The neural synapse is a fundamental component of the nervous system, enabling communication between neurons. In this process, information is transmitted from one neuron to another through electrical and chemical signals. The synapse's structure is complex, involving the presence of axon terminals, synaptic vesicles, neurotransmitters, and postsynaptic receptors. There are different types of synapses, such as excitatory and inhibitory, which play specific roles in transmitting information in the brain. The functioning of the neural synapse is essential for processes such as learning, memory, motor coordination, and other cognitive functions.
What are the different ways neurons connect in the nervous system?
Synapses are specialized connections between neurons in the nervous system, enabling the transmission of electrical and chemical signals. There are different types of connections between neurons, which can be classified into three main types: axodendritic synapses, axosomatic synapses, and axoaxonal synapses.
Axodendritic synapses are those in which the axon of one neuron connects with the dendrite of another neuron. This is the most common form of synapse in the nervous system and is responsible for transmitting information from one neuron to another. Axo-somatic synapses occur when the axon of one neuron connects directly with the cell body of another neuron. These synapses are important for regulating neuronal activity and integrating signals from different sources.
Finally, axoaxonal synapses are those where the axon of one neuron connects with the axon of another neuron. These synapses are less common, but they play an important role in modulating neuronal activity and regulating the flow of information within the nervous system.
Each type of synapse has specific functions and contributes to the complexity and effectiveness of the nervous system.
Understand how neural synapses work in 15 words.
A neural synapse is the connection between neurons, where the transmission of chemical and electrical information occurs.
Neurotransmitters are released into the synaptic cleft and bind to receptors on the postsynaptic membrane.
This generates a postsynaptic potential that can be excitatory or inhibitory, influencing the transmission of the nerve impulse.
There are different types of synapses, such as excitatory and inhibitory synapses , which perform specific functions in the nervous system.
Understanding how neural synapses work is essential to understanding how our brain processes information.
Understand how a synapse works and its importance in the nervous system.
A synapse is a fundamental structure in the functioning of the nervous system, responsible for transmitting information between neurons. It is essential for communication between nerve cells and for coordinating brain and body activities.
At a synapse, communication occurs through neurotransmitters, chemicals released by one neuron and taken up by another. These neurotransmitters are released from synaptic vesicles located at the end of the transmitting neuron's axon and bind to receptors on the receiving neuron's membrane, triggering a nerve impulse.
There are two main types of synapses: electrical and chemical. In electrical synapses, communication occurs through the direct passage of ions between neurons, allowing for faster transmission of the nerve impulse. In chemical synapses, communication is mediated by neurotransmitters, enabling greater modulation and control of the transmitted signal.
Synapses are extremely important for the functioning of the nervous system, as they enable the integration of sensory information, the coordination of muscle movements, and the regulation of vital functions such as breathing and blood circulation. They are also involved in cognitive processes such as memory, learning, and emotions.
Its proper functioning is essential for maintaining homeostasis and the individual's health.
Exploring the structures involved in transmitting signals between neurons: the parts of the synapse.
The synapse is the structure responsible for transmitting signals between neurons, enabling communication within the nervous system. It is composed of several parts that perform specific functions during this process.
One of the main parts of the synapse is the axon terminal, which releases neurotransmitters into the synaptic space. These neurotransmitters are chemicals that transmit signals from one neuron to another. Another important part is the synaptic cleft, which is the space between the axon terminal and the postsynaptic membrane.
The postsynaptic membrane contains specific receptors for neurotransmitters, which bind to these receptors and trigger a response in the postsynaptic neuron. This response may include the opening of ion channels, altering the neuron's action potential and propagating the signal.
There are different types of synapses, such as electrical synapses and chemical synapses. In electrical synapses, neurons are connected by gap junctions that allow the direct passage of ions between them. In chemical synapses, communication occurs through neurotransmitters.
Understanding how these structures work is essential to understanding how the brain processes information and controls the body's functions.
Neural synapse: structure, types and how it works
A neuronal synapse is the union of the terminal buttons of two neurons to transmit information. In this context, one neuron sends the message, while a part of the other receives it.
Thus, communication generally occurs in one direction: from the terminal button of one neuron or cell to the membrane of another cell, although there are some exceptions. A single neuron can receive information from hundreds of neurons.
Each neuron receives information from the terminal buttons of other nerve cells and, in turn, the terminal buttons of the latter synapse with other neurons.
Main concepts
The terminal button is defined as a small thickening at the end of an axon, which sends information at the synapse. An axon is a type of elongated, thin "cable" that conducts messages from the neuron's nucleus to its terminal button.
The terminal buttons of nerve cells can synapse with the membrane of the soma or dendrites.
