Difference Between Chemical and Electrical Synapse

Main Difference – Chemical vs Electrical Synapse

Chemical synapse and electrical synapse are two types of gaps that occur between nerve cells in the nervous system. The main difference between chemical synapse and electrical synapse is that in a chemical synapse, the nerve impulse passes chemically by means of neurotransmitters whereas an electrical synapse is connected through channel proteins. Nerve impulses pass through the membrane of the axon as an electrical signal. This electrical signal is converted into a chemical signal at a chemical synapse. But in an electrical synapse, the impulse can be transmitted as it is by means of ions. Therefore, electrical synapses are much faster than the chemical synapses. 

Key Areas Covered

1. What is a Chemical Synapse
      – Definition, Characteristic
2. What is an Electrical Synapse
      – Definition, Characteristics
3. What are the Similarities Between Chemical and Electrical Synapse
      – Outline of Common Features
4. What is the Difference Between Chemical and Electrical Synapse
      – Comparison of Key Differences

Key Terms: Channel Proteins, Chemical Signal, Chemical synapse, Electrical Signal, Electrical Synapse, Neurotransmitters

What is a Chemical Synapse

A chemical synapse refers to the cell junctions through which the nerve impulses are transmitted in one direction by means of neurotransmitters. The two plasma membranes are called pre-synaptic and post-synaptic membranes. The pre-synaptic membrane is in the pre-synaptic cell, and the post-synaptic membrane is in the post-synaptic cell. The synaptic cleft is between the pre-synaptic and post-synaptic membranes. A chemical synapse is shown in figure 1.

Figure 1: Chemical Synapse

The synaptic cleft is filled with interstitial fluid. When an action potential is received to the terminal of the pre-synaptic membrane, the voltage-gated calcium channels of the pre-synaptic membrane are opened. Generally, the concentration of the calcium ions in the synaptic cleft is 10-3 M, and that of the inside of nerve cells is 10-7 M. Therefore, calcium ions move from the synaptic cleft into the pre-synaptic nerve cell through calcium channels. This increases the calcium concentration inside the pre-synaptic nerve cell, allowing the synaptic vesicles to fuse with the pre-synaptic membrane to release neurotransmitters to the synaptic cleft by exocytosis. These neurotransmitters diffuse through the synaptic cleft to bind to the receptors in the post-synaptic membrane. They induce an action potential on the membrane of the post-synaptic neuron.  

What is an Electrical Synapse

An electrical synapse refers to cell junctions between nerve cells through which the transmission of nerve impulses occur by means of ions. The synaptic cleft of an electrical synapse is small, and the two plasma membranes of the neurons are connected together via a gap junction. One gap junction contains precisely aligned channel protein pairs in both pre-synaptic and post-synaptic membranes. Each channel pair forms a pore, which is much larger than the pore of a typical ion channel. Therefore, large molecules can be transported through these gap junctions in addition to the ions.  Hence, intracellular metabolites and second messengers can pass through two neurons. However, electrical synapses allow the passive transmission of the action potential through the pores of the gap junctions from one neuron to the second neuron. The structure of the electrical synapses is shown in figure 2.

Figure 2: Electrical Synapses

The transmission of the action potentials can occur in both directions through an electrical synapse. Moreover, the speed of transmission of the action potentials is very high. The electrical synapses are mainly involved in synchronizing the activity of a group of neurons. The neurons in the hypothalamus contain electrical synapses, firing the action potentials of many neurons at the same time. 

Similarities Between Chemical and Electrical Synapse

• Chemical and electrical synapses are made up of synaptic clefts.
• Both chemical and electrical synapses transmit nerve impulses between neurons or between neurons and effector organs.

Difference Between Chemical and Electrical Synapse

Definition

Chemical Synapse: A chemical synapse is a cell-to-cell junction through which nerve impulses are transmitted in one direction by means of neurotransmitters.

Electrical Synapse: An electrical synapse is a cell junction between two nerve cells through which rapid transmission of nerve impulses occurs by means of ions.

Occurrence

Chemical Synapse: Chemical synapses are found in higher vertebrates.

Electrical Synapse: Electrical synapses are found in lower vertebrates and invertebrates.

Mode of Transmission of  Nerve Impulses

Chemical Synapse: The nerve impulses transmit as a chemical signal by means of neurotransmitters at a chemical synapse.

Electrical Synapse: The nerve impulses transmit as an electrical signal through gap junctions or low resistance bridges at an electric synapse.

One-way/Two way

Chemical Synapse: The transmission of signals occurs in one-way at a chemical synapse.

Electrical Synapse: The transmission of signals occurs in two-way at an electrical synapse.

Size of the Synaptic Cleft

Chemical Synapse: The chemical synapse is large in size (10-20 nm).

Electrical Synapse: The electric synapse is smaller in size (0.2 nm).

Synaptic Vesicles and Mitochondria

Chemical Synapse: Chemical synaptic knobs contain synaptic vesicles and a large number of mitochondria.

Electrical Synapse: Electrical synaptic knobs contain no synaptic vesicles and very few mitochondria.

Chemoreceptors

Chemical Synapse: Chemoreceptors are present on the post-synaptic membrane in chemical synapses.

Electrical Synapse: Chemoreceptors are absent on the post-synaptic membrane in the electric synapses.

Speed of the Transmission of Signals

Chemical Synapse: The transmission of information at a chemical synapse is slow.

Electrical Synapse: The transmission of information at an electrical synapse occurs at high speed.

Vulnerability to Fatigue

Chemical Synapse: Chemical synapses are more vulnerable to fatigue.

Electrical Synapse: Electrical synapses are less vulnerable to fatigue.

Sensitivity to Hypoxia and pH

Chemical Synapse: Chemical synapses are sensitive to hypoxia and pH.

Electrical Synapse: Electrical synapses are insensitive to hypoxia and pH.

Location in the body

Chemical Synapse: Chemical synapses are found in most of the neuron junctions.

Electrical Synapse: Electrical synapses are found in the retina, olfactory bulb, cerebral cortex, lateral vestibular nucleus, and hippocampus.

Conclusion

Chemical synapses and electrical synapses are the two types of gaps that occur between nerve cells. Both types of synapses transmit nerve impulses. Chemical synapses transmit nerve impulses by means of neurotransmitters. But, electrical synapses transmit the action potentials by means of ions that through gap junctions.The main difference between chemical synapses and electrical synapses is the mode of the transmission of signals through the synaptic cleft in each type of synapses.

Reference:

1. Purves, Dale. “Chemical Synapses.” Neuroscience. 2nd edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here. Accessed 12 Sept. 2017.
2. Purves, Dale. “Electrical Synapses.” Neuroscience. 2nd edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here. Accessed 12 Sept. 2017.

Image Courtesy:

1. “1225 Chemical Synapse” By OpenStax –  (CC BY 4.0) via Commons Wikimedia
2. “Gap cell junction-en” By Mariana Ruiz LadyofHats – (Public Domain) via Commons Wikimedia