How Do Synapses Work in the Nervous System?
A synapse is a junction between two neurons that allows communication between them. There are certain chemical messengers called neurotransmitters released by the presynaptic neuron (neuron from which the communication is to be sent) in case of chemical synapses. The presynaptic neuron transmits the signal to the postsynaptic neuron (neuron where the communication is to reach). Thus, a synapse is neural junctions that help in the transmission of chemical or electrical signals known as nerve impulses. Depending on the signal transmission via the synapse, be it chemical or electrical, the synapse is named respectively as chemical synapse or electrical synapse. A detailed description given below helps to define synapse and understand what is the function of the synapse.
Synapse
In order to understand what is a synapse in biology, it is necessary to know what is the function of neurons in general. Neurons are specialised cells that are electrically excitable and communicate with one another through chemical messengers or electrical signals. These signals are generated by external stimuli such as touch, burning sensation, etc. These signals help to protect our body and react as fast as possible for survival.
Neurons are the main cells that comprise the nervous system and are widely known as brain cells. They help to form memory, learn and react in adverse conditions. But to carry out all these functions, the neurons need to communicate with one another and they do so via specialised junctions called synapses.
Thus, a synapse is a junction that permits the transmission of signals or information between either a neuron and another neuron or between a neuron and a muscle cell. The synapse between a neuron and another neuron is called a neuronal junction and the one between a neuron and a muscle is called the neuromuscular junction. It is not necessary that neurons can form synapses with only another neuron or a muscle cell. It can form a synapse with any different target cell for communication and that target cell is known as an effector cell.
A synapse image can be described as a knoblike structure emerging from the plasma membrane of a presynaptic neuron fitting into a curve shape formed by the plasma membrane of a postsynaptic neuron or an effector cell with a small gap in between. This gap known as the synaptic cleft is approximately 0.2 microns wide. Both the presynaptic and postsynaptic sites contain large assemblies of the molecular machinery that keep the two membranes together. These molecules are also in some cases known as the synaptic adhesion molecules (SAMs) and carry out the signalling process. The synapse image is clearly outlined in the diagram below.
(Image will be Uploaded soon)
Types of Synapse
A synapse can either be a chemical synapse or an electrical synapse depending upon the kind of signals it permits. It's important to understand that even though an electrically excitable neuron generates electrical impulses due to the voltage gradients across its membranes, the transmitting signal can either be chemical or electrical which in turn decides the type of synapse. The two synapses are briefly described as follows:
Chemical Synapse: When the electrical activity within the presynaptic neuron results in the release of chemical messengers known as neurotransmitters, then such neurotransmitters pass through a chemical synapse. The neurotransmitters released into the synaptic cleft bind to the receptors present on the postsynaptic neuronal membrane. These neurotransmitters then further may initiate a pathway of secondary messengers which can further pass the signal or inhibit it. This type of neuronal signalling is useful and essential when the signal is passed over complex and large routes.
Electrical Synapse: When the voltage changes in the presynaptic cell induce voltage changes in the postsynaptic cell it happens by the transmission of electrical current through the special channels known as gap junctions of an electrical synapse present on the membranes of the cells involved. The advantage of the electrical synapse is that it allows a very fast exchange of signals from one cell to another.
Signal Transmission Through the Synapse
The function of the neurons is largely owed to their cell polarity. In the case of neurons, it is the electrical polarity that allows and facilitates the transfer of electrical signals from presynaptic membranes to postsynaptic membranes or effector cell membranes. An action potential that arises by the large net flow of positively charged ions into a presynaptic neuron generates the electrical signal which is then transmitted to the postsynaptic cell. This transfer as mentioned above occurs by means of the chemical or electrical synapse. A synapse image shown below depicts the manner in which the signal is transmitted.
(Image will be Uploaded soon)
Around the chemical synapse, the electrical signal leads to the release of neurotransmitters. These neurotransmitters are present inside membrane-bound vesicles known as synaptic vesicles. These vesicles under the influence of the electrical impulse drive towards the synapse and fuse with the presynaptic neuronal membrane. As the vesicles fuse with the membrane they release the neurotransmitters into the synaptic cleft which then diffuse through the cleft and bind the receptor molecules on the postsynaptic membrane. These neurotransmitters as given above initiate secondary messenger pathways and excite the signal or inhibit the signal in the postsynaptic cell. Once the neurotransmitters have passed on the information they are deactivated by enzymes present in the synaptic cleft and are taken up by the presynaptic vesicles. Hence, there are brief transmission events taking place and each one takes place only for 0.5 to 4 milliseconds.
In the case of the electrical synapse, electrical current passes through the gap junctions. This direct communication in terms of electrical current happens to electrically charged ions that are permeated through these gap junctions. This allows for the rapid synchronisation of the nerve cells.
