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Overview of Skeletal Muscle

Overview of Skeletal Muscle

Skeletal muscles are composed of a bundle of muscle fibers and are attached to bones through tendons. Each skeletal muscle fiber is a single muscle cell. The sarcolemma, the plasma membrane of a skeletal muscle cell, consists of a lipid bilayer and glycocalyx that supports muscle fibers. The sarcolemma extends into the muscle cells to form tubular structures called transverse or T-tubules. Each side of the T-tubules consists of a membrane-bound structure called the sarcoplasmic reticulum,...

Relaxation of Skeletal Muscles

Relaxation of Skeletal Muscles

The period of muscle contraction primarily influences the duration of stimulation at the neuromuscular junction (NMJ), the presence of free calcium ions in the sarcoplasm, and the availability of energy or ATP to support contractions.
When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open.

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Effects of beta-escin and saponin on the transverse-tubular system and sarcoplasmic reticulum membranes of rat and

B S Launikonis¹, D G Stephenson

¹Department of Zoology, La Trobe University, Plenty Rd, Bundoora 3083, Victoria, Australia. zoobl@zoo.latrobe.edu.au

Pflugers Archiv : European Journal of Physiology

|June 17, 1999

Summary

Beta-escin and saponin permeabilize skeletal muscle membranes, affecting the transverse tubular system and sarcoplasmic reticulum. Beta-escin acts milder on surface membranes but more potently on sarcoplasmic reticulum function.

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Area of Science:

Muscle physiology
Cell membrane permeabilization
Skeletal muscle contractility

Background:

Mechanically skinned skeletal muscle fibers are used to study cellular mechanisms.
The transverse tubular system (t-system) and sarcoplasmic reticulum (SR) are crucial for muscle contraction.
Permeabilizing agents like beta-escin and saponin are used to investigate membrane function.

Purpose of the Study:

To compare the effects of beta-escin and saponin on the t-system and SR in rat and toad skeletal muscle.
To elucidate the differential mechanisms of action of these agents on muscle membranes.
To assess the extent of membrane permeabilization and its impact on calcium handling.

Main Methods:

Exposure of mechanically skinned skeletal muscle fibers to varying concentrations of beta-escin and saponin.
Measurement of force responses to t-system depolarization.
Quantification of fura-2 fluorescence in the sealed t-system.
Assessment of sarcoplasmic reticulum (SR) Ca2+ loading via caffeine-induced contractures.

Main Results:

Both beta-escin and saponin reduced t-system depolarization responses, but beta-escin showed a milder effect at lower concentrations.
A significant amount of fura-2 remained in the t-system even at high agent concentrations.
Beta-escin reduced SR Ca2+ loading capacity more effectively than saponin, with differing sensitivities between rat and toad muscle.
The differential sensitivities suggest distinct mechanisms of action on t-system and SR membranes.

Conclusions:

Beta-escin exhibits a less disruptive effect on the muscle surface membrane compared to saponin.
Beta-escin demonstrates greater potency in modulating sarcoplasmic reticulum (SR) function.
Simple membrane permeabilization does not fully explain the observed effects of beta-escin and saponin.
The transverse tubular system (t-system) network within muscle fibers is not a uniform compartment.