Functions of Muscles
The muscles of the body perform numerous functions. The major functions performed by human muscles are listed below.
1. The primary function of the muscles is to contract and to produce motion.
2. The muscles provide for ambulation (i.e. motion of the body from one place to another).
3. The muscles enable the body to take different postures.
4. The muscles help in developing muscular skills in various activities, for example. Games and Speech.
5. The muscles help in maintaining circulation of the body.
6. The muscles protect blood vessels.
7. Muscles help to maintain heat and fluid balance of the body.
8. Muscles help in conductance of respiration process inside the body.
9. Muscles help in defending ourselves. Through them and the CNS, we can express our face feelings and thoughts.
The muscular tissues are of different types. They can be classified on the basis of cross- striation and on the basis of nature of control.
1. Striations are the strips found on the muscular tissue. On the basis of cross striation found in muscular tissue, the muscle can be classified as:
a. Striated, and
b. Non Striated muscular tissue. The non striated muscle tissue can be either plain or smooth.
2. Different muscles are controlled by different means and different parts of the brain. Some are under our conscious control while others are not. On the basis of nature of control of the muscular tissue, the muscular tissue can be classified as:
a.Skeletal Muscle Tissue- The muscles of these tissues are called skeletal muscles. The skeletal muscular tissue is striated and voluntary (under Conscious Control).
b.Cardiac Muscle Tissue- The muscles formed of these tissues are called cardiac muscles. The cardiac muscle tissue is striated and involuntary (not under Conscious Control).
c.Visceral tissue- The visceral tissue is non striated and involuntary.
The muscle cells adopt themselves for the process of contraction and form a special fiber called muscle fiber for the contraction process.
Types of Muscles
The human body organs perform different functions and hence have peculiar shapes which facilitate the processes. Same is case with muscles. Muscles take different shapes for performing specific function. The types of muscular tissues that are found in human body are the Skeletal Muscles, the Cardiac Muscles and the Visceral Muscles. Different aspects of these muscle types are listed below.
The Skeletal Muscles
The Skeletal tissue forms the Skeletal Muscle Fiber. The length of the muscle fiber can vary from 1 to 40mm. The breath can vary from 0.01mm to 0.1 mm. The cell wall of the muscle fiber is called sacrolemma.These muscles form 40 to 50% of the total body weight of the body. The skeletal muscles contain many mitochondria (the power house of the cell) which are called sacrosomes. Myofibrils are found only in skeletal muscles. They are alternatively arranged dark and white striations with thick longitudinal strands. The skeletal fibers are striated and voluntary. The multi-nucleated skeletal muscle fibers are cylindrical in shape. They exhibit cross and longitudinal striations. All the voluntary movement of the human body (for example extending of arm or leg) is under the control of skeletal tissue. They are enabled to perform slow and sustained contractions (tonic contractions) and the rapid contractions.
If we study the skeletal muscle under electron microscope, we will find alternate dark and light shades that are collectively called transverse striations and the thick longitudinal strands that are formed of myofibrils. The myofibrils are separated by sacroplasm and the area comprising the sacroplasm and separated myofibrils is called Field of Cohnheim.
The dark bands of the myofibrils are called A band. They are doubly refractive. The light band is called the I band and it is monorefractive. The Z-line bisects the I band at its middle portion. Membranes that run across myofibrils form the Z line. Sacromere is the name given to the area enclosed by two Z lines. The length of sacromere varies from 2 to 3 µm. In some myofibrils, the A band has a pale color and is called the H band. The M band is located at the middle of the A/H band. At the M band the myosin filaments have more thickness. The N line is found on the sides of the Z line.
The Myofilaments are formed of protein filaments. The thinner filament is the Actin filament and the thicker filament is the myosin filament.
Distribution of skeletal muscle
The skeletal fiber and tissues are attached to the bones by tendons. This set-up provides for voluntary movements. The skeletal muscles have high vascularity and have muscle pigments. This gives them pink color. With variations, the skeletal muscles can have pale (white) or red color.
Features of Skeletal Muscle Tissue
The muscular framework of the body is supported by many other cells. These include epimysium or the outer most covering cells of the muscle fiber. The muscle bulk is divided in smaller bundles that are called fascicule. These fascicule are bounded by perimysium. Every single fasciculus is enclosed in a connective tissue, the endomysium. All of the muscle cells (fibers) have many myofibrils. Each muscle fibril has a blood capillary attached to it.
Mechanism of skeletal muscle contraction
The skeletal muscles contract by sliding of Myofilaments (Interdigitiation). The muscle protein consists of Actin -Myosin complex (formed by shortening of muscle proteins). When the contraction nerve impulse reaches the muscle fibers, the membrane of the skeletal muscle tissue de-polarizes. Calcium ion is released from the Sacroplasm. The Myosin ATP and the myosin filaments slide over the Actin filaments. During relaxation the calcium ion goes back to the Sacroplasm. Thus, the myosin ATP gets depressed and the myosin filament goes back to its initial resting site.
Constituents of Skeletal Muscles
Out of the total weight of skeletal muscles, 75% is water content and 25 % solid content. The solid content of skeletal muscles is made up of-
Proteins- They form 20% of the solid content of the muscular tissue. Actin and Myosin proteins form more than half of total protein content of muscle protein.
