Synapse: Definition, Meaning, Anatomy, Function, Types, Diagram

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:48 PM IST

Definition Of Synapse

A synapse is a small junction located at the end of a neuron, communicating and connecting with another neuron. The neuron is set into action in propagating electrical or chemical signals that lead it to release neurotransmitters into the synapse; these released messengers in turn excite the receptors inside the receiving neuron.

This Story also Contains

Definition Of Synapse
Structure Of A Synapse
Types Of Synapses
Synaptic Transmission
Neurotransmitters
Synapse Dysfunction And Associated Disorders
Synapse Studies
Recommended Video On ‘Synapse’

Synapse: Definition, Meaning, Anatomy, Function, Types, Diagram

Synapses form the core centre for the processing and transmission of information in the nervous system and thereby are a driving force for enabling complex functions corresponding to movement, perception, and thought processes.

Structure Of A Synapse

The synapse includes several components involved in the same, to pass the signals between the neurons.

Pre-synaptic Neuron:

Contains the synaptic vesicles full of neurotransmitters.
Synaptic Vesicles and Neurotransmitters: Dumps the neurotransmitters into the synaptic cleft.

Synaptic Cleft:

The minute gap or junction between neurons where neurotransmitters are dumped.

Post-synaptic Neuron:

Receptors and Signal Reception: Contains the receptors which are attached to the neurotransmitters leading to the propagation of response in the neuron.

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Diagram: Structure Of Synapse

Types Of Synapses

Depending on the mode of transmission of the signal, synapses may be either chemical or electrical.

Chemical Synapses

Neurotransmitters released into the synaptic cleft convey the signal.
Examples and Importance: More commonly found in the central nervous system. The principle behind complex signalling processes.

Electrical Synapses

Ions are transmitted directly from the presynaptic to the postsynaptic neuron through gap junctions.
Examples and Significance: Present in some neural circuits; enable fast transmission of signals.

Synaptic Transmission

The synaptic transmission is a process by which a signal is being forwarded from one neuron to another.

Arrival of Action Potential: This subsequently leads to the stimulation of the extrusion of neurotransmitters.
Neurotransmitter release: neurotransmitters are released into the synaptic cleft by synaptic vesicles.
Neurotransmitter Binding: Transmitters are chemicals that bind to a certain receptor in the post-syn.
Generation of Post-synaptic Potential: The post-synaptic neuron generates a response in it.

Neurotransmitters

Neurotransmitters are chemicals that diffuse an impulse conduction at the synapse.

Act as a communicator between the neurons.

Types Of Neurotransmitters:

Excitatory Neurotransmitters (ex. Glutamate): Generates action potentials.
Inhibitory Neurotransmitters (e.g. GABA): Inhibit the action potentials
Modulatory Neurotransmitters (e.g. Dopamine, Serotonin): Modulate multiple neural processes.

Synapse Dysfunction And Associated Disorders

Disorders in synapse dysfunction are associated with both neurological and psychiatric illnesses.

Neurological Disorders:

Alzheimer's Disease: Pathological in synaptic loss and dysfunction.
Parkinson's disease: Entails deficient synaptic transmission in motor pathways.

Psychiatric disorders:

Depression: Correlated with imbalances in neurotransmitter concentrations.
Schizophrenia: Correlated with defective synaptic connectivity and mechanisms.

Synapse Studies

There are various methods used in the studies that explore the status of synapses.

Microscopic Techniques:

Electron Microscopy provides very detailed images of the structures of synapses.
Live imaging can be done of the synapses through confocal microscopy.

Electrophysiological Techniques:

Through the Patch Clamp Technique recording of ion channel activities of neurons

Recommended Video On ‘Synapse’

Frequently Asked Questions (FAQs)

1. What is a synapse and how does it function?

A synapse is the junction between two neurons, in which electrical or chemical impulses are passed through, allowing communication to take place in the nervous system.

2. What are the different kinds of synapses and how do they work?

Synapses are both chemical in which the synapse has a neurotransmitter or electrical, in which case the synapse involves a direct ion flow through gap-like junctions, and both types allow the transmission of signals between the neurons.

3. How do neurotransmitters affect synaptic transmission?

Neurotransmitters are small molecules that bind to receptors on the post-synaptic neuron and may produce an excitatory or inhibitory response; thus, they influence the excitability of a neuron.

4. What is synaptic plasticity, and why is it important?

Synaptic plasticity can be defined as the ability of synaptic connections to strengthen, with the possibility of being weakened with time, critically involved in learning, memory, and adaptation.

