Muscular System

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Introduction

Skeletal muscle comprises about 40% of muscle mass. Skeletal muscles are organs of the locomotor system. They are under voluntary control and their activity is subconsciously regulated. Skeletal muscle are striated muscle because of their striped microscopic appearance resulting from the ordered and regular arrangement of sub-cellular contractile elements. Skeletal muscle fibres receive their nerve inputs at single fibers known as motor endplates.

Electrical events of muscle contraction

Muscle fibers are excitable cells and their cell membrane (sarcolemma) contains the ion channels and pump necessary to maintain a very negative resting membrane potential. They also contain the voltage gated ion channels necessary for generation of an action potential. The resting membrane potential in skeletal muscle cells is similar to that of neurons ( i. e., -70 to -90 mV). Skeletal muscle cell resting membrane potential receives a significant contribution from Cl- conductance. The physiological importance of the Cl- current lies in the need for maintaining muscle activity during repeated stimulation. When muscle contracts, there is leakage of K+ from the cell across the sarcolemma. Without the CL- current, the resting membrane potential is not maintain and the muscle would not repolarize sufficiently to regenerate the active state of the channels responsible for generation of succeeding action potentials.

Contractile structures

Muscle contraction occurs when the two filamentous proteins, actin and myosin interact with each others. Myosin and actin are organized into a regular pattern within the contractile units sarcomeres in skeletal muscle. Each sarcomere is formed between adjacent sheets of Z proteins running transversely across the fibre. In the center of the sarcomere, thin myofilament composed of two helically coiled actin filament projects perdendicurly form the sheest of Z proteins. The thin filament intercrossed with thick filaments that run parallel to the long axis of the fibre but lie in central part of the sarcomere. Thick filaments are formed by bundles of myosin filaments. The sarcomere is maintained by Z protein and other structural proteins. M line proteins form a structural mesh at the midline of the sarcomere. M line anchor the tails of myosin filaments and the central end of the thin filament. In a cross ...