Muscular tissue: skeletal, smooth and cardiac

The tissue that pulls

Think of a rope that can shorten itself. That is a muscle fibre. Muscular tissue is made of long cells, called fibres, that can contract and shorten. Most of the movement in the body comes from them: walking, breathing, the beat of the heart and the push of food along the gut.

Look at your arm. When you bend it, the biceps muscle shortens and pulls on a bone. When you eat, nobody tells your stomach to contract, yet it does. These two examples show two groups of muscle. One group is under your will, and the other is not.

• A muscle under the control of your will is voluntary.
• A muscle that works without your will is involuntary.

There are three kinds of muscle tissue.

Skeletal muscle

• It is closely attached to the skeletal bones, for example the biceps.
• Its fibres are long, cylindrical and unbranched. Each fibre has many nuclei (it is multinucleate), placed at the edge.
• It looks striated, which means striped, under a microscope. The stripes are the bands of the contractile proteins.
• It is voluntary.

Smooth muscle

• It lies in the walls of internal organs such as the blood vessels, the stomach and the intestine.
• Its cells are spindle shaped, which means wide in the middle and tapered at the ends. Each has one nucleus in the centre.
• It has no stripes. The word smooth means no striations.
• Cell junctions hold the cells together, and bundles of cells lie in a sheath of connective tissue.
• It is involuntary. It contracts slowly and can stay contracted for long.

Cardiac muscle

• It forms the wall of the heart.
• Its cells are striated like skeletal muscle, but they are branched, and each has one nucleus near the centre. (Some books say one or two.)
• Where the ends of two cells meet, there are dark lines called intercalated discs. Cell junctions fuse the membranes of the cells here, and gap junctions let a signal pass from cell to cell. So the cells contract as a unit.
• It is involuntary. It contracts again and again for a whole life without a rest.

A table to remember

• Place: skeletal muscle is attached to bones. Smooth muscle is in the walls of blood vessels, the stomach and the intestine. Cardiac muscle is in the wall of the heart.
• Stripes: skeletal yes, smooth no, cardiac yes.
• Cell shape: skeletal is long, cylindrical and unbranched. Smooth is spindle shaped. Cardiac is branched.
• Nuclei: skeletal has many in each fibre. Smooth has one. Cardiac has one.
• Control: skeletal is voluntary. Smooth and cardiac are involuntary.

Cardiac muscle is the only one that is striated and involuntary. This is a favourite question.

The work of the heart

The heart must never stop, so its muscle never rests. In a resting adult the heart beats about $72$ times in a minute and pumps about $70\ \text{mL}$ in each beat. The volume of one beat is the stroke volume.

The blood pumped in a minute is the cardiac output. At rest it is about $5\ \text{L}$. In exercise the rate and the stroke volume both rise.

Try it in Python

The program stores the three kinds of muscle with their features and asks a question.

Practise with new numbers

Common mistakes

• Saying that all striated muscle is voluntary. Cardiac muscle is striated and involuntary.
• Saying that smooth muscle has many nuclei. It has one. Skeletal fibres have many.
• Saying that cardiac cells are unbranched. They are branched.
• Thinking that involuntary means that a muscle is always weak. It only means that your will does not control it.
• Mixing up stroke volume and cardiac output. The first is for one beat, and the second is for one minute.

Remember these

• Skeletal: bones, striated, many nuclei, voluntary. Smooth: organ walls, no stripes, one nucleus, involuntary. Cardiac: heart, striated, branched, intercalated discs, involuntary.
• Cardiac output is the heart rate times the stroke volume.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the three kinds of muscle tissue.

2. Which muscle is striated and also involuntary?

3. What is the shape of a smooth muscle cell, and how many nuclei does it have?

4. What are intercalated discs, and what do they let the heart do?

5. A heart pumps 80 mL in each beat and beats 60 times in a minute. How much blood does it pump in a minute?

References

• OpenStax Biology 2e, chapter 33, The Animal Body: Basic Form and Function, section 33.2 Animal Primary Tissues.
• NCERT (2023). Biology, Textbook for Class XI, chapter 7, Structural Organisation in Animals.