Our ability to move our body on purpose is created by chemical or electrical communication between two cells of the brain.
The neuromuscular junction is the place in the human brain where the intent to make a movement is translated into action through communication between the neurons and the muscle fibers.
The commands our brain sends out to the muscles in order to make a movement are transmitted from the motor neurons to the chemical synapses of the corresponding muscle fibers.
The chemical synapse, or the gap, between a motor neuron and a muscle fiber is called a neuromuscular junction. This is the point of connection between the two cells where the information gained from the sensory stimuli is chemically transferred.
What Is the Neuromuscular Junction Comprised Of?
The structure of the neuromuscular junction can be divided into three separate elements:
- A presynaptic part (nerve terminal) of the neuromuscular junction
- A postsynaptic part (motor endplate) of the neuromuscular junction
- A synaptic cleft of the neuromuscular junction
Presynaptic part (nerve terminal) of the neuromuscular junction
The presynaptic part of the neuromuscular junction is a myelinated motor neuron that fires (sends away) the neurotransmitter when the action potential enters its nerve terminal. When the action potential reaches the nerve terminal, the electrical signal triggers the opening of calcium channels.
A nerve terminal is the part of the motor neuron which releases the neurotransmitter after being triggered by the axon’s electrical signals.
Postsynaptic part (motor endplate) of the neuromuscular junction
The neuron cell which receives the neurotransmitters after it has been transmitted via the synapse of the myelinated motor neuron is labeled as a postsynaptic part of the neuromuscular junction, i.e. motor endplate.
It establishes the specialized chemical synaptic connection of the motor neuron with the muscle fiber.
Synaptic cleft of the neuromuscular junction
The gap between the motor neuron and the muscle fibers at a chemical synapse is the synaptic cleft of the neuromuscular junction. It functions as a sort of bridge that aids the transport of neurotransmitters from one synapse to another.
This, in turn, helps the continuous transmission of the electrical impulse which carries the information gathered from the sensory stimuli until it reaches a targeted part of the brain.
What Types of Cells Comprise the Neuromuscular Junction?
There are several types of cells that comprise the neuromuscular junction:
- Motor neuron cell
- Terminal Schwann cell
- Skeletal muscle fiber
Motor neuron cell
The motor neuron cells are primarily located in the motor cortex but can be found in the brain stem and the spinal cord.
Their axons (or fibers) transmit both the chemical and electrical impulses from the central nervous system and the spinal cord to the skeletal and smooth muscles, glands, and the skin. In this way, they directly control and regulate the movement and contraction of our muscles.
Terminal Schwann Cell
Terminal Schwann cell (also labeled as Perisynaptic Schwann cell or Teloglia) is a glial cell that covers the surface of the nerve terminal. This type of neuron cell regulates the number of neurotransmitters that are released through the neuromuscular junction of the motor neuron and the muscle fiber.
The terminal Schwan cells also support the formation of the synaptic connections between the neurons cells. For this to happen, there is a reciprocal interaction between both the nerve terminal and the terminal Schwann cell at the synapse.
Skeletal muscle fiber
Skeletal muscle fiber is one of the three major classes of muscle fibers in our body, alongside the cardiac muscle fibers and smooth muscle fibers.
Its tissue is under the direct command of the cerebral cortex and comprises long, rod-like cells that contain large number of nuclei, called skeletal muscle fibers.
These specifically shaped bundles of cells give our muscles their strength. Also, the skeletal muscle fibers help us keep our body’s posture, control inner movement, and generate heat, as a result of the cellular metabolism.
According to the speed of contraction (the speed at which bundles of cells access the energy they need), there are three types of skeletal muscle fibers that can be distinguished:
- Skeletal slow-twitch muscle fibers (Type I – slow oxidative fibers) – This type of skeletal muscle fibers use oxygen in order to trigger muscle contraction. They have a very dense network of capillaries which gives them the characteristic red color.
The skeletal slow-twitch muscle fibers are activated by a contraction of the muscle. Since they provide their own source of energy from the breathing, the skeletal slow-twitch muscle fibers support the long-distance activities, such as marathon running.
Also, they help us maintain our body posture.
- Skeletal fast-twitch muscle fibers (Type IIa – fast oxidative fibers) – This type of skeletal muscle fibers contract with a larger force than the slow-twitch muscle fibers, but endure shorter distances and fatigue faster.
Since they also have a lower supply of oxygen in comparison to Type I, they fatigue more quickly. Their capillary network is somewhat less dense, yet they still maintain their characteristic red color.
- Fast glycolytic skeletal muscle fibers – (Type IIb – fast glycolytic fibers) – This type of skeletal muscle fiber requires greater endurance. It uses anaerobic glycolysis and has a less dense network of capillaries than the first two types.
The main function of the fast glycolytic skeletal muscle fibers is to produce strong contractions of the muscles. This, in turn, is the main reason why they fatigue sooner than the other two types of skeletal muscle fibers.
What Is the Function of the Neuromuscular Junction?
The neuromuscular junction (also labeled as myoneural junction) enables the motor neuron to send the signals to the muscle fiber. In fact, it transforms the action potential of the cell into muscle contractions in order to transmit the received data.
The axons which control the muscle fibers of the skeleton stretch out from the motor cell bodies. Their targets are the muscles. The surface of the muscle fibers is covered by densely positioned neurotransmitter receptors.
