2006B13 Briefly describe the structure of a mammalian skeletal muscle fibre and explain how
 its structure is related to its contractile function. DO NOT describe excitation-contraction coupling.



·         Definitions

·         Membrane structures

·         Contractile structures

·         Other intracellular structures



·         Muscle = entire structure

·         Myocyte = muscle fibre = single cell, 10-100 micrometres diameter

·         Myofibril = bunch of contractile elements, many in each myocyte

·         Myofilament = individual contractile protein


Membrane structures:

Neuromuscular junction

·  Link between somatic nervous system and skeletal muscle

·  Single interface allows co-ordinated activity

·  Junctional nAChR: for rapid depolarisation (millions of ACh molecules and receptors)

·  Peri- and extra-junctional VDNaC: to start and propagate action potential


·  Lipid bilayer

·  Invaginations (T tubules) extend towards sarcoplasmic reticulum, ensure synchronous contraction

·  Abundant ion channels, especially VDNaC, VDKC, Na+K+ATPase, ensure synchronous contraction


Contractile structures:


·  Actin = thin filament

·  Myosin = thick filament, has contractile heads

·  Tropomyosin = fits in groove between actin monomers, prevents interaction with actin

·  Troponin: controls tropomyosin, responds to change in ICF [Ca2+]
(every seven actin monomers)



·  Parallel relationship means additive force production in a single direction


Other intracellular structures:


·  Multiple due to fusion of myoblasts


·  For aerobic metabolism

·  More abundant in slow twitch oxidative

Sarcoplasmic reticulum

·  Analogous to the smooth endoplasmic reticulum

·  Calcium reservoir

·  Ion channels = ryanodine channel

·  Allows rapid change in ICF [Ca2+] to start and end contraction

Glycolytic enzymes

·  For anaerobic metabolism and start of aerobic ATP production


·  Binds oxygen with very high affinity (p50 2.8mmHg)

·  Preserves concentration gradient in peak exercise

·  More abundant in slow twitch fibres




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