Neurotransmission

Neurotransmission is the process by which information is transferred from one neuron to another through electrical and chemical signals.

Step 1: An action potential arrives at the axon terminal

There are a series of steps that take place during chemical synaptic transmission.

First, an action potential propagates down the axon until it arrives at the axon terminal, depolarizing the membrane of the presynaptic terminal. This results from transient changes in membrane permeability to different ions like Na, K and Ca.

• Resting membrane potential ≈ −70 mV
• Depolarization occurs due to opening of voltage-gated Na⁺ channels
• Repolarization occurs due to opening of K⁺ channels

The action potential propagates along the axon as a self-regenerating electrical signal.

Step 2: Membrane depolarization from action potential causes influx of calcium ions

When the action potential reaches the presynaptic terminal:

• Voltage-gated Ca²⁺ channels open
• Ca²⁺ enters the terminal according to its electrochemical gradient
• Increased intracellular Ca²⁺ triggers fusion of synaptic vesicles with the presynaptic membrane

This process converts an electrical signal into a chemical signal.

Step 3: Docking of synaptic vesicles at the membrane

• The synaptic vesicles fuse with the presynaptic membrane using SNARE proteins (v-SNARE's and t-SNARE's)
• Neurotransmitter molecules are released via exocytosis and diffuse across the synaptic cleft. Diffusion follows Fick's law: where:
  • J = diffusion flux
  • D = diffusion coefficient
  • dC/dx = concentration gradient

Step 4: Release of neurotransmitters into the synapse

The last step of neurotransmitter release is the fusing of the cell membrane. In order to release their chemical contents into the synapse, vesicles need to fuse with the cell membrane. As the vesicular membrane merges with the interior of the neuronal membrane, the membranes fuse and the contents of the vesicle become exposed to the extracellular space. The neurotransmitters then float across the aqueous synapse, giving them the opportunity to interact with postsynaptic receptors.

Step 5: Receptor Activation

Neurotransmitters bind to receptors on the postsynaptic membrane.

This causes changes in membrane conductance:

• Opening Na⁺ channels → depolarization (EPSP)
• Opening K⁺ or Cl⁻ channels → hyperpolarization (IPSP)Neurotransmission ends by:

• enzymatic degradation (e.g., acetylcholinesterase)
• reuptake into the presynaptic neuron
• diffusion away from the synapse

CITATION: https://openbooks.lib.msu.edu/introneuroscience1/chapter/neurotransmitter-release/