Steps in a Synapse

The process of synaptic communication involves two neurons connected at a synapse.
- Presynaptic Neuron: The neuron releasing neurotransmitters, located before the synapse.
- Postsynaptic Neuron: The neuron receiving the signal, located after the synapse.
The space between these two neurons is referred to as the synaptic cleft.

Components of a synapse can be divided into three main parts:
- Axon terminal: Part of the presynaptic neuron that releases neurotransmitters.
- Synaptic cleft: Gap between the presynaptic neuron and the postsynaptic neuron.
- End plate: Area on the postsynaptic neuron where neurotransmitter receptors are densely located.

When an action potential travels down the axon of the presynaptic neuron:
- Sodium ions (Na^+ ) flood into the cell through voltage-gated sodium channels at nodes of Ranvier, regenerating the action potential.
- This depolarization prompts the opening of voltage-gated calcium (Ca^2+ ) channels at the axon terminal.

Calcium plays a critical role as a signaling molecule within the presynaptic neuron;
- Entry of calcium ions is triggered when the voltage-gated calcium channels open in response to the action potential.
Calcium binding:
- Calcium binds to calbindin, a binding protein that facilitates the signaling process.
- The calcium-calbindin complex interacts with synaptogammon, a membrane protein that has a spring-like structure.

The interaction of the calcium-calbindin complex with synaptogammon enables it to become mobile.
- As the complex activates, it causes a spinning motion that pulls vesicles containing neurotransmitters closer to the presynaptic membrane.
Vesicles, composed of a lipid bilayer like the presynaptic membrane, fuse with the membrane when they make contact, allowing their contents to spill into the synaptic cleft.

After spilling into the cleft, neurotransmitters diffuse across and bind to specific receptors on the postsynaptic neuron.
- This interaction opens chemically gated (ligand gated) ion channels in the postsynaptic membrane, altering its permeability to specific ions.

Different ions can move in response to receptor activation:
- For instance:
- Sodium ions (Na^+ ): Enter the postsynaptic cell, leading to depolarization and potentially generating a positive graded potential.
- Chloride ions (Cl^- ): Enter the cell, causing hyperpolarization that inhibits action potential generation.
- Potassium ions (K^+ ): May leave the cell as a result of certain receptor activations, contributing to inhibition.

Postsynaptic potentials can be:
- Excitatory Postsynaptic Potentials (EPSPs): Resulting from sodium influx, which promotes action potential firing.
- Inhibitory Postsynaptic Potentials (IPSPs): Resulting from chloride influx or potassium efflux, which hinders action potential initiation.

Arrival of Signal: Action potential reaches the axon terminal.
Calcium Influx: Voltage-gated calcium channels open; calcium enters the presynaptic neuron.
Binding Protein Activation: Calcium binds with calbindin, activating it.
SNARE Complex Activation: The calcium-calbindin complex activates synaptogammon, enabling vesicles to move towards the membrane.
Vesicle-Membrane Fusion: Vesicles fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
Neurotransmitter Action: Neurotransmitters diffuse across to bind to receptors on the postsynaptic neuron.
Graded Potential Initiation: Binding opens channels, allowing ion flow that results in a graded potential.
Signal Integration: If sufficient, graded potentials reach the axon hillock and may initiate an action potential.
Inhibitory Effects: IPSPs may counteract excitatory signals to ensure balanced neural activity.
Termination of Signal: The process concludes with termination mechanisms, which need to be discussed further in future content.