Difference Between Action Potential and Synaptic Potential

The key difference between action potential and synaptic potential is that action potential is the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and endocrine cells, etc. while synaptic potential is the post-synaptic potential change in neurons.

The nervous system transmits signals among the different parts of the body and coordinates the actions and sensory information. It is comprised of a complex network of neurons and other cells.  Billions of nerve cells communicate with each other via nerve impulses. Neuronal action potential and synaptic potential are two electrical potentials that aid in the transmission of nerve impulses along the neurons. They are important for information processing, propagation and transmission.

In fact, action potentials are the fundamental units of communication between neurons. Action potential is the electrical potential difference across the plasma membrane of neurons. Synaptic potential is the electrical potential difference across the post-synaptic membrane. Action potential occurs as a result of the summation of many synaptic potentials across the membrane of the neuron.

CONTENTS

1. Overview and Key Difference
2. What is Action Potential 
3. What is Synaptic Potential
4. Similarities Between Action Potential and Synaptic Potential
5. Side by Side Comparison – Action Potential and Synaptic Potential in Tabular Form
6. Summary

What is Action Potential?

Action potential occurs within a neuron when it transmits electrical impulses. During this signal transmission, the membrane potential (the difference in electrical potential between the outside and inside of a cell) of the neuron (specifically the axon) fluctuates with rapid rises and falls. Actions potentials do not occur only in neurons. It occurs in various other excitable cells such as muscle cells, endocrine cells and also in some plant cells. During an action potential, the nerve transmission of impulses takes place along the axon of the neuron up to the synaptic knobs located at the end of the axon. The prime role of an action potential is to facilitate communication between cells.

Action potential generally rises to around +50 mV from its resting potential level of -70 mV and then rapidly returns to the resting level again as a result of a depolarizing current. In other terms, a stimulus that generates an action potential causes the resting potential of a neuron to decrease up to 0mV and further down up to a value of -55mV. This is referred to as the threshold value of excitation. Unless the neuron reaches the threshold value, an action potential won’t be generated.

Figure 01: Action Potential

Similar to resting potentials, action potentials occur due to the crossing of different ions across the membrane of the neuron. Initially, the Na+ ion channels are opened up in response to the stimulus. During resting potential, the inside of the neuron is more negatively charged and contains more Na+ ions outside. Due to the opening of the Na+ ion channels during an action potential, more Na+ ions will rush into the neuron across the membrane. Due to the + charge of sodium ions, the membrane becomes more positively charged and get depolarized

This depolarization is reversed by the opening of K+ ion channels that move a higher number of K+ ions out of the neuron. Once the K+ ion channels open up, the Na+ ion channels close. The opening of K+ ion channels for longer periods of time causes the voltage of the action potential to go past -70 mV. This condition is known as hyperpolarization. But when Na+ ion channels close, this value is brought back to -70mV. This is known as repolarization.

What is Synaptic Potential?

Synaptic potential is the potential difference across the post-synaptic membrane. This arises due to the action of neurotransmitters. This can also be defined as the incoming signal received by the post-synaptic neuron. There are two types of synaptic potentials as excitatory and inhibitory, based on the nature of the neurotransmitters and post-synaptic receptors. Excitatory synaptic potential depolarizes the membrane while inhibitory synaptic potential hyperpolarizes the post-synaptic membrane. Neurotransmitters such as glutamate and acetylcholine mostly carry the excitatory post-synaptic potential while neurotransmitters such as gamma-aminobutyric acid (GABA) and glycine carry the inhibitory post-synaptic potential. Synaptic potential is dependent on the release of neurotransmitters from the pre-synaptic neuron end.

Figure 02: Synaptic Potential

Synaptic potentials have a smaller amplitude. Hence, many synaptic potentials are needed to trigger an action potential. Moreover, they have a slower time course and do not have a refractory period. Unlike action potentials, synaptic potentials degrade quickly as they move away from the synapse.

What are the Similarities Between Action Potential and Synaptic Potential?

• Both action potential and synaptic potential are needed for neurons to communicate with each other and send nerve impulses.
• Many synaptic potentials are needed to generate an action potential.
• The occurrence of an action potential depends on the synaptic potential across the membrane of the neuron.
• Both action potential and synaptic potential travel or occur in one direction.

What is the Difference Between Action Potential and Synaptic Potential?

Action potential is the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells while synaptic potential is the potential difference across the post-synaptic membrane of a neuron. So, this is the key difference between action potential and synaptic potential.

Moreover, action potentials always lead to depolarization of the membrane while synaptic potentials can be depolarizing or hyperpolarizing the membrane. Besides, the amplitude is large in action potential while it is small in synaptic potential. Also, another major difference between action potential and synaptic potential is their refractory periods; refractory periods are associated with action potentials, but not with synaptic potentials.

Below is a summary of the difference between action potential and synaptic potential in tabular form.

Summary – Action Potential vs Synaptic Potential

Action potential is the sudden, fast, transitory, and propagating change of the resting membrane potential of neurons. It occurs when a neuron sends nerve impulses along the axon and depolarizes the cell body. Synaptic potential is the potential difference across the post-synaptic membrane. It depends on the release of neurotransmitters from the presynaptic terminal. Action potential actually occurs as a summation of synaptic potentials. Action potential occurs due to the flow of certain ions into and out of the neuron while synaptic potential occurs due to the neurotransmitters and post-synaptic receptors. Thus, this summarizes the difference between action potential and synaptic potential.

Reference:

1. “Neuron Action Potentials: The Creation of a Brain Signal (Article).” Khan Academy, Available here.
2. “Synaptic Potential.” Wikipedia, Wikimedia Foundation, 26 May 2020, Available here.

Image Courtesy:

1. “Action potential” By Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “IPSPsummation” By Gth768r (CC BY-SA 3.0) via Commons Wikimedia

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