Question
The diagram shows the bones and muscles of the human elbow joint.
(a) Identify structures X and Y.
(b) Outline the antagonistic action of the muscles of the elbow.
(c) State one reason that striated muscle cells are considered atypical.
(d) Explain how specialized cardiac muscle cells initiate each heartbeat.
▶️Answer/Explanation
(a) Identify structures X and Y.
X: Ulna
Y: Biceps (muscle)
(b)
- The muscles work in opposition: when one contracts, the other relaxes.
- The biceps contracts to flex (bend) the arm at the elbow.
- The triceps contracts to extend (straighten) the arm at the elbow.
(c) They are multinucleated, meaning each muscle fiber contains multiple nuclei, which is unusual for cells.
(d)
- Cardiac muscle is myogenic—it contracts on its own without nerve signals.
- The sinoatrial (SA) node acts as the pacemaker, starting each heartbeat.
- It sends out an electrical signal (action potential) that spreads across the heart muscle.
- This signal causes the atria and ventricles to contract in the correct rhythm.
- The heart rate can be influenced by nerves and hormones like epinephrine.