Module 1 The Nervous System

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Anatomy of the Synaptic Gap

 

 

Your retinal receptor cells registered that attractive person, the olfactory receptors detected the pizza, the pressure receptors were activated by the handshake. The receptor cells converted this information to a nerve impulse and information was on its way through a sensory neuron. But it came to a screeching halt at the end of the sensory neuron. Where to now? How does communication get to the next neuron? In the world of neurons, there’s a big gap to jump to the next neuron. To understand where the transmission goes next and the structures involved in the leap across the gap, study Figure 11.18 on page 379 of your textbook.

 

Notice that the impulse traveling down the axon reaches the axon ending or synaptic terminal. Recall from your study of the neuron that the tiny enlargement on each axon terminal is called a synaptic knob. Notice the synaptic vesicles in the synaptic knob. These tiny sacs contain chemicals called neurotransmitters which are represented by the little circles.

 

The synaptic knob is surrounded by the presynaptic membrane. The neuron that ends in the synaptic knob is called the presynaptic neuron. The synaptic cleft is the space between the presynaptic neuron and the next neuron, called the postsynaptic neuron. Remember that the receiving parts of the neuron are called the dendrites. Notice in the diagram that the membrane surrounding the dendrite is called the postsynaptic membrane. Find the sodium ion channels in the postsynaptic membrane. Notice how these channels are only open when the neurotransmitter fits into a receptor.

 

Crossing the Divide

 

Have you ever stood on a rock in a stream and struggled with whether or not you could leap to the next rock? You think you can do it: you’re strong and coordinated, but if you fall, your new shoes would get very wet. To understand how a nerve transmission takes the “big leap” across the synaptic gap, study figure 11.18 on page 379, and read pages 378-379.

 

From the diagram and from your readings you should now know 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 molecules on the postsynaptic membrane, causing the sodium gates to open and sodium ions from the synaptic cleft rush into the postsynaptic neuron.  You have also already learned that an inflow of sodium ions causes depolarization, and the start of an action potential. This now happens in the postsynaptic neuron (dendrite).

 

Watch and Listen

To visually explore synaptic transmission,

1. Go to Crash Course - Synapses

As you watch the video, answer the following questions. These questions will be an excellent reference for studying.

1. Outline the reward pathway.
2. What is a synapse?
3. What happens at a synapse during synaptic transmission?
4. What is a synaptic cleft? Is a synaptic cleft different from a synapse?
5. What is a synaptic vesicle?
6. What is the neurotransmitter in the reward pathway?
7. Where are dopamine receptors located?
8. What does the arriving nerve impulse do?
9. What happens to the released neurotransmitter?
10. Why is there a sucking sound in the video?
11. What happens to the dopamine?
12. How is the nerve impulse started in the second neuron?
13. Identify two things that stop nerve impulse transmission.
14. Does one neuron always synapse only with one other neuron? Suggest where this might be true.