5. How can synapses and their physiology be related to disorders or diseases of the nervous system or psychiatry?

By interfering with normal neural communication and signalling, synaptic dysfunction can cause such abnormalities as Alzheimer's, Parkinson's, depression, and schizophrenia.

6. What is a synapse?

A synapse is the junction between two neurons or between a neuron and another cell type where information is transmitted. It's not a physical structure, but rather a small gap where chemical or electrical signals pass from one cell to another.

7. What is synaptic convergence and divergence?

Synaptic convergence occurs when multiple presynaptic neurons synapse onto a single postsynaptic neuron. Divergence is when a single presynaptic neuron synapses onto multiple postsynaptic neurons. These patterns allow for complex information processing in neural circuits.

8. What is the difference between axoaxonic, axodendritic, and axosomatic synapses?

These terms describe the location of synapses on the postsynaptic neuron. Axoaxonic synapses occur between axons, axodendritic synapses are on dendrites, and axosomatic synapses are on the cell body. The location can influence the synapse's effect on the postsynaptic neuron.

9. What is synaptic facilitation?

Synaptic facilitation is a form of short-term synaptic plasticity where repeated stimulation leads to a temporary increase in synaptic strength. It often results from residual calcium in the presynaptic terminal enhancing neurotransmitter release.

10. What is the role of the postsynaptic density in synaptic function?

The postsynaptic density is a protein-rich region just beneath the postsynaptic membrane. It anchors and organizes receptors, ion channels, and signaling molecules, playing a crucial role in synaptic transmission and plasticity.

11. How do neurotransmitters affect the postsynaptic neuron?

Neurotransmitters bind to specific receptors on the postsynaptic neuron, which can either excite or inhibit the neuron. Excitatory neurotransmitters increase the likelihood of the postsynaptic neuron firing an action potential, while inhibitory neurotransmitters decrease this likelihood.

12. How do neurotransmitter receptors differ from ion channels?

Neurotransmitter receptors are proteins that specifically bind neurotransmitters, while ion channels are pore-forming proteins that allow ions to pass through the cell membrane. Some receptors, called ionotropic receptors, are also ion channels that open when a neurotransmitter binds.

13. How does reuptake affect synaptic signaling?

Reuptake is the process by which neurotransmitters are removed from the synaptic cleft, usually by being transported back into the presynaptic neuron. This process helps to terminate the synaptic signal and recycle neurotransmitters for future use.

14. How do neuromodulators differ from neurotransmitters?

Neuromodulators are substances that alter the effects of neurotransmitters rather than directly exciting or inhibiting neurons. They often act over larger areas and longer time scales than neurotransmitters, modulating the overall excitability of neural circuits.

15. What is the role of SNARE proteins in synaptic transmission?

SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) proteins are crucial for synaptic vesicle fusion. They form a complex that brings the vesicle membrane close to the presynaptic membrane, facilitating the release of neurotransmitters into the synaptic cleft.

16. What is the synaptic cleft?

The synaptic cleft is the narrow gap between the presynaptic and postsynaptic membranes in a chemical synapse. It's typically about 20-40 nanometers wide and is where neurotransmitters diffuse after being released from the presynaptic neuron.

17. How does the structure of a synapse relate to its function?

The structure of a synapse is specialized for its function of transmitting signals. The presynaptic terminal contains synaptic vesicles filled with neurotransmitters, the synaptic cleft allows for neurotransmitter diffusion, and the postsynaptic membrane has receptors to detect these neurotransmitters.

18. What is synaptic vesicle recycling?

Synaptic vesicle recycling is the process by which synaptic vesicles are reformed and refilled with neurotransmitters after fusion with the presynaptic membrane. This process is crucial for maintaining a supply of vesicles for ongoing synaptic transmission.

19. How do neurotransmitter transporters affect synaptic signaling?

Neurotransmitter transporters remove neurotransmitters from the synaptic cleft, terminating the synaptic signal and allowing for neurotransmitter recycling. They play a crucial role in regulating the duration and strength of synaptic transmission.

20. How do voltage-gated calcium channels contribute to synaptic transmission?

Voltage-gated calcium channels in the presynaptic terminal open in response to action potentials, allowing calcium influx. This calcium influx triggers synaptic vesicle fusion and neurotransmitter release, thus initiating synaptic transmission.

21. How do neurotoxins affect synapses?

Neurotoxins can affect synapses in various ways, such as blocking neurotransmitter release, interfering with receptor function, or disrupting the reuptake of neurotransmitters. These effects can lead to impaired neural signaling and potentially severe neurological symptoms.

