What Are Synaptic Vesicles?

Synaptic vesicle synaptic vesicles are formed in different parts of the neuron. The vesicles contain a high concentration of chemical transmission substances. As the nerve endings are excited, the contents of the vesicles are released to the synaptic space, causing synaptic transmission .

Chinese name: Synaptic vesicles
Foreign name: synaptic vesicle

Formed in: Different parts of the neuron
Make up: High concentration of chemical transfer substances in vesicles
Nature: private

Synaptic vesicles I. Overview

Synaptic vesicles refer to a large number of vesicles in the axoplasm of the nerve endings on the anterior side of the synapse, and the density near the presynaptic membrane is greater. The size of synaptic vesicles varies among different nerve endings. Synaptic vesicles are places where transmitters are stored and released.

Synaptic vesicles:

Synaptic vesicles are formed in different parts of the neuron through a variety of pathways, such as some directly formed by the presynaptic membrane, and some produced by the Golgi apparatus, endoplasmic reticulum, mitochondria, and microtubules. In chemically-transmitting synapses, they are found in nerve endings with many vesicles about 50 fiber diameter in diameter. The synaptic vesicles of motor nerve endings are all spherical, and the sympathetic nerve endings are mixed with synaptic vesicles with a diameter of about 100 fiber meters and a dark core. In addition to these two types in the central nervous system, there are also peripheral spheroid-shaped synaptic vesicles. The vesicles contain a high concentration of chemical transmission substances. When the neurons are stimulated, the synaptic vesicles will move toward the presynaptic membrane until they are fused, releasing the neurotransmitter.

Synaptic vesicles :

Synaptosomes are also called terminal nodules, which are a large part of terminal axons of anterior neurons. This body is small nodular or twist-like, containing mitochondria and synaptic vesicles. Mitochondria provide energy for the synthesis of chemical transmitters, synaptic vesicles can synthesize and release chemical transmitters, and enter the synaptic space through the presynaptic membrane to specifically bind to the postsynaptic membrane, acting on the postsynaptic nerve. .

Synaptic vesicle synapses:

A scholar suggested in 1897 that neurons are not connected to each other to form a network. They transmit information through contact points between them. He called these contact points synapses. In the early years of light microscopy studies, it was found that synapses are composed of small bulbs or nodules that branch and expand at the ends of neuron axons and are attached to the surface of other neuron cell bodies or dendrites. These little balls or nodules are called synaptic nodules. However, no structural details of the synapse have been seen. Electron microscopy studies provide a wealth of knowledge about the microstructure of synapses and their relationship to information transmission mechanisms. Many synapses are known to transmit nerve impulses, which require intervening effects by releasing special chemicals called neurotransmitters. This type of synapse is called a chemical synapse. Recent studies have found that some synapses can directly transmit electrical signals without the help of neurotransmitters. Such synapses are called electrical synapses. The microstructure and information transmission mechanism of the contact sites between motor nerve endings and effector cells are basically the same as chemical synapses, and they are often called synapses. In recent years, the contact site of sensory nerve endings and sensory cells is often called synapse. So today the term synapse has a broader meaning. Synapses have different styles. The most common is that the axon end is divided into multiple thin branches, and the ends of each thin branch swell up into small balls or nodules, which stop on the surface of another neuron. Such axon ends are called terminations or terminal feet. Some nodules are quite large and can surround most of the cell body of another neuron. Some axon thin branches have spaced or continuous swollen portions throughout the length, which are in contact with nerve cells or effector cells. Such axonal ends are called ganglia. The number of synapses on neurons varies widely. There are no synapses in sensory neurons in the brain ganglia and spinal ganglia. There are only a few granular cells in the cerebellum. There are about 2,000 large motor neurons in the anterior horn of the spinal cord. There can be hundreds of thousands of dendrites in Purkinje cells of the cerebellum.

Synaptic vesicles II. Location and morphological structure of synaptic vesicles:

Synaptic vesicles are a group of vesicle-like structures located in presynaptic cells that store acetylcholine and play a role in regulating acetylcholine in nerve cells.

Synaptic vesicles vary in size, with diameters ranging from 20 to 65 nm, with diverse morphology, containing various neurotransmitters. Transmitter-containing synaptic vesicles travel through the axon to the axon end in a fast forward direction.

Synaptic vesicles III. Synaptic structure:

A synapse is a point of functional contact between neurons, and neurons pass nerve impulses through this point. There are many types of synapses. Almost any part of a neuron can form a synapse, but a synaptic nodule formed by the axon twig end is attached to the surface of another neuron cell body or dendrite. Mostly. Synapses can be classified into chemical synapses and electrical synapses based on whether synaptic transmission of nerve impulses is aided by neurotransmitters and synaptic structural features. Many researches on chemical synapses have been made in depth, and they have quite complex and diverse structural features. The basic structure of electrical synapses is well understood, but the details are still unknown.

