Not all muscles are the same. In this article we will look at the three types of muscle: skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle
Skeletal muscle is under conscious control, so is sometimes called voluntary muscle. There are two types of skeletal muscle fibres: fast twitch and slow twitch. Fast twitch fibres contract quickly so are used for short bursts of speed and strength. Their main source of ATP is from anaerobic respiration. Slow twitch fibres contract slowly and are used for endurance activities and maintaining posture. Their main source of ATP is from aerobic respiration. Each of these muscle fibre types have different adaptations:
| Slow twitch | Fast twitch |
|---|---|
| Red in colour due to containing lots of myoglobin (a red protein which stores oxygen) | White in colour due to a much smaller amount of myoglobin |
| Contain many mitochondria for aerobic respiration | Only a few mitochondria |
| Many capillaries nearby to increase supply of oxygen for aerobic respiration | Less capillaries |
| Do not fatigue quickly as there is little lactate production | Fatigue quickly due to build up of lactate from anaerobic respiration |
Skeletal muscles are made up of different proportions of these two types of muscle fibres.
Under a microscope, skeletal muscle appears striated (stripy) if it is stained. This is due to the different bands in the sarcomere. Take a look here for a microscope image.
Cardiac muscle
Cardiac muscle is myogenic, meaning it contracts and relaxes without external stimulation. It is only found in the walls of the heart. The muscle fibre cells form a characteristic pattern as shown below. The cells are connected with intercalated discs – these allow electrical activity to be conducted between cells. Branching helps the electrical activity to spread throughout the muscle tissue. Under a microscope the muscle fibres appear slightly striated, but not as clearly as skeletal muscle. Thankfully, cardiac muscle does not fatigue!
Smooth muscle
Smooth muscle is found in the walls of hollow organs such as the blood vessels, the intestines, and the bronchioles. It is under unconscious control, so is sometimes called involuntary muscle. The smooth muscle fibre cells do not have the striated appearance, which is where they get their name. Smooth muscle does not fatigue, but contracts and relaxes to perform its function e.g. moving food along the lumen of the intestines.
Summary
- Skeletal muscle consists of fast twitch and slow twitch fibres. It is striated, and is voluntary.
- Cardiac muscle is only found in the heart. Electrical activity can spread through the muscle easily due to intercalated discs and branching.
- Smooth muscle is involuntary. It is found in the walls of hollow organs, and is not striated.
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