The nervous system of animals is made up billions of neurons. Neurons are electrically excitable cells that transmit nerve impulses throughout the nervous system. However, a junction known as the synapse can be identified between neurons. Neurons communicate each other through synapses. Two types of synapses can be identified based on the mechanism of transmitting the nerve impulses. They are chemical synapses and electrical synapses. Most synapses are chemical synapses. The transmission of the nerve impulses occurs by means of chemical messengers known as neurotransmitters. In electrical synapses, the nerve impulses are transmitted by means of an ion flow.
Key Areas Covered
1. What is a Synapse
– Definition, Facts, Types
2. How Do Neurons Communicate With Each Other Through Synapses
– Introduction, Chemical Synaptic Transmission, Electrical Synaptic Transmission
Key Terms: Chemical Synapses, Electrical Synapses, Ion Flow, Neurons, Neurotransmitters, Synapses
What is a Synapse
A synapse is a junction between two neurons. It serves as a site of functional contact between the neurons, aiding the transmission of nerve impulses between them. Synapses are found between two dendrites, dendrite/axon or dendrite/cell body of another neuron. The transmission of nerve impulses through neurons is shown in figure 1.
Figure 1: Transmission of Nerve Impulses
The two types of synapses are chemical synapses and electrical synapses. The three components of a typical synapse are the pre-synaptic membrane, synaptic cleft, and the post-synaptic membrane.
How Do Neurons Communicate With Each Other Through Synapses
Neurons transmit nerve impulses by means of nerve impulses or action potentials generated on the plasma membrane of the axon. This action potential should be transmitted through the synapse to a second neuron in order to transmit the nerve impulse to the target. However, the way of transmitting nerve impulses through the synapse is different. Furthermore, the two types of synapses transmit the action potentials in different ways.
Chemical Synapse: Synaptic Transmission
Chemical synapses are the junctions through which the action potentials are transmitted by means of chemical signals. Most mammalian cell junctions are composed of chemical synapses. A considerable gap known as the synaptic cleft occurs in chemical synapses. The distance of the gap could be 10-20 nm. The transmission of nerve impulses through chemical synapses occurs by means of chemical messengers known as neurotransmitters. These neurotransmitters are stored in synaptic vesicles near the pre-synaptic membrane. When an action potential reaches the terminal of the pre-synaptic neuron, the voltage-gated Ca2+ ion channels in the pre-synaptic membrane are activated to increase the Ca2+ influx to the cell. Generally, Ca2+ concentration is higher on the outside of the nerve cell. The Ca2+ ions facilitate the fusion of synaptic vesicles to the pre-synaptic membrane, releasing the neurotransmitters into the synaptic cleft. The synaptic transmission of a chemical synapse is shown in figure 2.
Figure 2: Chemical Synapse
These neurotransmitters diffuse through the synaptic cleft and bind to the receptors on the post-synaptic membrane. The activated post-synaptic receptors cause the opening or closing of various types of ion channels upon the binding of neurotransmitters. This leads to the depolarizing or hyperpolarizing of the post-synaptic membrane. The depolarization of the post-synaptic membrane causes the excitatory postsynaptic potential (EPSP), generating an action potential. The hyperpolarization of the post-synaptic membrane causes the inhibitory postsynaptic potential (IPSP), making it less likely to generate an action potential.
Electrical Synapse: Synaptic Transmission
Electrical synapses are the junctions through which action potentials are transmitted by means of an ion flow that occurs from the pre-synaptic to the post-synaptic neuron. They are mainly found in lower vertebrates and invertebrates. They are also found in the mammalian brain. Generally, electrical synapses transmit nerve impulses at a higher speed than a chemical synapse. Electrical synapses may contain no synaptic cleft or a tiny synaptic cleft. An electrical synapse is formed by a gap junction. In addition, the electrical synapse can transmit nerve impulses in both directions. The action of an electrical synapse is shown in figure 3.
Figure 3: Electrical Synapse
A – Pre-Synaptic Neuron, B – Post-Synaptic Neuron, 1 – Mitochondria, 2 – Ion Channels, 3 – Electrical Signal
However, electrical synapses cannot turn an EPSP to an IPSP or an IPSP to an EPSP as chemical synapses do.
Conclusion
Neurons are the structural and functional unit of the nervous system. Synapses are the gaps between neurons that are responsible for the transmission of nerve impulses between neurons. The two types of neurons in the nervous system are chemical synapses and electrical synapses. Chemical synapses transmit nerve impulses by means of chemical signals known as neurotransmitters. Electrical synapses transmit nerve impulses by means of an ion flow that occurs from the pre-synaptic to the post-synaptic neuron.
Reference:
1. “The synapse.” Khan Academy, Available here.
Image Courtesy:
1. “Chemical synapse schema cropped” By user:Looie496 created file, US National Institutes of Health, National Institute on Aging – (Public Domain) via Commons Wikimedia
2. “Figure 35 02 07” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
3. “Synapse diag2” (CC BY-SA 3.0) via Commons Wikimedia
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