Primary Somatosensory Cortex
Postsynaptic Neuron Resting Membrane Potential
- When a postsynaptic neuron is at rest (not “firing”), its membrane is polar-ized, meaning the interior is slightly more negative than the exterior.
- The polarity is referred to as the resting membrane potential (or RMP) and is due primarily to the unequal distribution of Na+, K+, and protein ions along the surfaces of the membrane.
- Na+ and K+ ions are actively moved to opposite sides of the postsynaptic membrane by Na+/K+ pump proteins.
- Each pump protein uses one molecule of ATP to transfer 3 Na+ ions out of the cell and 2 K+ ions in.
- As a result, Na+ ions are concentrated outside the postsynaptic membrane and K+ ions are concentrated inside the membrane. The opposing ion gradients created by the pumps are nearly equal and contribute only slightly to the RMP.
- Very few of the Na+ ions can diffuse back into the resting cell because most of the Na+ channels are closed.
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- The conditions are different for the K+ ions. The postsynaptic membrane contains many leak channels that allow K+ ions to freely exit the cell.
- The outward diffusion of K+ ions makes the interior of the membrane more negative and the exterior more positive, which significantly alters the RMP. Also contributing to the growing polarity are a large number protein anions that are confined within cell.
- An equilibrium is quickly reached between the outward diffusion gradient for K+ and the inward electrical gradient.
- The opposing forces slow the net outflow of K+ ions and the RMP stabilize around -70mV.