For each kind of muscle described A–E, select the most appropriate option from the following list.
1. Muscle in the wall of the heart.
2. Muscle that moves the skeleton.
3. Muscle in the wall of the small intestine.
4. Muscle in the iris of the eye.
- This type of muscle is striated and has multiple peripherally-placed nuclei.
- This type of muscle is not striated and is not spontaneously active.
- This muscle is spontaneously active and is richly endowed with B1 receptors
- This muscle is not striated and is spontaneously active.
- This type of muscle is not spontaneously active and is richly endowed with receptors that respond to acetylcholine and are blocked by ‘muscle relaxants’.
Medicine
A. Option 2 Muscle that moves the skeleton. Skeletal muscle has an orderly arrangement of actin and myosin molecules that gives it a striated appearance under the microscope.
B. Option 4 Muscle in the iris of the eye. This type of muscle contains actin and myosin in a more random arrangement and so appears smooth rather than striated; it requires stimulation of autonomic nerves to make it contract; sympathetic stimulation contracts radial fibres to dilate the pupil, parasympathetic stimulation contracts circular fibres and dilates the pupil.
C. Option 1 Muscle in the wall of the heart. The heart beats spontaneously due to impulses from the sinuatrial node; sympathetic stimulation of beta one receptors increases the force of contraction.
D. Option 3 Muscle in the wall of the small intestine. This type of smooth muscle shows rhythmical contractions, even when removed from the body.
E. Option 2 Muscle that moves the skeleton. Skeletal muscle contracts only when stimu-lated by somatic motor nerves releasing acetylcholine; during general anaesthesia, muscle relaxation is often provided (e.g. during abdominal operations) by drugs which interfere with the action of motor nerves on the muscle fibres.
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