Many people know that any movement made by the body isregulated by the nervous system. But most of them don't know the exact mechanism of how it happens. They are not aware of the connection between the nervous system and the muscular system.
The neuromuscular junction, as its name implies, acts as a bridge between the nervous system and the muscular system. It is a microstructure through which neurons start or stop the contraction process in muscles. Any change inneuromuscular junctionmay result in compromiseskeletal muscle contractions.
In this article we will study in detail the structure of the neuromuscular junction, the mechanisms by which it initiates the contraction mechanism, the drugs that act on it, as well as its clinical significance. By the end of this article, you will be able to understand the process of how contraction is initiated by theneuromuscular junction.
Oneuromuscular junctionIt is a chemical synapse between the motor neuron and the skeletal muscle fiber. It consists of a presynaptic terminal, a synaptic cleft, and a postsynaptic cell or membrane.
In the case of the neuromuscular junction, thepresynaptic terminalIt is an axonal terminal of a motor neuron. The axon terminal contains several synaptic vesicles. These vesicles contain the neurotransmitters that are released when receiving a nerve impulse.
The presynaptic terminal also has calcium channels. These channels are voltage-gated calcium channels that open when a nerve impulse reaches the presynaptic axon terminal.
It is the space between the presynaptic terminal and
the postsynaptic cell. It is approximately 30 nm in size. the synaptic cleft
allows neurotransmitters to diffuse and reach the other side of the synapse
or the neuromuscular junction. It also contains enzymes for the degradation of
the excess or excess of neurotransmitters.
Cell or postsynaptic membrane
Opostsynaptic cellin the case of the neuromuscular junction is theskeletal muscle fiber. Motor neurons synapse on the sarcolemma, or membrane, of skeletal muscle fibers.
At the neuromuscular junction, the sarcolemma of skeletal muscle displays a series of invaginations called postsynaptic folds. These folds greatly increase the surface area on which neurotransmitters act.
The walls of these folds haveacetylcholine receptors. These receptors are the most important functional part of the neuromuscular junction. A brief detail of these receptors is provided below.
Acetylcholine is the neurotransmitter used at the neuromuscular junction. Acetylcholine receptors are present in the walls of thepostunion folds. These receptors are also called cholinergic receptors. The receptor can also be activated by nicotine, so they are called nicotinic receptors.
Oacetylcholine receptors are the ionotropic receptors linked to ion channels. It is composed of two α subunits, one β, one ɛ and one δ. Acetylcholine binds to the alpha subunit. when a singleacetylcholine moleculebinds to the alpha subunit, it induces a conformational change that results in a higher affinity of the second subunit.
When both subunits are occupied by acetylcholine,opening of cation channels, resulting in internal diffusion of sodium and potassium ions.
When a nerve impulse reaches the presynaptic axon
terminal, causes depolarization. As a result, the voltage-dependent calcium
open channels. Calcium ions from the surrounding environment diffuse
the presynaptic axon.
These calcium ions activate the SNARE proteins. Those
Proteins mediate the fusion of synaptic vesicles with the cell membrane of the
neuron, resulting in the release of acetylcholine into the synaptic cleft.
Since acetylcholine isreleased into the synaptic cleft, diffuses through the synaptic cleft and binds to acetylcholine receptors. This results in the opening of the cation channels. These channels are open to sodium and potassium ions.
Since the concentration of sodium ions is higher in the
Into the extracellular space, sodium ions enter through these open cation channels.
As a result, depolarization of skeletal muscle occurs.
Odepolarization of the sarcolemmait results in the opening of voltage-gated calcium channels located in the sarcolemma, as well as in the smooth endoplasmic reticulum membrane.
while thecalcium ionsentering the cell, it initiates the skeletal muscle contraction cycle. Actin-myosin bridges are formed and result in the contraction of skeletal muscles.
Once acetylcholine is released at the synapse
cleavage, has a very short half-life. It is immediately metabolized by
acetylcholinesterase to its metabolites. The hill thus formed is absorbed by
presynaptic neurons. Metabolism of acetylcholine to its metabolites
results in the removal of all its effects and muscle contraction
Drugs acting at the neuromuscular junction
The normal mechanism and function of the neuromuscular
union is affected by the following drugs.
These drugs increase the amount of acetylcholine in the
the synaptic cleft. There are two types, direct action and indirect action.