The soma, or cell body, contains the neuron's nucleus; it has mechanisms that allow the cell to maintain itself. In contrast, dendrites are tree-like branches of neurons that extend from the soma.
When an action potential travels down a neuron's axon, the terminal buttons release chemicals. These chemicals can have excitatory or inhibitory effects on the neurons they connect to. Ultimately, the effects of these synapses give rise to our behavior.
An action potential is the product of communication processes within a neuron. A set of changes in the axon membrane causes the release of chemicals or neurotransmitters.
Neurons exchange neurotransmitters at their synapses as a way of sending information between them.
Neural synapse structure
Neurons communicate through synapses, and messages are transmitted through the release of neurotransmitters. These chemicals diffuse into the fluid space between the terminal buttons and the membranes that establish the synapses.
Presynaptic nerve
The neuron that releases neurotransmitters through its terminal button is called a presynaptic neuron, while the one that receives the information is called a postsynaptic neuron.
When the latter captures neurotransmitters, so-called synaptic potentials are produced—that is, changes in the potential of the postsynaptic neuronal membrane.
To communicate, cells must secrete chemicals (neurotransmitters) that are detected by specialized receptors. These receptors consist of specialized protein molecules.
These phenomena are simply differentiated by the distance between the neuron that releases the substance and the receptors that capture it.
Postsynaptic neuron
Thus, neurotransmitters are released by the terminal buttons of the presynaptic neuron and are detected by receptors located on the membrane of the postsynaptic neuron. Both neurons must be located within a short distance for this transmission to occur.
Synaptic space
However, contrary to what one might think, the neurons that produce chemical synapses are not physically connected. In fact, between them there is a space known as the synaptic space or synaptic cleft.
This gap appears to vary from synapse to synapse, but is typically about 20 nanometers wide. A network of filaments in the synaptic cleft keeps the pre- and postsynaptic neurons aligned.
Action potential
For there to be an exchange of information between two neurons or neuronal synapses, there must first be an action potential.
This phenomenon occurs in the neuron that sends the signals. The membrane of this cell has an electrical charge. In fact, the membranes of all cells in our body have an electrical charge, but only axons can cause action potentials.
The difference between the electrical potential inside and outside the neuron is called the membrane potential.
These electrical changes between the inside and outside of the neuron are mediated by the concentrations of existing ions, such as sodium and potassium.
When a very rapid reversal of membrane potential occurs, an action potential is produced. This consists of a brief electrical impulse that the axon carries from the neuron's soma or nucleus to the terminal buttons.
It should be added that the membrane potential must exceed a certain excitation threshold for the action potential to occur. This electrical impulse translates into chemical signals that are released through the terminal button.
How does a synapse work?
Neurons contain sacs called synaptic vesicles, which can be large or small. All terminal buttons contain small vesicles that carry neurotransmitter molecules.
Vesicles are produced in a mechanism located in the soma called the Golgi apparatus. They are then transported near the terminal bud. However, they can also be produced in the terminal bud from "recycled" material.
When an action potential is sent along the axon, the cell depolarizes (excites). As a result, the neuron's calcium channels open, allowing calcium ions to enter.
These ions bind to the membrane molecules of synaptic vesicles located in the terminal bouton. This membrane is disrupted, fusing with the terminal bouton membrane. This results in the release of the neurotransmitter into the synaptic space.
The cell's cytoplasm captures the remaining membrane fragments and carries them to the cisternae, where they are recycled, creating new synaptic vesicles.
The postsynaptic neuron has receptors that capture substances in the synaptic space. These are known as postsynaptic receptors, and when activated, they open ion channels.
When these channels open, certain substances enter the neuron, causing a postsynaptic potential. This can have excitatory or inhibitory effects on the cell, depending on the type of ion channel that was opened.
Normally, excitatory postsynaptic potentials occur when sodium enters the nerve cell, while inhibitory postsynaptic potentials are produced by the release of potassium or the entry of chloride.
Calcium entry into the neuron causes excitatory postsynaptic potentials, but it also activates specialized enzymes that produce physiological changes in the cell. For example, it triggers the displacement of synaptic vesicles and the release of neurotransmitters.
It also facilitates structural changes in the neuron after learning.
End of synapse
Postsynaptic potentials are generally very short and terminated by special mechanisms.
One of these is the inactivation of acetylcholine by an enzyme called acetylcholinesterase. Neurotransmitter molecules are removed from the synaptic space by reuptake or reabsorption by transporters located in the presynaptic membrane.
Thus, both presynaptic and postsynaptic neurons have receptors that capture the presence of chemicals around them.
There are presynaptic receptors called autoreceptors that control the amount of neurotransmitter that the neuron releases or synthesizes.
Types of synapses
Electrical synapses
Electrical neurotransmission occurs within them. The two neurons are physically connected through protein structures known as gap junctions.
