10. Animal Tissue - part 04 - Muscular tissue

 

10. Animal Tissue - part 04 - Muscular tissue


Muscular tissue :

  • The cells of this tissue are elongated and are called muscle fibres. 
  • Each muscle fibre is covered by a membrane sarcolemma.
  • Cytoplasm of muscle cell is called sarcoplasm.
  • Large number of contractile fibrils called myofibrils are present in sarcoplasm. 
  • One or many nuclei are present in muscle cell depending on the type. 
  • Myofibrils are made up of proteins, actin and myosin. 
  • Muscle fibres contract and decrease in length on stimulation. Hence, muscular tissue is known as contractile tissue. 
  • It is vascular tissue and is innervated by nerves too. 
  • Muscle cells contain large number of mitochondria.

A. Types of Muscular Tissue
1. Skeletal muscles : 
  • These muscles are found attached to bones. 
  • Skeletal muscles consists of large number of fasciculi which are wrapped by connective tissue sheath called epimysium or fascia. 
  • Each individual fasciculus is covered by perimysium. 
  • Each fasiculus in turn consists of many muscle fibres called myofibres.
  • Each muscle fibre is a syncytial fibre that contains several nuclei. 
  • The cell membrane called sarcolemma delimits the cytoplasm called sarcoplasm. 
  • Sarcoplasm contains large number of parallely arranged myofibrils hence nuclei get shifted to periphery. 
  • Each myofibril is made up of repeated functional units called sarcomeres. 
  • Each sarcomere has a dark band called anisotropic or 'A' band in the centre. 
  • In the centre of 'A' band is light area called 'H' zone or 'Hensen's Zone'. 
  • In the centre of 'H' zone there is 'M' line.
  •  'A' bands are made up of myosin as well as actin. 
  • On either side of 'A' band are light bands called isotropic or 'I' bands that contain only actin. 
  • Myosin are thick and dark coloured while actin filaments are thin and light coloured. 
  • Adjacent light bands are separated by 'Z' line (Z - Zwischenscheibe line). 
  • Dark and light bands on neighbouring myofibrils correspond with each other hence the muscle gets striated appearance.
  • Skeletal muscles show quick and strong voluntary contractions. They bring about voluntary movements of the body. 
Red and white muscles : 
  • On the basis of amount of a red pigment, skeletal muscles are of two main types – 
  1. Red muscles 
  2. White muscles 
  • Red muscles contain very high amount of myoglobin while white muscles contain very low amount of this pigment.
Myoglobin :
  • It is an iron containing red coloured pigment only in muscles. 
  • It consists of one haeme and one polyepeptide chain. 
  • It can carry one molecule of oxygen.
  • Due to presence of myoglobin, the muscles can obtain their oxygen from two sources, myoglobin and haemoglobin.
2. Smooth or Non-striated muscles :

  • These muscles are present in the form of sheets or layers. 
  • Each muscle cell is spindle shaped or fusiform. 
  • The fibres are unbranched having single nucleus at the centre. 
  • Sarcoplasmcontains myofibrils. Myofibrils are made up of contractile proteins actin and myosin. 
  • Smooth muscles contain less myosin and more actin as compared to skeletal muscles. 
  • Striations are absent. 
  • These muscles undergo slow and sustained involuntary contractions. 
  • They are innervated by autonomous nervous system.
  • These are found in the walls of visceral organs and blood vessels. Hence they are also called as visceral muscles. 
  • They may be arranged lengthwise (longitudenal muscles) or around circumference (circular muscles) of any organ.

3. Cardiac Muscles : 


  • Muscles of this tissue show characters of both striated and non-striated fibres. 
  • Sarcolemma is not distinct. Hence uni-nucleate muscle fibres appear to be multi-nucleate. 
  • Adjacent muscle fibres join together to give branched appearance to the tissue. 
  • Points of adhesion of muscle fibres are formed by transverse thickenings of sarcolemma called intercalated discs. 
  • These junctions at places allow cardiac muscles to contract as a unit. i.e. It helps in quick transfer of stimulus. 
  • The cardiac muscles are striated involuntary muscles.
  • Some mammalian cardiac muscles are modified are capable of generating impulse on their own. Hence mamalian heart is a myogenic heart. 
  • In some animals, cardiac muscles need neural stimulus to initiate the contraction. Such a heart is called neurogenic heart. 
  • Cardiac muscles form myocardium of the heart wall.

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