Differences Between Hormones and Neurotransmitters

Hormones are secreted in the blood and neurotransmitters in the synaptic cleft. Their functions are different, but they are more similar than it might appear at first.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 09 September, 2021

The human body requires the synergistic work of many substances to function properly. The endocrine system is an essential area to explain systemic communication within the body, since hormones that are released into the blood reach its target organs and allow perfect communication between different parts of the body. So, what are the differences between hormones and neurotransmitters?

Hormones belong to the endocrine system, but neurotransmitters are encompassed in the nervous system. Although both groups of compounds have communication within the body as their main functionality, in the following lines you’ll see that their differences are very extensive. If you want to see a detailed comparison between hormones and neurotransmitters, read on.

What are the differences between hormones and neurotransmitters?

Hormones and neurotransmitters are two types of chemical signaling molecules that are synthesized inside the bodies of humans and animals. Their common function is to establish communication between cells, but as you’ll see below, there are more disparities than similarities between them. Without wasting any more time, we’ll show you all the differences between hormones and neurotransmitters.

1. Endocrine system and nervous system

As obvious as this distinction may seem, it’s necessary to make it clear in the first instance that neurotransmitters are part of the nervous system, whereas hormones are encompassed in the endocrine system. In the very definition of each of the terms you can see this vital difference: for example, a hormone is any substance secreted by the endocrine glands.

Hormones are conceived as key elements for cellular communication, as their main function is to send signals between distant tissues. The endocrine glands are responsible for releasing them and differ from the rest (apocrine or paracrine) by lacking an excretory duct. Thus, the hormones are dumped directly into the blood capillaries and travel in the blood to their target.

Instead, neurotransmitters locate their functional bulk in the nervous system and their action is much more immediate. These compounds are released from a presynaptic neuron and cross the synapse (space) that separates the nerve cell from its target. Upon release, they’re received by the postsynaptic neuron and allow the transmission of the nerve impulse.

Thus, the first of the differences between hormones and neurotransmitters lies in the organic system they’re part of. Hormones are born from the endocrine glands (endocrine system) and carry signals through the blood, while neurotransmitters cross neuronal synapses and allow the transmission of impulses (nervous system).

2. Hormones are universal, but neurotransmitters are only produced by animals

As we have said, in vertebrate animals, hormones are responsible for communicating organs and tissues. Thus, they play an essential role in physiological processes such as digestion, metabolism, respiration, sensory perception, excretion, rest, response to stress, and much more. However, these aren’t the only living beings that express them.

Plants also contain these compounds, known in biology as phytohormones. According to professional sources, these molecules exert their work in very low concentrations and change the growth patterns of vegetables, allowing their control. They also modulate the fall of the leaves, flowering, the formation of the fruit, and the germination of the seeds.

Invertebrate animals have components similar to hormones, but it should be noted that these are not secreted by endocrine glands. When released by basic nerve cells, they are known as neurohormones and modulate growth and reproduction patterns in insects and other living things.

With all these lines, we want to explain that hormones are present in animals and plants. On the other hand, neurotransmitters are only present in animals, at least in their strict function: transmitting a message from neuron to neuron. As they lack neural networks, the term “neurotransmitter” can’t be applied to any plant compound.

3. The types within each category are different

So far, we have described that hormones are endocrine and are in animals and plants, while neurotransmitters are nervous and are only found in vertebrates. Next, we’re going to review the categories within each of these groups of molecules in separate ways.

Types of hormones

The types of hormones can be summarized in the following list:

• Derived from amino acids: as their name suggests, these hormones are derived from amino acids, organic molecules that make up the backbone of proteins. The bases of these hormonal compounds are tyrosine and tryptophan.
• Peptides: Peptide hormones are composed of peptides (chains of amino acids) of variable length. Since they can’t cross the cell membrane, these compounds present target receptors on the surface of the target organ/tissue. As a curiosity, it should be noted that they form an important part of doping practices in sports.
• Lipids: as their name suggests, they’re organic compounds of a lipid nature. Because they’re lipophilic (they have an affinity for lipids), they can cross the cell membrane (which is abundant in phospholipids), so their target receptors are found inside the cell, specifically in the cytosol.