These structures allow changes in the electrical properties of one neuron to directly influence the other, and vice versa. In this way, the two neurons would act as if they were one.
Chemical synapses
Chemical neurotransmission occurs in these neurons. The pre- and postsynaptic neurons are separated by the synaptic space. An action potential in the presynaptic neuron would trigger the release of neurotransmitters.
These reach the synaptic cleft, making them available to exert their effects on postsynaptic neurons. In the following video, although in English, you can see how a chemical synapse works.
Excitatory synapses
An example of an excitatory neuronal synapse would be the withdrawal reflex when we burn. A sensory neuron would detect the hot object by stimulating its dendrites.
This neuron would send messages through its axon to its terminal buttons, located in the spinal cord. The terminal buttons of the sensory neuron would release chemicals known as neurotransmitters that would excite the neuron with which it synchronizes, specifically, an interneuron (the one that mediates between sensory and motor neurons).
This would cause the interneuron to send information along its axon. In turn, the interneuron's terminal buttons would secrete neurotransmitters that excite the motor neuron.
This type of neuron sends messages along its axon, which connects a nerve to reach the target muscle. Once neurotransmitters are released by the motor neuron's terminal buttons, the muscle cells contract to move away from the hot object.
Inhibitory synapses
This type of synapse is a bit more complicated. This would be illustrated in the following example: imagine taking a very hot tray out of the oven. You use oven mitts to avoid burning yourself, but they're a bit thin, and the heat begins to overwhelm them. Instead of throwing the tray on the floor, try to withstand some heat until it rests on a surface.
Our body's withdrawal reaction to a painful stimulus would make us release the object, yet we control this impulse. How does this phenomenon occur?
The heat from the tray is perceived, increasing the activity of excitatory synapses in motor neurons (as explained in the previous section). However, this excitation is counteracted by inhibition coming from another structure: our brain.
This sends information indicating that if we let go of the tray, it could be a total disaster. Therefore, messages are sent to the spinal cord that prevent the withdrawal reflex.
To do this, an axon from a brain neuron reaches the spinal cord, where its terminal buttons synapse with an inhibitory interneuron. This interneuron secretes an inhibitory neurotransmitter that reduces motor neuron activity, blocking the withdrawal reflex.
Importantly, these are just examples. The processes are actually more complex (especially inhibitory ones), with thousands of neurons involved.
Classes of synapses according to the locations where they occur
– Axodendritic synapses: in this type, the terminal button connects to the surface of a dendrite. Or, with dendritic spines, which are small swellings located on the dendrites in some types of neurons.
– Axosomatic synapses: in these, the terminal button synchronizes with the soma or nucleus of the neuron.
– Axoaxonic synapses : the terminal button of the presynaptic cell connects to the axon of the postsynaptic cell. This type of synapse functions differently from the other two. Its function is to reduce or reinforce the amount of neurotransmitter released by the terminal button. Thus, it promotes or inhibits the activity of the presynaptic neuron.
Dendrodendritic synapses have also been found, but their exact role in neuronal communication is currently unknown.
Substances released in the neuronal synapse
During neuronal communication, not only neurotransmitters such as serotonin, acetylcholine, dopamine, norepinephrine, etc. are released. Other chemicals, such as neuromodulators, can also be released.
They are so named because they modulate the activity of many neurons in a given area of the brain. They secrete in greater quantities and travel longer distances, spreading more widely than neurotransmitters.
Another type of substance is hormones. These are released by cells in the endocrine glands, located in different parts of the body, such as the stomach, intestines, kidneys, and brain.
Hormones are released into the extracellular fluid (outside the cells) and are subsequently captured by capillaries. They are then distributed throughout the body through the bloodstream. These substances can bind to neurons that have special receptors to capture them.
Thus, hormones can affect behavior by altering the activity of the neurons that receive them. For example, testosterone appears to increase aggression in most mammals.
References
- Carlson, N.R. (2006). Physiology of Behavior 8th Ed. Madrid: Pearson. pp: 32-68.
- Cowan, W. M., Südhof, T. & Stevens, C. F. (2001). Synapses. Baltimore, MD: Johns Hopkins University Press.
- Electrical synapse. (n.d.). Retrieved February 28, 2017, from Pontifical Catholic University of Chile: 7.uc.cl.
- Stufflebeam, R. (n.d.). Neurons, synapses, action potentials, and neurotransmission. Retrieved February 28, 2017, from CCSI: mind.ilstu.edu.
- Nicholls, JG, Martín, AR, Fuchs, PA & Wallace, BG (2001). From Neuron to Brain, 4th ed. Sunderland, MA: Sinauer.
- The Synapse (n.d.). Retrieved February 28, 2017, from University of Washington: faculty.washington.edu.