3. Explain how an excitatory post-synaptic potential (EPSP) triggers an action potential in the post-synaptic cell. Use the following key terms: excitatory neurotransmitter, Na+ ions, Na+ channels, resting membrane potential, threshold voltage, depolarization, action potential.

Answer 1

An excitatory post-synaptic potential (EPSP) is a critical process in neuronal communication that can lead to the generation of an action potential in the post-synaptic cell. Here’s how this process unfolds using the specified key terms:

1. Release of Excitatory Neurotransmitter: When an action potential reaches the axon terminal of a presynaptic neuron, it triggers the release of excitatory neurotransmitters (such as glutamate) into the synaptic cleft.

2. Binding to Receptors: These neurotransmitters bind to specific Na+ channels on the post-synaptic membrane. This binding causes the channels to open.

3. Influx of Na+ Ions: With the Na+ channels open, Na+ ions (sodium ions) flow into the post-synaptic cell. This influx occurs because the concentration of Na+ is higher outside the cell than inside, and the inside of the cell is more negatively charged relative to the outside.

4. Depolarization: The entry of Na+ ions causes a decrease in the negativity of the resting membrane potential (which is typically around -70 mV) as the membrane becomes more positive. This change in membrane potential is referred to as depolarization. If sufficient Na+ ions flow in, the membrane potential may approach a critical level.

5. Reaching Threshold Voltage: If the depolarization is strong enough to bring the membrane potential to a specific threshold voltage (usually around -55 mV), this triggers a cascade of events.

6. Action Potential Generation: Once the threshold is reached, voltage-gated Na+ channels open rapidly, leading to a rapid influx of more Na+ ions and causing a full action potential. This results in a sharp rise in the membrane potential, which constitutes the action potential that propagates along the neuron.

In summary, an EPSP is initiated by the binding of an excitatory neurotransmitter, facilitating Na+ influx through Na+ channels. This influx causes depolarization of the post-synaptic membrane. If depolarization reaches the threshold voltage, an action potential is triggered, allowing the signal to continue along the neuronal pathway.