Chemical Synapse

Mechanism of Action Potentials and Neurotransmitter Release

Action Potentials Arrival

  • Action potentials arrive at the presynaptic terminal.
    • This is an essential step in neurotransmission, which is the process of signal transmission between neurons.

Opening of Calcium Ion Channels

  • The arrival of action potentials causes voltage-gated calcium ion channels to open.
    • Voltage-gated channels respond to changes in membrane potential, allowing specific ions to enter the cell.

Calcium Ion Diffusion

  • Following the opening of these channels, calcium ions (Ca²⁺) diffuse into the cell.
    • The influx of calcium ions is critical for the next step in the neurotransmission process.

Synaptic Vesicles Release

  • The entry of calcium ions triggers synaptic vesicles to release their contents.
    • These vesicles store neurotransmitters, and their release is a key event in signal transmission.

Acetylcholine Release

  • The neurotransmitter released is acetylcholine (ACh).
    • Acetylcholine is a crucial neurotransmitter involved in various functions, including muscle contraction and signal transmission in nerves.
    • After release, acetylcholine molecules diffuse across the synaptic cleft.

Binding to Receptor Sites

  • Acetylcholine binds to receptor sites on ligand-gated sodium ion channels located on the postsynaptic neuron.
    • Ligand-gated channels require a specific molecule (ligand) to bind to them in order to open.

Opening of Sodium Ion Channels

  • The binding of acetylcholine causes the ligand-gated sodium ion channels to open.
    • This allows sodium ions (Na⁺) to diffuse into the postsynaptic cell.
    • The influx of sodium ions alters the membrane potential.

Membrane Potential Change

  • As sodium ions enter, the membrane potential becomes more positive.
    • This is also known as depolarization of the membrane.

Threshold Level and Action Potential Production

  • If depolarization reaches a certain point, known as the threshold level, an action potential will be generated.
    • This is a critical concept in neural signaling, indicating an all-or-nothing response once the threshold is crossed.

Summary of Sequence

  • Sequence of Events in Neurotransmission:
    1. Action potential arrives at presynaptic terminal.
    2. Voltage-gated calcium ion channels open.
    3. Calcium ions diffuse into the cell.
    4. Synaptic vesicles release acetylcholine.
    5. Acetylcholine diffuses across synaptic cleft.
    6. Acetylcholine binds to receptors on ligand-gated sodium channels.
    7. Sodium ion channels open, allowing Na⁺ to enter the cell.
    8. Membrane potential becomes more positive.
    9. If threshold level is reached, an action potential is generated in the postsynaptic neuron.