Z lines: mus­cles, fil­a­ments and flesh

The Weekend Australian - Travel - - Health -

What are they? Re­peat­ing dark discs in mus­cle cells. The ba­sic con­trac­tile unit of a mus­cle cell — a sar­com­ere — is de­fined as the seg­ment be­tween two Z lines. Sar­com­eres are small: a mus­cle cell may con­tain 100,000 of them, and Z lines can only be seen un­der a mi­cro­scope. How do they work? When a nerve stim­u­lates a mus­cle, it con­tracts by short­en­ing its fi­bres. It does this by con­strict­ing each sar­com­ere so the dis­tance from one Z line to the next de­creases. Be­tween the Z lines are thin fil­a­ments made of a pro­tein called actin, and thick ones, made of an­other pro­tein, myosin. When mus­cles con­tract, the actin and myosin fil­a­ments slide over each other, forc­ing the Z lines closer to­gether. Why the name? Z stands for Zwis­chen­scheibe, which is Ger­man for ‘‘ be­tween disc’’. Sar­com­ere comes from the Greek word sarx, mean­ing flesh. A re­lated word is sar­coph­a­gus, which lit­er­ally means flesh-eat­ing.

Il­lus­tra­tion: Nathalie Gar­cia

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