Primary Somatosenory Cortex
Depolarization of the Postsynaptic Neuron Membrane
- While the postsynaptic neuron is at rest, Na+ / K+ pump proteins actively concentrated Na+ ions outside the cell and K+ ions inside the cell. Many of the K+ ions diffused back across the membrane through leak channels, making the cell’s interior relatively negative (polarized to -70mV).
- Depolarization of the postsynaptic membrane begins when ACh is released by the presynaptic neuron and binds to receptors in the postsynaptic membrane.
- Chemical gates open in response and the receptors form water-filled channels, which Na+ and K+ ions pass through.
- Because of favorable diffusion and electrical gradients, more Na+ ions enter the cell than K+ ions exit.
- A gain in positive ions inside the postsynaptic neuron causes it to depolarize (excitatory postsynaptic potential or EPSP).
- The amount of depolarization is graded (variable) and depends on how many ACh molecules bind to the postsyaptic receptors and how many ions are exchanged.