What is a Synapse?

Synapses are structures in which the impulses of one neuron pass to another neuron or the contact between cells.

Synapse: between two neurons or between neurons and

The transmission of nerve impulses between neurons is unidirectional, that is, the nerve impulses can only be transmitted by one neuron.

C. 1896 S. Sherrington (

Chemical synapses or electrical synapses are composed of the presynaptic and posterior membranes and the narrow gap between the two membranes, the synaptic space (see figure), but there are obvious differences between the two. There is synapse formation between the soma and the soma, dendrites and dendrites, and axons and axons, but the axon of one neuron and the other are common

Synaptic structural parameters were significantly different between the CON group and the LS group, suggesting that PNS has caused morphological changes in the hippocampal synapses of offspring, which may have an impact on their plasticity.

Classification by way of nerve impulses through synapses

1.

It is found that more types of electrical synapses are transmitted in two directions, that is, regardless of pre-synaptic or post-synaptic, signals from either party can be transmitted. Electrical synapses only function as resistors and have low resistivity. This type of synapse is transmitted by electrical tension potential, so it is called electrical tension synapse.

Post-synaptic tip: contains neurotransmitters

For example, the space present in the lateral giant fibers of the crawfish ventral nerve cord is synapse. In fact, the lateral giant fiber is formed by axons belonging to multiple neurons in series, and the interval exists at the junction of them. It is composed of axon membranes that belong to two neurons.

In the experiment, a positive or negative current (not exceeding the threshold) is passed to either side of the interval, and the corresponding electrical potential can be recorded on the other side. Electron microscopy observations indicate that giant axons connected by interstitial synapses also exist in the nerve cords of other crustaceans and link animals. Excitable bidirectional electrical synapses between cells are also mainly found in invertebrates, such as lobster heart ganglion pacemaker cells, between two giant cells of leech, etc., but this also exists in the vertebrate brain between myocardium and smooth muscle cells Kind of synapse.

Such transmissions are non-directional, and some people do not recognize them as true synapses. Later, one-way transmission of electrical synapses was discovered one after another, both excitatory and inhibitory, thereby confirming the existence of electrical synapses. For example, in the crawfish ventral nerve cord, the giant synapse formed between the lateral giant fiber and the motor giant fiber can only allow excitement to be transmitted from pre-synaptic to post-synaptic in the form of electrical tension, which is a rectifying synapse. This type of synapse is also found in the nervous system of sea hares and leech. Inhibitory electrical synapses were also found on the mast cells axis mounds in some fish brainstems.

Synaptic neurotransmitter

There are still many difficulties in studying the chemical properties of neurotransmitters, especially central transmitters. Many synaptic neurotransmitters

Synaptic analysis

The quality is uncertain. To determine that a substance is a transmitter of an organization, certain criteria must be met, such as the following:

(1) When applied to the post-synaptic membrane, it causes the same physiological effects as those induced by pre-synaptic cells.

(2) This substance must be released when presynaptic neurons are active.

(3) Its effect must be blocked by a blocking agent capable of blocking normal transmission.

Acetylcholine is the most familiar neurotransmitter. Acetylcholine is released from vertebrate motor axonal ends, vertebrate vegetative nervous system preganglionic ends, parasympathetic postganglionic ends, and some neurons of the vertebrate central nervous system. The transmitter of certain invertebrate neurons is also acetylcholine. Acetylcholine is released from the axon to bind to the receptor, and is then hydrolyzed by cholinesterase to choline and acetic acid on the post-synaptic membrane, terminating the effect of acetylcholine on the post-synaptic membrane. Choline is absorbed by the presynaptic terminals to resynthesize acetylcholine.

(Acetylcholine receptors can also be divided into two types: one is muscarinic receptor (M-type receptor); the other is nicotinic receptor (N-type receptor) ). M-type receptors are present on all parasympathetic ganglia-fiber-dominated effectors, as well as on post-sympathetic ganglia-fiber-dominated sweat glands, and on sympathetic vasculature-dominated skeletal muscle vessels. Acetylcholine and M-type Receptor binding can produce a similar effect to muscarinic binding, including cardiac arrest, contraction of bronchial gastrointestinal smooth muscle and pupil sphincter, digestive gland secretion, sweat gland secretion, skeletal muscle vasodilation, and so on.