Direct acting drugs
These drugs increase the amount of acetylcholine by acting as its precursor. These includebetanecol,metacolina, etc.
Indirect acting drugs
These are acetylcholinesterase inhibitors.
enzyme. They inhibit the metabolism of acetylcholine, resulting in a
increased amount of acetylcholine in the synaptic cleft. These drugs include
neostigmina, fisostigmina, etc.
These are the antagonists ofnicotinic acetylcholine receptorspresent at neuromuscular junctions. Blockade of these receptors results in relaxation of skeletal muscles. These drugs are used as skeletal muscle relaxants.
They are further divided into two types,
nondepolarizing and nondepolarizing.
These drugs are antagonists ofnicotinic receptors. They block receptors and prevent depolarization, resulting in skeletal muscle relaxation. These drugs have few side effects and are used more often than depolarizing drugs. These include atracurium, tubocurarine, etc.
These drugs are potent nicotinic receptor agonists. They cause excessive depolarization, which cannot be reversed. Prolonged depolarization causes A1 blockade, resulting inrelaxation of skeletal muscles. These include suxamethonium and other drugs.
Important clinical conditions associated with
neuromuscular junction are as follows.
It's aautoimmune diseasein which antibodies are formed against acetylcholine receptors. As a result, the neuromuscular junction is unable to initiate skeletal muscle contraction.
This results in varying degrees of muscle weakness. The muscles most commonly affected include the muscles of the eyes, face, and pharynx that help with swallowing.
and otherautoimmune diseaseof the neuromuscular junction. However, it affects presynaptic neurons. In this disease, antibodies are formed against voltage-gated calcium channels present on presynaptic neurons. TO
As a result, the nerve impulse arriving at the presynaptic terminal fails to release the neurotransmitter into the synaptic cleft. The muscles are unable to contract. It also causes varying degrees ofskeletal muscle weakness.
The most commonly affected muscles include those of the legs and arms. The person has trouble walking, climbing stairs, etc.
This disease of the neuromuscular junction produces hyperarousal of skeletal muscles. It is due to the downward regulation ofvoltage-gated postsynaptic potassium channels.
As a result, potassium ions are unable to leave skeletal muscle and hyperpolarization occurs. This hyperpolarization leads to hyperarousal of skeletal muscle and muscle spasm. It is also believed to be an autoimmune disorder of the neuromuscular junction.
neuromuscular junctionIt is a microstructure present at the junction of motor neurons with skeletal muscle fibers. It acts as a bridge connecting the skeletal system and the nervous system.
The neuromuscular junction is a chemical synapse.
The presynaptic terminal is the axonal terminal of the
motor neuron containing synaptic vesicles.
These vesicles are released into the synaptic cleft.
when a nerve impulse arrives.
The postsynaptic sarcolemma has the synaptic clefts
have acetylcholine receptors on their walls.
Oacetylcholine moleculesReleased from the presynaptic terminal, they bind to these receptors and cause the opening of cation channels.
Sodium ions diffuse through these channels,
resulting in depolarization of skeletal muscles. This depolarization initiates
the process of muscle contraction.
Acetylcholine is rapidly metabolized by
acetylcholinesterase, which eliminates all its effects.
The normal mechanism of the neuromuscular junction is
affected by cholinergic drugs, as well as skeletal muscle relaxants.
Ocholinergic drugs, which can be of direct or indirect action, increase the activity of acetylcholine.
Skeletal muscle relaxants areneuromuscular blockers. They block the neuromuscular junction by inhibiting depolarization or causing excessive depolarization.
The important pathological conditions associated with
the neuromuscular junction include:
- Myasthenia gravis
- Lambert-Eaton Syndrome
These are all autoimmune conditions. the first two
result in muscle weakness, while the third causes hyperextension of skeletal muscles
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The neuromuscular system includes all the muscles in the body and the nerves serving them. Every movement the body makes requires communication between the brain and the muscles. The nervous system provides the link between thoughts and actions by relaying messages from the brain to other parts of the body.What are the 7 steps of the neuromuscular junction? ›
- 1) An AP travels down the axon. to the axon terminal.
- 2) Electrical gated calcium channels open. ...
- 3) Calcium causes the vesicles to. ...
- 4) ACH diffuses across the synaptic cleft. ...
