Skeletal muscle experiments

0:00 If we were doing this experimental with live tissue, this is what the setup would look like. What the camera is showing you is a muscle together with a white nerve that runs to it, held in place in a perspex holder.

0:14 The muscle here is the muscle extending from the knee, down to the heel. Its mainly the Gastrocnemius muscle, and the nerve which is attached to it which controls it is the Sciatic nerve.

0:25 The muscle is held in place because we've dissected it in a way so that the knee has also been removed with it. So at the bottom of this perspex dish is where you can see the knee, and extending up from it is the muscle. That allows us to apply electrodes to the nerve or to the muscle and stimulate it.

0:45 But then, how do we record the responses?

0:47 You will notice that from the top of the muscle is a string (a black thread) extending all the way up to a small silver box. The thread is attaching the muscle to what is known as a Force transducer.

1:01 When the muscle contracts (when we stimulate the nerve and cause the muscle to contract), it's going to pull on that string. Because the string is tied up at both ends, the string length doesn't actually change, but the tension that's generated within that string is detected by the force transducer.

1:24 The electronics within the force transducer, together with an amplifier, allow us to display this contraction (and the pull on the thread) on a computer screen, as the muscle contracting itself.

1:44 And that's what we will show you in the rest of these videos. You will be able to see, on a computer screen, the results of applying electrical stimulation. In our case to the nerve, but in your case we'll be applying it in the virtual prac, as well as in the wet prac, directly to the muscle itself.

2:07 We're now going to apply a single electrical pulse to the nerve. It's going to be applied at a very high voltage, so we can guarantee that the muscle will indeed contract, because we will be activating the nerve fibres.

2:22 We will first show you the muscle contracting, and then show you what the response looks like on the computer screen that's recording the tension generated in the string.

2:35 (muscle contracts)

2:36 And there you'd have seen the muscle contracting.

2:41 Now you've seen the response with the muscle exposed to the air, and this is not something we want to do for any length of time, because of course the muscle will dry out. So in fact, in practice when you do the wet lab, we will ask you to put the muscle into a beaker.

2:57 It's a beaker of solution that's optimal for muscle environments, and you will notice it's being bubbled through with oxygen, to make sure the muscle remains active and alive.

3:07 Okay, so here is the response that you've just seen when the muscle contracted. There are a few features of this that we need to point out. This green line here represents the time zero when the electrical stimulus was applied to the nerve.

3:21 You will notice that when the nerve was stimulated, it actually took about 40 to 50 milliseconds before the muscle contracted. This is known as the Latent period, it's the time taken for the nerves to produce action potentials, for the action potentials to travel down the nerve, to arrive at the terminal, cause the release of transmitter, for the transmitter to bind to the muscle receptors, and so on and so forth.

3:48 So there is a period or a delay between the nerve being activated, and the muscles starting to contract.

3:54 The next thing you'll notice is that the muscle contraction takes place over at least 50 to 60 milliseconds to reach the peak. Here's the point at which it starts, it keeps increasing for about 50 milliseconds and then it declines. It slowly recovers, and then the whole thing is over in about roughly 200 milliseconds.

4:18 This is the reason why summation can occur. This is a single muscle twitch. If you were to apply a second stimulus at about this point, somewhere while that first twitch was occurring, you would get a second twitch, which could sum on to the first twitch. This has got to do a lot with the availability of calcium, as you would recall from the lectures.