Neuromuscular junction and motor unit

Definitions & Key takeaways

The neuromuscular junction is a site where a motor neuron meets a skeletal muscle fiber. It is where a nerve impulse is transmitted from a motor neuron to a muscle fiber. A neuromuscular junction is made up of a nerve terminal, which contains the neurotransmitter acetylcholine, and a muscle fiber, which has receptors for acetylcholine.

When an action potential reaches the nerve terminal, it causes the release of acetylcholine, which diffuses across the synapse and binds to receptors on the muscle fiber. This activates an ion channel, allowing ions to flow into the muscle fiber and causing it to depolarize. This depolarization triggers the release of calcium ions from intracellular stores, which initiates the contraction of the muscle.

Chapters:

Watch video only

Introduction0:00–0:35

In order for a skeletal muscle to contract, your brain sends a signal, in the form of an action potential in an upper motor neuron.
The upper motor neuron is part of the cerebral cortex, and it activates a lower motor neuron, which is located in the anterior horn of the spinal cord.
From here, the action potential is sent through an axon down to its ending branches, called axon terminals, to muscle fibers which they innervate.
The place where an axon terminal meets the muscle fiber is the neuromuscular junction. The neuromuscular junction has three main parts: a presynaptic membrane, which is the membrane of an axon terminal; a postsynaptic membrane, which is the membrane of a skeletal muscle fiber and is also called a motor end-plate; and a synaptic cleft, which is the gap between the presynaptic and postsynaptic membrane.

Neuromuscular Junction0:35–0:56

Action potential0:56–3:39

When an action potential reaches the axon terminal, it stimulates voltage-gated calcium channels in the membrane to open and extracellular calcium ions flow into the lower motor neuron.
Inside the axon terminal are synaptic vesicles that contain neurotransmitters called acetylcholine. The calcium that enters the axon terminal binds to the vesicles, which allows them to fuse with the cell membrane of the axon terminal, releasing the acetylcholine into the synaptic cleft.
The acetylcholine then diffuses over to the motor end plate on the muscle fiber - and because it’s a short distance, that diffusion happens really quickly.
Here, two acetylcholine molecules will bind to one ligand-gated ion channel, also called nicotinic receptor. When that happens, these ligand-gated ion channels, which are selective for positively charged ions, open up.
When they open, lots of sodium ions rush into the skeletal muscle fiber, and a few potassium ions leak out of the cell. But overall there’s an increase in positive charge on the inside of the muscle fiber and therefore on the inside of the membrane, relative to the outside of the membrane - and this is called depolarization.
This local depolarization on the postsynaptic membrane is called an end-plate potential and it makes the resting potential of the cell membrane which is usually around -100mV, more positive, or in this case, less negative.
The local depolarization causes the cell membrane’s resting potential to rise up to about -60mV, and that’s the threshold for voltage gated sodium ion channels.
So when the threshold potential is reached, lots of voltage gated sodium ion channels open up, and there’s a huge influx of sodium ions into the muscle fiber.
This leads to a generation of an action potential which rapidly spreads along the entire membrane and that gets the entire muscle fiber to contract.
When the signal sent from the lower motor neuron stops, voltage-gated calcium channels on the presynaptic membrane close and influx of calcium ions stops.
This causes synaptic vesicles full of acetylcholine to stop fusing with the membrane and instead they remain within the cytoplasm of the axon terminal.
Molecules of acetylcholine that are left behind within the synaptic cleft, are degraded by an enzyme called acetylcholinesterase into choline and acetate.
The acetate diffuses out from the synaptic cleft, while choline is taken back into the axon terminal where it’s reused by the enzyme acetylcholine transferase to make new acetylcholine molecules.

Motor Unit3:39–4:37

Now, each skeletal muscle fiber has only one neuromuscular junction, so that means that each muscle cell is controlled by one lower motor neuron.
However, the axon of a lower motor neuron splits into many branches and each of these branches can innervate multiple adjacent muscle fibers.
This lower motor neuron and all the muscle fibers it innervates form a single “motor unit,” and on average, one motor neuron innervates 150 skeletal muscle fibers.
The number of skeletal muscle fibers that are innervated by one motor neuron depends on the precision of the muscle they control.
For example, muscles that control precise eye movements, consist of many relatively small motor units, where every lower motor neuron innervates only 10 to 15 muscle fibers.
On the other hand, large muscles like the biceps brachii muscle found in your upper arm, can have up to 2000 muscle fibers within a single motor unit.

Review4:37–5:03

Alright, as a quick recap. The neuromuscular junction is a site where a motor neuron meets a skeletal muscle fiber.
When a lower motor neuron is activated, there’s an influx of calcium ions into a presynaptic membrane, followed by release of acetylcholine into the synaptic cleft.
Acetylcholine binds to nicotinic receptors, and that leads to generation of an action potential and eventual muscle contraction.