Generation of an Excitatory Postsynaptic Potential
- Recall that a synapse is a junction between two neurons or between a neuron and an effector cell (like a muscle or gland).
- When a nerve impulse reaches the end of a neuron, it can’t simply jump to the next cell.
- Instead, it relies on neurotransmitters, chemical messengers that carry the signal across the synaptic gap.
The synaptic gap is incredibly narrow, only about 20–40 nm wide, which is just two to four times the thickness of a typical cell membrane.
Steps in the Generation of an EPSP Using Acetylcholine
Release of Acetylcholine
- When an action potential reaches the presynaptic terminal of a motor neuron, acetylcholine is released into the synaptic cleft through exocytosis.
- Acetylcholine travels across the synaptic cleft and binds to acetylcholine receptors located on the postsynaptic membrane of the muscle cell or next neuron.
Binding to Transmembrane Receptors
- Acetylcholine binds to specific ligand-gated ion channels (also called nicotinic receptors) on the postsynaptic membrane.
- This binding causes the ion channels to open, allowing sodium (Na⁺) ions to flow into the postsynaptic cell.
- In the case of acetylcholine, binding to its receptor opens sodium ($\text{Na}^+$) channels.
- Sodium ions then flow into the postsynaptic cell, making the inside of the cell less negative.
Depolarization of the Postsynaptic Membrane
- The influx of positively charged sodium ions (Na⁺) makes the inside of the postsynaptic cell more positive.
- This results in depolarization of the postsynaptic membrane, which is the characteristic feature of an EPSP.
Generation of EPSP
- The EPSP is a small, localized depolarization that may be enough to bring the membrane potential closer to the threshold for an action potential.
- If the depolarization reaches the threshold at the axon hillock of the postsynaptic neuron, it will trigger the generation of an action potential.
Termination of the Signal
- The acetylcholine signal is terminated by the enzyme acetylcholinesterase, which breaks down acetylcholine in the synaptic cleft, preventing continuous stimulation of the postsynaptic membrane.
- The breakdown products are taken back into the presynaptic neuron for recycling.
- Diffusion is a passive process, meaning it doesn’t require energy.
- This allows neurotransmitters to cross the synaptic cleft rapidly.
Neurotransmitter Receptors Open Ion Channels to Alter Membrane Potential
- Many neurotransmitter receptors are linked to ion channels.
- When a neurotransmitter binds to its receptor, it causes the ion channel to open, allowing ions to flow into the postsynaptic cell.
EPSPs Reduce Membrane Potential to Trigger Action Potentials
- The influx of positive ions (like sodium) reduces the negative charge inside the postsynaptic neuron.
- This change in charge is called an excitatory postsynaptic potential (EPSP).
- If the EPSP is strong enough, it can trigger an action potential in the postsynaptic neuron, allowing the signal to continue.
