9.2 Synaptic transmission and neurotransmitter release
4 min read•august 1, 2024
Synaptic transmission is the cornerstone of neural communication. Neurons release neurotransmitters into synapses, which bind to receptors on target cells, triggering electrical or chemical changes. This process is crucial for information processing in the nervous system.
Neurotransmitter release is a complex, calcium-dependent process. When an action potential reaches a synapse, calcium influx triggers with the cell membrane, releasing neurotransmitters. This mechanism allows for precise control of neural signaling and forms the basis for synaptic plasticity.
Chemical synapse structure and function
Synaptic components and their roles
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Chemical synapses are specialized junctions between neurons that transmit information via neurotransmitters
The presynaptic terminal contains synaptic vesicles filled with neurotransmitters, mitochondria, and other organelles necessary for energy production and vesicle recycling
The postsynaptic membrane contains neurotransmitter receptors, which can be ionotropic (directly opening ion channels) or metabotropic (activating second messenger cascades)
The is a narrow space between the pre- and postsynaptic membranes that allows for rapid diffusion of neurotransmitters (typically 20-40 nm wide)
Neurotransmitter release and binding
The presynaptic neuron releases neurotransmitters into the synaptic cleft through of synaptic vesicles
Released neurotransmitters bind to receptors on the postsynaptic neuron, causing changes in its membrane potential
Ionotropic receptors directly open ion channels, leading to rapid changes in postsynaptic membrane potential (e.g., AMPA and for )
Metabotropic receptors activate second messenger cascades, resulting in slower and longer-lasting changes in postsynaptic neuron excitability (e.g., mGluR receptors for glutamate)
Neurotransmitter release and calcium
Calcium-dependent exocytosis
Neurotransmitter release occurs when an action potential reaches the presynaptic terminal, causing voltage-gated calcium channels to open
The influx of triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft
Calcium binds to specific proteins, such as , which are involved in the docking and fusion of synaptic vesicles with the presynaptic membrane
The amount of neurotransmitter released is proportional to the concentration of calcium ions in the presynaptic terminal (typically requiring ~100 μM calcium for release)
Neurotransmitter clearance and signal termination
After release, neurotransmitters are quickly removed from the synaptic cleft by into the presynaptic neuron or degradation by enzymes to terminate the signal
Reuptake is mediated by specific (e.g., SERT for , DAT for ) that use ion gradients to drive neurotransmitter uptake
Enzymatic degradation of neurotransmitters occurs in the synaptic cleft or surrounding glial cells (e.g., acetylcholinesterase breaks down acetylcholine)
Excitatory vs Inhibitory transmission
Excitatory synaptic transmission
Excitatory synaptic transmission occurs when neurotransmitters cause the postsynaptic membrane to depolarize, increasing the likelihood of an action potential in the postsynaptic neuron
Excitatory neurotransmitters, such as glutamate, typically bind to ionotropic receptors that allow the influx of positively charged ions (e.g., sodium or calcium), leading to depolarization
Examples of excitatory ionotropic receptors include AMPA and NMDA receptors for glutamate, and
Inhibitory synaptic transmission
Inhibitory synaptic transmission occurs when neurotransmitters cause the postsynaptic membrane to hyperpolarize, decreasing the likelihood of an action potential in the postsynaptic neuron
Inhibitory neurotransmitters, such as and , typically bind to ionotropic receptors that allow the influx of negatively charged ions (e.g., chloride), leading to hyperpolarization
Examples of inhibitory ionotropic receptors include GABAA receptors for GABA and glycine receptors
Some neurotransmitters, such as acetylcholine and serotonin, can have both excitatory and inhibitory effects depending on the receptor subtypes they activate (e.g., nicotinic vs. )
Synaptic strength and plasticity
Short-term synaptic plasticity
includes (increased neurotransmitter release with repeated stimulation) and depression (decreased neurotransmitter release with repeated stimulation)
Facilitation occurs when residual calcium in the presynaptic terminal enhances subsequent neurotransmitter release (e.g., paired-pulse facilitation)
Depression occurs when the readily releasable pool of synaptic vesicles is depleted faster than it can be replenished (e.g., synaptic fatigue)
Long-term synaptic plasticity
Long-term synaptic plasticity includes (LTP) and (LTD), which involve persistent changes in synaptic strength
LTP is induced by high-frequency stimulation and is associated with an increase in the number of at the postsynaptic membrane, enhancing synaptic transmission
LTD is induced by low-frequency stimulation and is associated with a decrease in the number of AMPA receptors at the postsynaptic membrane, reducing synaptic transmission
LTP and LTD are thought to be cellular mechanisms underlying learning and memory formation in the brain (e.g., hippocampal LTP in spatial memory)
Neuromodulation of synaptic plasticity
Neuromodulators, such as dopamine and norepinephrine, can modulate synaptic plasticity by altering the properties of neurotransmitter receptors or the excitability of neurons
Dopamine has been shown to modulate LTP and LTD in various brain regions, such as the striatum and prefrontal cortex, influencing reward-based learning and decision-making
Norepinephrine can enhance LTP in the hippocampus and amygdala, contributing to the formation of emotional memories and stress responses