Atropine can bind to M-type receptors to block the effects of acetylcholine. N-type receptors are present on the post-synaptic membrane of neuromuscular junctions and on the post-synaptic membrane of visceral ganglia (sympathetic, parasympathetic ganglia). Acetylcholine binds to N-type receptors to produce an effect similar to that associated with a small amount of nicotine, which is the excitation of skeletal muscle and postganglionic neurons. Arrow poison can bind with N-type receptors on the post-synaptic membrane of neuromuscular junctions to block the effect of acetylcholine; hexahydrocarbon diamine can bind with N-type receptors on the post-synaptic membrane in sympathetic and parasympathetic ganglia to block acetylcholine Role.

Norepinephrine, dopamine and serotonin are similar compounds. These compounds are found in neurons of certain vertebrates and invertebrates. Most of them are concentrated in nerve endings. Norepinephrine is an excitatory transmitter of cells in the sympathetic nervous system.

After norepinephrine is released from the presynaptic terminals, it combines with the adrenergic receptors on the postsynaptic membrane to exert physiological effects. Most of them are reabsorbed and used by the terminals. Shift enzyme inactivation. Stop acetylcholine

Synaptic site structure

The effect depends on the cholinesterase hydrolyzing acetylcholine, and the effect of terminating norepinephrine mainly depends on the reabsorption of the transmitter by the peripheral. Adrenaline receptors can also be divided into alpha-type adrenaline receptors (referred to as alpha receptors) and beta-type adrenaline receptors (referred to as beta receptors). The combination of norepinephrine and epinephrine with alpha receptors causes excitement of the effector, but there are also cases of inhibition, such as smooth muscle of the small intestine; binding with beta receptors causes inhibition of the effector, but the effect on the heart is excitation. Certain amino acids, such as glutamic acid, are transmitters released from excitatory synapses in the central nervous system of vertebrates and excitatory neuromuscular junctions in insects and crustaceans.

Such as -aminobutyric acid (GABA), NH2-CH2-CH2-CH2-COOH, are inhibitory transmitters of the central nervous system (cerebral cortex, cerebellum) of vertebrates and play a very important role. Crustacean inhibitory motor synaptic transmitters are also gamma-aminobutyric acid.

Neuropeptides are a group of polypeptide molecules other than the few "classic" transmitters mentioned above. They are produced and released in the nervous system and function as transmitters or synaptic modulators that affect synaptic transmission. The first neuropeptide was U. S. von Euler and J. H. Gaddum was discovered in 1931. When they tested acetylcholine in rabbit brain and small intestine extracts, they found that this extract caused a contraction of the free intestine similar to acetylcholine, but this contraction could not be blocked by acetylcholine antagonists. They found that the contraction was caused by a polypeptide, named substance P. Since then a series of neuropeptides have been found in the central nervous system, peripheral nervous system, autonomic nervous system and invertebrate nervous system in vertebrates. Interestingly, some neuropeptides were originally found in the internal organs, such as glucagon and cholecystokinin in gastrointestinal hormones.

In recent years, fluorescent peptides can be used to locate neuropeptides in tissue sections. There are dozens of neuropeptides currently known, including substance P, enkephalin, vasoactive intestinal polypeptide (VIP), antidiuretic, oxytocin, adrenocorticotropic hormone-releasing factor and the like. Some neuropeptides act as both neurotransmitters and neurohormones, just like norepinephrine. In recent years, it has also been found that some neuropeptides can coexist in the nerve endings with classic transmitters such as acetylcholine and norepinephrine, and are released as cotransmitter of classic transmitters.

Enkephalins and endorphins are two interesting types of neuropeptides because they have analgesic and opioid euphoria. Unlike other neuropeptides, the first thing found in the body is the opioid receptor. In 1973

Schematic post-synaptic molecular mechanism

Three laboratories independently discovered the presence of opioid receptors in mammalian brains. These receptors can bind to opioids and initiate their effects. The presence of opioid receptors indicates that there are also endogenous opioids in the body, because the natural role of opioid receptors is not to interact with extracts of plants such as poppy, but to interact with certain opioids in the body. A few years later, peptides with opioid activity, opioid peptides, were found in mammalian brains.

