Locomotor System: Muscular Function

Introduction

Muscles are responsible for providing the bodily movements. There are also muscles that do not move bones, but provide involuntary movements of internal organs, such as the peristaltic movements of the intestine and the contraction of blood vessels. Finally, when the muscles contract they produce heat (consuming energy).

There are three different types of muscle: smooth, cardiac and skeletal. When we are talking about the locomotor system, however, we are only referring to skeletal muscles, that provide general movements of the skeleton.

The muscular system is an important part of our body. It is 40 % of our total weight.

Structure of muscular fibres

All the skeletal muscles are surrounded by a layer made up of connective tissue called epimysium. The muscle is divided into fascicles by a connective membrane called perimysium. The fascicles are made up of several cells called muscular fibre. Each muscular fibre is surrounded by a thin connective membrane called endomysium. These three membranes join at the edge of the muscle. After the fusion of these membranes, the connective tissue becomes richer in elastic and cartilaginous fibres, forming the tendon. The tendon firmly connects the muscle to the bone.

Skeletal muscle.

The muscle cells that make up the skeletal muscles, called myocytes, are cylindrical and extremely long. Indeed, they can be more than five centimetres long. They have many nuclei, even more than one hundred nuclei per cell.

The myocytes have transversal striations. They are a repeated series of dark and light bands that come from the extremely ordered distribution of microfilaments. These microfilaments are responsible for the contraction of the myocyte.

As we have just studied, there are light bands and dark bands; the light bands are called I-bands (because they are isotopic). They are divided into two by a thin, black band called Z-line. The thick, dark bands between I-bands are called A-bands. These bands are anisotropic, and in its central part they have a slightly clear band called H-band. The H-band has a darker band in its central part called M-band.

The striated structure located between the Z-lines, made up of one half A-band, one A-band and half of other A-band is called sarcome.

Sarcome - Bands

The I-bands are regions where there are only actin filaments. These filaments are the rails that fix the structure. The Z-line is the structure where the actin filaments are linked. The movable filaments are the myosin  filaments. The darker region of the A-band is made up of both actin and myosin filaments. The M-line is the structure where the myosin filaments are linked. The little H-band is the region where there are myosin fibres, but not actin fibres.

When the muscle contracts, the myosin pulls the actin. Due to this, the distance between the Z-lines reduces and the H band disappears, making the sarcome shorter. 

Physiology of muscular contraction

The muscle contracts after receiving the nerve impulse from the motoneurons, that are responsible for ordering the muscular contraction and movement.

Each motoneuron does not only stimulate one simple fibre, but a group of fibres called motor-unit.

The muscular groups that must carry out stronger movements have bigger motor-units, merely because one simple neurone orders the contraction of many muscular fibres, causing a quick and strong muscular contraction. The muscular groups that carry out fine and precise movements, however, have smaller motor-units, because they can control the contraction more easily.

Motoneurons and muscular fibres

Physical exercise and the type of training can change the unit-motors' size. Due to this, a violinist could not be a body builder at the same time: the training necessary to improve the strength of the muscle makes it much more imprecise.

The muscular contraction is a complex process. It starts when the nerve impulse reaches the neuromuscular junction that connects the neurone with the muscle. The motor- neurone releases neurotransmitters that are received by the muscle, causing special calcium canals of this cell to open, allowing the ion to pass from the exterior to the interior. When the amount of calcium in the cytoplasm rises, a special organelle called sarcoplasmic reticulum, that stores calcium, releases the ion to the cytoplasm. Due to this, the concentration of calcium in the cytoplasm of the muscular cell increases drastically.

The high amount of calcium brings about temporary structural changes in two proteins associated with the actin and myosin fibres, called troponin and tropomyosin. In their original conformation, these proteins block the binding points between the actin and the myosin, but the structural changes allow the myosin to bind to the actin. The myosin pulls the actin consuming energy and contracting the sarcome. The actin is the rail and the myosin is the motor that causes contraction. 

When the nerve impulse stops, the muscular cells release the internal calcium to the exterior or store this ion in the sarcoplasmic reticulum, reducing its concentration. Then, tropomyosin and troponin return to their original conformation, blocking again the binding point between actin and myosin and stopping the movement.

Muscular function

Due to the physiology of the muscular contraction, muscles can only cause a force when they contract, but never when they relax. As a result, when one muscle causes a concrete movement, another muscle must exist in order to cause the opposite movement to undo the first movement.

The principal muscle that causes a concrete movement is called agonist muscle. The principal muscle that undoes this movement (it causes the opposite movement) is called antagonist. 

There must be, besides, other muscles that aid the movement, avoiding lateral or inappropriate movements and fixing the articulation to promote the movement. They are called synergetic muscles.

Each movement has its own agonist, antagonist and synergetic muscles. One muscle that is the agonist for one movement would be the antagonist or synergetic for other movements.

Movements of the human body

The movements of the human body can be classified as:

• Flexion: this movement reduces the angle between the bones of a joint.
• Extension: this movement increases the angle between the bones of a joint.
• Circumduction: the distal part of a limb moves in circles.
• Abduction: movement away from the middle line of the body.
• Adduction: movement towards the middle line of the body.
• Supination: movement to put a part of the body face-up.
• Pronation: movement to put a part of the body face-down.
• Internal rotation: rotation of a limb towards the body.
• External rotation: rotation of a limb away from the body.