MCAT Question A Day - 6/24/13 - Answer!
When a neurotransmitter released from one neuron (pre-synaptic neuron) makes it more difficult for the next neuron (post-synaptic neuron) to depolarize, this phenomenon is called inhibition. What might be happening at the synapse to cause inhibition?
A. The neurotransmitter causes extra Na+ to immediately enter the post-synaptic neuron.
B. The neurotransmitter causes extra K+ to immediately enter the post-synaptic neuron.
C. The neurotransmitter causes extra Na+ to immediately leave the post-synaptic neuron.
D. The neurotransmitter causes extra K+ to immediately leave the post-synaptic neuron.
The correct answer is (D). Prior to stimulation, membranes in the polarized state are more (-) inside and more (+) outside. They have more Na+ outside and more K+ and (-) proteins inside. The difference in charge is approximately 270mv. For normal excitation at a synapse or neuromuscular junction to occur, the pre-synaptic neurotransmitter makes it easier for Na+ to enter the post-synaptic cell, making the inside (+). Inhibition usually involves the neurotransmitter allowing K+ to leave the postsynaptic cell, making the transmembrane voltage difference more than 270mv (280mv or more), thus making it more difficult to cause depolarization. If extra Na+ entered (choice A), depolarization would occur more easily. The movement of K+ into the post-synaptic cell (choice B) and the movement of Na+ out of the postsynaptic cell (choice C) are not good answers since both would be against concentration gradients and would require active transport and the expenditure of ATP.
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