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The Structure of Skeletal Muscle.

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Introduction

The Structure of Skeletal Muscle Skeletal muscles are all muscles that are attached to the skeleton such as the biceps and the hamstring. Within each muscle cell (also known as a muscle fibre) are structures called myofibrils as shown in the picture below: (Ref. The picture above was found at www.google.com) Myofibrils are made up of tiny units called sarcomeres. Sarcomeres are the smallest structures in a muscle that can contract; they are long filament-like structures, arranged in series - end to end - that run lengthways in the myofibril. Within the sarcomeres are two types of protein filaments that are actin and myosin - running lengthways, parallel to each other. The myosin filaments have 'cross-bridges' across to the actin filaments, which during contraction allow them to bond with the actin filaments. The source of energy for this bonding is the molecule adenosine triphosphate (ATP). During the bonding, energy is released by the breaking down of ATP into adenosine diphosphate (ADP) and Pi at another site - the ATPase site - on the myosin cross-bridge (by the action of the enzyme ATPase). ...read more.

Middle

A picture of skeletal muscle is as follows: (www.google.com) Motor Neurone The Motor Neurone forms synaptic junctions with either extrafusal muscle fibres (skeletal muscle) or intrafusal muscle fibres (thread-like muscle that adjusts tension). Stimulation of these motor neurones induces contraction or shortening of the muscle fibres. Alpha motor neurones induce the contraction of extrafusal muscle fibres upon stimulation, whereas gamma motor neurones induce the contraction of intrafusal muscle fibres upon stimulation. Alpha motor neurones control muscle contraction involved in voluntary movement, whereas gamma motor neurones control muscle contraction in response to external forces acting on the muscle. In response to these external forces, the gamma motor neurones induce the involuntary, reflexive movement called the stretch reflex. Intrafusal motor neurones adjust the length of intrafusal muscle fibres to maintain an appropriate level of tension on the muscle spindle receptor. The control of intrafusal muscle fibres occurs independently of the length of skeletal muscle fibres. This independent function allows the spindle to maintain a high degree of sensitivity over a wide range of muscle lengths, and in effect acts as a means of encoding muscle length. ...read more.

Conclusion

FT fibres are brought into play by either the effort to more a heavy load or by the need to move an object faster than is possible with ST fibres. Type IIB fibres can twitch three times faster (and therefore, more often) than ST fibres. Type IIAs can also twitch faster and more often than ST fibres. Because of this, and the recruitment pattern, a FT fibre may begin its contraction after a ST fibre but actually finish at the same time or before. This leads to another contributor to the FT fibres abilities to produce greater force - their enhanced frequency of firing. Because they complete the firing sequence more quickly they can fire more often than ST fibres, thus developing more tension. The force developed by a muscle depend on the number of fibres that are forced to contract (the more units contracting, the more force developed). A sudden increase in force is met by the involvement of more motor units. So a weight lifter will be using more motor units than a cross-country runner at any one time. If the weight lifter increased his weights to a degree that the units are under fatigue then they will increase the frequency of firing therefor meeting the muscles demand. 1 ...read more.

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