Differences between an electrical and chemical synapse?

Question

What are the differences? Which is better? Which is more common?

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Answer 1

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An electrical synapse
An electrical synapse is a mechanical and electrically conductive link between two abutting neuron cells that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. Each gap junction contains numerous gap junction channels which cross the membranes of both cells. With a lumen diameter of about 1.2 to 2.0 nm, the pore of a gap junction channel is wide enough to allow ions and even medium sized molecules like signaling molecules to flow from one cell to the next thereby connecting the two cells' cytoplasm. Thus when the voltage of one cell changes, ions may move through from one cell to the next, carrying positive charge with them and depolarizing the postsynaptic cell.
Gap junction channels are composed of two hemi-channels called connexons in vertebrates, one contributed by each cell at the synapse.

A chemical syapse
The space between a chemical syapse is much larger than an electrical synapse.
The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion, also known as exocytosis: Within the pre-synaptic nerve terminal, vesicles containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels. Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic-membrane and releasing their contents to the synaptic cleft.

An electrical synapse is faster than a chemical synapse but chemical synapases are far more common.

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Answer 2

An electrical synapse is a physical connection between two neurons with a pore allowing charged particles (ions) to pass from one neuron to the other. Unless there are a large number of pores, or the pores are very large, the connection is weak, and doesn't cause an action potential in one neuron to result in an action potential in the other. Since electrical synapses can cause either neuron to influence the other, these are bidirectional synapses.

A chemical synapse is a junction between two neurons. The first (presynaptic) neuron releases a chemical (a neurotransmitter) which rapidly (in as little as 50 microseconds) crosses the small space across the synapse and binds to a receptor on the second (postsynaptic) neuron. This receptor may cause the direct or indirect opening of an ion-specific pore, allowing influx of sodium, calcium, or chloride or outflux of potassium ions. Since the presynaptic neuron can affect the postsynaptic neuron, these are unidirectional synapses.

Originally synapses were thought to be largely electrical, but it turned out that the vast majority are chemical. A large number of neurotransmitter and receptors exist in the brain (the main transmitters being glutamate and GABA). As for which are better, the chemical synapses allow a wide variety of responses, varying in duration and intensity (depending on the properties of the postsynaptic receptor, where the response can last from milliseconds to minutes or longer), so are almost certainly superior. Electrical synapses appear to be used to allow large networks of neurons to act in a more unified manner (for example, a single neuronal subtype in the cortex is wired to it's identical neighbors).

Source(s):

Kandel and Schwartz, Principles of Neural Science, 2000

Answer 3

The difference between an electrical and chemical synapse is the way conduction is passed from one cell to another.

Chemical conduction occurs mainly in the nervous system at the terminals of nerve fibers. Neurotransmitters are released on the pre-synaptic side of the cleft upon stimulation of the action potential and diffuse across the cleft where they may bind to a receptor on the post-synaptic terminal of the cleft.

Electrical synapse occur mainly in cells such as the cardiomyocytes that make up the heart muscle. Gap junction proteins known as connexins form direct pathways between two corresponding cells. Ionic currents such as those caused by the action potential can pass directly from one cell to another.

The important thing to remember is that one is not more important than another, but they serve different purposes. In the heart, you want the propagation of the action potential to proceed smoothly and uninterrupted from one cell to another to provide a uniform contraction of the heart. In the nervous system, however, the action potential serves as a pathway to conduct a signal, which is often modified and sometimes not conducted through the synapse because it does not have the intensity to be received on the post-synaptic terminal of the cleft.

Source(s):

Alberts, The Molecular Biology of the Cell, 4th Edition.
Vander, Human Physiology, 9th Edition
Boron and Boulpyaep, Medical Physiology, Updated Edition