The Muscles / What Are Muscles?

submitted by Dr. Gary Farr Last Updated June, 25, 2002

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The muscles are the active part of the apparatus of locomotion. Their task is to move the body. The muscles comprise 40% of the body's weight. The body has 300 individual muscles (musculi) in different shapes and sizes.

The basis of the muscles are muscle cells, capable of contracting longitudinally in response to nervous stimulation. The muscle cells contain small contractile strands of protein (so-called myofibrils). They contract when stimulated, returning to their original state as soon as the stimulation ceases. Muscle fibers and muscle cells are distinguished according to their construction:

• Striped muscle
• Smooth muscles
• Cardiac muscle (myocardium)

The smooth muscles consist of smooth muscle fibers. Their myofibrils (muscle fibers) run through the muscle fibers longitudinally and are of equal length. Consequently, the muscle fibers look smooth and not striated. These cells are 40-500µm long and 4-20µm thick.

The smooth muscles carry out the involuntary movements of the inner organs, e.g. the (peristaltic) bowel movement. The nerves are supplied exclusively by the vegetative (autonomous) nervous system.

The striped muscles are composed of different kinds of tissue: most of them striped muscle fibers. They make up all the skeletal muscles, facilitating our voluntary movements and reflexes. These voluntary movements are driven by stimuli within the cerebral cortex (cortex cerebri) and reach the muscles via nerve paths in the spinal cord (medulla spinalis) and myokinetic nerves. In contrast, involuntary movements stem from myokinetic centers in the brain stem (truncus cerebri) and take place automatically through a voluntary impulse within the cerebral cortex.

The muscle fibers of the striped muscles are multinuclear, cylindrical cells, which can be up to 12 cm (4,68 in) long and 100µm thick. Their striations are due to myofibrils within the cells. The cells and their nuclei are directly under the surface, with their longitudinal axes running in the same direction as the fibers. A microscopic view reveals alternating double and single light-refracting sections. Here it is clear that the myofibrils are made up of thick and thin myosin filaments. Thicker filaments are categorized as double-refracting, thinner filaments as single-refracting sections.

On entering the muscle, the myokinetic nerves split up into individual fibers and make contact with every single muscle cell. An electric impulse from the brain (cerebrum) runs down the nerve to the motor end-plate (synapse), forming the transition between nerve and muscle. This stimulus is transferred from the nerve to the muscle by acetylcholine, triggering the contracting of the muscle.

This process occurs as follows: In each muscle fiber (filament) there are two different proteins (actin & myosin). They are connected by transverse bridges.

At rest, the pattern is loosely meshed, with few bridges. If the filaments move across each other, the muscle shortens. Myosin and actin filaments join up, form additional trans- verse bridges and create a much more closely meshed pattern.

Once enough muscle fibers have contracted, the entire muscle shortens and the bone (os) is moved. The strength of the muscle contraction depends on the strength and frequency of the stimulation which reaches it. If the link between nerve and muscle is interrupted, the muscle movement is out of order or paralysed.

Muscular activity generates lactic acid (organic hydroxy acid which forms when carbohydrates divide) within the muscle. If too much acid is generated, the muscle becomes harder, its contractions and therefore its movements become smaller, the muscles ache.

Some muscles are always contracted at any point in time. They are in a state of tension (tonus), giving the body posture. The assumption of a particular posture (e.g. standing or lying down) is also a result of muscular tension, a force which can only be maintained through a high consumption of energy. This force without movement is also known as isometric contraction. When the body moves, the muscular tension must be increased and the muscle shortened (isotonic contraction). Muscles working in unison during a movement are termed synergists and the muscles working against them are called antagonists.

Depending on the movement, the combination of synergists and antagonists changes. When the hand (manus) is flexed, for example, several muscles work together as synergists, yet when the lower arm (antebrachium) is moved outwards, the same muscles function as antagonists. Synergists and antagonists must cooperate correctly for specific movements to be carried out.

Our natural movements usually involve not only the activity of a few muscles, but the use of numerous muscles one after the other. The muscles may be divided up according to their main movements:

• Flexors and extensors
• Adductors and abductors
• External rotators (exorotators) and internal rotators (endorotators)

The origin (origio) and insertion (sertio) of the striped muscles are constructed from high-tensile connective tissue. Loose connective tissue (endomysium) may be found between the individual muscle fibers, facilitating their movements during contractions.

Stronger connective tissue (perimysium) holds several bundles (fasciae) of muscle fibers together, connecting them to each other. They can be recognized as fleshy fibers with the naked eye.

The myocardium reveals cell features of the striped and the smooth muscles. It consists of muscle fibers which form a cell bond without fixed borders between the cells. Cardiacmuscle (myocardial) fibers are 20-30µm thick. The myofibrils of one fiber run into the other fibers in a marked longitudinal direction, giving the myocardial fiber a clear longitudinal striation. The myofibrils of the cardiac muscles are striped, albeit more finely than those of the skeletal muscles. The myocardium is controlled by the 10th cranial nerve (nervus cranialis).

The myocardium has a high energy requirement, supplied by numerous blood vessels. Larger vessels run through the strong connective tissue, whereas capillaries branch off to form a network in the epimysium (moving layer). Each muscle contraction is driven by a myokinetic nerve, leading to the motor end-plate (synapse). This is the point of contact between nerve fiber and muscle fibers, in which stimuli (from nerves within the spinal cord or the brain) are transferred. Anything from one muscle fiber up to 100 (through branching) can be supplied simultaneously.

The auxiliary organs of the skeletal muscles include tendons (tendo), fasciae, tendon sheaths (vagina tendinis), synovial sacs (bursa synovialis), sesamoid bones (os sesamoideum) and tendinous cartilage.

Muscular traction is transferred to the bones by tendons. At the muscle origin (origo) and the muscle insertion (insertio), they run into the collagen fibers of the bone.

Tendons are composed of highly tensile collagen fiber bundles, holding the ends of the muscle fiber bundles together in a fixed structure in the manner of a rope. Short tendons form the insertion of a muscle, as can be seen in the large pectoral muscle (musculus pectoralis major), for example. In contrast, the tendons within the hand and foot muscles, for example, are very narrow and long. Surface tendons (aponeuroses) are to be found in oblique abdominal muscles.

The tendons can be additionally divided up into pressure (push) and traction (pull) tendons.

Pressure tendons change their course of direction by running around bones, and are strained by pressure on the side facing the bone. One example of this is the insertion tendon of the long calf muscle (musculus peronaeus longus). It runs around the side of the cuboid bone and then inserts on the underneath side of the foot.

Traction tendons run in the main direction of the muscle and are only strained by traction. Fasciae are connective tissue covers surrounding individual muscles or muscle groups. This enables several muscles to glide over each other without any friction.

Tendon sheaths are lubricated covers improving the gliding abilities of tendons. A tendon sheath is constructed similarly to an articular capsule.

The outer layer (stratum fibro- sum) consists of connective tissue, the inner layer (stratum synoviale) excretes a kind of synovia to improve lubrication.

Synovial sacs (bursae syno- viales) have the task of protecting muscles which run directly around bones.

Sesamoid bones can be found wherever tendons are subjected to particular pressure. They are embedded at the deflection point of a tendon. There they form a synovial joint with the bone below, in order to reduce friction. The largest sesamoid bone is the knee cap (patella). Sesamoid cartilage serves to cartilage over a tendon without bone deposits. The individual muscles are separated by adipoids (corpora adiposa). These fatty bodies also serve to improve lubrication.

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