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

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

Sarcolemma

· Lipid bilayer

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

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

Components

· 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)

Sarcomere

· Parallel relationship means additive force production in a single direction

Nuclei	· Multiple due to fusion of myoblasts
Mitochondria	· 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
Myoglobin	· Binds oxygen with very high affinity (p50 2.8mmHg) · Preserves concentration gradient in peak exercise · More abundant in slow twitch fibres