V

T system (transverse tubules)

Sarcoplasmic reticulum

(longitudinal tubules)

T system (transverse tubules)

Sarcoplasmic reticulum

(longitudinal tubules)

Porter Franzini Armstrong

Sarcolemma (cell membrane)

Triads

-Mitochondrion

(After Porter and Franzini-Armstrong)

Sarcolemma (cell membrane)

Triads

-Mitochondrion

(After Porter and Franzini-Armstrong)

i— B. Ca2+ as mediator between electrical stimulus and contraction

Tsystem

DHPR, AP

2 Skeletal muscle i— B. Ca2+ as mediator between electrical stimulus and contraction

Tsystem

1 Ca2+ release

1 Ca2+ release

DHPR, AP

2 Skeletal muscle

vJ J J

B Myocardium

- C. Sliding filaments

Actin-myosinII binding I Strong I Weak | Strong

Actin-myosinII binding I Strong I Weak | Strong

Actin 1 Strong binding

4nm |- 36nm or multiple

2 Work phase 3 Resting phase (ca. 90% of time; other sin he.

myosin heads are meanwhile active)

Actin 1 Strong binding

4nm |- 36nm or multiple

2 Work phase 3 Resting phase (ca. 90% of time; other sin he.

myosin heads are meanwhile active)

Contraction cycle C and D). Each of the two myosin heads (M) of a myosin-!! molecule bind one ATP molecule in their nucleotide binding pocket. The resulting M-ATP complex lies at an approx 90° angle to the rest of the myosin filament (^ D4). In this state, myosin has only a weak affinity for actin binding. Due to the influence of the increased cytosolic Ca2+ concentration on the troponin - tropomyosin p complex, actin (A) activates myosin's ATPase, 5 resulting in hydrolysis of ATP (ATP ^ ADP + Pi) ¡3 and the formation of an A-M-ADP-Pi com-£ plex (^ D1). Detachment of Pi (inorganic £ phosphate) from the complex results in a con-<u formational change of myosin that increases ^ the actin-myosin association constant by four powers of ten (binding affinity now strong). -o The myosin heads consequently tilt to a 40° r5 angle (^ D2a), causing the actin and myosin g filaments to slide past each other. The release ¡5 of ADP from myosin ultimately brings the my-z osin heads to their final position, a 45° angle ™ (^ D2b). The remaining A-M complex (rigor complex) is stable and can again be transformed into a weak bond when the myosin heads bind ATP anew ("softening effect" of ATP). The high flexibility of the muscle at rest is important for processes such as cardiac filling or the relaxing of the extensor muscles during rapid bending movement. If a new ATP is bound to myosin, the subsequent weakening of the actin-myosin bond allows the realignment of the myosin head from 45° to 90° (^ D3,4), the position preferred by the M-ATP complex. If the cytosolic Ca2+ concentration remains > 10-6 mol/L, the D1 to D4 cycle will begin anew. This depends mainly on whether subsequent action potentials arrive. Only a portion of the myosin heads that pull actin filaments are "on duty" (low duty ratio; see p. 62) to ensure the smoothness of contractions.

The Ca2+ ions released from the sarco-plasmic reticulum (SR) are continuously pumped back to the SR due to active transport by Ca2+-ATPase (^ pp. 17 A and 26), also called SERCA (^ p. 16). Thus, if the RYR-mediated release of Ca2+ from the SR is interrupted, the cytosolic Ca2+ concentration rapidly drops below 10-6 mol/L and filament sliding ceases (resting position; ^ D, upper left corner).

Reducing Blood Pressure Naturally

Reducing Blood Pressure Naturally

Do You Suffer From High Blood Pressure? Do You Feel Like This Silent Killer Might Be Stalking You? Have you been diagnosed or pre-hypertension and hypertension? Then JOIN THE CROWD Nearly 1 in 3 adults in the United States suffer from High Blood Pressure and only 1 in 3 adults are actually aware that they have it.

Get My Free Ebook


Post a comment