Anatomy of the NMJ

Anatomy of the NMJ

The neuromuscular junctions are places on muscle fibers where a motor neuron comes almost into contact with the muscle fibers. That’s the picture that you see here. The blue structure is a neuron. We can imagine that the structure is either in your brain or your spinal cordThe central nervous system structure that relays signals between the brain and body.. As this neuron travels through the epimysium, it branches out. It reaches into all of these little dead ends. Each little dead end is called a neuromuscular junctionThe connection between a motor neuron and a muscle fiber..

It is important to realize that there can be hundreds of neuromuscular junctions in one muscle. There are muscle fibers that have more than one neuromuscular junction. The muscle and the neuron do not touch. They are separated by a very small space called a synapseThe junction between two neurons where communication occurs.. In order for the muscle to contract events must take place here at the neuromuscular junction that excite the muscle.

The neuron will have an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. This brings information from the brain to the muscle. The neuron will pass that action potential to the muscle, exciting it. I can’t stress enough how this word excitation now means something very specific to you. It even has a location at these neuromuscular junctions. Students go to take the quiz and find questions about excitation. They wonder if they even talked about that. This is because the term excitation is so familiar, but it takes on a very specific meaning here.

The neuron does not touch the muscle fiber. Electricity, like an action potential, cannot travel through open space. Molecules flow from the neuron to the muscle fiber. These moleculesGroups of atoms bonded together. carry the message of excitation. Molecules that are capable of traveling across synapses between neuronsThe functional cells of the nervous system that transmit signals. and muscle or between neurons are called neurotransmittersChemicals released by neurons to transmit signals across a synapse.. I encourage you to use this abbreviation NXT in your notes to stand for neurotransmitters. Here at the neuromuscular junction there is only one neurotransmitterChemicals that transmit signals across synapses. and its name is acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. and we abbreviate it ACh.  That is correct that the A and the C are capitalized and the H is lower case.

Anatomy of the NMJ

We’ve zoomed in on the muscle fiber where it comes very close but does not touch a neuron. This big yellow thing is coming down from the top of the picture. It is the tip or the dead end of that motor neuron. We call it a motor neuron because it is carrying information to a muscle that will cause motion. The dead end of any neuron is called an Axon terminal. This structure here is the Axon terminal of a motor neuron.

It is shoved full of vesicles of acetylcholine.  There are also these voltage gated calcium channelsProtein passages in the cell membrane that allow specific molecules to pass through. on the sides of the Axon terminal. We covered cell membrane proteinsLarge molecules made of amino acids with various functions in the body. in chapter 3. If these are voltage gated channels that means that they open with a change in voltage. That is what an action potential is. An action potential is a moving change in voltage from the brain to the end of a neuron. It excites the muscle across the synapse. These are voltage gated calcium channels. When they open, only calcium ionsCharged atoms or molecules. can move into them.

Anatomy of the NMJ

Here is the muscle fiber. The place that comes into contact with the neuron is called the motor end plateThe part of the muscle fiber membrane involved in neuromuscular transmission.. This area on the sarcolemma has these folds or WAVY lines here called junctional folds. These folds increase the surface area of the sarcolemma at the motor end plate. This allows faster transmission of an action potential from a neuron to the muscle fiber. Shoved into the sarcolemma at the motor end plate are ligand gated sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. potassium(K⁺): Major ICF cation; essential for muscle and nerve function. channels.

Recall from chapter 3 that ligand gated channels open up when a physical molecule makes a connection with the channel. The ligand that opens up the channel or the key that opens up the door in this instance is acetylcholine. Acetylcholine does not actually enter into a muscle fiber. It just connects with the receptorA structure that detects stimuli.. This connection opens up the ligand gated channel. When this channel opens up two events happen sodium diffuses into the muscle fiber. Then potassium diffuses out of the muscle fiber. It’s similar to when people exit an elevator. You have to let everyone off so others can enter. The same thing happens. Sodium diffuses first. Then it lets potassium go.