Opioid peptides vary widely in size from enkephalins of 5 amino acids to -endorphins of 31 amino acids, but they all share 5 common amino acid sequences, namely tyrosine-glycine-glycine-phenylalanine Acid-methionine (or leucine). This sequence is a hallmark of opioid peptides and is necessary for them to bind to opioid receptors and to exhibit opioid pharmacological activity. A neuron is a unified whole, and the transmitters released by its various terminals should be the same.

In 1957 J. Eccles is further summarized as a neuron releasing only one transmitter, known as the Dale principle. Therefore, neurons are named after the released transmitters, such as acetylcholine neurons, epinephrine neurons, and so on. Recent studies have shown that a neuron can contain more than one transmitter, such as neuropeptides in addition to classic transmitters. In some cells of the submandibular ganglia that dominate the salivary glands of cats, vasoactive intestinal peptide (VIP) and acetylcholine coexist. Stimulation of the parasympathetic nerve that innervates the submandibular gland can detect acetylcholine and vasoactive intestinal peptide from the venous blood of the submandibular gland, indicating that two neurotransmitters are released from nerve endings. Low-frequency electrical stimulation (2 Hz) causes vasodilation and saliva secretion. These effects can be enhanced with physostigmine and blocked by atropine, suggesting the effects of acetylcholine. However, vasodilation caused by high-frequency electrical stimulation (15 Hz) is not blocked by atropine, which is the effect of vasoactive peptides.

In addition to the role of neurotransmitters in neuromodulation, neuromodulators are also playing a role. Neuromodulators are substances released by nerve cells and certain endocrine cells. They do not directly cause changes in the function of the dominated cells, but rather modulate the activity of classic transmitters released by presynaptic terminals and the effects of post-synaptic cells on transmitters. reaction.

The conduction velocity of impulses on nerve fibers is relatively constant, but they all show a certain time delay-synaptic delay when they pass through chemical synapses. Synaptic delay refers to the time interval from the transmission of excitement to the presynaptic tip to the appearance of post-synaptic potential. Synaptic delays in mammalian central synapses are about 0.2 to 0.3 milliseconds, and frog ganglia are up to 2 to 3 milliseconds in duration; synaptic delays through electrical synapses do not appear. Chemical synaptic transmission is prone to fatigue due to the limitation of transmitter metabolism; electrical synaptic transmission is as fatigue-free as fiber conduction. Chemical transmission is susceptible to inhibition and promotion by environmental factors such as blood flow, metabolism, and drugs that can affect transmitter synthesis, breakdown, release, and receptor function; electrical synaptic transmission is less susceptible to these factors, but it has also been found Some factors that modulate electrical synapses. Those neurons that need fast and synchronized activity are mostly electrical synapses. For example, the rapid stereotyped activities that dominate the shrimp's bow body to avoid reflection are mainly achieved by means of electrical synapses; as for those detailed and coordinated activities, especially those that require previous activities to affect subsequent activities, such as learning, memory, etc. , It should be achieved by chemical synapses.

Synaptic integration

Neurons are not connected in a single line, but are connected in multiples to form an intricate network. Each neuron is always connected to multiple neurons. An intermediate neuron, on the one hand, forms many synapses with the axons of multiple neurons (higher animals can form 100 to 10,000 synapses), and on the other hand, it uses multiple branches of its own axon and multiple neurons ( Intermediate neurons and motor neurons) form multiple synapses. Generally speaking, the amount of stimulation of a pre-synaptic cell is not enough to cause a response from a post-synaptic cell, that is, not enough to produce sufficient transmitters to reverse the polarity of a post-synaptic cell membrane; Co-stimulation causes multiple synapses to produce transmitters. The combined effect of these transmitters can excite post-synaptic cells. A post-synaptic cell can connect with several pre-synaptic cells simultaneously to form two types of excitatory and inhibitory synapses. The effects of these two synapses can cancel each other out. If inhibitory synapses work, stronger excitatory stimuli are needed to excite post-synaptic cells.

A neuron is an integrator, receiving hundreds or thousands of information at any time, and processing the received information at any time, so that the same information is added together, and the opposite information cancels each other out, and then decides whether to excite or maintain Silence (suppression) is the integration of neurons. This is probably the basic mechanism for processing and processing of incoming information by the neural network in vivo. More than 90% of the nerve cell bodies in the body are distributed in the brain and spinal cord, and the remaining 10% are in ganglia outside the central nervous system. Therefore, it is not difficult to understand that neural integration is mainly performed in the brain and spinal cord. ^[5]