How does acetylcholine slow heart rate?

In this post we are going to answer the question ‘’How does acetylcholine slow heart rate?’’ We will explain how this neurotransmitter participates in the decrease in heart rate as well as highlight other functions of acetylcholine.

How does acetylcholine slow heart rate?

Acetylcholine slows down the heart rate by binding to M2 receptors, this allows a decrease in the depolarization rate and the speed of conduction through the atrioventricular node.

Acetylcholine is the most abundant neurotransmitter in the nervous system. This chemical that our brain produces mainly from sugar and choline, is that essential messenger that facilitates communication between neurons. Thanks to it, we regulate attention and memory, assimilate new information and take care of our state of mind.

We will start by commenting that acetylcholine was the first neurotransmitter identified. It was done by Henry Hallett Dale in 1915, and later, Dr. Otto Loewi would describe much of his functions.

Both doctors would receive the Nobel Prize for it in 1936. As we see, we are not facing one more chemical compound, the implication of this small element in our lives and in much of the biological functions is immense.

If we want to understand the relevance of acetylcholine in our body, let’s think for example that without it, the muscles of our heart would stop contracting and expanding. That is, this organ would stop beating …

It is known, for example, that there are specific cells in our body that react exclusively with acetylcholine. So much so that the basal forebrain and hippocampus need this specific neurotransmitter to be able to carry out their tasks.

It not only acts as a messenger, but it also increases the intensity of the signals between neurons through theta waves. Optimizes memory, promotes neuroplasticity, communication … Let’s see more information about it below.

Acetylcholine: a neurotransmitter

Acetylcholine is a substance classified as an ester, made by compounds of an oxygenated acid and an organic radical. It is, as I have already mentioned, the first neurotransmitter to be discovered, in 1914, and the different elements that are responsible for its synthesis and elimination make up the so-called cholinergic system.

Acetylcholine is mainly seen as an excitatory neurotransmitter, but it can also exert an inhibitory action depending on the type of synapse in which it acts.

On the other hand, acetylcholine is considered to be one of the main neurotransmitters in the nervous system and one of the most common, being found throughout the brain and in the autonomic nervous system.

Synthesis

Acetylcholine synthesis occurs inside neurons, specifically in their cytoplasm, through the binding of acetic acid or acetyl-CoA and choline thanks to the enzyme choline acetyltransferase.

After that, acetylcholine is sent along the axon to the terminal button, where it will be stored until its use and release in the synaptic space.

Acetylcholine receptors

The action of acetylcholine occurs through its interaction with a series of receptors that react to its presence in the different locations where this neurotransmitter acts. Specifically, we can find two main types of cholinergic receptors in the nervous system.

Muscarinic receptor

It is a type of metabotropic receptor, that is, it requires the use of chains of second messengers to allow the opening of ion channels. This implies that its action is usually slow and has a longer effect over time.

This type of receptor is usually the one with the highest level of presence in the brain, as well as in the parasympathetic nervous system. They can have both an excitatory and an inhibitory action.

Nicotinic receptor

This type of receptor, which also has an affinity for nicotine, is ionotropic, which generates a rapid response from the receptor that allows the immediate opening of the channel. Its effect is fundamentally excitatory. They are usually found in the connections between neuron and muscle.

Neurotransmitter degradation

Most neurotransmitters are received by the presynaptic neuron after being released. In this sense, acetylcholine has the particularity that it is not retained but is degraded by the acetylcholinesterase enzyme present in the synapse itself.

Acetylcholine has a very short lifetime at synapses because it degrades very rapidly.

Acetylcholine: what functions does it have?

Most of us have heard of acetylcholine for one reason: to enhance memory and concentration. Hence, it is one of the most common components in nootropics, those supplements used to improve our cognitive functions. Now, beyond this area we know that it is essential for other functions that we will see below.

However, it is necessary to remember first that its field of action in our body is multiple. It is found in both the central and peripheral nervous systems, and also has both excitatory and inhibitory functions.

That is, it can facilitate the electrical impulse in a neuron or it can inhibit, for example, the heart rate at a given moment. It is like that conductor who directs and ensures that each performance has harmony, rhythm and balance.

Functions in the central nervous system

In the central nervous system, aceticoline acts excitatory. Thanks to its interaction between neurons and nerve cells, it promotes the processes of motivation, excitement and attention.

Not only does it stimulate the activity of the hippocampus to carry out these processes, it also acts on the cerebral cortex so that we shape those higher executive functions, such as problem solving or reflection.

On the other hand, the main cause that cholinergic pathways lose their functionality in the central nervous system and stop communicating with each other is Alzheimer’s disease.

Acetylcholine and REM sleep

This data is interesting. Acetylcholine promotes REM sleep in our brain, and it does so by concentrating on a very special structure: the basal forebrain. Thanks to this, thanks to the fact that we enter this phase of paradoxical sleep or rapid sleep, we are able to better store the memories and information obtained during the day.

