Excitation-contraction Coupling and Neuromuscular Junction (Nursing)

00:00 If you recall from earlier in the lecture, the muscle tissue is an excitable tissue.

00:07 So the way that a muscle contraction occurs starts with the excitability or the depolarization of the sarcolemma of the muscle fiber.

00:20 This depolarization leads to the opening of voltage-gated calcium channels that are going to be found in the transverse tubules or the T tubules.

00:32 From here, this allows an influx of calcium from the sarcoplasmic reticulum into the sarcoplasm of the muscle fiber.

00:43 This is the same calcium that will then bind to the troponin and start the contraction cycle.

00:50 Now what goes up must come down.

00:53 So how do we turn this off? Well, there is also an ATPase that is found inside of the membrane of the sarcoplasmic reticulum.

01:03 This is going to remove the calcium from the sarcoplasm back into the sarcoplasmic reticulum.

01:11 As the concentration of calcium decreases in the sarcoplasm, then the muscle contraction no longer is able to occur.

01:21 At the same time repolarization also occurs at the membrane which closes the calcium-gated channels.

01:33 So, if we think about the way that the sliding filament mechanism works, the force of a muscle contraction will be dependent on the length of the sarcomere in a muscle prior to the contraction occurring.

01:47 So there will be an optimal length where you have the optimal or the most amount of potential cross bridges that are able to be formed.

01:57 If it is too small, this decreases that number of potential cross bridges.

02:04 Also, overstretching can also decrease the potential for muscle tension, as we now decrease the potential for crossfitters to form between the thick and the thin filament.

02:16 This is displayed here in this chart.

02:20 So we've just discussed how the excitation contraction cycle happens.

02:26 Now, let's talk a little bit about what exactly starts this excitation process? The excitation of the muscles occur at the neuromuscular junction.

02:38 The way this happens is there are voltage-gated calcium channels at the axon terminal end of the neuron.

02:46 The synaptic bulbs in the neuron are going to then open in response to an electrical impulse being sent down the neuron, the other type of excitable cell in the body.

02:58 This results in an influx of calcium into the synaptic in bulb.

03:04 Once this happens exocytosis of the neurotransmitter, in this case acetylcholine is going to enter into the synaptic cleft.

03:15 The synaptic cleft is the space between where an axon of a neuron ends and the muscle fiber begins.

03:25 This neurotransmitter or acetylcholine is then going to bind to ligand-gated sodium channels, which are going to be found on the muscle side of the cleft.

03:37 This is referred to as the motor end-plate.

03:41 Once this happens, this causes an influx of sodium into the muscle fiber.

03:48 This influx of sodium then depolarizes or excite the muscle and this results then, in the opening of the voltage-gated calcium channels associated with the sarcoplasmic reticulum and the transverse tubules that we discussed before.

04:08 So as I said in previous slides, what goes up must come down.

04:13 So how do we get rid of the neurotransmitter that is in the synaptic cleft? Well, there's an enzyme referred to as acetylcholinesterase, which is going to go in and breakdown the acetylcholine in the cleft.

04:29 Once the concentration of acetylcholine goes down enough that there's not enough to bind to the receptors on the motor end-plate we now are going to be able to stop the muscle contraction.

04:44 So, let's put it all together.

04:47 We're going to start at the neuromuscular junction and end with a contraction.

04:52 We start when acetylcholine is released from the axon terminal of the neuron.

04:58 This acetylcholine is going to crossover the synaptic cleft and bind to the receptors on the muscle fiber.

05:05 From here, we are going to now depolarize the muscle, which excites the muscle and opens voltage-gated channels on the muscle side, which causes an influx of calcium into the sarcoplasm of the muscle fiber.

05:21 This influx of calcium is then going to trigger the contraction cycle and we're going to trigger a contraction.

05:29 From there, the calcium is going to be return to the sarcoplasmic reticulum using a calcium ATPase.

05:37 And as well, acetylcholinesterase is going to be removing acetylcholine from the synaptic cleft.

05:43 Both of these two events will then lead to the ability of the muscle to relax or the end of a contraction.