By transferring the information received from the sensory stimuli via chemical impulses, the axons of the motor neurons command the muscles to contract in order to produce a movement.
What Is Acetylcholine and What Is Its Function?
The chemical which is found between the synapses of the nerve cells is called acetylcholine. The activation of this neurotransmitter triggers the contraction of the skeletal muscles. That, in turn, activates the production of hormones in some of the glands of the endocrine system.
The role of acetylcholine is to signal the contraction of the muscles. By generating nerve impulses across the neuromuscular junction, it helps the information pass from the motor neuron to the muscle fiber.
Furthermore, this neurotransmitter contributes to the regulation and control of the sleep/wake cycles. It also helps with the translation of the sensory stimuli of pain, and supports both the learning and formation of memory in the cerebral cortex.
Which Toxins Can Affect the Neuromuscular Junction?
There are a number of events that can occur during neuromuscular transmission as an effect of some toxins. The list includes:
- Nerve gasses
- Botulinum toxin
- Tetanus toxin
- Latrotoxin
- Snake venom
How Do Nerve Gasses Affect the Neuromuscular Junction?
Nerve gasses are organic chemicals that interfere with the process of transmission occurring between the neuron cells and the muscle fibers. Due to the fact that they can be easily inhaled, they can cause paralysis of the lung muscle and lead to fatal consequences as a result of suffocation.
How Does the Botulinum Toxin Affect the Neuromuscular Junction?
Botulinum toxin is one of the most lethal neurotoxins, and it’s produced by the Clostridium botulinum bacteria often found in improperly canned or stored foods. It blocks the release of acetylcholine. Without this neurotransmitter, the muscles are unable to contract, which practically leads to paralysis of the affected individual.
However, this neurotoxin has some applications in medicine, as it’s used for relaxation of a target group of muscles in cases where they show dysfunctionality.
How Does the Tetanus Toxin Affect the Neuromuscular Junction?
Tetanus is a neurotoxin that binds to the lower motor neurons of the neuromuscular junction, at the presynaptic membrane.
The tetanus toxin blocks the release of the neurotransmitters that control muscle movement, which can be fatal in some cases. Namely, this can lead to severe problems with breathing and can cause strong muscle spasms.
How Does Latrotoxin Affect the Neuromuscular Junction?
Latrotoxin is a neurotoxin that binds to the nerve receptors of the neuromuscular junction. By producing an excessive amount of acetylcholine in the synapse, it triggers the nerve cells to release their chemical signals at once. This leads to excruciating pain in the head.
This neurotoxin is detected in the venom of the black widow spiders. The back, the shoulders, and the thighs are the first groups of muscles that suffer the strong contraction that is the effect of this neurotoxin.
How Does Snake Venom Affect the Neuromuscular Junction?
Snake venom causes paralysis of the skeletal muscles by blocking the neurotransmission in several parts of the neuromuscular junction.
It can also affect the respiratory function of the affected individual’s lugs, resulting in respiratory paralysis.
Which Disorders Can Compromise the Neuromuscular Junction?
There are some autoimmune and genetic diseases that can disable the normal function of the neuromuscular junction. The list includes:
- Myasthenia gravis
- Neonatal MG
- Lambert-Eaton Myasthenic syndrome
- Neuromyotonia
- Congenital myasthenic syndrome
- Neuromyotonia
- Bulbospinal muscular atrophy
- Duchenne muscular dystrophy
- Congenital myasthenic syndromes
To Sum Up
The neuromuscular junction is a type of synapse where the motor neuron signals pass in order to reach the skeletal muscle fibers. When a cluster of muscle fibers are activated, they make the muscle contract and create a movement.
In this way, the neuromuscular junction becomes the most important element in the neurological process of producing the movements of our body. This is due to the fact that without the contraction of our muscles, we wouldn’t be able to walk or breathe.
Thus, the neuromuscular junction plays one of the most vital roles in the proper functioning of our body, despite its microscopic dimensions.
Fun Facts
Did you know?
- The processes of transmission of sensory information that occur at neuromuscular junctions are so fast, that there is no lag or delay in the movements of our body.
- Slow-twitch muscle fibers contain blood-carrying myoglobin which gives them their distinguishing dark color.
- By training, we can build the right group of muscle fibers in order to achieve the desired athletic shape and improve our performance in sports.
- If the muscles stop contracting, they will gradually decline in their effective performance. The medical condition that occurs as a result of muscle inactivity is called atrophy.
- Slow-twitch fibers can be trained with exercises, such as the front plank, side, plank, or side leg balance.
- Fast-twitch fibers are activated only when the slow-twitch fibers cannot endure the imposed force or activity.
- These fast-oxidative fast-twitch muscle fibers can be trained with exercises of strength and power.
- Unlike the other two types, the fast-twitch fibers are white in color, due to the lesser amount of blood they contain.
- Because they fatigue faster, fast-twitch fibers need a longer period of rest in order to replenish their strength and energy.
- The king cobra (Ophiophagus hannah), black mamba (Dendroaspis polylepis), and the saw-scaled viper (Echis carinatus), are one the most venomous snakes in the world. Their venom contains a super-strong neurotoxin that causes paralysis of the lungs, inhibiting breathing and leading to fatal consequences.
As for the spiders, even though there are more than 43.000 species in the world, only 30 of them are dangerous for humans. Among them are the black widow spider (Latrodectus geometricus), and the Brown Recluse Spider (Loxosceles reclusa).