What is Muscle? Structure, Types and its function

Muscle is a contractile tissue. Muscle cells are specialized for contractile ability.  The characteristic property of muscular tissue is its ability to contract when excited. The property of conductivity is also well developed in muscular tissue through not to the degree as that of nervous tissue. This topic covers the types of muscle, skeletal muscle structure, and function.

The fundamental unit of a muscle is the muscle fibre of which many fuses and constitute a whole muscle. The size of muscle fibres has no direct relationship with the muscle they constitute. Some of the muscle fibre is small and short, while other muscles consist of large muscle cells running the whole length of the muscle.

Generally, muscle fibres are covered over by a layer of collagen fibres and connective tissue. According to Bowman (1940), muscle fibres are covered by a membrane, Sarcolemma, consisting of a typical plasma membrane with a trilaminate structure and an outer basement membrane. The fluid portion of muscle fibre is called as Sarcoplasm.

Types of Muscles


Three types of Muscles are recognized in the higher animals and man. They are

  1. Skeletal muscle (or) Striated (or) Voluntary Muscles
  2. Smooth Muscles (or) Unstriated (or) Involuntary Muscles
  3. Cardiac muscle (or) Striated involuntary muscles

1. What is Skeletal Muscle?


The skeletal (voluntary and striated) muscle fibres are multinucleated cylindrical structures having a clear display of longitudinal and cross-striations. This muscular tissue is responsible for the voluntary movement of the living system.

The skeletal muscle is composed of numerous muscle fibres (or Cells). Each muscle fibre (cell) is cylindrical, multinucleated, varying 1-120mm. In length and 10 to 100 microns in diameter. It is surrounded by a tough membrane called sarcolemma, and outer basement membrane which is continuous with endomysial connective tissue. The muscle fibres are united together by connective tissue to form a muscle bundle. The muscle fibres taper at both ends and are firmly united at their blunt ends forming tendons, which in turn are connected with the bones at their secondary ridges.

Each muscle cell has numerous myofibrils, 1 micron in diameter which runs in an axial direction along the length of muscle fibre and is contained in the sarcoplasm. A myofibril, under the electron microscope, is seen to consist of about 1500 myosin (thick) filaments and twice its number of actin (thin) filaments. These two types of filaments are made up of proteins, lie side by side and are instrumental for muscle contraction.

Skeletal muscle location

Smooth muscle fibres are located in walls of hollow visceral organs, except the heart, appear spindle-shaped, and are also under involuntary control. Skeletal muscle fibers occur in muscles which are attached to the skeleton. They are striated in appearance and are under voluntary control.

Structure of Muscle

Muscle is the single largest tissue of the human body. Muscle constitutes about 20% of body mass at birth, 40% in young adults and 30% in aged adults. There are three types of muscles are present in vertebrates. They are mentioned above. The skeletal and cardiac muscles are striated and smooth muscles are non-striated.

The structure of striated muscle is explained in the diagram.  It is composed of bundles of multinucleated muscle fibre cells. Each cell is surrounded by an electrically excitable plasma membrane, called sarcolemma. The muscle fibre cells are long which may extend the entire length of the muscle. The intracellular fluid of fibre cells is the sarcoplasm into which the myofibrils are embedded. The sarcoplasm is rich in glycogen, ATP, Creatine phosphate, and the enzymes of glycolysis.

When the myofibril is examined under the electronic microscope, alternating dark bands (anisotropic or A-bands) and light bands (isotropic or I Bands) are observed. The less dense central region of A-band is referred to as H-band (or H-line). A narrow and dense Z-line bisects the I-band. The region of the muscle fibre between two Z-lines is termed as Sarcomere. The sarcomere is the functional unit of muscle.

In the electron microscopy, it is further observed that the myofibrils are composed of thick and thin longitudinal filaments. The thick filaments contain the protein myosin and are confined to A-band. The thin filaments lie in the I-band and can extend into A-Band (but not into H-line). These thin filaments contain the proteins actin, tropomyosin, and troponin.

The composition of Skeletal muscles

The composition of skeletal muscle is given below.

  1. Water – 75%
  2. Solids – 25%
    1. Proteins (20%) – These are Actin and Myosin (Contractile proteins)
    2. Fats (0.2%) – They are Cholesterol, lecithin, and Natural fat
    3. Carbohydrates (1.0%) – They are Glycogen (0.5 to 1.0%) and Hexose phosphate (0.05%)
    4. Inorganic Salts (1.0 to 1.5%) – Contains potassium phosphate, Ca, Na, Mg, Fe, Cl and sulfate
  3. Extractives
    1. Non-Nitrogenous: 
  • Lactic acid (0.02%) – It is present in fresh resting muscle
  • Inositol (0.25%) – Muscle sugar, (Hexahydroxycyclohexane)
    1. Nitrogenous
      • ATP (0.25%) and AMP (0.15%)
      • Creatin phosphate (0.35%)
      • Xanthine, Hypoxanthine and Inosinic acid – These are derived from an adenylic acid of the tissues and the nucleoprotein of the nuclei.
  1. Carnosine (0.3%): It is a dipeptide (beta-alanyl histidine)
  1. Pigments
    1. Myoglobin (Myohemoglobin) – It is an iron containing chromoprotein found in red muscles.
    2. Cytochrome (Myohematin): It is an iron-porphyrin pigment found in three forms
    3. Flavines and Others
  2. Enzymes

Structure of Skeletal Muscle


Skeletal muscle is responsible for the movement of the skeleton (which is why we usually find it attached to bones through tendons) and is the most abundant in the human body (it accounts for approximately 40% of body weight). It is characterized by contracting only in response to a signal transmitted by a somatic neuron.

