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Signaling across a synapse

Page history last edited by Joe Redish 3 years, 5 months ago

4.3.3.P13

 

Neurons are animal cells that connect with each other using electrical signals. The connection between neurons is a synapse – a small gap between the membranes of the two cells. A schematic of a synapse is shown in the sketch at the right. When an electrical signal from the upper cell reaches the synapse, it releases neurotransmitter signaling molecules (e.g., acetylcholine) into the gap between the cells. The signaling molecules then diffuse across the gap where they are detected by receptors on the second (lower) cell. Answer the following questions about the diffusion process by choosing all correct answers.

 

1. The way an individual molecule knows to go from the top cell to the bottom rather than somewhere else is:

  1. There is a long-distance attraction between the receptor and the signaling molecule.
  2. The molecule knows to go from a region where there is a higher density of signaling molecules (near the upper cell) to a lower density of signaling molecules (near the lower cell).
  3. It doesn’t. Each signaling molecule just wanders randomly.
  4. The molecules are pulled across the gap by gravity.
  5. None of the above.

 

2. The reason a signaling molecule moves the way it does is

  1. Since there are more signaling molecules near the upper cell, it has more collisions and therefore moves away from a region of high density.
  2. It is colliding frequently with the molecules of the fluid it is in. Although on the average they cancel, fluctuations in the collisions drive it in random directions.
  3. It is colliding frequently with the molecules of the fluid it is in. A bias in those collisions tends to drive it more frequently towards the lower (receptor) cell.
  4. None of the above.

 

3. The distance between the membranes of the two cells is rather small (though not on a molecular size scale). What would happen to the time it takes a signal to diffuse across the gap if the distance between the membranes were twice as big?

  1. The time to diffuse would be twice as big.
  2. The time to diffuse would not change.
  3. The time to diffuse would be four times as big.
  4. The time to diffuse would be bigger by a factor of the square root of two (40% bigger).
  5. The time to diffuse would get smaller.

 

 

Joe Redish 12/16/15

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