A neuron in a mouse\'s brain (neuron A) continuously generates action potentials

Question

A neuron in a mouse's brain (neuron A) continuously generates action potentials (APs) at a constant rate of 50 action potential per second (50 Hertz (Hz)). Neuron A receives presynaptic input from three other neurons, B, C, and D. Neuron B's axon tip forms a synapse on neuron A's axon near A's cell body. When neuron B is active, its action potentials transmit EPSPs (excitatory post-synaptic potentials) to neuron A at a rate of 500Hz. When B is active, neuron A increases its AP-firing rate from 50Hz to 100Hz. Neuron C forms a synapse on A very close to neuron B's synapse and, when C is active (and B is inactive), A receives EPSPs at a rate of 50Hz but the rate at which A generates APs remains unchanged by neuron C, it stays at 50Hz. Neuron D's synapse on A is located on one of A's dendrites. When neuron D is active it causes neuron A to receive EPSPs at 500 Hz but the output of neuron A does not change, it stays at 50Hz. The EPSPs in each of the three situations are the same magnitude, 5 millivolts (5mV). Explain how neurons B, C, and D affect neuron A differently.

Explanation / Answer

The synapse of neuron B is close to A and is very energetic (500Hz), whereas neuron C is not energetic enough and neuron D is too far away. These two things prevent an excitation from being transmitted to neuron A, while B, which does not have these two problems, can indeed excite neuron A.

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