22. What is the role of adenosine in synaptic function?

Adenosine is a neuromodulator that generally inhibits synaptic transmission. It accumulates during periods of high neural activity or metabolic stress and can act as a neuroprotective agent by reducing excitatory transmission.

23. What is synaptic scaling?

Synaptic scaling is a form of homeostatic plasticity where a neuron adjusts the strength of all its synapses up or down to maintain a stable level of activity. This process helps prevent excessive excitation or inhibition in neural networks.

24. How do neurotrophic factors affect synapses?

Neurotrophic factors, such as BDNF (Brain-Derived Neurotrophic Factor), promote the growth, survival, and plasticity of synapses. They can enhance synaptic transmission, promote the formation of new synapses, and play a role in long-term potentiation.

25. What is the difference between presynaptic and postsynaptic inhibition?

Presynaptic inhibition reduces neurotransmitter release from the presynaptic neuron, while postsynaptic inhibition directly hyperpolarizes the postsynaptic neuron or reduces its excitability. Both forms of inhibition can regulate synaptic signaling and neural circuit function.

26. How do drugs affect synaptic transmission?

Drugs can affect synaptic transmission in various ways, such as by mimicking neurotransmitters, blocking their reuptake, enhancing their release, or modifying receptor sensitivity. These effects can lead to changes in neural signaling and behavior.

27. How do astrocytes contribute to synaptic function?

Astrocytes, a type of glial cell, contribute to synaptic function by regulating neurotransmitter levels in the synaptic cleft, providing nutrients to neurons, and participating in synaptic plasticity. They can also release their own signaling molecules, called gliotransmitters.

28. What is synaptic fatigue?

Synaptic fatigue is a temporary decrease in synaptic strength that occurs with repeated stimulation. It's often due to depletion of readily releasable synaptic vesicles or desensitization of postsynaptic receptors, and serves as a form of short-term synaptic plasticity.

29. Why are synapses important for learning and memory?

Synapses are crucial for learning and memory because they can change their strength over time, a process called synaptic plasticity. This allows for the formation and storage of memories and the acquisition of new skills through the strengthening or weakening of specific neural connections.

30. What is synaptic pruning?

Synaptic pruning is the process of eliminating unnecessary synaptic connections in the brain, particularly during development and adolescence. This process helps to refine neural circuits, improve efficiency, and is crucial for normal brain development and function.

31. How does information flow across a synapse?

Information flows across a synapse in one direction, from the presynaptic neuron to the postsynaptic neuron. The presynaptic neuron releases neurotransmitters, which bind to receptors on the postsynaptic neuron, potentially causing it to fire an action potential.

32. What is synaptic delay?

Synaptic delay is the brief time lag between the arrival of an action potential at the presynaptic terminal and the generation of a response in the postsynaptic neuron. This delay, typically about 0.5-1 millisecond, is due to the time required for neurotransmitter release, diffusion, and binding.

33. What is the role of calcium in synaptic transmission?

Calcium plays a crucial role in synaptic transmission by triggering the fusion of synaptic vesicles with the presynaptic membrane, leading to neurotransmitter release. The influx of calcium occurs when an action potential reaches the axon terminal.

34. What is synaptic vesicle fusion?

Synaptic vesicle fusion is the process where vesicles containing neurotransmitters merge with the presynaptic membrane, releasing their contents into the synaptic cleft. This fusion is triggered by an influx of calcium ions when an action potential reaches the axon terminal.

35. What is synaptic integration?

Synaptic integration is the process by which a neuron combines multiple synaptic inputs, both excitatory and inhibitory, to determine whether it will fire an action potential. This integration occurs primarily in the cell body and dendrites of the neuron.

36. What's the difference between chemical and electrical synapses?

Chemical synapses use neurotransmitters to transmit signals, while electrical synapses allow direct electrical current flow between neurons through gap junctions. Chemical synapses are more common and allow for signal modulation, while electrical synapses provide faster transmission.

37. How do gap junctions in electrical synapses work?

Gap junctions in electrical synapses are channels made of connexin proteins that directly connect the cytoplasm of adjacent neurons. They allow for the rapid, bidirectional flow of ions and small molecules, enabling electrical coupling between neurons.

38. How do silent synapses differ from active synapses?

Silent synapses contain NMDA receptors but lack AMPA receptors, making them unresponsive to typical synaptic transmission. They can be "unsilenced" through the insertion of AMPA receptors, a process important in synaptic plasticity and learning.

39. What is the difference between fast and slow synaptic transmission?

Fast synaptic transmission involves ionotropic receptors and occurs within milliseconds, directly altering the postsynaptic neuron's membrane potential. Slow synaptic transmission involves metabotropic receptors, takes seconds to minutes, and often leads to longer-lasting changes in the neuron's properties.

