If you've ever wondered your brain cells communicate with each other, it's through something called a synapse. A synapse is where two cells meet and exchange information. Scientists use special microscopes to see these connections, and they've found that the average neuron has 1000 of them! But get this: the outer layer of your brain, called the cortex, has a whopping 125 trillion synapses! That's more than all the stars in our galaxy! There are different types of synapses, too. They can be classified by how they attach to other cells, what kind of neurotransmitter they release, and what effect they have on the receiving cell.

What is the function of a synapse?

A synapse acts as a bridge that helps transmit information between neurons or between a neuron and another cell, depending on the type of synapse. Essentially, synapses are the interfaces that connect the specialized cells of the nervous system with each other or with other cells outside of the nervous system. This communication is essential for various functions such as learning, memory, movement, sensation, and more.

How are synapses named?

Synapses are commonly named after the main neurotransmitter that is passed on at the synapse, and the suffix "-ergic" is added to indicate this relationship. For example, a synapse that transmits dopamine is called dopaminergic, while a synapse that transmits adrenaline is called adrenergic. Similarly, a synapse that transmits the primary inhibitory neurotransmitter GABA is called GABA-ergic.

However, there is one exception to this naming rule, which is the cholinergic synapse. This synapse transmits the neurotransmitter acetylcholine, but it does not follow the -ergic naming convention. Instead, it is named after the neurotransmitter itself, which is acetylcholine. This is because acetylcholine is such an important neurotransmitter that it was discovered and named before the -ergic naming convention was established.

What is the structure of a synapse?

The synapse consists of three main parts:

1. The pre-synapse - this is the axon terminal of the neuron that is sending information. It releases neurotransmitters into the synaptic cleft to communicate with the next neuron or cell.
2. The synaptic cleft - this is a tiny gap (about 20-30 nanometers wide) between the pre-synapse and the post-synapse. It is filled with a fluid called the interstitium, which helps facilitate the transmission of neurotransmitters.
3. The post-synapse - this is the membrane of the neuron or cell that is receiving the information. It contains protein channels called receptors, which bind to the neurotransmitters released by the pre-synapse. The post-synapse can be another neuron, a gland, an organ or a muscle.

The terms pre- and post- refer to the positions of the neuron relative to the synaptic cleft. The pre-synapse is the neuron before the synaptic cleft, while the post-synapse is the neuron after the synaptic cleft.

What are the two main types of synapses?

There are two major types of synapses: electrical synapses and chemical synapses. There are more chemical synapses in the human body than electrical, but both have important functions.

What is an electrical synapse?

Electrical synapses are a type of synapse that directly connect or other cells through a protein channel called a gap junction, connexon or pore. These protein channels are made of connexin proteins and allow charged ions and messenger proteins to pass through uninhibited.

One of the main advantages of electrical synapses is that they allow for fast and efficient communication between cells. Because the transmission of information is direct and uninhibited, electrical synapses are able to transmit signals much faster than chemical synapses, which rely on the diffusion of neurotransmitters across the synaptic cleft.

While electrical synapses are more common in animals like squid and zebrafish, they are also present in the central nervous system, retina, and olfactory bulbs of humans, where they play an important role in synchronizing and coordinating the activity of neurons. Additionally, because the transmission of information in electrical synapses can flow in both directions, they can also allow for bi-directional communication between cells.

What is a chemical synapse?

Electrical synapses are a type of synapse that directly connect or other cells through a protein channel called a gap junction, connexon or pore. These protein channels are made of connexin proteins and allow charged ions and messenger proteins to pass through uninhibited. One of the main advantages of electrical synapses is that they allow for fast and efficient communication between cells. Because the transmission of information is direct and uninhibited, electrical synapses are able to transmit signals much faster than chemical synapses, which rely on the diffusion of neurotransmitters across the synaptic cleft. While electrical synapses are more common in animals like squid and zebrafish, they are also present in the central nervous system, retina, and olfactory bulbs of humans, where they play an important role in synchronizing and coordinating the activity of neurons. Additionally, because the transmission of information in electrical synapses can flow in both directions, they can also allow for bi-directional communication between cells.

Comparison between electrical and chemical synapses

Table 1. Differences between the electrical and chemical synapses.

How can synapses be classified?

Synapses can be grouped and classified in several ways.

Cell attachment

Synapses can also be classified based on how they connect to other neurons or cells. Here are some different types of synaptic connections:

1. Axodendritic: This is the most common type of synapse in the human body. It occurs when the axon of one neuron connects to the dendrites of another neuron.
2. Axosomatic: This type of synapse occurs when the axon of one neuron connects to the cell membrane or body (soma) of another neuron.
3. Axo-axonic: This type of synapse occurs when the axon of one neuron connects to the axon of another neuron. These synapses are usually inhibitory.
4. Dendro-dendritic: This type of synapse occurs between the dendrites of two different neurons.
5. Neuromuscular: This type of synapse occurs when the axon of a neuron connects to a muscle. These synapses are highly specialized and convert electrical impulses in the motor neuron into the electrical activity that causes muscle contractions. All neuromuscular junctions use acetylcholine as a neurotransmitter.

It's also worth noting that neurons can connect to all parts of the body, not just other neurons. For example, neurons can send axons into the interstitial spaces or to blood vessels.

Type of neurotransmitter released.

Synapses can be classified on the type of neurotransmitter released. Examples of neurotransmitters include dopamine, adrenaline, GABA, acetylcholine and others. These help name the synapses accordingly (except for acetylcholine).

Effect on the postsynaptic membrane

A synapse is the site where a neuron or a neuron and another cell meet. The presynaptic neuron/cell is the transmitting cell, while the postsynaptic neuron/cell is the receiving cell. There are two main types of synapses - electrical and chemical.

Electrical synapses use protein channels called gap junctions to directly connect two neurons and enable fast, bidirectional transmission of electrical impulses and molecules. Chemical synapses, on the other hand, use neurotransmitters that are diffused into the synaptic cleft to bind to receptors that open gates and allow ions to flow into the postsynaptic cell.

Synapses can have different interfaces, with the most common being axodendritic (presynaptic axon to postsynaptic dendrite), axosomatic (presynaptic axon to postsynaptic cell body), and axo-axonic (axon to axon). Additionally, synapses can be classified based on the type of ion channel or enzyme present in the postsynaptic neuron/cell, which can either excite or inhibit the postsynaptic neuron/cell.

Types of Synapse

What are the 3 types of synapses?

There are more but the main ones we focus on are electrical synapses, neuromuscular junctions and inhibitory ion channel synapses.

What is the difference between presynaptic and postsynaptic?

The terms presynaptic and postsynaptic refer to either side of the gap or synaptic cleft, with the presynaptic side being axon terminal of the sending neurone and the postsynaptic side being the specialised membrane of the receiving cell (neurone, muscle or other cell).

How can synapses be classified?

Synapses can be classified in three ways:  according to how they attach to other cells (axo-axonic, axodendritic, axosomatic, etc.) according to what kind of neurotransmitter is released by them (dopaminergic for dopamine-releasing synapses)what kind of effect they have on the postsynaptic membrane (excitatory ion channel, inhibitory ion channel or non-channel synapse)

Which is not a common type of neuronal synapse?

Electrical synapses are much less common in higher invertebrates.

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