Inorganic Salts- The skeletal muscles contain inorganic salts like calcium salts, phosphate salts and salts of the other metals. The total inorganic salt content in the solid part of muscles is 1 to 1.5 %.
Carbohydrates- They constitutes 1% of the total solid part.
Muscles also contain non nitrogenous (ex. lactic acid) and nitrogenous (ex.ATP) extractives. They do not exceed 1% of the total solid part of the skeletal muscle.
Skeletal muscles also contain xanthine, isosinic acid, hypoxanthine, carnosine, enzymes, coenzymes and pigment Myoglobin. Myoglobin binds oxygen similar to Haemoglobin in RBC.
The Cardiac Muscles
The Cardiac Muscle is formed of Cardiac Muscle Tissues which are striated and involuntary. They perform some key functions vital for maintaining life. These include:
Supply of oxygen and food nutrients to different body parts, and Removal of their metabolic wastes that get generated during different metabolic processes.
The rhythm and contraction of muscles is not under voluntary /conscious control. The cells of cardiac muscle cells are singularly nucleated. The cardiac muscle fiber bifurcates and combines with the adjacent and neighboring other fiber to form a 3D network. The cardiac muscles are the main muscles of the heart. They also found in the vessels that take the blood away from the heart and in the muscles that bring blood to the heart.
Contraction of Cardiac Muscles
The nerve impulse for contracting the cardiac muscles is generated in the Pace Maker of the heart. After reaching the cardiac muscles, the impulse is carried form one cell to another through junctions of the intercalated discs. The intercalated discs run along the transverse section of the myofibril. In structure, the discs are the areas where extensive cell contact occurs. The sacrotubular system is the bridge through which the impulses reach the myofibrils. The coronary arteries supply blood vessels to the cardiac muscles.
Composition of Cardiac Muscles
The cardiac muscles contain less protein, less glycogen, less ATP and phosphates than skeletal muscles. They contain more nucleoproteins, cholesterol, phospholipids and more nucleoprotein than skeletal muscles.
The visceral muscles are smooth, involuntary, non striated and plain in character. Thus, they are not under conscious control. These muscles are found in the hollow viscera of the body. Viscera are that part of the body that contains soft internal organs. For example, the thoracic and abdominal cavities are visceral cavities. Thus, the visceral muscles are found in:
The ducts of exocrine glands,
Urogenital system, and
Visceral cells are also found in the iris of the eye, in ciliary bodies and in the dermis of the skin.
Mechanism of Contraction in Visceral Muscles
The visceral smooth muscles contract in the same way as the cardiac muscles do, automatically. But they contract slowly in comparison to the cardiac muscles. Stretching of a muscle (for example the stretching of GI tract due to passage of food) is a cause of generation of contraction impulses. The stimulation of motor nerve causes the modification of actions of visceral and skeletal muscles. The visceral muscles also undergo rhythmic contractions the way cardiac muscles do.
Composition of Visceral Muscles
The cardiac muscles contain less protein, less glycogen, less ATP and phosphates than skeletal muscles. They contain more nucleoproteins and sodium than skeletal muscles.
Different physiological changes that occur during contraction of a muscle
Various physiological changes occur inside the human body during the contraction of muscles. The changes that accompany contraction of a muscle are categorized and explained below.
Chemical changes- The ATP of the muscles breaks down into ADP, thereby releasing energy and phosphate. Glycogen and creatinine phosphate break down and the energy liberated is used to re-synthesize ATP.
Mechanical changes- when contraction occurs, the muscle fibers increase in thickness. But their length is shortened. Thus their resultant volume remains the same.
Thermal changes- Heat is produced during contraction of muscles. The initial heat is the name given to heat that is formed at the start of contraction of the muscle fiber. Heat is also produced after the contraction process is over. It is called recovery or delayed heat.
Electrical changes- Electrical impulses are also produced which can be measured.
Types of muscle contractions
1.Isotonic contractions: The muscle is said to be in Isotonic contraction, when it contracts with constant load that it can lift. In this type the muscle maintains an equal tension or tonus.
2.Isometric contractions: Where a muscle contracts against a weight that it cannot lift, it is said to be isometric condition and it maintains uniform length. In this case tension increases sharply but no mechanical work is done.
Rigor mortis: It is the contraction of muscles that occur after death. When muscles passes into rigor mortis its ATP and the glycogen content disappear and Lactic acid accumulates in it.
Muscle fatigue: If energy is not continuously supplied to a contracting muscle, actomysin- ATP complex cannot be regenerated and muscle becomes fatigued.This condition is a state where a muscle experiences a diminished ability to contract as a result of prolonged stimulation. Several factors like lack of oxygen, accumulation of Lactic acid and decreased level of phosphocreatine are responsible for this.
Muscle treppe: When muscle is stimulated repeatedly at regular intervals, the first 5-15 contractions increase progressively. This is called treppe or staircase phenomenon.
Muscle twitch: Contractions offered by a skeletal muscle in response to single stimulus. It has latent period of 0.1 second, contraction period of 0.4 second and relaxation period of 0.5 second.