Thus, from the above discussion, the synapse meaning and function is clear. Overall if one is to define synapse or briefly describe what is a synapse in Biology then one can say that a synapse is a junction that exists majorly between neurons for the transmission of electrical impulses and action potentials. But it cannot just be classified as a neuron synapse as the synapse can be between a neuron and another excitable cell such as a muscle cell known as an effector cell.
FAQs on Synapse: Definition, Types & Functions
1. What is a synapse and where is it found in the nervous system?
A synapse, also known as a neuronal junction, is a specialized structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to a target effector cell, such as a muscle or gland cell. Synapses are found throughout the nervous system, located at the point of contact between the axon terminal of one neuron and the dendrite or cell body of another neuron, or at a neuromuscular junction between a neuron and a muscle fibre.
2. What are the main types of synapses?
There are two primary types of synapses based on the mechanism of signal transmission: chemical synapses and electrical synapses. Chemical synapses transmit information using chemical messengers called neurotransmitters. Electrical synapses, which are less common, allow the direct flow of ions from one neuron to another through channels called gap junctions, resulting in a much faster transmission.
3. What is the primary function of a synapse in neural communication?
The primary function of a synapse is to control the transmission of nerve impulses between neurons, ensuring that signals flow in a single direction. It acts as a processing point where signals can be modified, amplified, or inhibited. This regulation is crucial for all nervous system functions, including thought, memory, learning, and coordinating movement.
4. How does a chemical synapse transmit a signal from one neuron to another?
In a chemical synapse, the arrival of an action potential at the axon terminal of the pre-synaptic neuron triggers the opening of voltage-gated calcium channels. The influx of calcium ions causes synaptic vesicles, which contain neurotransmitters, to fuse with the pre-synaptic membrane and release their contents into the synaptic cleft. These neurotransmitters then diffuse across the cleft and bind to specific receptors on the post-synaptic neuron, causing a new electrical signal to be generated.
5. What are the key differences between a chemical and an electrical synapse?
The key differences between chemical and electrical synapses are:
- Transmission Method: Chemical synapses use neurotransmitters, while electrical synapses use a direct flow of ions through gap junctions.
- Speed: Electrical synapses are almost instantaneous, whereas chemical synapses involve a slight delay (synaptic delay) due to the release and diffusion of neurotransmitters.
- Synaptic Cleft: The gap between neurons, the synaptic cleft, is much wider in a chemical synapse (20-40 nm) compared to an electrical synapse (around 3.5 nm).
- Flexibility: Chemical synapses are more flexible and can be excitatory or inhibitory, allowing for complex signal processing. Electrical synapses are typically only excitatory.
6. Why are chemical synapses more common than electrical synapses in the human nervous system?
Chemical synapses are more common because they offer greater plasticity and regulatory control. They can convert an electrical signal into a chemical one and back, which allows for signal modulation, such as amplification or inhibition. This ability to modify the signal is fundamental for complex processes like learning and memory. While slower, this flexibility is more advantageous for the sophisticated functions of the human brain than the simple, rapid transmission of electrical synapses.
7. What are some examples of excitatory and inhibitory neurotransmitters?
Excitatory neurotransmitters increase the likelihood that a neuron will fire an action potential. Common examples include Acetylcholine (at the neuromuscular junction) and Glutamate. Inhibitory neurotransmitters decrease this likelihood. Key examples are GABA (gamma-aminobutyric acid), which is the main inhibitory neurotransmitter in the brain, and Glycine, primarily found in the spinal cord.
8. How is a neural synapse different from the term 'synapsis' used in cell division?
This is a common point of confusion. A neural synapse is a junction between two nerve cells for signal transmission. In contrast, synapsis is a biological process that occurs during the prophase I stage of meiosis, where homologous chromosomes pair up to form a bivalent. The two terms are unrelated in function and context; one relates to the nervous system, and the other to genetic recombination during sexual reproduction.
9. What is the role of the synaptic cleft in a chemical synapse?
The synaptic cleft is the small, fluid-filled space that separates the pre-synaptic neuron from the post-synaptic neuron. Its role is crucial: it prevents the direct propagation of the electrical signal, forcing the communication to occur via chemical messengers (neurotransmitters). This gap ensures unidirectional signal flow and provides a space where neurotransmitter concentration can be regulated, for instance, by enzymes that break them down or by reuptake mechanisms, thus terminating the signal.
10. How can a single neuron receive both excitatory and inhibitory signals at the same time?
A single post-synaptic neuron can have thousands of synapses with many different pre-synaptic neurons. Some of these synapses are excitatory, and others are inhibitory. The neuron integrates these simultaneous signals in a process called summation. If the total excitatory input outweighs the inhibitory input and reaches the threshold potential, the neuron will fire an action potential. If the inhibitory input is stronger, the neuron will be prevented from firing. This integration is the basis of neural computation and decision-making at the cellular level.
