SARCOMERE, SARCOTUBULAR SYSTEM

DR.NILESH KATE. M.D.

ASSOCIATE PROFESSOR,DEPARTMENT OF PHYSIOLOGY,

ESIC MEDICAL COLLEGE & HOSPITAL,

GULBARGA.

OBJECTIVES. Introduction Classification of Muscles Structure of Skeletal Muscle Myofibrils Sarcomere Molecular level of Myosin and Actin Action of Actin, Myosin and Calcium in contraction Walk – Along –Theory Energy source of Muscle contraction Iso metric and Iso Tonic contraction

Introduction We are born with about 300 to 350 bones but

some of the bones fuse together as we grow and by the time we reach young adulthood we have on an average about 206 bones.

There are about 650 muscles in the body, and they are divided into three different types, skeletal, visceral and cardiac.

Introduction About 40% of your body weight is accounted for by

muscles. largest --The gluteus maximus muscle, located in the

buttocks. Longest– Sartorius. Smallest – stapedius. Strongest – Masseter in the jaw. Shortest– styloglossus. Tensor tympani (skeletal) Contrary to what people assume, muscles do not

push, but can only pull.

Muscle Classification

Functional anatomy & organization.Structural organization•Endomysiym.

•Fasciculi – Perimysium.

•Muscle belly – Epimysium.

•Tendons.

Structural organization

Structure of muscle fibre.Long cylindrical.

Sarcolemma.

Sarcoplasm.Cell organelle.Sarcoplasmic reticulum.Myofibrils.Sarcotubular system.

Myofibrils

Diameter- 1-2 μm Length – 1-4 cm. Made up of

Myofilaments. Thick – Myosin.(1500) Thin – Actin.(3000)

Myofilaments

Arrangement of Myofilaments under light microscope. Alternate light & dark

bands.

Striations of muscle fibres.

Due to difference in the refractive index. ‘A’ Band – Dark band

Anisometropic to polarized light. Length --- 1.5 μm ‘H’ zone – centre of A band

H means Henson or “Hell” – light.

Striations of muscle fibres.

“M” Line – centre of H zone. Pronounced during muscle contraction.

I Band - Light band. Isotropic to polarized light. “Z” Line – Dark

Z Wischenscheibe – German- Between disc.

Sarcomere A portion of myofibril

that lies between two successive Z discs is called sarcomere.

Sarcomere = ½ I band + A band +

½ I band. The structural &

functional unit of muscle fiber.

Myofilaments. Contractile apparatus.

Contractile proteins. Myosin Actin

Regulatory proteins. Troponin Tropomyosin.

Anchoring proteins. α Actinin Titin. Nebulin Dystrophin

Thick filaments. Twice the diameter. Each thick filament

surrounded by 6 thin flaments.

Myosin . Mol.wt – 4,80,000. 6 polypeptide chains 4 – light 2 – heavy.

Myosin .

Globular head. Myosin II The tails of myosin molecules

bundles to form the body of myosin filament with heads hangs to the side.

The part of head hangs from body form the arm

Thus the protruding arms and heads -- cross- bridges

Participate is actual muscle contraction.

Thin filaments. Head end – extend into

A band Tail end – anchored to

the Z line. Contractile protein

Actin Regulatory proteins

Tropomyosin Troponin.

Actin 300-400 molecules/ thin

filaments.

Long double helix of 2

chains of globular units.

Globular molecules-G

Actin.

Chain formed – F Actin.

Tropomyosin 40-60 molecules/ thin

filaments. Long filaments. Lie in groove between

2 actin filaments. Covers binding sites

on actin. Thus regulatory

proteins.

Troponin Small globular units.

3 subunits.

Troponin ‘I’ – Affinity for

Actin.

Troponin ‘T’ – Affinity

for Tropomyosin.

Troponin ‘C’- Affinity for

Calcium

Anchoring proteins. α Actinin – cross link the Actin

filaments in the area of Z line.

Titin. – Interconnect Z line.

Nebulin.– connect α actinin with

troponin- tropomyosin complex.

Dystrophin-glycoprotein

complex.– provide structural

support & strength.

Sarcotubular system. Sarcotubular system –

with sarcoplasmic reticulum

Imp role in internal conduction of depolarization within the muscle fibre.

Formed by Transverse tubular

system (T) Longitudinal

sarcoplasmic reticulum.

Transverse tubular system (T) Invagination of

sarcollema into muscle fibre at junction of A & I bands.

Rapid transmission. Its membranes contains

voltage gated Ca channels Dihydropyridine receptors.

Activate longitudinal SR.

Longitudinal sarcoplasmic reticulum.

Sarcoplasmic tubules of sarcoplasmic reticulum.

Run in long axis of muscle fibres.

Do not open to exterior. Terminal cisterns. Triad- T tubule + 2

terminal cisterns. Ryanodine receptors.

Energy source for Muscle Contraction The energy source -- ATP

Large amount of ATP are cleaved during contraction process. When greater the work is done more ATP is cleaved. This is called Fenn effect.

ATP – 1 to 2 seconds

Phosphocreatine – 5 to 8 seconds

Glycogen – 1 minute

Oxidative metabolism – for many hours.

Isotonic Contraction Isotonic contraction:

In this muscle contract with change in length of muscle fibre but no change in tension.

Eg., Walking, Running,

Lifting a load

Isometric contraction:In this type the muscle contract

with change in tension but no change in length of the muscle fibre.

Eg., Muscle which helps in

maintaining posture against gravity.

Contraction of the arm muscles when trying to push a wall.

Summary Introduction Classification of Muscles Structure of Skeletal Muscle Myofibrils Sarcomere Molecular level of Myosin and Actin Action of Actin, Myosin and Calcium in contraction Energy source of Muscle contraction Iso metric and Iso Tonic contraction

Thank you.