- 5) ACH binding opens ion channels. ...
- 6) If the muscle reaches the threshold (-55mv) at the motor end plate. ...
- 7) ACH is broken down by.
The neuromuscular junction comprises four cell types: the motor neuron, terminal Schwann cell, skeletal muscle fibre and kranocyte, with the motor neuron and muscle fibre separated by a gap called the synaptic cleft.What is the correct order of the neuromuscular junction? ›
The anatomy of a neuromuscular junction can be divided into three parts: the presynaptic terminal (i.e. the motor neuron) the synaptic cleft. the postsynaptic membrane (i.e. the membrane of the muscle cell).What is the neuromuscular junction structure and function? ›
The neuromuscular junction (NMJ) is a synaptic connection between the terminal end of a motor nerve and a muscle (skeletal/ smooth/ cardiac). It is the site for the transmission of action potential from nerve to the muscle. It is also a site for many diseases and a site of action for many pharmacological drugs.How do you assess neuromuscular function? ›
The most reliable method of measuring neuromuscular function is to stimulate an accessible peripheral motor nerve and measurement of the evoked response of the skeletal muscle or muscles innervated by the stimulated motor nerve.What is neuromuscular transmission summary? ›
The definition of neuromuscular transmission is the information flow from the motor nerve ending to the muscle fibre via the neuromuscular junction. It is the process by which the motor nerve impulses start the contraction of the muscles.What are the 5 steps of muscle contraction in order? ›
- Depolarisation and Calcium Ion Release. An action potential from a motor neuron triggers the release of acetylcholine into the motor end plate. ...
- Actin and Myosin Cross-Bridge Formation. ...
- Sliding Mechanism of Actin and Myosin. ...
- Sarcomere Shortening.
Neuromuscular transmission is dependent on a coordinated mechanism involving (1) synthesis, storage, and release of acetylcholine from the presynaptic motor nerve endings at the neuromuscular junction; (2) binding of acetylcholine to nicotinic receptors on the postsynaptic region of the muscle membrane, with consequent ...What affects the function of a neuromuscular junction? ›
Presynaptic neurotoxins, commonly known as β-neurotoxins, affect the presynaptic regions of the neuromuscular junction. The majority of these neurotoxins act by inhibiting the release of neurotransmitters, such as acetylcholine, into the synapse between neurons.
One muscle fiber has one NMJ, which is innervated by one motor nerve terminal.What is the neuromuscular junction called? ›
neuromuscular junction, also called myoneural junction, site of chemical communication between a nerve fibre and a muscle cell. The neuromuscular junction is analogous to the synapse between two neurons.What are the 3 functions of cell junctions? ›
Many cells in tissues are linked to one another and to the extracellular matrix at specialized contact sites called cell junctions. Cell junctions fall into three functional classes: occluding junctions, anchoring junctions, and communicating junctions.What is neuromuscular junction conclusion? ›
The neuromuscular junction (NMJ) is the specialized site that connects the terminal of a motor neuron axon to skeletal muscle. As a synapse NMJ integrity is essential for transducing motor neuron signals that initiate skeletal muscle contraction.What are the different types of cell junctions and their functions? ›
- occluding junctions (zonula occludens or tight junctions)
- adhering junctions (zonula adherens).
- desmosomes (macula adherens). There are also 'hemidesmosomes' that lie on the basal membrane, to help stick the cells to the underlying basal lamina.
- Gap junctions.
The neuromuscular junction—where nerves and muscle fibers meet—is an essential synapse for muscle contraction and movement. Improper function of these junctions can lead to the development of progressive neuromuscular diseases, some of which have no effective treatment (like Lou Gehrig's disease).What are 3 main functions of the muscular systems? ›
The muscular system is a complex network of muscles vital to the human body. Muscles play a part in everything you do. They control your heartbeat and breathing, help digestion, and allow movement.How do you increase neuromuscular function? ›
Exercise has beneficial effects on the nervous system, including at the neuromuscular junction (NMJ). Exercise causes hypertrophy of NMJs and improves recovery from peripheral nerve injuries, whereas decreased physical activity causes degenerative changes in NMJs.What are the 4 main functions of the muscular system? ›
Four functions of muscle contraction are movement, posture, joint stability, and heat production. Three types of muscle are skeletal, smooth, and cardiac.