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Locomotion in Humans

In humans, locomotion is brought about by the muscles. Such muscular movements is brought about by the contraction of muscle fibres. 

Types of Muscles
  1. Skeletal Muscles
  2. Cardiac Muscles
  3. Smooth Muscles
Principle Types of Muscles
Three principle types of muscles occur in animals. These are striated, smooth and cardiac muscles. These three different types of muscles differ in their structure (histologically), in location (anatomically), in function (physiologically), and in the manner of their innervations (neurologically).

Striated Muscles

These muscles are multinucleate and display longitudinal and cross striations of alternating light (isotropic) and dark (anisotropic) bands and are thus normal striated muscles. In higher animals, these muscles are attached to the skeleton of the body and are involved in skeletal movements. Therefore, these muscles are also called skeletal muscles. These are also called voluntary muscles because they are under the control of the central nervous system rather than the autonomic nervous system.

Structure of skeletal muscles
(A) muscle fibres
(B) cross-sectional view

The skeletal muscle is made up of a large number of muscle fibres arranged in bundles. Each muscle fibre is made up of smaller units called myofibrils. The detailed structure of these myofibrils is seen only under the electron microscope.

Ultrastructure of the Skeletal Muscle

Under the microscope each myofibril shows cross striations due to alternating light and dark bands. The dark bands are called A or anisotropic bands: the light bands are called I or isotropic bands. Dense Z lines cross the centre of each I band. The distance between two I lines is called a sarcomere. In other words, the myofibrils are formed of numerous sarcomeres arranged in a pile. The light and dark bands give the muscle fibre its characteristic striated appearance. The light and dark bands are due to regularly arranged thick and thin protein filaments. The thick filaments consist of the protein, myosin, and the thin filaments of actin. The actin and myosin help in the contraction of shortening of muscle by the formation of cross bridges. The sarcomere is the functional unit of the contractile system in muscle. The events occurring in a sarcomeres are duplicated in the other sarcomeres along the myofibrils.

Smooth Muscles

In the vertebrates these muscles are also known as involuntary and visceral muscles. They are called involuntary because the activities of these muscles are not under the control of will and , thus, are controlled by the autonomic nervous system These are called visceral muscles because they are found predominantly in visceral tissues such as in the walls of the alimentary, circulatory, respiratory and urinogenital tracts. They are also found in the interior of the eye ciliary muscles and the muscles in the iris) in skin and in the ducts of glands.

Structure of Smooth Muscle: A-TS and B-LS

Muscle fibres of the smooth muscle are spindle-shaped and each fibre contains one centrally placed nucleus (uninucleate). The smooth muscle fibres are innervated by the sympathetic and parasympathetic nervous system. The smooth muscle fibres are unstriated. These muscle fibres also have actin and myosin which are not aggregated into filaments and so striations are not seen.

Cardiac Muscles

These muscles are found in the heart of animals (vertebrates). The fibres of these muscles are involuntary in nature as their activities are not under the control of will. The fibres of these muscles are both longitudinally and transversely striated. The cardiac muscle fibres exhibit branching and contain myofibrils and filaments of actin and myosin arranged similarly as it is in the skeletal muscle fibre. But the cardiac muscle fibres are branched and each contains one or two centrally placed nuclei.

The branching of the cardiac muscle enables the contraction wave to spread to all fibres throughout the heart. Specialised regions of cell membranes of two adjacent cardiac muscle fibres, are called intercalated discs. These function as boosters of the contraction wave and allow the contraction to be transmitted from one cardiac muscle to another. The cardiac muscle is innervated by the autonomic nervous system.

Structure of the Cardiac Muscle

Thus the three types of muscles differ in their structure, location in the body, innervation and function. The important similarities and differences of these muscles are summarised in Table below. 


Table - Difference among skeletal, smooth and cardiac muscles

Skeletal Muscle

Smooth Muscle

Cardiac Muscle

Structural Differences 

(i) Fibres are long thread like and unbranched.

Comparatively shorter, unbranched and thread like.

Fibres are branched.

(ii) Striated and multinucleate

Non-striated, uninucleate.


(iii) Thick myosin and thin actin filaments in the sarcomere.

Myosin and actin are not aggregated into thick and thin filaments.

As in skeletal muscle fibres.

(iv) Intercalated discs are not present.

Not present.

Intercalated discs are present which act as boosters to contraction.


Attached to bones, extrinsic eyeball muscles and upper part of oesophagus.

Present in walls of blood vessels and large lymphatics, tubular viscera of digestive, Respiratory and urino- genital tracts.

Present in the wall of heart.


Functional Differences 

Movement of bones and different parts of the body, e.g., hands, legs, eyes and facial muscles, etc.

Contraction of stomach and intestine i.e. peristalsis. Expulsion of urine from the urinary bladder, pushing out of foetus during childbirth.

Contraction of heart.

Nerve Supply

Receives nerve supply from the central nervous system. They are under voluntary control.

Supplied with nerves from autonomic nervous system. They are involuntary in nature.

Supplied with autonomic nervous systems , involuntary.

The Muscular System

The variety of movements that animals perform are done with the help of an extensive system called the muscular system. Muscles consist of thousands of elongated fibres or cells organised in a variety of ways and bound together by connective tissue.

The bones of the skeleton are passive structures to which the active skeletal muscles are attached and by means of which body parts are moved, or the body as a whole is moved from place to place.

A muscle functions by contracting. This is initiated by the transmission of a nerve impulse, which may be voluntary. The muscles of the skeleton are under the control of the will, and those of the viscera (stomach, intestines, etc.) and heart are not.

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