NUTRITION IN SPORT

coplasmic reticulum, and in the absence of
calcium, the myosin-binding sites on actin are
physically blocked by the tropomyosin rods (Fig.
2.1). Electrical excitation passing as an action
potential along the sarcolemma and down the T-
tubules leads to calcium release from the sar-
coplasmic reticulum into the sarcoplasm and
subsequent activation and contraction of the fila-

18 nutrition and exercise

ment array (Frank 1982). The calcium ions bind

to troponin, causing a change in its conformation

that physically moves tropomyosin away from

the myosin binding sites on the underlying actin

chain. Excitation is initiated by the arrival of a

nerve impulse at the muscle membrane via the

motor end plate. Activated or ‘cocked’ myosin

heads now bind to the actin, and as this happens

Thick filament H-zone Thin filament

Z-line Z-line

(a)

Myosin heads containing

ATPase activity and

actin-binding sites

(b)

Ca2+ binding sites

Troponin complex Tropomysin G-actin

(c)

Fig. 2.1(a) Molecular

components of the myofilaments

and the arrangement of the thick

and thin filaments in longitudinal

cross section within one

sarcomere (the region between

two successive Z-lines in a

myofibril). (b) The thick filaments

are composed of myosin

molecules; each of these

comprises a rod-like tail and a

globular head. The latter contains

ATPase activity and actin-binding

sites. (c) The thin filaments are

composed of actin molecules and

several regulatory proteins.

Globular (G)-actin monomers are

polymerized into long strands

called fibrous (F)-actin. Two F-

actin strands twisted together

form the backbone of each thin

filament. Rod-shaped

tropomyosin molecules spiral

about the F-actin chains. The other

main protein present in the thin

filaments is troponin, which

contains three subunits. One of

these, troponin I, binds to actin;

another, troponin T, binds to

tropomyosin; and the other,

troponin C, can bind calcium ions.