T system of tubules

The thin (8 nm) outer cell membrane or "plasma-lemma" (Fig. 1-7) controls the flow of materials into and out of cells, conducts impulses in nerve cells and along muscle fibrils, and participates in chemical communication with other cells. Deep infoldings of the outer membrane sometimes nm into the cytoplasm. An example, is the "T system" of tubules which functions in excitation of muscle contraction (Figs. 19-7, 19-21). Surfaces of cells designated to secrete materials or to absorb substances from the surrounding fluid, such as the cells lining kidney tubules and pancreatic secretory cells, are often covered with very fine projections or microvilli which greatly increase the surface area. 

T-system membrane results in an interaction between the T-tubules and the SR at specialised junctions, known as excitation-contraction coupling units. Here the two membranes are finked by a foot process consisting of two Ca ion channels that function in concert. 

As in skeletal muscle, the action potential is transmitted into the cardiomyocytes by a T-system. The depolarisation opens Ca ion channels in the T-system (i.e. the plasma membrane) which leads to a small increase in the cytosolic ion concentration. This activates Csl ion channels in the sarcoplasmic reticulum (a process known as a Ca -induced Ca ion release) which results in the movement of more Ca ions into the cytosol. The amount of Ca ion released from the reticulum varies according to the stimulation of the ion channel, i.e. the amount of Ca ions released from the T-tubule. This provides for variations in the cytosolic Ca ion concentration and hence in the force of contraction. 

Sarcoplasm - contains multiple nuclei, located peripherally beneath sarcolemma, sarcoplasmic reticulum, mithocondria, which are specialised into an intense oxidative mechanism, working into a medium rich in hemoglobin that fixes the oxygen. All these peculiarities just reflect the specialisation of muscle fiber with a view to perform the contractile function. Sarcoplasmic reticulum and T tubules is a membranous system of longitudinal tubules, in the zone of H band, and terminal cisterns (flattened reservoirs for Ca ) that forms closely meshed network around each myofibril. 

In the zone of Z membrane, invaginations of sarcolemma regularly occur, constituting the secondary system of T tubules or transversal, which is not connected to the sarcoplasmic reticulum. Functionally, these ones intervene by a trap system in the caption of Ca ions released during the muscle excitation and in metabolic exchanges of the fibrillar apparatus T tubules system conveys the excitation from the sarcolemma to the fibrillar apparatus, causing the fibres contraction, by releasing Ca. ... 

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

According to Schneider and Chandler (1973), depolarization of the T-tubules affects sensors which open Ca " channels in the SR. The sensors are modified Ca channels which act as voltage sensors (Tanabe et al., 1987). The signal from the sensor reaches the SR and opens the Ca channels with the release of Ca to the myoplasm. The Ca channels in the SR system are opened by micromolar [Ca ], mM [ATP], and caffeine but are inhibited by Mg (Smith et al., 1986 Rosseau et al., 1988). The channels are closed in resting muscle and are opened when the voltage sensor is activated. 

In the sarcoplasm of resting muscle, the concentration of Ca + is 10 to 10 mol/L. The resting state is achieved because Ca + is pumped into the sarcoplasmic reticulum through the action of an active transport system, called the Ca + ATPase (Figure 49-8), initiating relaxation. The sarcoplasmic reticulum is a network of fine membranous sacs. Inside the sarcoplasmic reticulum, Ca + is bound to a specific Ca -binding protein designated calsequestrin. The sarcomere is surrounded by an excitable membrane (the T tubule system) composed of transverse (T) channels closely associated with the sarcoplasmic reticulum. 

When the sarcolemma is excited by a nerve impulse, the signal is transmitted into the T tubule system and a release channel in the nearby sarcoplasmic reticulum opens, releasing Ca + from the sarcoplasmic reticulum into the sarcoplasm. The concentration of Ca in the sarcoplasm rises rapidly to 10 mol/L. The Ca -binding sites on TpC in the thin filament are quickly occupied by Ca +. The TpC-4Ca + interacts with Tpl and TpT to alter their interaction with tropomyosin. Accordingly, tropomyosin moves out of the way or alters the conformation of F-actin so that the myosin head-ADP-P (Figure 49-6) can interact with F-actin to start the contraction cycle. 

Using the reconstitution approaches described above, we have demonstrated that phosphorylation of the skeletal muscle Ca channels by PKC results in activation of the channels [108], In the fluo 3-containing liposomes, channels phosphorylated by PKC exhibited a two-fold increase in the rate and extent of Ca " influx [108], Using the lipid bilayer-T-tubule membrane reconstitution system we are currently analyzing the effects of PKC-catalyzed phosphorylation at the single channel level [133], The demonstration that these channels undergo phosphorylation as a result of activation of PKC in intact skeletal muscle cells has not yet been achieved. 

Ca ion concentration As an action potential travels along the muscle fibre and into the interior of the fibre, via the T-tubule system, ions are released from the sarcoplasmic reticulum (Chapter 13). This increases the concentration of Ca + ions in the cytosol which is followed by an increase in concentration within the mitochondria. 

Figure 13.13 A diagrammatic three-dimensional view of part of a single muscle fibre showing the T-tubule system and the sarcoplasmic reticulum. The T-tubules are located within the fibre and are attached to the reticulum. This is a sheet of anastomosing flattened vesicles that surround each myofibril like a net stocking.

At the ultrastructural level, flatworm muscle resembles smooth muscle with individual, non-striated myofibrils being delimited by the sarcolemma and interconnected by gap junctions. Also, flatworm muscles lack a T-tubule system that is characteristic of striated muscle in other animal groups. The contractile portion of flatworm myofibrils contains thick myosin and thin actin filaments that connect with the sarcolemma via attachment plaques or desmo-somes. Actomyosin cross-bridges have been reported and where overlap has been observed, ratios that vary from 9 1 to 12 1 have been observed. Although flatworm muscle is mostly non-striated, pseudo-striated (e.g. in the tail of schistosome cercariae Dorsey et al., 2002 Mair et al., 2003) and obliquely striated (e.g. tentacular bulb of the trypanorhynch, Crillotia eri-naceus Ward et al., 1986) muscles have been reported. It is presumed that the role played by these structures has demanded the development... 

A closer look at striated muscle fibers shows that they themselves are assemblies of fine, hairlike structures known as myofibrils (Fig. 1A, B). Myofibrils may be about 2 to 5 jxm in diameter, with cell organelles such as mitochondria and membranous systems called T-tubules and the sarcoplasmic reticulum (SR) sandwiched between them (Fig. 2B). 

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