We said the first steps of muscle contraction are going to be all about how the muscle fiber receives a signal from the nervous system, and then how it spreads that signal throughout the entire cell. So, to understand how that works, we need to talk about neurotransmitters and action potentials. Now, neurotransmitters and action potentials, we're going to talk about in a lot more detail when we get to the nervous system. But to understand how the muscular system works and to be able to answer some questions about it, you need to know the basics. So, neurotransmitters, they're going to be these chemical messengers that are used at a synapse. And a synapse, we're just going to say, is a really small space between the axon and the muscle. And so, if we look over here, we have the small image here. This is the axon, that end of a neuron, that highly specialized nervous tissue cell, and it comes down, and this is going to be our membrane of the muscle fiber there. And you can see that there's a small space between the two. They don't actually quite touch. So to get a message across that synapse, the neuron dumps these neurotransmitters into the synapse. You can see all these little dots in here. It's dumping those neurotransmitters into the synapse. They're going to diffuse across the synapse, and they're going to bind to receptors in the membrane of the muscle fiber. Now when enough of them bind, that'll stimulate an action potential. Now, before we get to the action potential though, we just need to know that acetylcholine is the neurotransmitter of the neuromuscular junction. In the nervous system, there's all sorts of different neurotransmitters that are used, but there's only one neurotransmitter that is used to stimulate muscle fibers, and that is acetylcholine. Alright. So, we pass the message on to the muscle fiber. Now it's spreading. It's going to spread through an action potential. An action potential is going to be this wave of electric signal that moves along a membrane, and in this case, our membrane is the sarcolemma, or the membrane of the muscle fiber. And to understand how this works, we need to know that muscle fibers are polarized. And by that, we mean that they have a negative charge inside and a positive charge on the outside. Polarized means they have a separation of charge. And an action potential is just going to be this really brief, and by brief, I mean milliseconds change of polarization, and it's going to be caused by the movement of two ions, sodium and potassium. The ions Na+ and K+ ions. And I really suggest that you just remember that sodium is Na+ and K+ is potassium if you don't know that yet. It's a really important ion for anatomy and physiology. Okay. So let's see how this works. We have some illustrations here. We're going to start with our polarized cell here, and this is our muscle fiber at rest. We have all these sodium ions, Na+ ions on the outside of the cell, and we have a lot of potassium ions K+ in high concentration on the inside of the cell. And that results in the cell being positively charged on the outside and negatively charged on the inside. Now one way to remember this, we can say that the cell is swimming in a salty sodium sea. That's something that we're going to say again in the nervous system. Your cells are swimming in a salty sodium sea, and the sodium is positively charged, so you have a positive charge on the outside of the cell. Now what confuses people sometimes a little bit, this potassium, you'll note, also has a positive charge. Don't worry about that too much. We're just talking about net charges. There's a lot of other stuff going on here, and so we really just want to remember the sodium's on the outside, potassium's on the inside, positively charged outside, negatively charged inside. So we said we want to flip that charge. So when we flip it, we're going to look at this image here. We are going to call that depolarization. So, to first depolarize the cell, we said we have a really high concentration of sodium on the outside. It's swimming in a salty sodium sea. So we open the sodium channel, and this high concentration of sodium causes the sodium ions to diffuse into the cell. As the sodium, or as I'll write here, Na+ moves inside the cell, well, it's going to bring along with it its positive charge, and that's going to cause the inside of the cell to become more positive. So, we've now flipped the charge. It's become just a little bit more positive on the inside and negatively charged on the outside. But the action potential, we're going to flip it and then we're going to flip it back. So to flip it back, we're going to talk about repolarization. Well, the sodium came in to make the inside of the cell positive, so now we're going to open the potassium channels. We have a really high concentration of potassium inside the cell, so when you open that potassium channel, that high concentration is going to cause them to diffuse out of the cell. So I’m going to write here the K+ moves outside the cell, and with that, it's going to bring its positive charge with it. It's going to turn the outside of the cell positive and the inside negative, and the charge is restored. Alright. So that flip and flip back of the charge, that's the action potential, and it's going to be passed in this wave down the cell membrane. It's going to go really fast. Fast enough that essentially your entire muscle fiber, which can be inches or, in some cases, longer than a foot long, is going to basically all start contracting at the same time. Alright. Again, we're going to go into all of this in a lot more detail in the nervous system, but for the muscular system, you should be familiar with it at about this level. We'll practice it some more in the example to follow. I'll see you there.
