Types of Muscles and Muscle Fibre NEET Notes | EduRev

MUSCULATURE OF LIMBS
Study of muscles known as Myology.
Myology also known as Sarcology.

Development of muscle
Except Iris & ciliary body all muscles of body  develop from mesoderm.

• Muscle of Iris, ciliary body & myoepithelial cell of sweat gland develop from ectoderm. Conductivity & contractility are the two main characteristics of muscle.

Three types of muscles are found in the body.
(i) Voluntary or skeletal muscles.
(ii) Involuntary or smooth muscles.
(iii) Cardiac muscles.

Voluntary Muscle

Fig: Voluntary Muscle

It develops from the epimeric layer of myotome.
Skeletal muscles constitutes around 40% of adult body mass in an average healthy person.

• They are related to the skeletal system. So also called as skeletal muscles.
• Transverse lines are found at regular interval. Hence these muscles are also called as striped or striated muscle 
• Their contractions are controlled by will power of animal so also called voluntary muscle.

STRUCTURE OF MUSCLE FIBRE 

Fig: Structure of muscle fibre

Fine structure of muscle fibre:- 

•  The outer membrane of muscle fibre is called sarcolemma.
•  This cell membrane contain collagen fibres.
•  Each muscle fibre contain multinucleated sarcoplasm.
•  Nucleus & sarcoplasm are found in peripheral part.
•  Myofibril are arranged in parallel row & form the dark & light line. These lines are found in alternate order. These lines are made up of actin & myosin protein. Both proteins are filamentous proteins. 
• Actin filaments are thin while myosin filaments are thick.
• Light line or band is made up of only actin filament, these band are mono refractive in polarised light so it is called Isotropic band (I band).
• Actin filaments are connected with a disc of Z–line protein (Actinin) which is called as Z line (Zweichenschiebe) or (Doble's membrane) or (Krause's membrane). 
• Dark line is made up of actin & myosin filament so it is double mono refractive in polarised light due to overlapping so it is called Anisotropic band.
• The terminal end of actin filament are embedded among the myosin filament so peripheral part of A band is darker as compared to the middle part of A band which is called as H-Zone or Hensen zone.
• A dark line is also found in the central part of H-zone which is called as M line or mesogramme line. The distance between two Z–lines is called as Sarcomere. 
• It is structural unit or functional unit of voluntary muscle fibre.
  Sarcomere = 1A band + two half  Iband
  The Length of Sarcomere is 2.5 µm.
  (I band = 1µm, myosin = 1.5 mm) 
• 1 Myosin filament is surrounded by 6 Actin filaments & 1 Actin filament is surrounded by 3 Myosin filaments.
• Muscle fibre is covered by a layer of connective tissue which is called Endomysium.
• Many muscle fibres are combined to form a group which is called fasciculi.
• Each fasciculi is covered by a layer of connective tissue which is called Perimysium.
• Many fasciculi combined to form a muscle.
• Muscle is also covered by a layer of connective tissue which is called as Epimysium.
• Muscle fibre is a cylindrical or tubular shape.
• The muscle fibres attached to a tough cord of connective tissue called Tendon & Tendon is further attached with a bone.

FORCE GENERATING PROTEIN
Actin – The Actin or thin filament is a double helix made up of protein molecule called as G–Actin. (Globular actin) Many G-actin combined to form a filament like structure, which is called as filamentous-actin. G-actin contain a active site where myosin head is attached.

Myosin
Each myosin molecule consist of a tail & a head. Tail is made up of two chains intertwined with each other like double helix.
The myosin head has sites for attachments with
(i) The actin filament

 (ii) ATP molecules.
Each myosin (thick) filament is also a polymerised protein.

Fig: Myosin

Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM component, i.e.; the head and short arm projects outwards at regular filament and is known as cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

REGULATING PROTEIN
Tropomyosin – It is one type of contractile protein. In the relaxed state of the muscle situated in such a way, that the active sites remain covered by the tropomyosin & attached at the terminal end of actin.

Fig: Tropomyosin

Troponin – It is one type of protein which attached with one of ends of the tropomyosin molecules.
Troponin is made up of three sub unit.
(a) Troponin I    (Inhibitory site)  
(b) Troponin T   (Tropomyosin site)        
(c) Troponin C  (Ca⁺² binding site)

Fig: Troponin

STRUCTURAL PROTEIN
Actinin – It is one type of protein which found in Z–line.

Fig: Actinin