Muscle tissue

General Information
Muscle tissue is one of the four basic tissue types. According to their function and their morphological characteristics muscle tissue can be distinguished in three different types.

Smooth muscle: fusiform cells without striations, slow and involuntary contractions

Cardiac muscle: involuntary, rhythmic and synchronised contractions

Skeletal muscle: multinucleated cells, quick and strong contractions, cross-striations

The major characteristic is the improved cell ability to contract using actin filaments and their associated Proteins. All muscle cells origin in mesoderm and during their differentiation they elongate and increase their synthesis of actin and myosin proteins.

Skeletal ( or striated ) muscles
The muscle cell itself ( myocyte ) is called a myofibre with many nuclei due to the developmental mechanism of muscle tissue from embryonic mesenchyme and is therefore a syncytium of many progenitor cells. It has a diameter of 10 - 100 micro meter and a length up to 15 cm.

It shows cross-striations of alternating light and dark bands.

Every Myofibril consist of so called Sarcomers which are about 2,5 micro m long and are separated through Z-discs. They are the functional units of the muscle cell. There are roughly 2000 actin filaments in one sarcomere and they are connected to the Z-Plate. The Actin filament is a double helix of two f-actin filaments and in each of the two groves of the filaments is a long tropomyosin molecule attached. It provides stability and consists of small troponincomplexes which cover the myosin binding site of the actin filament. It is made up of three subunits with different functions.

Troponin-Tropomyosin ( Tn-T ) is the connector to the Tropomyosin.

Troponin-inhibiting subunit ( Tn-I ) prevents the gliding of the filament during rest.

Troponin-calcin subunit ( Tn-C ) is the binding site for the Calcium Ions. The actin filament is polarised with the + end attached to the Z-Plate. They are also referred as the thin myofilament. The + end is capped and the - end contains troponinmodulinat.

The Myosin II filaments, which are also called thick myofilaments, ( about 1000 per sarcomere ) are situated in the middle of the Sarcomere and thickened to the M-plate. Their heads have two hinges and can be bend under the assistance of ATP. They are composed of an actin binding site and a ATP pocket which contains ATPase, but the splitting of ATP occurs only when the Myosin II head is bound to the Actin filament to avoid an overuse of ATP when it’s not needed ( during rest ). The heads are arranged in the opposite direction ( to each Z-Plate ) and therefore in the direction of the +end of the thin myofilament so that the sarcomere can be shortened and contraction of the muscle can occur.

Each muscle fibre is surrounded by a thin layer of reticular fibres and fibroblasts, which is called endomysium. Many muscle fibres in a bundle are covered by a layer of connective tissue, the perimysium. The epimysium encircles the whole muscle. These structures transmit the mechanical forces which are generated by the individual muscle fibres during the contraction from one end of the muscle to the other end and allow joint movements and postural control.

Proprioception and sensory organs of the muscle tissue
Proprioception is the ability to recognise movements and tension of muscles and joints. The two sensory organs connected directly to muscle tissue are muscle spindles and golgi-tendon-organs. The muscle spindle is arranged parallel to the muscle itself and controls and regulates the length of the muscle involuntarily over the spinal cord through reflexes. The golgi-tendon-organ is arranged serial in the tendon of the muscle and is the main sensor for tension. Both work together to control and regulate movements and are important for the postural control of the body.

Types of contraction
If the force of the muscle increases but the length remains the same it is called isometric contraction. The opposite is the isotonic contraction where the force remains constant and the length changes. If you combine both of these types it is called an auxotonic contraction which usually occurs in the human body during movements and posture control, because the force and the length have to change fast and accurate to enable complex and economic movements.

Muscle extensibility
A resting muscle can be stretched until a certain point. At the beginning of the stretching process less force is needed which increases during greater elastic strain exponential. This occurs due to the surrounding structures ( fascias ) and a huge molecule called Titin which is integrated into the sarcomere to prevent the muscle to be overstretched so that the myosin heads are always able to connect to a part of the actin filament.

Types of muscle tensions
During resting position, the muscle develops only a small tension and only a few myosin and actin complexes are active. If a voluntary movement occurs the total tension is the sum of the passive and the active tension of the muscle. The isometric force of the muscle is strongly related to the length of the sarcomeres. The highest force can be developed in a distance of 2.0 to 2,2 micro meter between to Z-plates because all myosin heads are able to connect to the actin filaments. If the Z-plates are closer to each other or if they are wider apart the isometric force decreases. The force can be regulated through the summation of simple contractions which means that more impulses from the motoric endplate are following each other, this is called superposition. The maximal frequents of all impulses and therefore the maximal muscle tension is called tetanus. All myosin heads are active.

Smooth muscle
This type of muscle tissue consists of spindle shape cells which lacks troponin, myofibrils and therefore sarcomeres ( no cross-striations ). The cells are connected via gap junctions and can contract spontaneously as well as through excitation from a vegetative nerve. They mainly function as regulators for the vascular system in vessels and as contractual part of many inner organs. The filaments form a more or less loose contractual apparatus which is orientated longitudinal inside the cell. It is possible to distinguish smooth muscles into Single and Multi-unit.

Cardiac muscle
Cardiac muscles are not that flexible as skeletal muscles which means that the passive tension of the cardiac muscle is higher during the same elongation compared to skeletal muscles. The heart is usually working underneath its maximum length and is able to develop a higher force if the chambers are filled with a higher volume of blood during the diastole. This is called Frank-Starling mechanism. The sensitivity of Troponin for Calcium also increases during elongation. There is no possibility of a Tetanus in the cardiac muscles because the refraction-period of the Action-potential ends when the contraction is nearly finished. There are also no motoric units in the myocard so that a contraction occurs over the whole heart.