We said that the first major step of muscle contraction is that the skeletal muscle fiber needs to receive a signal from the nervous system, and then it has to spread that signal throughout the muscle fiber. Receiving that signal from the nervous system, we're going to call the events at the neuromuscular junction. To remind ourselves that this is the first of 3 major steps, we have it labeled here as a. So the neuromuscular junction, that's the connection between the nervous system and the motor end plate of the muscle fiber. Now remember the cell membrane of the muscle fiber we call the sarcolemma, and there's going to be one small region of the sarcolemma that's going to be specialized for receiving this signal from the neuron.

That specialized region is going to be the motor end plate. Alright. Now remember, these don't actually connect to each other, they have this real small space between each other that we call the synapse. So to get this signal across the synapse, we need to use a neurotransmitter, and the neurotransmitter used for muscles is acetylcholine, or as we're going to write here, ACh. So I'm going to write ACh going forward just because it's easier, but whenever you see ACh, just know that means acetylcholine.

It's the neurotransmitter used at the neuromuscular junction. Alright. So let's go through this step by step. First up, the action potential is going to arrive at the axon terminal. Alright.

The axon is the extension of the neuron, that highly specialized nervous tissue cell, and neurons send these electrical messages using action potentials, that flipping of the charge, using the sodium and potassium ions. So this action potential is going to arrive at the terminal, and we can see here in yellow is our axon terminal, and we see this action potential coming in with those arrows there. Now the word terminal just means the end of something, but the way I remember that this yellow structure here is called the terminal, is that if I get off a train, I get off at a train terminal. How do we get the message off of the axon? The message gets off the axon at the axon terminal.

Alright, so we've gotten this electrical signal down into the terminal. What that's going to do is cause voltage-gated calcium channels to open, and we can see that here in our image. We have the calcium on the outside here, and we can see these channels here. And when that action potential comes in, that's going to open the channels, and the calcium is going to flow into the axon terminal. Now you may remember in the sarcomere, calcium entering the sarcomere is going to cause the sarcomere to start contracting.

So, in both cases, an action potential is going to stimulate the release of calcium, and the release of these calcium ions is going to start the process we're talking about. In this case, the release of the calcium ion as they enter the axon is going to release that acetylcholine, ACh. It's going to release it into the synapse. So you can see here in our illustration, we have all these vesicles, and in these vesicles, you see these little sort of blue dots. That's the acetylcholine.

This calcium enters the axon terminal and these vesicles sort of dump by exocytosis this acetylcholine into the synapse there. That acetylcholine is just going to sort of diffuse across the synapse. It's going to diffuse across the synaptic cleft, that's just another word for the synapse, and it's going to bind to the receptors in the sarcolemma. And we can see that here. This sort of pinkish membrane here is our sarcolemma.

It's sort of this wavy membrane at this neuromuscular junction. And you can see here we have these receptors that are going to bind to the acetylcholine. Now when the acetylcholine binds to the receptors, the sodium ion channels are going to open in the sarcolemma. Alright. So that binding of the neurotransmitter acetylcholine causes sodium ion channels to open.

Now you remember, opening sodium ion channels, that's how we start an action potential. So the action potential is going to start. Those sodium ions are going to enter the sarcolemma. They're going to bring their positive charge with it. That's going to depolarize the membrane.

We're then going to have the potassium ions leave, that'll repolarize, and that's just going to happen like a switch flipping back and forth, going down like a wave down the membrane. Alright. So I'll just show here that's going to cause this action potential to go out and spread in both directions down the muscle fiber and that's going to be the signal in the muscle fiber to start contracting. Okay. But we have this acetylcholine now in the membrane, and we got to get rid of it because if this acetylcholine just keeps binding to these receptors here well, if it keeps binding then the muscle is just going to keep getting action potentials and it's just going to keep contracting.

So there are 2 ways that we get rid of the acetylcholine. First off, the acetylcholine can just sort of diffuse out of the synapse, and some acetylcholine will do that. But more importantly, the acetylcholine is going to be broken down by an enzyme, and that enzyme is Acetylcholinesterase. Acetylcholinesterase is going to be in the synapse and it's going to just be breaking down acetylcholine into acetic acid and choline, and when that happens, when that acetylcholine is gone, the signal stops. Now that choline actually gets taken back up by the axon terminal.

It gets recycled into more acetylcholine so that we can do this whole process again. But for our purposes, we started an action potential. That was our goal. Our membrane's now excited, and now we have to figure out how do we couple that excitation to the contraction. That's what we'll be talking about next, but first, we have an example in practice problems.

Give them a try.