Table of contents
- 1. Introduction to Anatomy & Physiology5h 40m
- What is Anatomy & Physiology?20m
- Levels of Organization13m
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- Introduction to Organ Systems27m
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- 4. Tissues & Histology10h 3m
- Introduction to Tissues & Histology16m
- Introduction to Epithelial Tissue24m
- Characteristics of Epithelial Tissue37m
- Structural Naming of Epithelial Tissue19m
- Simple Epithelial Tissues1h 2m
- Stratified Epithelial Tissues55m
- Identifying Types of Epithelial Tissue32m
- Glandular Epithelial Tissue26m
- Introduction to Connective Tissue36m
- Classes of Connective Tissue8m
- Introduction to Connective Tissue Proper40m
- Connective Tissue Proper: Loose Connective Tissue56m
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- Specialized Connective Tissue: Cartilage44m
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- Introduction to Muscle Tissue7m
- Types of Muscle Tissue45m
- Introduction to Nervous Tissue8m
- Nervous Tissue: The Neuron8m
- 5. Integumentary System2h 20m
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- An Introduction to Bone and Skeletal Tissue18m
- Gross Anatomy of Bone: Compact and Spongy Bone7m
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- Gross Anatomy of Bones - Structure of a Long Bone23m
- Microscopic Anatomy of Bones - Bone Matrix9m
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- Microscopic Anatomy of Bones - Trabeculae9m
- 7. The Skeletal System2h 35m
- 8. Joints2h 17m
- 9. Muscle Tissue2h 33m
- 10. Muscles1h 11m
- 11. Nervous Tissue and Nervous System1h 35m
- 12. The Central Nervous System1h 6m
- 13. The Peripheral Nervous System1h 26m
- Introduction to the Peripheral Nervous System5m
- Organization of Sensory Pathways16m
- Introduction to Sensory Receptors5m
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- Adaptation of Sensory Receptors8m
- Introduction to Reflex Arcs13m
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- 14. The Autonomic Nervous System1h 38m
- 15. The Special Senses2h 41m
- 16. The Endocrine System2h 48m
- 17. The Blood1h 22m
- 18. The Heart1h 42m
- 19. The Blood Vessels3h 35m
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- Introduction to the Immune System10m
- Introduction to Innate Immunity17m
- Introduction to First-Line Defenses5m
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- Introduction to Cells of the Immune System15m
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- Introduction to Cell Communication5m
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- Introduction to T Lymphocytes38m
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- Review of Cytotoxic vs Helper T Cells13m
- Introduction to B Lymphocytes27m
- Antibodies14m
- Classes of Antibodies35m
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- Antibody Class Switching17m
- Affinity Maturation14m
- Primary and Secondary Response of Adaptive Immunity21m
- Immune Tolerance28m
- Regulatory T Cells10m
- Natural Killer Cells16m
- Review of Adaptive Immunity25m
- 22. The Respiratory System3h 20m
- 23. The Digestive System2h 5m
- 24. Metabolism and Nutrition4h 0m
- Essential Amino Acids5m
- Lipid Vitamins19m
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- Introduction to Cellular Respiration22m
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- Cellular Respiration: Glycolysis19m
- Cellular Respiration: Pyruvate Oxidation8m
- Cellular Respiration: Krebs Cycle16m
- Cellular Respiration: Electron Transport Chain14m
- Cellular Respiration: Chemiosmosis7m
- Review of Aerobic Cellular Respiration18m
- Fermentation & Anaerobic Respiration23m
- Gluconeogenesis16m
- Fatty Acid Oxidation20m
- Amino Acid Oxidation17m
- 25. The Urinary System2h 39m
- 26. Fluid and Electrolyte Balance, Acid Base Balance Coming soon
- 27. The Reproductive System2h 5m
- 28. Human Development1h 21m
- 29. Heredity Coming soon
9. Muscle Tissue
Steps of Muscle Contraction
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Steps of Muscle Contraction practice set
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