MUSCLE STRUCTURE AND FUNCTIONS

Mr. Sunil Mehta -DNEP(INK)
Neuroelectrophysiologist
Neotia Getwel Healthcare Centre








MUSCLE STRUCTURE AND FUNCTIONS


INTRODUCTION:

•Human body contains over 400 skeletal muscles

40-50% of total body weight.

Functions of skeletal muscle

•Body movement (Locomotion)

•Maintenance of posture

•Respiration:-

Diaphragm and intercostal contractions

•Communication (Verbal and Facial)

•Constriction of organs and vessels:-

 Peristalsis of intestinal tract
 Vasoconstriction of blood vessels and other structures (pupils)

•Production of body heat (Thermogenesis)

Skeletal muscles

·Most are attached by tendons to bones

·Cells are multinucleated

·Striated – have visible banding

·Voluntary – subject to conscious control

·Cells are surrounded and bundled by connective tissue =

 great force, but tires easily


•Sarcolemma = Cell membrane

•Sarcoplasm = Cytoplasm

•Sarcoplasmic Reticulum = Endoplasmic Reticulum

•Sarcosomes = Mitochondria


Structure of skeletal muscle: connective tissue covering


•Epimysium

     Surrounds entire muscle

•Perimysium

        Surrounds bundles of muscle fibers

•Endomysium

      Surrounds individual muscle fibers

Muscle proteins

Contractile proteins 

•Actin- thin myofilament

•Myosin- thick filament

Regulatory proteins

•Tropomyosin

•Troponin

Attachment proteins 

•Titin, nebulin, alpha actinin, dystrophin

Skeletal muscle structure


•Composed of muscle cells (fibers), connective tissue, 

blood vessels, nerves

•Fibers are long, cylindrical, and multinucleated

•Tend to be smaller diameter in small muscles and larger in 

large muscles. 1 mm- 4 cm in length



Muscle fiber anatomy


•Sarcolemma - cell membrane

 Surrounds the sarcoplasm (cytoplasm of fiber)

•Contains many of the same organelles seen in other 

   cells

•An abundance of the oxygen-binding protein myoglobin

Punctuated by openings called the transverse tubules 

(T-tubules)

•Narrow tubes that extend into the sarcoplasm at right 

angles to the surface

•Filled with extracellular fluid

•Myofibrils -cylindrical structures within muscle fiber

Are bundles of protein filaments (myofilaments)

•Two types of myofilaments

1.Actin filaments (thin filaments)

2.Myosin filaments (thick filaments)

-At each end of the fiber, myofibrils are anchored to the

inner surface of the sarcolemma.

-When myofibril shortens, muscle shortens (contracts).

•Thin Filament: composed of 3 major proteins

1.F (fibrous) actin

2.Tropomyosin

3.Troponin

•Two strands of fibrous (F) actin form a double helix 

extending the length of the myofilament; attached at 

either end at sarcomere

–Composed of G actin monomers each of which has a myosin-

  binding site

–Actin site can bind myosin during muscle contraction.


•Tropomyosin: an elongated protein winds along the groove

 of the F actin double helix. 

•Troponin is composed of three subunits:

–Tn-A : binds to actin

–Tn-T :binds to tropomyosin

–Tn-C :binds to calcium ions.

•Many elongated myosin molecules shaped like golf clubs.

•Single filament contains roughly 300 myosin molecule

•Molecule consists of two heavy myosin molecules wound 

together to form a rod portion lying parallel to the myosin 

myofilament and two heads that extend laterally. 

•Myosin heads

1.Can bind to active sites on the actin molecules to form cross-

bridges. (Actin binding site)

2.Attached to the rod portion by a hinge region that can bend 

and straighten during contraction.


3.Have ATPase activity: activity that breaks down adenosine 

triphosphate (ATP), releasing energy. Part of the energy is used

 to bend the hinge region of the myosin molecule during 

        contraction.

•Sarcomere - repeating functional units of a myofibril

 About 10,000 sarcomeres per myofibril, end to end

 Each is about 2 µm long

•Differences in size, density, and distribution of thick and thin

 filaments gives the muscle fiber a banded or striated appearance.

1. A bands: a dark band; full length of thick  (myosin) filament.

2. M line - protein to which myosins attach.

3.H zone - thick but NO thin filaments.

4. I bands: a light band; from Z disks to ends of thick filaments.

•Thin but NO thick filaments.

•Extends from A band of one sarcomere to A band of the next sarcomere.

5. Z disk: filamentous network of protein. Serves as attachment for actin 

   myofilaments.

6. Titin filaments: elastic chains of amino acids; keep thick and thin 

        filaments in proper alignment.

Sarcoplasmic Reticulum (SR)

•SR is an elaborate, smooth endoplasmic reticulum. 

–runs longitudinally and surrounds each myofibril.
–Form chambers called terminal cisternae on either side of the          T-tubules.

•A single T-tubule and the 2 terminal cisternae form a triad.

•SR stores Ca++ when muscle not contracting.

–When stimulated, calcium released into sarcoplasm.
– SR membrane has Ca++ pumps that function to pump Ca++ out 
of the sarcoplasm back into the SR after contraction.

Muscle contraction
•The sliding filament model

–Muscle shortening occurs due to the movement of the actin      
  filament over the myosin filament
–Formation of cross-bridges between actin and myosin filaments
–Reduction in the distance between Z-lines of the sarcomere

                                                                                                                            By- Mr. Sunil Mehta