At the level of structure and anatomy of the skeletal muscle, we have to imagine that it is as if it were a thick cord formed by multiple finer cords and these by thinner threads still:

Organization of individual fibrils that form a muscle and layers of connective tissue

Skeletal muscle is made up of muscle fibers (it is the name given to muscle cells ), covered with a layer of connective tissue that separates them from adjacent fibers ( endomysium ) and groups them together creating bundles or fascicles , among which we find another layer of tissue denser connective, the perimysium(formed by collagen, elastic fibres, nerves, and blood vessels) and all of them are covered by a fibrous sheath, the epimysium .

The muscle cells are cylindrical and elongated and can have hundreds of nuclei: they are the largest in the body.

In the physiology of muscle cells a specialized terminology is used, according to which the cell membrane is called sarcolemma(outer surface of the muscle fibre), the cytoplasm  is known as  sarcoplasm (semifluid material containing soluble proteins, glycogen particles, fat, and mitochondria) and the endoplasmic reticulum is called a sarcoplasmic reticulum (a network of transverse tubes ( T tubules ) and terminal cisterns that move the action potentials from the cell surface into the fibre).

Inside each fibre we find more than a thousand myofibrils, intracellular structures responsible for muscle contraction (1-2 μm in diameter), which are arranged in the longitudinal direction and are formed, in turn, by filaments of large protein molecules: actin(thin filaments) and myosin (thick filaments).

In the microscopic vision, the position of thin and thick filaments along the myofibril generates a pattern of dark and clear bands that is repeated and called the sarcomere, in which we distinguish:

  • A clear area occupied only by thin filaments (called Band I), a darker zone ( Band A ), which occupies the entire length of the thick filaments and in which we find an area in which they overlap with the thin filaments and a central zone composed only of thick filaments ( Zone H ) Insertion points of the thick filaments (line M) and thin filaments (Z disks).
  • The sarcomere comprises the segment between two Z disks.
  • Accessory giant protein titin (with the function giving elasticity and stabilize the thick filaments) and nebulin(attached to the thin filaments, to keep them aligned).
  • Muscle contraction would be visualized at the molecular level by the filament sliding theory. According to muscle contraction, there is sliding on each other of thin and thick filaments, bringing the Z disks closer together.

2. SMOOTH OR VISCERAL MUSCLE


It is located in the excretory and reproductive system, in the blood vessels, in the skin and in the internal organs (in the walls of the hollow viscera). They form the involuntary muscles of the esophagus, stomach, and intestines. It is located forming part of the walls of the viscera such as the stomach, intestines, bladder, and uterus. It is found in all hollow organs except the heart. They are involuntary and their cells are elongated. It is characterized by its fibres that have only 1 core and its Myofibrils are smooth. It produces involuntary movements that are not controlled by the Central Nervous System.

CHARACTERISTICS:

  • Your cells form muscular aces and they are of fast contraction because they have myofibrils in the cytoplasm.
  • They are composed of uni-nucleated and spindle-shaped cells and do not show striations.
  • Its cells measure between 20 and 500 mm.

FUNCTION:

Operates independently of the will of the individual.

MUSCLE STRICT or SKELETAL

They are attached to the skeleton of a man, forming voluntary muscles. It differs from smooth muscle tissue by its fibers that are polynucleated and its myofibrils have lines or striate oriented transversely to the fiber. This tissue forms the muscles that mobilize the skeleton. The movements are voluntary and controlled by the SNC.

CHARACTERISTICS:

  • Its cells are polynuclear, they do not have intercellular space, and they are cylindrical and elongated.
  • Some experts say that they measure up to 3 cm and others say that it extends throughout the muscle.
  • It presents the striations.

Skeletal muscle function

  • Covers and protects the bones.
  • It gives movement to the axial and appendicular skeleton.
  • Maintains body posture.

3. CARDIAC MUSCLE


It constitutes the myocardium or contractile part of the heart. MIOCARDIO is formed by cardiac muscle tissue made up of striated muscle fibers that cross transversally with inter fibrillary connective tissue. It is responsible for pumping blood through the circulatory system by contraction. The heart muscle works involuntarily and rhythmically, without having nerve stimulation. It is a myogenic muscle, that is, self-excitable.

Source: Wikipedia

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