What is the Difference Between Neurotransmitter and Neuromodulator

The main difference between neurotransmitter and neuromodulator is that neurotransmitter is a chemical messenger released by a neuron to affect either one or two post-synaptic neurons or another specific effector organ whereas neuromodulator is another chemical messenger released by a neuron to affect a group of neurons or effector organs with a specific receptor. Furthermore, a neurotransmitter directly affects the postsynaptic partner to produce a quick, rapid effect while a neuromodulator indirectly affects the post-synaptic partner, especially through a second messenger to produce a slow but, long-lasting effect. 

Neurotransmitter and neuromodulator are two types of chemical messengers produced by neurons in the nervous system. 

Key Areas Covered  

1. What is a Neurotransmitter
     – Definition, Structure, Function
2. What is a Neuromodulator
     – Definition, Structure, Function
3. What are the Similarities Between Neurotransmitter and Neuromodulator
     – Outline of Common Features
4. What is the Difference Between Neurotransmitter and Neuromodulator
     – Comparison of Key Differences

Key Terms 

Excitatory Neurotransmitter, Iinhibitory Neurotransmitters, Neuromodulator, Neurotransmitter, Post-synaptic Neuron, Synapse

What is a Neurotransmitter 

A neurotransmitter is a chemical messenger released by the end of a stimulated pre-synaptic neuron in order to transmit the nerve impulse through the synaptic cleft to the post-synaptic neuron or to an effector cell. Moreover, the synaptic vesicles at the terminus of the pre-synaptic neuron store neurotransmitters. Generally, the release of neurotransmitters occurs in the direct opposition to the target, which contains specific receptors for binding. Upon binding, the change of the trans-membrane ion flow generates a nerve impulse on the post-synaptic neuron.

Figure 1: General Action of a Neurotransmitter

Therefore, based on the type of change in the trans-membrane ion flow that occurs by each, there are two types of neurotransmitters; the excitatory and inhibitory neurotransmitters. As excitatory neurotransmitters increase the trans-membrane ion flow and generate an action potential while inhibitory neurotransmitters decrease the trans-membrane ion flow to make it difficult to generate an action potential. Furthermore, acetylcholine and glutamate are the main excitatory neurotransmitters while the main inhibitory neurotransmitters of the nervous system are GABA and glycine.  

What is a Neuromodulator 

A neuromodulator is another type of chemical messenger released by the end of a stimulated neuron, affecting a diverse group of neurons or effector cells with the appropriate receptor. In contrast, a neurotransmitter directly affects only one or two post-synaptic neurons. Due to the very long range of action of neuromodulators, their mechanism of action occurs through volume transmission. Here, prolonged activation of target cells occurs due to the lack of rapid degradation or taking up of neuromodulators.

Figure 2: Acetylcholine

Moreover, neuromodulators can alter the transmission of nerve impulses by controlling the amount of neurotransmitters synthesized and released by neurons. Also, the site of action of the neuromodulators can be either near to the site of release or far away from the site of release. Moreover, some neurotransmitters including substance P, octopamine, serotonin, and acetylcholine can serve as neuromodulators. 

Similarities Between Neurotransmitter and Neuromodulator 

• Neurotransmitter and neuromodulator are two types of chemical messengers released by the nervous system. 
• The terminus of the presynaptic neuron stores them in vesicles and releases them into the synapses. 
• They transmit neuron impulses through the synapse. 
• Moreover, they bind to specific receptors on the post-synaptic neuron or effector cells.  
• And, their effect can be either excitatory or inhibitory.  

Difference Between Neurotransmitter and Neuromodulator 

Definition 

Neurotransmitter refers to a chemical substance released at the end of a nerve fiber by the arrival of a nerve impulse and, by diffusing across the synapse or junction, effects the transfer of the impulse to another nerve fiber, a muscle fiber or some other structure while neuromodulator refers to a substance, other than a neurotransmitter, released by a neuron and transmitting information to other neurons, altering their activities. Thus, this explains the main difference between neurotransmitter and neuromodulator.

Number of Post-Synaptic Targets 

Moreover, the number of post-synaptic targets is also an important difference between neurotransmitter and neuromodulator. Neurotransmitters affect one or two post-synaptic targets at once while neuromodulators affect a group of post-synaptic targets. 

Mechanism 

Furthermore, neurotransmitters affect directly on their post-synaptic target while neuromodulators indirectly effect on their post-synaptic targets via second messengers.  

Location 

Additionally, neurotransmitters affect the adjacent post-synaptic targets while neuromodulators affect targets that are quite far away from the point of release. 

Degradation or Taken up by Neurons 

Moreover, while neurotransmitters are degraded or taken up by neurons rapidly, neuromodulators are not degraded rapidly or taken up by neurons. 

Type of Effect 

Another difference between neurotransmitter and neuromodulator is their effect. Neurotransmitters produce a rapid effect, which lasts for a short time period, while neuromodulators produce a slow but, long-lasting effect. 

Examples 

Some examples of neurotransmitters are serotonin, acetylcholine, dopamine, GABA, glycine, and norepinephrine while some examples of neuromodulators are opioid peptides such as enkephalins, endorphins, and dynorphins.

Conclusion 

A neurotransmitter is a chemical messenger released by the end of the pre-synaptic neuron to transmit nerve signals through the synaptic cleft either to the post-synaptic neuron or an effector cell. Generally, it produces a rapid and short effect on the nearby target. On the other hand, neuromodulator is another type of chemical messenger released by the ends of the pre-synaptic neurons. However, it affects a group of post-synaptic targets, which can be far away from the point of release. Therefore, it uses second messengers during the process. Moreover, the effect of a neuromodulator is slow and long-lasting. Hence, the main difference between neurotransmitter and neuromodulator is their mechanism of action. 

References:

1. “What Are Neurotransmitters?” Queensland Brain Institute, 9 Nov. 2017, Available Here.
2. Nadim, Farzan, and Dirk Bucher. “Neuromodulation of neurons and synapses.” Current opinion in neurobiology vol. 29 (2014): 48-56. doi:10.1016/j.conb.2014.05.003  

Image Courtesy:

1. “Generic Neurotransmitter System” By NIDA(NIH) (Public Domain) via Commons Wikimedia   
2. “Acetylcholine” By Harbin – Own work (Public Domain) via Commons Wikimedia