The word muscle is derived from Latin word mus which means a mouse. Most of muscles of vertebrates resemble a mouse in their external appearance, the tail of the mouse representing the tendon of the muscle.
Usually the muscle tissue forms about one half of the total mass of an average vertebrate. Functionally the muscles are also extremely important due to following reasons:
1.They take part in locomotion.
2.They maintain the posture and stability.
3.They conserve the internal heat of the body.
4.Even the highly sophisticated activities of human brain are expressed in the form of muscular contraction, e.g., the sorrow and grief is expressed by contraction of muscles of face.
The property of contractility, which is one of the fundamental properties of the living substance, is significantly developed and highly specialized in muscle tissue. The later is composed of highly excitable and contractile elongated cells called muscle cells. The mesenchyme is mainly responsible for forming muscle cells. By differentiation three important varieties of muscle-cells emerge :
1.Skeletal muscle cell.
2.Smooth muscle cell.
3.Cardiac muscle cell.
These three varieties of cells are quite different from each other in their structure and function.
When the anatomists are talking of muscular system, they are referring to skeletal muscles. There are over 650 skeletal muscles in the human body.
However, it is important and realistic for all beginners’ to know about 75 pairs of muscles involved in maintenance of posture and body movement. In addition, they should have knowledge of muscle in movements of eye, facial expression, speech, mastication and deglutition (swallowing).
CLASSIFICATION:
The muscles could be classified as follows:
According to Structure:
Depending upon the microscopic structure of muscle cells, the muscles are classified into following subgroups:
A)Striated muscle or skeletal muscle :
It is composed of long, cylindrical, multi-nucleated contractile cells which show rapid and unsustained contraction, and are under the control of somatic nervous system.
These cells show dark and light bands under light microscope. These bands are responsible for striated appearance of skeletal muscles.
1.Muscles attached to long bones.
2.Diaphragm.
3.Tongue muscles.
4.Most of the muscles of eye ball.
5.Muscles of facial expression.
6.Muscles of pharynx and upper third of oesophagus.
7.Muscles of middle ear.
Smooth Muscle:
It is composed of long spindle-shaped contractile cells, which show slow and sustained contraction and are under the control of autonomic nervous system.
These cells are devoid of any type of striations when seen under light microscope. Hence these cells are called smooth muscle cells.
1.Most of the viscera (Organs).
2.Blood vascular system.
C)Arrector pili muscle of skin.
Cardiac Muscle :
It is composed of cylindrical, branching and anastomosing contractile cells, which show myogenic rythmic contraction and relaxation, however, their activity is modified by autonomic nervous system.
Microscopically they show striations (a feature of skeletal muscle) and a single central nucleus (a feature of smooth muscle). Thus the cardiac muscle cells show characteristics in between skeletal and smooth muscle. Due to their unique mode of contraction and structure, they are grouped as a separate entity.
1.Myocardium of heart.
2.In the walls of great veins, near their entrance into the heart.
II.According to function:
A)Functionally the muscles are classified into three groups.
Voluntary Muscles :
These are those muscles whose contraction is under the control of our will, e.g., we can move our hand whenever we like we can produce wrinkle over the forehead whenever it is desired. Such muscles are called voluntary muscles. They form about 43% of body weight and are engines or the motors of the body.
i)All skeletal muscles except the muscles of:
ii) Upper part of oesophagus, which are skeletal in character but involuntary in their action.
Smooth muscles of ciliary body which are concerned with accommodation because focus of the eye can be changed by effort of will (Clark, 1975).
Involuntary muscles :
These are those muscles whose contraction is automatic, not under the control of our will and is regulated by autonomic nervous system, e.g., our intestine are moving without our desire.
Example:
1. All smooth muscles
2.Skeletal muscles of larynx, pharynx and upper part of oesophagus.
Cardiac muscle :
It is a unique muscle in the body, in which the control of contraction is inherent myogenic activity. It should be remembered that autonomic nervous system is not controlling the activity of heart. However, it takes part in modification of activity.
Embryology offers an explanation of the control. The heart starts beating in the third week of intra-uterine develop- ment, when the nervous system is in the process of early differentiation. Due to this reason the cardiac muscle is grouped separately according to function.
Examples:
- Myocardium of heart.
- Great veins near their entrance into the heart.
- According to Development
- Developmentally the muscles are of two types Mesodermal muscles
- These are those muscles which are derived from the mesodermal tissue in the embryo.
Example:
Almost all skeletal, smooth and cardiac muscles are mesodermal in origin.
Ectodermal muscles :
These are those muscles which are derived from embryonic ectoderm by differentiation.
Examples:
- Smooth muscles of the iris.
- Arrector pili muscle of skin.
- Myo-epithelial cells associated with certain glands of body, e.g., sweat glands.
- IV. According to Phylogenetic History :
The muscles are most appropriately classified on the basis of Phylogenetic history of vertebrates into following groups:
Somatic muscles :
These muscles are characterised by following features:
1.They are universally striated.
2. They typically develop from mytomes.
3.They are controlled by somatic nervous system..
4. They serve to adapt the organism to its external environment.
They are sub-divided into two sub-groups:
Somatic axial muscles:
They are limited to the central or axial skeleton of the body.
Examples:
1.Trunk muscles.
2. Muscles of eye ball.
3)Somatic appendicular muscles:
4)They are limited to appendicular skeleton.
Examples:
1.Muscles of upper limb.
2.Muscles of lower limb.
Visceral Muscles :
These muscles are characterised by the following features:
1. They are mainly connected with the primitive gut.
2.They are derived from mesenchyme surrounding the endodermal gut tube and not from myotomes.
3.They are controlled or modified by autonomic nervous system..
4.They adjust the internal environment of the individual organism by providing motive power in the process of:
i)Digestion
ii)Circulation
iii)Secretion
iv)Excretion
They are sub-divided into two sub groups:
a)Visceral Pharyngeal muscles or muscles of pharyngeal arch apparatus:
They are associated with structures developing from pharyngeal arch apparatus, e.g., middle ear, and are striated in appearance.
Examples:
1.Muscles of mastication.
2.Tensor tympani and stapedius muscles of the middle ear.
Visceral smooth muscles:
They are associated with endodermal gut tube and are non-striated in appearance.
Examples:
1.Muscles of stomach
2.Muscles of intestine
3.Muscles of urinary bladder
FUNCTIONS OF MUSCLES IN GENERAL:
- They are concerned with locomotion and maintenance of balance, e.g., muscles of limbs and trunks.
- They give form and shape to the body.
- They support other organs of the body e.g., muscles of the abdominal wall are supporting the various abdominal viscera.
- They give attachments to muscles and ligaments, e.g., mylohyoid muscles.
- They help in speech, e.g., muscle of larynx and tongue. They conserve heat.
- They take
- part in respiration, e.g., muscles of chest wall.
- They play a role in maintenance of fluid balance, e.g., muscles of lower limb and abdominal wall promote venous return and indirectly the fluid balance.
- The muscles of heart help in pumping the blood to whole of the body.
- They help in digestive process, e.g., muscles of mastication, muscles of gastro-intestinal tract.
- They help in micturition, e.g., muscles of urinary bladder.
- They help in sexual act, e.g., muscles of genital tract, pelvic diaphragm, muscles of anterior abdominal wall, muscles of inner side of Thigh, etc.
- They help in facial expressions and thereby act as agents of brain, e.g., muscles of facial expression.
- They prepare the individuals for fright.