Synapse – Class 11 | Chapter – 21 | Biology Short Notes Series PDF

Synapse: It play a crucial role in the transfer of nerve impulses from one neuron to the next. Synapses are the connections between neurons that allow them to send messages to specific target cells. The signal-passing neuron’s plasma membrane (the presynaptic neuron) comes into close contact with the membrane of the target (postsynaptic) cell at a synapse. Both the presynaptic and postsynaptic locations have a large amount of molecular machinery that connects two membranes and permits the signaling process to occur. 


A synapse connects two neurons or a neuron to a target or effector cell like a muscle cell. It allows electrical or chemical impulses to be transmitted. Between presynaptic and postsynaptic neurons, a synapse is produced. The neuromuscular junction is the connection between a neuron and a muscle. A synapse is a structure that allows nerve impulses to pass from one neuron’s axon terminal to the dendrites of the next neuron. It could be electrical or chemical.

The presynaptic portion of many synapses is found on an axon, whereas the postsynaptic portion is found on a dendrite or soma. Astrocytes also communicate with synaptic neurons, responding to synaptic activity and altering neurotransmission as a result. Synaptic adhesion molecules (SAMs) extend from both the pre-and post-synaptic neuron and stick together where they overlap stabilize synapses (at least chemical synapses); SAMs may also aid in the development and function of synapses.

Chemical Synapse

Synapses formed by chemicals are more common. Neurotransmitters are responsible for the transmission of nerve impulses through chemical synapses. Between the two neurons is a fluid-filled region known as the synaptic cleft. The nerve impulse is unable to jump from one neuron to the next.

Chemical Synapse

Synaptic vesicles are found in a knob-like shape at axon terminals. Synaptic vesicles from the terminal of the presynaptic neuron release neurotransmitters at the synaptic cleft when the action potential reaches the terminals. The receptors in the postsynaptic membrane bind neurotransmitters. This causes voltage-gated channels to open, allowing ions to flow. The polarity of the postsynaptic membrane changes as a result and the electric signal is sent across the synapse.

Inhibitory and excitatory neurotransmitters exist. Various cells can respond to the same neurotransmitter in different ways. The neurotransmitter is excitatory if there is a net flow of positively charged ions inside the cell, which leads to the generation of the action potential. EPSP, or excitatory postsynaptic potential, is the term for this.

When the membrane potential falls below zero, the membrane becomes hyperpolarized, and the neurotransmitter’s activity becomes inhibitory. IPSP, or inhibitory postsynaptic potential, is produced by them. Once connected to the receptor, neurotransmitters are either acted on by enzymes or taken back and recycled to end the signal after it has been transmitted forward.

Electrical Synapse

The membranes of the presynaptic and postsynaptic cells are joined in an electrical synapse by unique channels called gap junctions, which can pass an electric current and cause voltage changes in the presynaptic cell to generate voltage changes in the postsynaptic cell. The fundamental benefit of an electrical synapse is the speed with which signals are transferred from one cell to the next.

Electrical Synapse

Synaptic communication varies from ephaptic coupling, which happens when neurons interact by indirect electric fields. When the axon of one neuron synapses onto the dendrites of another neuron, it produces an autapse, which is a chemical or electrical synapse.

  • Chemical synapses take longer to form than electrical synapses.
  • Gap junctions arise when presynaptic and postsynaptic neurons are in close contact. 
  • Protein channels form a physical link between pre-and postsynaptic neurons at the gap junction.
  • The transmission of an electric signal across the electrical synapse is analogous to the conduction of impulse in an axon because these gap junctions enable direction passage.
  • Electrical synapses are less adaptable than chemical synapses since they can’t switch from excitatory to inhibitory signals.
  • It can be found in a variety of lower invertebrates. It is discovered between glial cells in humans.
  • In this way, the synapse ensures that nerve impulses are transmitted in the correct direction and that random stimulation is avoided.

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By Team Learning Mantras