Short Notes on Synapse

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The presence of fodrin and microtubules in addition to F-actin and synapsin are significant in presynaptic terminal. The area in presynaptic ends that are formed by the membrane and a dense collection of proteins that communicate neurotransmitter release is called active zone. It is here where the initial events of synaptic transmission occurs. The postsynaptic section has a higher concentration of receptor proteins or ligand ion channels to receive neurotransmitters.

The association of neurotransmitters to the receptors induce the flow of ions to the post synaptic neurons that eventually leads to the formation of IPSP and EPSP. In the case of glutamate (excitatory synapse neurotransmitters), the acetylcholine associates with its corresponding receptors to cause the flow of positive ions towards the post synaptic cell wherein the association of inhibitory neurotransmitter with its receptor causes the opening of ion channels.

Based on anatomy, synapses can be classified into three types:

1. Axoaxonic synapses are those where the axons of two different neurons meet. 
2. An axodendritic synapse is where the axon of one neuron terminates on the dendrite of another neuron. 
3. Axosomatic synapse is where the axon of one neuron attaches to the soma (cell body) of another neuron. 

Based on function or physiology, synapses are of two types – electrical synapses and chemical synapses. 

Also Check: Difference between Electrical and Chemical Synapses

Chemical Synapse and Its Functions

Signal transmission at synapse is a chemical process. Here, let us see how the pre and post synaptic neurons aid in nerve impulse transmission. 

• Chemical synapses are more common. The transmission of nerve impulses through chemical synapses is mediated by neurotransmitters.
• There is a fluid-filled space between the two neurons called the synaptic cleft. The nerve impulse cannot jump from one neuron to another.
• Axon terminals have a knob-like structure, which contain synaptic vesicles.
• When the action potential reaches the terminals, the synaptic vesicles from the terminal of the presynaptic neuron release neurotransmitters at the synaptic cleft.
• Neurotransmitters bind to the receptors present in the postsynaptic membrane.
• This results in the opening of voltage-gated channels and the flow of ions. This causes a change in the polarisation of the postsynaptic membrane and the electric signal is conducted across the synapse.
• Neurotransmitters can be inhibitory or excitatory. One neurotransmitter can initiate different responses in different cells.
• If there is a net flow of positively charged ions inside the cell, then the neurotransmitter is excitatory and it results in generating the action potential. This is called EPSP or excitatory postsynaptic potential.
• When the membrane potential becomes more negative, the membrane becomes hyperpolarized and the action of neurotransmitter is inhibitory. They generate IPSP or inhibitory postsynaptic potential.
• Once the neurotransmitters get attached to the receptor they are either acted on by enzymes or taken back and recycled to terminate the signal after it is conducted forward.

Electrical Synapse and Its Functions

• Electrical synapses are faster than chemical synapses.
• The presynaptic and postsynaptic neurons are in close proximity and they form gap junctions. There is a physical connection between pre and postsynaptic neurons at the gap junction by protein channels.
• These gap junctions allow the direct flow of ions and the transmission of an electric signal across the electrical synapse is similar to the conduction of impulse in an axon.
• Electrical synapses are not as flexible as chemical synapses as they cannot turn the excitatory signal to the inhibitory signal.
• It is found in some lower invertebrates. In humans, it is found between glial cells.

The synapse, in this way, ensures the conduction of nerve impulses in the correct direction and prevents random stimulation.

Properties of Synapse

The following are the 5 basic properties of a synapse:

1. One-way conductions – At synapse, impulses can only travel in one direction which is from the presynaptic neuron to the postsynaptic neuron.
2. Synaptic delay – A brief delay in transmitting impulses through the synapses is called synaptic delay. This is due to the time taken for the release and action of neurotransmitters.
3. Fatigue – Synapses become the site of fatigue during prolonged muscular activity, along with Betz cells found in the motor area of the frontal lobe of the cerebral cortex. The depletion of acetylcholine is what causes fatigue at synapses.
4. Electrical property – Electrical properties of the synapse are the IPSP (inhibitory postsynaptic potential) and EPSP (excitatory postsynaptic potentials.
5. Summation – When multiple presynaptic excitatory terminals are stimulated simultaneously or when a single presynaptic terminal is repeatedly stimulated, the effects are combined or the EPSP gradually increases.

Synaptic transmission

Synaptic transmission is a biological process by which a neuron communicates with a target cell across a synapse. A neurotransmitter is released from the pre-synaptic neuron during chemical synaptic transmission, and the neurotransmitter then binds to particular post-synaptic receptors. Electrical synapses allow direct, passive current flow from one neuron to the next. 

Electrical synapses are special ion channels connecting the pre and postsynaptic cells which provide a low-resistance route for the flow of electrical current between two cells. On the other hand, in the chemical synapse, an action potential in the presynaptic neuron results in the release of neurotransmitters which trigger the current flow in the postsynaptic cell.

Differences between Electrical and Chemical synapse

This was a brief note on Neurons and Synapse. Explore notes on neural communication and other important concepts related to NEET, only at BYJU’S.

Also Check:

• Function Of Pons
• Medulla Oblongata
• Thalamus Function
• Process Of Neural Communication
• Flashcards Of Biology For NEET Neural Control And Coordination

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