Skeletal muscles consist of a number of different parts which all contribute to the muscle contracting and towards growth, repair and protection of the muscle.
The best way of explaining muscular anatomy is by imaginging cutting through a muscle and the different layers and structures you would see.
The epimysium is a protective covering which surrounds the muscle and holds it all together. It reduces friction between the muscle and surrounding bones and other tissues.
The epimysium also extends at either end to form the tendons of the muscle.
The fasciculi are bundles of muscle fibres which can be seen on the image above. The fasciculi of larger muscles have more fibres per bundle and smaller muscles contain less. They range between 10 and 100 fibres per fasciculi.
The fasciculi are contained by another connective tissue called perimysium.
3. Muscle Fibres
Individual muscle fibres are covered in yet another connective tissue known as endomysium. This acts to insulate each fibre. Muscle fibres again vary in size but can be up to 35cm long and 10 micrometers in diameter.
Each muscle fibre has a cell membrane called the Sarcolemma. Beneath this is the Sarcoplasm - a gelatinous fluid which fills most types of cells. Contained within the Sarcoplasm are mitochondria - the powerhouse of any cell, which produce it's energy.
Within each muscle fiber are organelles known as Myofibrils. These are cylinder shaped structures and each muscle fiber contains hundreds or even thousands of them! Myofibrils actually consist of bundles of two proteins called Actin and Myosin which run the length of the muscle fiber and are vital for muscle contraction.
Another important feature for muscle contraction is the Sarcoplasmic Reticulum - a network of channels which surround the myofibrils. Calcium is stored within these channels which is used to trigger and then stop muscle contraction. Transverse tubules pass inwards from the Sacrolemma throughout the Myofibril, through which nerve impulses travel.
Each Myofibril can also be broken down into functional repeating segments along it's length. These are called Sarcomeres. It is these segments the cause the muscle shortening seen during concentric contractions as the filaments slide over each other.
Please visit the sliding filament theory of muscle contraction page for more information!