As you can see, hormones are classified according to their chemical composition and their ability to enter (or not) inside the target cell. More than 50 different hormonal compounds have been detected in humans and other animals, as indicated by the United States Environmental Protection Agency.

Types of neurotransmitters

The types of neurotransmitters are summarized in the following list:

• Amino Acids: Amino acids like glutamate, aspartate, and glycine act as neurotransmitters. They are also part of proteins; these are made up of between 100 and 300 amino acids ordered in a specific way.
• Gaseous signaling molecules: For example, nitric oxide and carbon monoxide are gaseous substances capable of passing through the cell membrane and acting as neurotransmitters.
• Trace amines: these are endogenous compounds considered as monoamine neuromodulators.
• Peptides: within this group are the most famous neurotransmitters, such as oxytocin, somatostatin, and opioid peptides. Many of them are related to the reward mechanisms of the brain circuit and addictions.
• Purines: Purines (nitrogenous bases) can also rarely act as neurotransmitters.

The differences between hormones and neurotransmitters on this front can be summed up in a very simple point: neurotransmitters report a greater variety in terms of chemical composition. About 40 neurotransmitters have been recorded in the human brain, but it’s estimated that more than 500 exist in nature.

4. Remote work and immediate work

As the US National Library of Medicine indicates, hormones are very powerful messengers and it takes only a small amount to cause very large changes in the body.

In any case, these compounds act slowly, since they have to be released by the glands, travel through the blood system, reach the receptor of the target cell and stimulate it.

For these reasons, hormonal treatments usually don’t start to report positive effects until 1 or 2 weeks after they start. On the other hand, the action of neurotransmitters is much more immediate, since they only have to be released and travel through the synaptic space before reaching their target. The gap between neurons doesn’t exceed 20-30 nanometers.

Due to the immediacy of the action of neurotransmitters, a message sent from the brain to any part of the body takes less than 2 tenths of a second, reaching a speed of 360 kilometers per hour. The differences between hormones and neurotransmitters are that neurotransmitters exert their effect much faster.

5. Other differences between neurons and neurotransmitters

In the points cited we have stated the most important differences between neurons and neurotransmitters, but there are many more. We’ll briefly explain them in the following list:

1. Neurotransmitters cross only the synaptic cleft to reach the target, while hormones must travel through part of the bloodstream.
2. Hormones control growth, development, and reproduction. On the other hand, neurotransmitters are limited to the field of nerve signal transmission.
3. Hormones regulate distant tissues through intracellular or extracellular receptors, whereas neurotransmitters only affect the postsynaptic neuron immediately next to where they were released.

Although the differences between the two molecules seem very clear, it is necessary to emphasize that some neurotransmitters act like hormones in certain situations. These are called neurohormones, since they are released into the blood instead of the presynaptic cleft despite not being hormones to use. However, this sporadic behavior is not the norm.

Criteria of neurotransmitters

Although no hormone is capable of being transported as a neurotransmitter, there are situations in which the opposite occurs. For this reason, the line that divides both compounds is getting weaker and weaker. In an attempt to better circumscribe neurotransmitters and separate them from other molecules, the following requirements are postulated:

1. The substance that is released into the synaptic space must be within the neuron. If it were to come from somewhere other than the neuronal body, it could not be considered a neurotransmitter.
2. The enzymes that allow the synthesis of the neurotransmitter must also be found naturally within the neuron. Creating these compounds is very complex, so for a neurotransmitter to be considered as such, all the tools to assemble it must be found in the neural environment.
3. The effect of the neurotransmitter must be the same in all possible scenarios.

Differences between neurons and neurotransmitters: not as many as it might seem

Differentiating a neurotransmitter from a hormone is simple, since a hormonal compound will never be secreted by a neuron into the synaptic space, nor will it produce an electrical impulse. However, the existence of neurohormones does complicate matters a lot, since various neurotransmitters can be released into the blood and stimulate distant cells.

Thus, the differences that we have mentioned are general, but there are always exceptions to the rule. In the end, what defines every substance is its behavior at a physiological level and its chemical composition, not the category to which it belongs.