Muscle Tissue: The Core Concepts
Muscle Tissue: The Core Concepts
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Your body has three kinds of muscles:
Skeletal muscle – attached to bones; you control it voluntarily.
Cardiac muscle – only in the heart; works automatically.
Smooth muscle – in organs and blood vessels; also automatic.
Even though they look different, all three types share important traits:
What All Muscles Can Do
What All Muscles Can Do
1. Excitability: Muscle cells can respond to electrical signals. Their membranes can switch from negative (polarized) to less negative (depolarized) and send an action potential, a tiny electrical wave, down the cell.
1. Excitability: Muscle cells can respond to electrical signals. Their membranes can switch from negative (polarized) to less negative (depolarized) and send an action potential, a tiny electrical wave, down the cell.
2. Contractility: They can shorten and pull with force.
2. Contractility: They can shorten and pull with force.
3. Extensibility: They can stretch without being damaged.
3. Extensibility: They can stretch without being damaged.
4. Elasticity: They can return to their original shape after stretching.
4. Elasticity: They can return to their original shape after stretching.
How Muscles Actually Contract
How Muscles Actually Contract
All muscle contraction comes down to two proteins: actin (thin filament) and myosin (thick filament). Myosin “grabs” actin and pulls it, which makes the muscle shorter (contraction).
Contractions are triggered by calcium ions (Ca++).
In skeletal & cardiac muscle: Calcium moves special proteins (troponin and tropomyosin) out of the way so myosin can attach to actin.
In skeletal & cardiac muscle: Calcium moves special proteins (troponin and tropomyosin) out of the way so myosin can attach to actin.
In smooth muscle: Calcium starts a chemical reaction that activates myosin heads differently.
In smooth muscle: Calcium starts a chemical reaction that activates myosin heads differently.
All muscles require adenosine triphosphate (ATP) to continue the process of contracting, and they all relax when the Ca++ is removed and the actin-binding sites are re-shielded.
Skeletal Muscle
Skeletal Muscle
Skeletal muscle has visible stripes, called striations, because its actin and myosin proteins are arranged in a very organized pattern. Its cells are long and tube-shaped, and each one contains many nuclei.
Skeletal muscle can only contract when it receives signals from the nervous system. It helps you move your body, maintain posture, protect certain organs, and control voluntary openings such as those used for swallowing, urination, and defecation. Skeletal muscle also produces heat, which is why your body warms up during exercise or when you shiver in the cold.
Cardiac Muscle
Cardiac Muscle
Cardiac muscle is also striated, giving it a striped appearance similar to skeletal muscle.
Its cells typically contain one or two nuclei.
Cardiac muscle cells are electrically connected to each other, which allows the entire heart to contract as one coordinated unit, called a syncytium.
This type of muscle can respond not only to signals from nerves but also to hormones and its own internal electrical activity.
Smooth Muscle
Smooth Muscle
Smooth muscle does not have striations because its actin and myosin proteins are arranged in a less organized, more random pattern.
Each smooth muscle cell contains only one nucleus.
This muscle type is found in the walls of blood vessels, the stomach, intestines, bladder, and many other internal organs.
Smooth muscle controls the movement of food through the digestive system, helps regulate blood pressure by affecting blood vessel diameter, and moves materials through various internal passageways.
References
References
J Gordon Betts, Desaix, P., Johnson, E., Johnson, J. E., Korol, O., Kruse, D., Poe, B., Wise, J., Womble, M. D., & Young, K. A. (2013). Anatomy & physiology. Openstax College, Rice University. https://openstax.org/details/books/anatomy-and-physiology
Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.
Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.
Content paraphrased; adaptations were made.
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