CDS (I) 2021 General Knowledge · Question 112 of 120

Correct — D, Cylindrical, Branched and Uninucleate. Cardiac muscle is the tissue of the heart wall and it borrows features from both of the other muscle types. Its cells are cylindrical and striated like skeletal muscle fibres, which is why the heart contracts with the same rapid, forceful mechanism. But they branch, so that the cells interlock into a continuous network, and each cell normally carries a single central nucleus rather than the many nuclei of a skeletal fibre. Where two cardiac cells meet end to end they are joined by an intercalated disc, and the gap junctions in that disc pass the electrical signal directly from cell to cell, which is how the whole heart contracts as one unit without a nerve reaching every fibre. Involuntary, striated, branched and uninucleate — that combination belongs to cardiac muscle alone.

• (a)Cylindrical, Unbranched and Multinucleate — This describes skeletal muscle, the voluntary tissue attached to bone. Its long fibres form by the fusion of many cells, which is why one fibre carries many nuclei, and they run in parallel without branching.
• (b)Spinal shaped, Unbranched and Uninucleate — This describes smooth muscle — the involuntary tissue of the gut wall, blood vessels and iris. Its cells taper at both ends, carry one nucleus and never branch, and they show no striations.
• (c)Spinal shaped, Branched and Uninucleate — A manufactured combination. Nothing in the body is tapered and branched at once; the option takes the shape of smooth muscle and the branching of cardiac muscle and joins them.

The body has three kinds of muscle tissue. Skeletal muscle is voluntary, striated, cylindrical, unbranched and multinucleate, and moves the bones. Smooth muscle is involuntary, unstriated, spindle-shaped, unbranched and uninucleate, and lines the hollow organs. Cardiac muscle is involuntary and striated like skeletal muscle but branched and uninucleate, and it works continuously without fatigue for the whole of life.

Three features are being tested at once, and the four options are permutations of them, so the reliable method is to settle each feature separately rather than to look for a familiar-sounding phrase. Shape rules out both of the tapered options at a stroke, since a heart-muscle cell is cylindrical. Branching then separates cardiac from skeletal, and nucleus count confirms it. One printed detail to be clear about: the paper spells the shape 'Spinal shaped' in options (b) and (c) where the intended word is spindle-shaped, the tapering form of a smooth-muscle cell. The slip is in the spelling and does not change which option is correct.

• Cardiac muscle cells are cylindrical, branched, striated and normally have a single central nucleus.
• Adjacent cardiac cells are joined end to end by intercalated discs, whose gap junctions pass the impulse from cell to cell.
• Skeletal muscle fibres are cylindrical, unbranched and multinucleate, and are under voluntary control.
• Smooth muscle cells are spindle-shaped, unbranched, uninucleate and unstriated.
• Cardiac muscle is involuntary yet striated — the combination that makes it distinct from the other two.

• Matching cardiac muscle to skeletal muscle because both are striated, and taking the many nuclei with it.
• Reading the misspelt shape in options (b) and (c) as something other than spindle-shaped.
• Assuming involuntary must mean unstriated — cardiac muscle is involuntary and striated.

As a three-feature description to be matched to a muscle type, or as a match-the-list item pairing each muscle with its location.

• practice — not a real PYQ

  Which one of the following muscle types is involuntary and striated?

  • (a)Skeletal muscle
  • (b)Smooth muscle
  • (c)Cardiac muscle
  • (d)All three

  Answer(c) Cardiac muscle — skeletal muscle is striated but voluntary, smooth muscle is involuntary but unstriated, and only the heart muscle is both.

• practice — not a real PYQ

  Intercalated discs are found in

  • (a)smooth muscle
  • (b)cardiac muscle
  • (c)skeletal muscle
  • (d)nervous tissue

  Answer(b) cardiac muscle — the junctions where adjacent heart-muscle cells meet end to end, carrying the gap junctions that pass the impulse along.