Assignment on excitation contraction coupling

MAT NO: UJ/2010/MD/0448

COURSE: PHYSIOLOGY ASSIGNMENT

DEPARTMENT: MEDICINE AND SURGERY

QUESTION

            Write short note on excitation contraction coupling (ie; the sliding theory of muscle contraction)

Introduction

Excitation Contraction Coupling is the process that occurs in between the excitation of a muscle and the contraction of a muscle.

Processes Involved in the contraction of excited muscle:

** Impulse passes through a motor neuron and then reaches the neuromuscular junction. Acetylcholine (ACH) is released from the motor endplate. This ACH causes the opening of the gated sodium channels, so that the sodium ions can enter the neuromuscular junction and this leads to the development of the endplate potential.

** This endplate potential causes the generation of action potential in the muscle fibre. This action potential spreads through the sarcolemma and the tubules, and then moves to the cisternae of the other tubules. This leads to the release of calcium ions from the cisternae into the sarcoplasm; the calcium released binds to Troponin C position. The exposure of active sites of F actin is seen and there is binding of myosin head with the F actin and there is a power stroke in the myosin head. These whole processes lead to the sliding of actin filament over myosin and this leads to the muscular contraction.

** The sliding theory of muscle contraction shows the relaxed state of a sacromere (top) and the contracted state (bottom). In the relaxed state, the ends of the actin filaments derived from two (2) successive Z discs barely begin to overlap one another, while at the same time, lying adjacent to the myosin filament. The contracted actin filaments have been pulled inwardly among the myosin filaments so that they now overlap one another to a major extent. The Z discs have been pulled by the actin filaments up to the ends of the myosin filaments buckle. This implies that the muscle contraction occurs by the sliding filament mechanism.

The Molecular Characteristics of the Contractile Filaments:

The Myosin Filament: This filament is composed of multiple myosin molecules. The myosin molecule is made up of six (6) polypeptide chains; two heavy chains and four light chains wrapped spirally around each other to form a double helix. One end of each of these chains is folded into a globular polypeptide structure called the myosin head. There are two other free heads lying side by side at one end of the double helix myosin molecules, the elongated portion of the coiled helix is called the tail. The four light chains are also parts of the myosin, two to each head. Each light chain has two binding sites; one for ATPase enzyme (for energy for the process) and the other for actin. These light chains help to control the functions of the head during muscle contraction.

The Actin Filament: This filament is complex; it consists of three protein components- Actin, Tropomysin, Troponin. The backbone of the actin filament is a double stranded F-actin protein molecule. These double strands are wound in a helix in the same manner as the myosin molecule but with a complete revolution every seventy (70) minutes in each revolution of each strand of helix.