Endocrine functions

A part of our neurotransmitters, such as oxytocin, also act as hormones. In the case of acetylcholine, it must be said that it also has a very important endocrine function: it acts on the pituitary. In this way, you can control the amount of urine excreted, stimulate the production of thyroid hormones, etc.

Functions in the peripheral nervous system

Our peripheral nervous system could not carry out a large part of its functions if it did not have the presence of this neurotransmitter. The tasks it carries out are as many as relevant to our subsistence and well-being:

• It transmits the signals between our brain and the heart muscles.
• Likewise, it is that bridge between the brain, nerves, muscles, and bones that shape each of our movements.
• In the cardiovascular system, it almost always acts as a vasodilator, that is, it reduces and balances the heart rate.
• Also, in the gastrointestinal system, it favors digestive contractions.
• In the urinary tract, it prompts the voluntary sensation of evacuation.
• In addition, and as a curiosity, it should be said that acetylcholine also mediates in that process that also guarantees our survival: the perception of pain.

How does acetylcholine slow heart rate?

This neurotransmitter stimulates the production of nitric oxide, a compound that controls blood pressure by relaxing blood vessels (vasodilation) throughout the cardiovascular system. Muscarinic receptors are especially relevant for the cardiovascular functions of acetylcholine.

Muscarinic receptors are acetylcholine receptors that complex with G proteins on the membranes of certain neurons and other cells of the nervous system. They fulfill various functions, the main receptors being stimulated by acetylcholine released by postganglionic fibers in the parasympathetic nervous system.

They are called muscarinic because they are more sensitive to muscarine than nicotine, unlike their counterpart nicotinic receptors, which are very important in the autonomic nervous system. Many substances, such as scopolamine and pilocarpine, influence these two types of receptors by activating them as selective agonists or antagonists.

Muscarinic receptors are found in various places in the body, both organs and tissues, and within the central nervous system. Among the most notable tissues where these receptors can be found we have smooth muscle and heart tissue, as well as some exocrine glands.

Muscarinic receptors belong to the group of metabotropic receptors that use G proteins as a signaling mechanism. In these receptors, the molecule or ligand used to give the signal binds to the receptor, which has seven transmembrane regions. In the case of muscarinic receptors, the ligand is acetylcholine.

Up to five different types of muscarinic receptors have been discovered, which are called “M” followed by a number between 1 and 5. Receptors M1, M3 and M5 bind to Gq proteins, while M2 and M4 do so. They make with Gi / o proteins.

M2 Receptors

Acetylcholine acts as an agonist at muscarinic and nicotinic receptors. However, the affinity for muscarinic receptors is greater than the affinity for nicotinic receptors. Therefore, when ACh is administered, unless muscarinic receptors are blocked, a response mediated by nicotinic receptors is almost never seen.

Muscarinic receptors that promote vasodilation are located on endothelial cells. The receptors are not innervated, but when encountering an agonist they induce nitric oxide secretion by endothelial cell cells.

Nitric oxide diffuses through vascular smooth muscle cells and activates guanylate cyclase which, through a series of steps, causes vascular relaxation.

M2 receptors are found in the heart, where they are responsible for slowing down the heartbeat, keeping it below normal rhythm. They do this by slowing down the rate of depolarization.

In humans, when we are resting, vagal activity dominates over sympathetic activity. If M2 receptors are inhibited, then the heart rate increases.

FAQS: How does acetylcholine slow heart rate?

How does acetylcholine decrease heart rate?

Acetylcholine can decrease the L-type Ca2 + current in heart cells, but only if previously this current has been increased by the action of adrenergic agonists on the adrenergic receptor.

What effect does acetylcholine have on the heart rate?

These channels, initially called K (Ach), slow the depolarization of the pacemaker cell and decrease the heart rate. Furthermore, acetylcholine can initiate and maintain slow modulation mechanisms, with soluble intracellular messengers, kinase activation, and phosphorylation.

How does ACh inhibit cardiac muscle?

In cardiac tissue, neurotransmission of acetylcholine has an inhibitory effect that reduces the heart rate. However, at neuromuscular junctions of skeletal muscle, acetylcholine also acts like an excitatory neurotransmitter.

How does the sympathetic system affect the heart?

The sympathetic nervous system accelerates the heart rate; the parasympathetic decreases it. The sympathetic system provides the heart with a network of nerves, called the sympathetic plexus. The parasympathetic system reaches the heart through a single nerve: the vagus or pneumogastric nerve.

What if acetylcholine was not released?

Inhibition of the enzyme acetylcholinesterase causes devastating effects on nerve agents, resulting in continuous stimulation of the muscles, glands and the central nervous system.

In this post we answered the question ‘’How does acetylcholine slow heart rate?’’ We explained how this neurotransmitter participates in the decrease in heart rate as well as highlighted other functions of acetylcholine.

If you have any questions or comments please let us know!

References

Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S., Hudspeth, A. J., & Mack, S. (Eds.). (2000). Principles of neural science (Vol. 4, pp. 1227-1246). New York: McGraw-hill.

Beers, W. H., & Reich, E. (1970). Structure and activity of acetylcholine. Nature, 228(5275), 917-922.