Module 1 Lesson 7 - 2

Anatomy of the Synaptic Gap

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Read pages 378 - 379

In the world of neurons, the jump to the next neuron is a big gap. The synapse is the gap between neurons.

All synapses involve the following elements:

• Presynaptic axon terminal
  
• Neurotransmitters (such as norepinephrine, acetylcholine, and cholinesterase)
  
• Synaptic cleft
  
• Receptor proteins
  
• Postsynaptic dendrite or muscle
  
• Neurotransmitter re-uptake

The impulse travelling along the axon reaches the end of the axon called the axon terminal (or synaptic terminal). The tiny bulb on each axon terminal is called a synaptic knob. When the impulse reaches the axon terminal, it must cross the synapse to reach the next neuron. The impulse can cross the synapse using chemical neurotransmitters. The most common neurotransmitter in the human body is acetylcholine.

The neurotransmitters are released only by the axon terminals. Notice the synaptic vesicles in the diagram. These tiny sacs contain chemicals called neurotransmitters that are represented by the little green dots.

The neuron that ends in the axon terminal is called the presynaptic neuron. The synaptic cleft is the space between the presynaptic neuron and the postsynaptic neuron. The dendrites of the postsynaptic neuron receive the signal.

Summary of Signal Transmission at the Synapse

Crossing the Divide

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From the diagram and from your readings, you should be aware that, when the nerve impulse arrives at the synapse, it stimulates several reactions that end with the movement of the synaptic vesicles toward the presynaptic membrane and then fusing with it.

A neurotransmitter is released into the synaptic cleft, and it quickly diffuses across the synapse. Neurotransmitter molecules lock into receptor protein on the postsynaptic membrane, causing the sodium gates to open. Sodium ions from the synaptic cleft rush into the postsynaptic neuron.  The sodium gates are open only when the neurotransmitters are attached to the receptor proteins.  

The inflow of sodium ions causes depolarization and starts an action potential. This now occurs in the dendrite of postsynaptic neuron.

Watch and Listen

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For more visual exploration of synaptic transmission, complete the following activities.

1. Complete the following interactive on synaptic structures and functions.
   
2. Watch Crossing the Divide: How Neurons Talk to Each Other by clicking the image below.
   
   
   
   Summary of Crossing the Divide: How Neurons Talk to Each Other:
   
   The reward pathway detects a positive stimulus and involves dopamine neurotransmitter. Nerve cells communicate with each other at a junction called a synapse. Nerve signals move from one neuron to another neuron. The synaptic cleft is the space between the two cells. The presynaptic knob has round sacs called vesicles that are filled with neurotransmitters. The neurotransmitter used in the reward pathway is dopamine. The dopamine receptors are located in the post-synaptic dendrite.
   
   When the nerve impulse arrives in the presynaptic knob, it triggers the vesicles to fuse with the membrane and dump the dopamine neurotransmitter into the synaptic cleft. The dopamine molecules lock into the receptors in the post-synaptic dendrite. The receptors are specific to the shape of the dopamine molecule and do not allow other molecules to lock into the receptor protein. The dopamine triggers a cascade of events in the post-synaptic neuron through a second messenger system.
   
   After triggering the signal in the post-synaptic neuron, dopamine is released from the receptor and returns to the presynaptic neuron through a re-uptake transporter. Two things can stop nerve impulse transmission. First, if too few neurotransmitters bind to receptors, the neuron will not transmit a nerve impulse. Second, inhibitory neurotransmitters can stop the impulse.
   
   
3.    Watch the following segment of Biologix-03 on synaptic transmission.