properties of the receptors on the postsynaptic membrane determine the effect on the postsynaptic cell. A neurotransmitter may also influence the cell that just released it by acting on receptors called autoreceptors. Autoreceptors are important for providing information about whether adjustments should be made in response to synaptic communication.

Step 4: Neurotransmitter and Receptor Interaction

1.9 Check Your Understanding

Check Your Understanding

Question 1.2

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Neurotransmitters diffuse across the synaptic cleft and bind to a receptor site on the postsynaptic cell. The neurotransmitter must fit into the receptor like a key fits into a lock. After the neurotransmitter has bound to the receptor, it can cause a number of possible responses in the postsynaptic cell.

1.10 Step 5: Neurotransmitter Inactivation

Step 5: Neurotransmitter Inactivation

The fifth and final step in neurotransmission is inactivation of the neurotransmitter. There are four ways the neurotransmitter can be inactivated:

  1. it can diffuse away from the synaptic cleft,
  2. it can be degraded by enzymes in the synaptic cleft,
  3. it can be taken back into the presynaptic terminal, or
  4. it can be taken up by neighboring glial cells.

Click on the animation below to view the different ways that neurotransmitters are inactivated.

Step 5: Neurotransmitter Inactivation

1.11 Check Your Understanding

Check Your Understanding

Question 1.3

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Neurotransmitters can be returned to the presynaptic cell in a process known as reuptake. Matching membrane transporters bring the neurotransmitter back to the presynaptic axon terminal for reuse. The by-products of degradation by enzymes can also be taken back into the terminal for reuse in the cell. Reuptake can be blocked by certain drugs, such as selective-serotonin reuptake inhibitors (SSRIs), which are commonly prescribed for depression. For example, blocking the reuptake of serotonin, which is thought to regulate mood, allows that neurotransmitter to stay in the synaptic cleft longer.

1.12 Check Your Understanding

Check Your Understanding
Finish
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The following statements relate to the basic steps in the process of anterograde neurotransmission in action. Put them in the correct sequence. You will have 3 attempts to correctly arrange these steps.

Neurotransmitters are synthesized, packaged, and stored in vesicles in the presynaptic terminal.
An action potential arrives at the axon terminal, creating a change in voltage potential.
Voltage-activated calcium channels open on the presynaptic membrane.
Ca2+ ions flow into the presynaptic cell and bind to proteins, forming a complex that binds to vesicles.
Neurotransmitter-containing vesicles are transported to and bind to the presynaptic membrane.
Vesicles release their contents by exocytosis.
Neurotransmitters diffuse across the synaptic cleft.
Neurotransmitters bind to the appropriate receptors on the postsynaptic membrane, leading to several possible outcomes.
Neurotransmitters are released from the receptor sites.
Neurotransmitters are inactivated through either diffusion, degradation, reuptake, or astrocyte uptake.

1.13 Summary

Summary

Congratulations! You have successfully completed this activity. You have examined the structures of the presynaptic membrane and the postsynaptic membrane and seen the five basic steps of anterograde neurotransmission in action. Throughout the activity, you tested your knowledge of how neurons communicate with each other.

Your instructor may now have you take a short quiz about this activity. Good luck!

Question

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Viewed final slide of activity