The Muscular System

Hey everyone! This is the first stem talks post in our series of posts about body systems! This is going to be a good one so kick back, relax, enjoy the sunlight, and get ready to learn! 😀

The muscular system is one of the most important fiber systems in the animal body. The system is responsible for controlling our every internal and external movement. This is possible when the muscles go through the process action of contraction. Muscles are attached to skeletons and work as antagonistic pairs, meaning each muscle works against each other. Contraction is possible because of several micro-filaments in our muscles and bodies. The skeletal muscles are small units that consist of fiber bundles where each fiber is a cell with multiple nuclei. The muscle fibers consist of even smaller bundles called myofibrils.

Muscle Fibers

Myofibrils have two different types of myofilaments: one is a thin one called actin, and the other is thick called myosin. The sections of myofibrils are called sarcomeres, and these are the contractile units of our muscle. The area where thin filaments only exist when the muscle is at rest is called the I band. The A band is the region where the length of the thick filament spans.

Sarcomere and the Sliding Filament Model

Muscle contraction happens by myosin actin interactions and the sliding filament model explains this. The sliding filament model states that the thin filaments and thick filaments do not change in length when their sarcomere compresses, but they actually slide past each other and overlap.

Myosin-actin interactions begin when a myosin head has an ATP molecule attached to it and has low energy. The head uses a water molecule to hydrolyze the ATP into ADP and a phosphate which has high energy. The head attached to the actin filament above, forming a cross bridge. The ADP and phosphate are released and the myosin is back to low energy. This process slides the thin filament to the middle of the sarcomere and results in contraction. A new ATP molecule can rebind to the myosin head to release the contraction.

Myosin-Actin Contraction

Regulatory proteins and calcium play roles in contraction as well. The protein, tropomyosin, blocks myosin from binding sites where the head attaches to the actin. Calcium ions bind to another protein complex of troponin which moves the tropomyosin away from blocking sites which allows contraction.

A motor neuron is responsible for stimulating contraction. The neuron releases acetylcholine, a neurotransmitter, which spreads an action potential into the muscle fibers and the foldings within their membrane called transverse tubules. The sarcoplasmic reticulum is an endoplasmic reticulum outside a myofibril that stores Calcium ions when a muscle is at rest. When active the ions are pumped out allowing the ions to enter myofibril cytosol and activate contraction.

Types of muscles include cardiac and smooth. Cardiac muscles trigger action potentials without a motor neuron, unlike skeletal muscles. Smooth muscles makes up muscle detached from skeletons such as blood vessels and the digestive tract. Muscles are used for our everyday actions. Next time you flex your muscles, remember how cool they are!

Thank you and catch the next system talk! 😀

Written By: Neil 5/26/2019


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