The synapse is a specialized junction between two neurons, characterized by the exchange of information through the release of neurotransmitters from the presynaptic neuron. This process involves both electrical and chemical signaling, and the synapse acts as a crucial site for modifying neural circuit strength and for learning and memory. In this section, we'll delve deeper into the components that make up the synapse.
Pre Synaptic Terminal
The pre-synaptic terminal, also known as the axon terminal or pre-terminal, is the site where the presynaptic neuron communicates with the postsynaptic neuron. Within this terminal are thousands of synaptic vesicles, which store neurotransmitters such as dopamine, serotonin, acetylcholine, or GABA. Upon receiving an action potential from thepostsynaptic cell, the presynaptic neuron Releases neurotransmitters through a specialized gateway called the active zone, a structure with protruding filaments of specialized proteins.
Active Zone
The active zone is a specialized zone of the presynaptic plasma membrane where the synaptic vesicles are clustered. The presynaptic terminal's surface has protruding structures known as active zones, which have filaments that anchor to and bundle together the vesicles. These filaments form a dense matrix that is essential for the release of neurotransmitters.
Synaptic Cleft
The synaptic cleft is the space that lies between the presynaptic and postsynaptic membranes. It is filled with a small fluid-like substance known as the synaptic cleft fluid that aids in the diffusion of neurotransmitters. The width of the synaptic cleft is typically 10–20 nm, which is less than the diameter of a single neurotransmitter molecule, which is 5 nm.
Postsynaptic Membrane
The postsynaptic membrane is the membrane that receives neurotransmitters at the synapse. It is a specialized postsynaptic plasma membrane that contains neurotransmitter receptors. Different neurotransmitters bind to specific receptors on the postsynaptic membrane, which then either activate or inhibit the neuron. For example, dopamine can activate both D1 and D2 dopamine receptors on the postsynaptic membrane.
Postsynaptic Density (PSD)
The postsynaptic density is a group of proteins that cluster in the postsynaptic plasma membrane at the site where the synapse occurs. It includes receptors, adhesion molecules that help maintain proper positioning of the synapse, and regulatory proteins that modulate receptor function. The PSD is often found at the base of the postsynaptic dendrite, and its composition changes during postsynaptic plasticity.
Presynaptic Terminal Reforms
Presynaptic terminals can change their shape in response to activity levels. When activity levels drop, presynaptic terminals can expand to increase the surface area available for neurotransmitter release by extending and adding more filaments to the active zone. Conversely, when activity levels rise, terminals can缩回到原有的 size, or expand, to accommodate more neurotransmitters to be released.
Synaptic Plasticity
Synapses exhibit a form of synapse plasticity, meaning their ability to strengthen or weaken with repeated use. This is thought to play a critical role in learning and memory的形成 by allowing synapses to adjust their strength in response to their pattern. This can be observed as a changes in the number of functional neurotransmitter receptors at the postsynaptic membrane, leading to an enhanced response to future neurotransmitter release.
Synapse Transmission Steps
Synapse transmission occurs through a series of steps:
- Action potential propagation along the axon to the pre-synaptic terminal.
- Increase in intracellular Ca2+ levels triggered by action potential entry.
- Release of neurotransmitters from the presynaptic terminal through the active zone.
- Diffusion of neurotransmitters across the synaptic cleft to bind to their postsynaptic receptors.
- Activation or inhibition of postsynaptic neurons leading to generation of action potentials, which then travel back along the axon.
- Neurotransmitters removed from the synaptic cleft by reuptake into presynaptic neurons, degradation by enzymes, or摄取 by glial cells.
The Electronmicrograph of a Synapse
The synapse is often visualized using Electron Microscopy to observe synaptic ultrastructure. Images of synapses reveal that they contain a dense array of structural elements, including:
- presynaptic termini with hundreds of synaptic vesicles.
- active zones with filaments.
- The synaptic cleft, which is filled with synaptic cleft fluid.
- Postsynaptic density at the base of the postsynaptic dendrite.
- Often, clear dark bands of microtubules and neurofilaments extend beyond the PSD, perpendicular to the axis of the axons.
Summary
The synapse is a multi-component complex that plays a pivotal role in neurotransmitter release and the transmission of signals between neurons in the brain. Understanding the composition and function of synapses is crucial for grasping how the nervous system communicates and adapts to new experiences.