40. What is synaptic tagging?

Synaptic tagging is a process where synapses that have undergone short-term strengthening are "tagged" for subsequent long-term potentiation. This mechanism helps explain how short-term memories can be converted into long-term memories.

41. How do endocannabinoids affect synaptic transmission?

Endocannabinoids are retrograde messengers that are released from postsynaptic neurons and bind to receptors on presynaptic terminals. They typically inhibit neurotransmitter release, providing a mechanism for feedback regulation of synaptic strength.

42. What is the difference between ionotropic and metabotropic receptors?

Ionotropic receptors are ligand-gated ion channels that open directly when a neurotransmitter binds, allowing ion flow. Metabotropic receptors, when activated, trigger intracellular signaling cascades that can have more diverse and longer-lasting effects on the neuron.

43. How does synaptic transmission differ in the peripheral vs. central nervous system?

While the basic principles are similar, synapses in the peripheral nervous system often involve neuromuscular junctions or autonomic ganglia, while central nervous system synapses are primarily between neurons. The neurotransmitters and receptors involved can also differ.

44. What is the role of protein kinases in synaptic plasticity?

Protein kinases play a crucial role in synaptic plasticity by phosphorylating various synaptic proteins. This can alter the properties of ion channels, receptors, and other signaling molecules, leading to changes in synaptic strength and function.

45. How do neuromodulators like dopamine affect synaptic transmission?

Neuromodulators like dopamine can alter the excitability of neurons and the strength of synaptic connections. They often act through G-protein coupled receptors to influence intracellular signaling cascades, affecting processes like synaptic plasticity and neuronal excitability.

46. How does synaptic transmission at the neuromuscular junction differ from typical neuron-to-neuron synapses?

The neuromuscular junction uses acetylcholine as its primary neurotransmitter and has a specialized structure called the motor end plate. It's also larger and more reliable than most neuron-to-neuron synapses, ensuring consistent muscle activation.

47. How do AMPA and NMDA receptors differ in their roles at excitatory synapses?

AMPA receptors mediate fast excitatory transmission and are always active. NMDA receptors require both glutamate binding and membrane depolarization to open, making them coincidence detectors. NMDA receptors are crucial for certain forms of synaptic plasticity.

48. How do neurotransmitter release probability and quantal size affect synaptic strength?

Release probability is the likelihood of a vesicle fusing upon arrival of an action potential, while quantal size is the amount of neurotransmitter in a single vesicle. Both factors influence the amount of neurotransmitter released and thus the strength of the synaptic signal.

49. How do second messenger systems contribute to synaptic plasticity?

Second messenger systems, activated by metabotropic receptors, can lead to long-lasting changes in synaptic strength. They can activate protein kinases, alter gene expression, and modify the properties of synaptic proteins, contributing to various forms of synaptic plasticity.

50. What is the role of zinc in synaptic transmission?

Zinc is co-released with glutamate at some synapses and can modulate both NMDA and AMPA receptor function. It can have both potentiating and inhibitory effects on synaptic transmission, depending on its concentration and the specific synapse involved.

51. How do astrocytes participate in the "tripartite synapse"?

In the tripartite synapse model, astrocytes are active participants in synaptic function alongside the pre- and postsynaptic neurons. They can detect neurotransmitters, release gliotransmitters, regulate neurotransmitter and ion concentrations, and influence synaptic plasticity.

52. What is synaptic specificity and why is it important?

Synaptic specificity refers to the precise targeting of synaptic connections between neurons. It's crucial for the proper wiring of neural circuits and the formation of functional networks that underlie complex behaviors and cognitive functions.

53. How do cannabinoids affect synaptic transmission?

Cannabinoids, both endogenous and exogenous, typically act through CB1 receptors on presynaptic terminals to inhibit neurotransmitter release. This can lead to various effects on neural signaling and behavior, depending on the specific circuits involved.

54. What is the difference between homosynaptic and heterosynaptic plasticity?

Homosynaptic plasticity involves changes in the strength of a synapse based on its own activity. Heterosynaptic plasticity involves changes in one synapse based on the activity of other, nearby synapses. Both forms contribute to the dynamic regulation of neural circuits.

55. How do synapses contribute to the computational properties of neurons?

Synapses allow neurons to integrate multiple inputs, perform complex computations, and adapt their responses over time. Through processes like spatial and temporal summation, synaptic integration, and various forms of plasticity, synapses enable neurons to perform sophisticated information processing tasks.