The Neuromuscular Junction: Function, Structure & Physiology
- Track Progress
- 0:07 Synaptic Transmission
- 1:24 Anatomy of a Synapse
- 2:18 Physiology of a Synapse
- 4:47 Neuromuscular Disease
- 5:38 Lesson Summary
A neuromuscular junction is a synapse between a motor neuron and skeletal muscle. This lesson describes the events of synaptic transmission leading to contraction of skeletal muscle. Myasthenia gravis is described as a neuromuscular disease.
In order for skeletal muscle to contract, it must first be stimulated by a motor neuron. The space between the motor neuron and the skeletal muscle cell is simply referred to as a synapse. More specifically, the synapse between a motor neuron and a skeletal muscle cell is referred to as a myoneural or neuromuscular junction. If you break down these terms, the names will make better sense. 'Myo'- means muscle, and 'neuro'- refers to nerves. Regardless of the name, the synapse is a real space across which the excitatory impulse must travel before the muscle contracts.
Synaptic transmission includes all the events within the synapse leading to excitation of the muscle. Let me make a quick note that other synapses occurs between other cells - for example, nerve to nerve and nerve to gland. For example, the adult human brain is thought to contain 100-500 trillion synaptic connections, and those are in between neurons. In this lesson, we will describe the anatomy of a neuromuscular junction and then discuss the events of synaptic transmission.
Anatomy of a Synapse
The image you see on the screen illustrates a synapse between a neuron and a muscle cell. The neuron is sending the transmission and is thus referred to as the pre-synaptic cell, while the muscle is receiving the transmission and is referred to as the post-synaptic cell. Neurotransmitters are molecules stored in the pre-synaptic cell that are secreted into the synapse. Neurotransmitters, in turn, bind to receptors on the post-synaptic cell membrane, and these receptors are specific for that neurotransmitter.
Think of the neurotransmitter-receptor relationship as a lock and a key, where only one key will fit that lock. The synaptic cleft refers to the space between the two cells and is only about 20 nanometers wide. That's pretty thin!
Physiology of a Synapse
Now that we know what makes up a synapse, we're ready to describe the function of the neuromuscular junction. It's important to keep the big picture in mind. So what is that big picture? Synaptic transmission carries the excitatory signal from the neuron to the muscle cell, much like a bridge could connect two land masses.
Let's start with the motor neuron. Calcium enters the excited neuron, and the calcium stimulates exocytosis of the neurotransmitter. Where does the neurotransmitter go but into the synaptic cleft. The neurotransmitter secreted by the somatic motor neurons is acetylcholine.
So the acetylcholine diffuses across the cleft and binds to acetylcholine receptors within the muscle cell membrane. Like a key unlocking a door, acetylcholine opens ion channels, and sodium ions diffuse into the muscle cell. It's important to note that acetylcholine does not remain in the synaptic cleft forever, but rather an enzyme called acetylcholinesterase catalyzes the breakdown of acetylcholine, and where is it located - in the synaptic cleft. This enzyme breaks down acetylcholine and therefore prevents overcontraction, or prevents contraction for lasting longer than necessary.
Since sodium is a positive ion, it depolarizes and thus excites the skeletal muscle cell membrane as it enters. Now the excitatory impulse has transferred from the motor neuron to the muscle cell, much like a car would cross a bridge from one land mass to the next.
Invaginations of the cell membrane referred to as T-tubules will then carry that excitatory impulse deep into the muscle cell's interior. The excitatory impulse then triggers release of calcium into the cell's interior coming from the sarcoplasmic reticulum or simply the SR. Recall that calcium triggered secretion of acetylcholine in the motor neuron. What's going on here? In the muscle, calcium will trigger contraction of the muscle cell.
Neuromuscular disorders can result from problems occurring at the neuromuscular junction. For example, myasthenia gravis is an autoimmune disorder in which muscle weakness is caused by antibodies that target the acetylcholine receptor on the skeletal muscles. Those antibodies will target the receptor, and then the receptor is destroyed by own our immune system (that's why it's called an autoimmune disorder). Without sufficient receptors for acetylcholine, neurotransmission is slowed and contraction is inhibited. Now there is no current cure for this disease; however, acetylcholinesterase inhibitors, such as neostigmine, can improve muscular function by at least slowing the degradation of acetylcholine in the synapse.
In summary, synaptic transmissions carry excitatory impulses from motor neurons to skeletal muscle cells. Calcium triggers the release of acetylcholine from the neuron. Acetylcholine then crosses the synapse and binds to acetylcholine receptors on the muscle cell membrane. Like a key in a lock, acetylcholine opens sodium ion channels that depolarize and excite the muscle membrane. The excitatory impulse is then carried to the sarcoplasmic reticulum along T-tubules. Calcium is released by the sarcoplasmic reticulum, and that calcium triggers muscular contraction. Myasthenia gravis is a neuromuscular disease resulting from destruction of those acetylcholine receptors and thus inhibiting neurotransmission.
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