What is Acetylcholine (ACh) and what does it do?

Acetylcholine (Acetylcholinum, ацетилхолин, الأستيل كولين) is an ester of Choline (C5H14NO) and Acetic acid (CH3COOH) released by parasympathetic nerve cells, it functions as a neurotransmitter in the brain and autonomic nervous system, which is a part of the nervous system that regulates homeostatic functions such as the heart rate, respiratory rate, digestion, urination, pupillary response, and sexual arousal.[2]

Acetylcholine (ACh) is the first known neurotransmitter, as it was discovered in the 1920s. [10][11][12]

What is the function of acetylcholine?

Acetylcholine has many functions: It stimulates muscle contraction, decreases heart rate and plays an essential role in encoding of new memories. It is found in the brain and autonomic nervous system, and plays an essential role in switching the brain from one sleep stage to another and in scheduling REM sleep.

How does acetylcholine stimulate muscle contraction?

Acetylcholine molecules bind to cholinergic receptors (nicotinic ion channel receptors) on sarcolemma (the muscle cell membrane) leading to changes in ion permeability of sarcolemma and causing the voltage gated calcium channels to open. Calcium ions (cations) then rushes into the muscle cell, which leads to a change in membrane voltage (local depolarization, as cations move into the cell and cations or anions move outside the cell), initiating an action potential in sarcolemma and leading to muscle contraction.[2]

ACh receptors (ionotropic receptors) are proteins found on the post synaptic membrane on the target cells, they react with acetylcholine (extracellular signal) and convert it into intracellular effects. ACh receptors are transmembrane ion channel proteins (ligand gated ion channels ) that open to allow ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chlorine (Cl−) to pass through the membrane in response to the binding of acetylcholine.

What is the role of acetylcholine at the neuromuscular junction?

When action potential (the nerve impulse) reaches the axon terminal at neuromuscular junction (a chemical synapse between a motor neuron and a muscle fiber), a neurotransmitter (Acetylcholine) is released. Acetylcholine binds to cholinergic receptors on sarcolemma leading to changes in ion permeability of sarcolemma (the cell membrane that envelops each muscle fiber cells), which leads to a change in membrane voltage (local depolarization). Local depolarization propagates action potential in sarcolemma, which, in turn, leads to muscle fiber contraction.[2][9]

The neuromuscular junction (NMJ) is a chemical synapse between a motor nerve axon and a muscle fiber. It is composed of four specialized cell types: motor neurons, muscle fibres, Schwann cells, and the kranocytes.

How does acetylcholine affect the heart and How acetylcholine decreases heart rate?

Effect of acetylcholine on heart rate and force of contraction

Acetylcholine is released by the vagus nerve (pacemakers) at the sinoatrial node (SA node) after parasympathetic and vagus nerve activation (SA node action potential) triggered by movement. Acetylcholine binds to cholinergic receptors on sarcolemma leading to changes in ion permeability of sarcolemma, which leads to a change in membrane voltage. This decreases the pacemaker rate (bradycardia) by increasing potassium movement outside of the cell (potassium leaving the cell) and decreasing sodium and calcium movement into the cell (voltage gated calcium channels open and calcium flows in to the cell). As the pacemaker slows, so does your heart rate (bradycardia).[2]

At rest the heart is under control of the parasympathetic nervous system that releases acetylcholine at the sinoatrial node (SA node), acetylcholine slows the resting heart rate to between 60 – 80 bpm (beats per minute).

How Acetylcholine enhances hypotension?

Normally, Acetylcholine acts as a vasoconstrictor and vasodilator in coronary vessels, where it acts on the endothelial cells to release nitric acid (NO), which, in turn, propagates vasodilation of coronary arteries.

Acetylcholine propagates vasoconstriction of vascular smooth muscle cells by direct mechanism, as it stimulates the endothelial cells to produce NO (nitric acid) that diffuses out of the endothelial cells leading to relaxation of the nearby smooth muscle cells thus in healthy arterial walls, indirect vasodilation by acetylcholine mediated by NO (nitric oxide) is bigger than the direct vasoconstriction effect.

How does acetylcholine affect memory and the role of acetylcholine in the brain.

The Role of Acetylcholine in Learning and Memory

Cholinergic receptors (muscarinic and nicotinic) play an essential role in encoding of new memories. Acetylcholine propagates physiological affects that enhance different types of memory encoding in different cortical structures. The modulatory effects of acetylcholine in the hippocampus (the hippocampus acts as a kind of temporary transit point for long-term memories), entorhinal and perirhinal cortices may be important for encoding of new episodic memories. However, these effects can result in peak memory performance during alert waking, as they usually last for many seconds or minutes. [2][4][5] Acetylcholine may enhance memory encoding by various means such as:

1. Increasing the strength of afferent input relative to feedback.[1]
2. Contributing to theta rhythm oscillations.
3. Activating intrinsic mechanisms for persistent spiking.
4. Increasing the modification of synapses.

High acetylcholine levels suppress the excitatory feedback connections which are believed to be responsible for the reactivation of memories and the impact of hippocampus on cerebral neocortex.

What is encoding in memory?

Encoding is the very first step to creating a new memory that allows us to process information in visual, acoustic, or semantic forms, which is necessary for creating memory representations and storing them in the extracellular matrix at synapses in the brain. It is the process of receiving and registering information or events that you experience and converting the perceived data into a construct that can be stored within the brain for later recall (short-term or long-term memory). Episodic and semantic long-term memories can be better recalled when both encoding and retrieval are conducted using the same language.[2][8]

What are the three stages of memory?

Encoding, storage, and retrieval are the three stages of memory involved in storing information for later recall.

The role of acetylcholine in REM sleep

Acetylcholine plays an important role in switching the brain from one sleep stage to another. REM sleep (rapid eye movement) occurs in ∼90-min cycles when activity in the aminergic system (a class of modulatory neurons in the central nervous system) has decreased enough to allow the ascending reticular activating system (RAS) to escape its inhibitory influence.[15] The escape from aminergic inhibition stimulates cholinergic neurons of the reticular activating system in the brain stem (the midbrain, the pons, and the medulla oblongata) and switches the sleeping brain from dreaming sleep (rapid eye movement, REM) into the highly active REM sleep (slow-wave sleep, deep sleep, SWS), in which acetylcholine levels are as high as in the waking state. [2][6]

REM sleep is considered an important factor for the consolidation of procedural memory (motor skills) that is a long-term memory responsible for knowing how to do things. Moreover, it plays an important role in the storage of some types of emotional information.

Dreams and episodic memory content

Episodic memories are rare in REM sleep dreams. When examining REM sleep dreams, one finds that they emerge as disconnected fragments that often have bizarre content. Moreover, REM sleep dreams are often difficult to relate to waking life events.[16][17][18]

 

Symptoms of high acetylcholine

 

 

References

Verified by: Dr.Diab (July 13, 2017)

Citation: Dr.Diab. (July 13, 2017). What is Acetylcholine (ACh) and what does it do?. Medcoi Journal of Medicine, 30(2). urn:medcoi:article18804.