Sarcoplasmic reticulum junctional

Calsequestrin is the major calcium storage protein of the sarcoplasmic reticulum in skeletal and cardiac muscles. It is highly acidic and has a large capacity for Ca2+. Calsequestrin functions to localize calcium near the junctional face of the terminal cistemae from which calcium can be released into the cytosol via the ryanodine receptor. 

Voluntary muscle contraction is initiated in the brain-eliciting action potentials which are transmitted via motor nerves to the neuromuscular junction where acetylcholine is released causing a depolarization of the muscle cell membrane. An action potential is formed which is spread over the surface membrane and into the transverse (T) tubular system. The action potential in the T-tubular system triggers Ca " release from the sarcoplasmic reticulum (SR) into the myoplasm where Ca " binds to troponin C and activates actin. This results in crossbridge formation between actin and myosin and muscle contraction. 

Explain the functions of the following myosin crossbridges, troponin, tropomyosin, sarcomeres, Z lines, neuromuscular junction, transverse tubules, and sarcoplasmic reticulum... 

In tissue culture, astrocytes, hippocampal neurons and arterial myocytes all express al diffusely in their plas-malemma. In contrast, immunocytochemically stained a2 in astroglia and a3 in myocytes and neurons display reticular patterns and colocalize with the Na+/Ca2+ exchanger (NCX) (Fig. 5-6). These plasmalemma reticular patterns coincide with the patterns of staining for junctional complexes on sarcoplasmic reticulum and, in astroglia and neurons, junctional complexes on endoplasmic reticulum [23]. 

Campbell, K. P., Knudson, C. M., Imagawa, T. et al. Identification and characterization of the high affinity [1 3H]ryanodine receptor of the junctional sarcoplasmic reticulum Ca2+ release channel. /. Biol. Chem. 262 6460-6463,1987. 

FIG. 1. Surface coupling in a portion of a smooth muscle cell from the chicken amnion. An element of junctional sarcoplasmic reticulum (SRj) separated by an 18 nm junctional gap between the plasma membrane and the SR is traversed by periodic bridging structures. 

Figure 13.16 A summary of the control of muscle contraction by the motor neurone. When an electrical impulse arrives at the junction between a nerve axon and a muscle fibre, a small amount of acetylcholine is released. This initiates an action potential which is transmitted throughout the fibre via the T-tubules. This causes the sarcoplasmic reticulum to release Ca ions which initiate contraction of the myofibrils via changes in troponin and tropomyosin. Thus sites on the actin for binding of the myosin cross-bridges are exposed.

Pharmacology In isolated nerve-muscle preparation, dantrolene produced relaxation by affecting contractile response of the skeletal muscle at a site beyond the myoneural junction and directly on the muscle itself. In skeletal muscle, the drug dissociates the excitation-contraction coupling, probably by interfering with the release of calcium from the sarcoplasmic reticulum. 

Zhang, L., Kelley, J., Schmeisser, G., Kobayashi, Y. M., and Jones, L. R. (1997). Complex Formation Between Junctin, Triadin, Calsequestrin, and die Ryanodine Receptor. Proteins of die Cardiac Junctional Sarcoplasmic, reticulum membrane. J Biol Chem 272(37) 23389—97. 

In many tissues such as brain, heart, and liver, CaM-kinase II is mainly cytosolic, whereas in others a variable amount has been reported to be associated with membranes or particulate fractions (Colbran and Soderling, 1990). CaM-kinase II has been localized in junctional sarcoplasmic reticulum from skeletal... 

At the neuromuscular junction, ACh activates the nicotinic AChR, resulting in a Na+ influx and a depolarisation. This event generates an action potential which spreads along the membrane via voltage-gated Na+ channels (discussed in above) Muscle cells and neurons possess Ca2+ channels [57], the so-called ryanodine reptors (regulated by the alkaloid ryanodine Table 10), and IP3-sensitive Ca2+ channels. In skeletal muscle cells, ryanodine receptors are located in the sarcoplasmic reticulum (SR)... 

Calcium ion (Ga +) is involved in another ubiquitous second-messenger scheme. Much of the calcium-mediated response depends on release of Ga + from intracellular reservoirs, similar to the release of Ga from the sarcoplasmic reticulum in the action of the neuromuscular junction. A component of the inner layer of the phospholipid bilayer, phosphatidylinositol 4,5-hisphosphate (PIPg), is also required in this scheme (Figure 24.12). 

There are two possible applications of the approach The concept of high proton mobility implies that will be the first among the ions to react to a change in the electrochemical potential in the different compartments of a muscle fiber. Thus H" are expected to move into the T system upon the arrival of the propagated action potentials and thence into the sarcoplasmic reticulum via the junctions between the two. Their interaction with calsequestrin in the terminal cisternae may effect an enhancement in the activity of Ca and these may also enhance the permeability... 

Since the work of Porter and Palade (1957), the numerical relation of this junctional sarcoplasmic reticulum to transverse tubule has been expressed in the terms triad and dyad, respectively. Cardiac muscle can have couplings singly at a transverse tubule (two interior coupUngs = triad), or at opposite sides of a transverse tubule (two interior couplings = triad) instead, or in addition, it may... 

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