The Essentials of Muscular Contraction: Part 1 – Skeletal Muscle Structure & Function
There is no two ways about it, the following two part series is going to be an absolute nerd fest of classic muscle physiology. At WTW we pride ourselves on actionable-advice, however sometimes it is important to go back to basics and outline exactly what is happening when we are trying to achieve our performance goals: whether your playing sport, lifting weights or running a marathon, your muscles are contracting and in this series, you will learn the complexities of this process.
Today we will discuss the fundamental structure & composition of skeletal muscle and in part two we will progress into the how muscles contract, the different types of contraction and the effect they have on our performance goals.
In order to understand how a muscle contracts we need to understand the structure and function of skeletal muscle; muscle proteins provide the framework of the contractile machine, hence the role of these biomolecules is need to know information!
Muscle tissue is one of four primary tissue types in the body: others being nervous, connective and epithelial tissue. There are also three main types of muscle found in the body: cardiac, found only in the heart; smooth muscle, located in the walls of blood vessels, airways, gut and bladder, and skeletal muscle, which links parts of the skeleton.
Â It is only skeletal muscle that is under direct voluntary control and allows for movement of limbs and postural maintenance via contraction.
Skeletal Muscle Structure
Skeletal muscle is separated from its surroundings by a layer of membranous connective tissue know as the perimysium of fascia, which then extends in to the belly of the muscle of at a decreasing thickness sub-dividing the muscle into smaller compartments known as fasciculus; which contains a number of muscle fibres bound together and the endomysium.
At both ends of the muscle this connective tissue converges to form tendonsIndividual muscle are made up of many parallel muscle fibres which are innervated by one nerve ending located near the middle of each muscle fibre; this nerve is called the motor end plate and its neurotransmitter initiates muscular contraction. Each fibre is surrounded by a membrane know as the sarcolemma that contains collagen fibres which connect it to the intramuscular connective tissue.
Individual muscle are made up of many parallel muscle fibres which are innervated by one nerve ending located near the middle of each muscle fibre; this nerve is called the motor end plate and its neurotransmitter initiates muscular contraction. Each fibre is surrounded by a membrane know as the sarcolemma that contains collagen fibres which connect it to the intramuscular connective tissue.
Muscle Fibre Structure
The interior of muscle fibres is filled with sarcoplasm; a red fluid containing nuclei, mitochondria, myoglobin and a bucket load of myofibrils, which are surrounded by the sarcoplasmic reticulum. The striated appearance of skeletal muscle is due to dark A bands alternating with light I bands along the length of each myofibril; these bands are specifically important as they are contractile elements!
Check out the figure above, each A band is interrupted by a H zone (only visible in relaxed muscle) which itself is bisected by a dark line called the M line; which contains M-protein and the muscle isoform of creatine-kinase (often used in scientific experiments as an indicator of muscle damage). The I band also has an interruption called the Z line, with each sarcomere defined as the distance between two successive Z lines in a myofibril.
Each myofibril is a chain of sarcomeres laid end to end
At a molecular level, within each sarcomere are two types of protein filaments. The thin filament is composed of the proteins actin, tropomyosin and troponin and extends across the I band & part-way into the A band, while the thick filament which contains the protein myosin, extends the entire length of the A band! The Z line or disc is a protein sheet at right angles to the fibre serving as a point of attachment for the thin filament and is the point of myofibril connection within the muscle fibre. Within the sarcomere actin is joined to the Z disc by a large elastic protein called a-actinin and the distance from the M line to the Z disc is spanned by titin, a giant protein that helps to maintain thick filament (myosin) alignment.
The architecture of the sarcomere is maintained by several cytoskeletal proteins that keep the contractile proteins in correct spatial arrangement. Desmin is the protein which links adjacent Z discs in a longitudinal and lateral manner. Five other proteins anchor the cytoskeleton to the sarcolemma, called costameres; actin, a-actinin, vinculin, talin and dystrophin… all of which are though to play a vital role in force transmission to the endomysium
When calcium and ATP are present the filaments interact to form actomyosin and shorten by sliding over each over – But more on this next week as we go into detail of how muscles actually contract!