Imagine if you could tinker with any component of the signal transduction pathway involved in vision in vertebrates. In this problem, you will predict the effect of a change in the activity of a single component of the signal transduction pathway in vision. The rod cell secretes the neurotransmitter glutamate onto the bipolar cell as a function of its membrane potential – the more depolarized, the more it secretes; the more hyperpolarized, the less it secretes. The bipolar cell in this case (ON bipolar) has glutamate receptors that cause closure of cation channels, hyperpolarizing their membrane. They synapse with ON ganglion cells and secrete acetylcholine into the synapse at a rate that increases with increased depolarization of the bipolar cell membrane. The ON ganglions have acetylcholine receptors that are ligand-gated ion channels. The ganglion cell responds by sending a train of action potentials to the brain; the frequency of these action potentials is a function of the amount of acetylcholine that they sense. In your experiment, you will expose the rod cells to a controlled light pulse (same length and intensity every time) and measure the output of the ON ganglion cells to which they are connected. Then you will repeat this in each perturbed system.
- The acetylcholine receptor binds acetylcholine, causing it to
a. pump k+ ion out on the cell
b. change into the open conformation allowing Na+ ion to inter the cell
c. change into the closed conformation, stopping the leak of Na+ ions into the cell
b. change into the open conformation allowing Na+ ion to enter the cell
Explanation: The binding of acetylcholine to the acetylcholine receptor opens the sodium ion channels and allowing more sodium inside the cell. This leads to the depolarization of the membrane potential, which results in more glutamate secretion by bipolar cell and hence more acetylcholine secretion. This will keep signal ON.
Explanation for wrong answers
a. The role of K+ channel is not reported in the literature for the acetylcholine signaling response. At synapse, more Na+ ion channels are present than K+
c. acetylcholine plays a role in the propagation of signal. If the channel will be closed, then hyperpolarization of the membrane takes place. This leads to less acetylcholine production and hence signal will turn OFF.
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