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.
Anatomy of a Synapse
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.
Action Potential Propagation
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.
Role of Calcium in Neurotransmitter Release
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.
Mechanism of Vesicle Fusion
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.
Neurotransmitter Binding and Effect
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.
Ion Movement
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
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.
Summary of Steps in Synaptic Transmission
Arrival of Signal: Action potential reaches the axon terminal.