Muscles: MDCAT Biology notes
Muscles MDCAT notes: skeletal, cardiac and smooth muscle compared, striations, nuclei, control, intercalated discs, myogenic heart, flexors and extensors.
Three types of muscle
Muscle tissue is specialized for contraction. Humans have three types, which differ in structure, location and nervous control.
| Feature | Skeletal | Cardiac | Smooth |
|---|---|---|---|
| Location | Attached to bones (biceps, triceps, tongue) | Heart wall only | Walls of hollow organs: stomach, intestine, blood vessels, trachea, bronchi, bronchioles, uterus |
| Shape | Long, cylindrical, unbranched fibres | Branched cells | Long, spindle-shaped cells |
| Nuclei | Multinucleate | One (sometimes two) per cell | One per cell |
| Striations | Striated | Striated, but irregular | Non-striated |
| Control | Voluntary (somatic nervous system) | Involuntary | Involuntary (autonomic nervous system) |
| Special feature | Contracts quickly and tires quickly | Intercalated discs; myogenic (self-excitatory) | Slow, sustained contraction |
Shared features
- Skeletal and cardiac muscle are both striated, meaning they show light and dark bands made by sarcomeres.
- Cardiac and smooth muscle are both involuntary.
- Cardiac muscle is unusual in being striated and involuntary.
Cardiac muscle
Cardiac muscle is found only in the heart. Its branched cells are joined end to end by intercalated discs, which let impulses pass quickly so the heart contracts as a unit. It is myogenic: the heartbeat starts within the muscle itself, at the SA node, without needing a nerve impulse. Nerves only change the rate. It does not tire. Cardiac cells are not multinucleate, and that is the feature missing from cardiac muscle.
Smooth muscle
Smooth muscle cells are spindle-shaped, uninucleate and lack striations. Smooth muscle lines the digestive tract, blood vessels and airways (trachea to bronchioles, but not the alveoli, which are thin epithelium). It is controlled by the autonomic nervous system. If smooth muscle fails, organs such as the stomach and intestine cannot churn and move food. Smooth muscle does not line the heart.
Skeletal muscle and antagonistic pairs
Skeletal muscles are voluntary, multinucleate and controlled by the somatic nervous system. They work in antagonistic pairs, because a muscle can only pull, not push:
- Biceps is a flexor: it bends the elbow.
- Triceps is an extensor: it straightens the elbow.
- Adductors move a limb toward the body's midline; abductors move it away.
Worked example
Question: Which two organs both contain only non-striated muscle: intestine and biceps, intestine and blood vessels, or heart and stomach?
Answer:Intestine and blood vessels. Both are hollow organs with smooth muscle. The biceps is skeletal and the heart is cardiac, and both of those are striated.
Common MDCAT traps
- Cardiac muscle is striated, so "has no striations" is the incorrect statement about it.
- Smooth muscle cells are uninucleate, while skeletal fibres are multinucleate.
- Intercalated discs are found only in cardiac muscle.
- Myogenic means cardiac.
- Triceps extends and biceps flexes.
Quick revision
- Skeletal muscle is voluntary, striated and multinucleate.
- Cardiac muscle is involuntary, striated and branched.
- Smooth muscle is involuntary, non-striated, spindle-shaped and uninucleate.
- Cardiac and smooth muscle are both involuntary.
- The somatic nervous system controls skeletal muscle.