Chemical synapses:

Typical chemical synapses are composed of pre-synaptic components, synaptic clefts, and post-synaptic components. The presynaptic component is the synaptic nodule of the axonal tip. The postsynaptic component is the local area where the cell body or dendrite of another neuron contacts the presynaptic component. The membrane of the presynaptic component is called the presynaptic membrane, and the membrane of the postsynaptic component is called the postsynaptic membrane. The structures of the two membranes are specialized, with dense substances and special structures of varying thickness attached to the cytoplasmic surface. There is a gap between the two membranes with a width of 10-20 nm, which is called synaptic space or synaptic seam. The gap contains a medium-density substance and filaments connecting the two membranes. They are probably proteins and mucopolysaccharides, and they may have other functions besides firmly adhering to the presynaptic membrane and the posterior membrane. Synaptosomes obtained by centrifugal separation of brain tissue are presynaptic components and post-synaptic membranes attached to it. The detailed chemical analysis of these bodies provides valuable knowledge for exploring the chemical composition of synapses and the mechanism of information transmission.

(1) Presynaptic components:

Light microscope observation of the silver-stained specimen showed that it was often nodular or spherical, stained brown or black, and had a diameter of 1 to 2 m. It can be seen to contain neurofibrils and mitochondria, but no more structural details have been seen. Electron microscopy revealed that it contained mitochondria, small capsules and tubules in the endoplasmic reticulum of the sliding surface, bundled nerve filaments and microtubules. An important feature of this part is that it contains many membrane-coated vesicles, called synaptic vesicles, which are the main storage of neurotransmitters. Synaptic vesicles vary in size, shape, and density of contents within each synapse. The content of some vesicles is very small, showing clear vesicles. The content of some vesicles is dense and presents larger particles, which are dense particle vesicles or dense core vesicles. Qingming vesicles are round, flat or irregular. Circular clear vesicles with a diameter of 40-50 nm are found in the motor endplates of skeletal muscles, pre-sympathetic fiber terminals and parasympathetic fiber terminals, and are found in many synapses in the central nervous system. Most of these vesicles contain acetylcholine, and some contain gamma-aminobutyric acid. Oblate qingming vesicles are found in the central nervous system and are known to contain glycine. Dense particle vesicles vary in size. Some vesicles have a diameter of 40 to 60 nm and contain particles of 15 to 25 nm. Most of these vesicles contain catecholamines, especially norepinephrine, found in the fiber endings after the sympathetic ganglia. Some synapses in the brain contain vesicles with a diameter of 80 to 90 nm with a particle diameter of 50 nm; some synapses contain vesicles with a diameter of 80 to 150 nm with a particle diameter of 50 to 70 nm; most of these vesicles contain serotonin and dopamine, and some contain peptides .

Neurotransmitters are mainly present in synaptic vesicles. There can be as many as thousands of synaptic vesicles in one axonal tip. Each vesicle contains 10,000 to 200,000 transmitter molecules. When nerve impulses reach the axonal tip, the calcium channel of the presynaptic membrane is triggered to open, and a large amount of calcium ions enter, causing the synaptic vesicles to fuse with the presynaptic membrane, and release neurotransmitters to the synaptic space by exocytosis. Only a part of the released transmitters binds to the receptors of the postsynaptic membrane, producing physiological effects. The excess of acetylcholine and monoamine can be taken up by the axonal tip, and it seems that it cannot be recovered after the peptides are released. Excess acetylcholine can also be broken down by acetylcholinesterase in the synaptic cleft. Norepinephrine is degraded by catechol-oxygen-methyl transposase in the synaptic cleft and post-synaptic membrane, or decomposed by mitochondrial monoamine oxidase, and the decomposed products can be used in the distal end to synthesize transmitters.

(2) Post-synaptic components:

This part of the cytoplasm often aggregates mitochondria. Thicker dense substances are attached to the cytoplasmic surface of the postsynaptic membrane, and type I synapses are thicker. Some dendritic spines have spines in their cytoplasm. The spine consists of several flat sacs stacked in parallel and a plate-like compact located between the sacs. Structures like spines are also found in dendrites and axons. The meaning of this device is unknown, and some have speculated that it may be related to learning and memory. Postsynaptic membranes have special proteins that function as receptors, ion channels, and pumps, respectively. The receptor is coupled to a channel or adenylyl cyclase and can bind to specific neurotransmitters to excite (depolarize) or inhibit (hyperpolarize) the post-synaptic membrane. When the receptor binds to the transmitter molecule, the channel is activated, the ion concentration inside and outside the membrane changes, causing a rapid change in membrane potential. Receptors can also activate adenylate cyclase, allowing cells to synthesize cyclic adenylate. The latter acts as a second messenger, activates intracellular protein kinases, regulates the phosphorylation of membrane proteins or acidic nucleoproteins, regulates the production and breakdown of membrane receptors, pumps and channel proteins, and then affects the binding of receptors to transmitters, Affects the membrane's permeability to ions, causing slow and long-term excitation or inhibition of the post-synaptic membrane. It is known that nerve cells bind to more than one receptor of a certain neurotransmitter. Some receptors are excitatory and some are inhibitory. Therefore, a neurotransmitter has different effects at different synapses.