Muscle structure – muscle under the microscope

Does all muscle look the same? If you were to look at skeletal, smooth and cardiac muscle using a microscope, you would see differences in their structure.

Skeletal muscle

Skeletal muscle looks striped or "striated" – the fibres contain alternating light and dark bands (striations) like horizontal stripes on a rugby shirt. In skeletal muscle, the fibres are packed into regular parallel bundles.

Cardiac muscle

Cardiac muscle tissue, like skeletal muscle tissue, looks striated or striped. The bundles are branched, like a tree, but connected at both ends. Unlike skeletal muscle tissue, the contraction of cardiac muscle tissue is usually not under conscious control , so it is called involuntary.

Smooth muscle

Compared to skeletal muscle, smooth muscle cells are small. They are spindle shaped and have no striations. Instead, they have bundles of thin and thick filaments.

Looking closer at skeletal muscle

All skeletal muscle fibres are not the same. They differ in structure and function, for example, in the speed they can contract.

Skeletal muscle fibres contract at different speeds depending on:

• their ability to split ATP (an energy-releasing chemical)
• the way they produce ATP
• how quickly they get tired

So skeletal muscle is classified into two broad types – fast twitch and slow twitch.

Slow twitch (also called Type I):

• has lots of tiny blood vessels called capillaries (and so looks red)
• has many mitochondria (sites of energy production)
• has lots of myoglobin (the oxygen transporting and storage protein of the muscle)
• carries more oxygen
• doesn’t get tired easily (can sustain aerobic activity)
• can contract slowly
• is found in large numbers in the postural muscles of the neck

Nature of science

Have you noticed that when you look at something under a microscope it can be very confusing, but once you look at a reference diagram or picture, you can see a lot more detail under the microscope? When scientists observe, they already have some understanding of what they are looking at. Their observations are influenced by their experience, knowledge and understanding of existing theories.

Fast twitch (also called Type II):

Type IIa

• is aerobic like slow muscle
• is rich in capillaries
• looks red

Type IIx

• has fewer mitochondria and less myoglobin
• can contract more quickly than Type IIa
• can contract with more force than aerobic muscle
• can sustain only short, anaerobic bursts of activity before muscle contraction
• becomes painful (e.g. getting the stitch)
• is the fastest muscle type in humans

Type IIb

• is anaerobic white muscle
• is even less dense in mitochondria and myoglobin
• can contract even more quickly
• is the major fast muscle type in small animals like rodents or rabbits (which explains why their meat is so pale)

Looking inside a muscle cell

Skeletal muscles are made up of hundreds of thousands of muscle cells (also called muscle fibres). These muscle cells act together to perform the functions of the specific muscle they are part of.

Unlike other tissue, skeletal muscle cells contain myofibrils – these are shaped like long cylinders and extend along the full length of the muscle fibre/cell.

Each myofibril consists of two types of protein filaments called thick filaments and thin filaments. The thick filaments and the thin filaments within myofibrils overlap, and the sections where they overlap and occur together are called sarcomeres. When muscle contraction occurs, the thin filaments and the thick filaments slide past each other.

Nature of science

Scientists make observations - and develop their explanations using inference, imagination and creativity. Often they use 'models' to help other scientists understand their theories. A diagram is an example of an explanatory model. These diagrams demonstrate the creativity required by scientists to use their observations to develop models and to communicate their explanations to others.