MICROANATOMY OF MUSCLE FIBRE

Thick filaments

Thin filaments

Sarcomere

Each thick filament is made up of hundreds of molecules of myosin

Sarcotubular system Plays an important role in internal conduction of depolarisation and storage of calcium. Consists of :

Transverse tubular system – formed by invagination of sarcolemma into the muscle fibre in the region of junction of A and I bands, lumen has ECF

Longitudinal Sarcoplasmic reticulum – sarcoplasmic tubules running along long axis of muscle fibres

Longitudinal tubules on either side of T- tubules are dilated to form the so called terminal cisterns. A T-tubule with two terminal cisterns lying in close proximity constituite a triad at the junction of A and I bands. Thus there are two triads in each sarcomere. The cisternae store calcium

Excitation and contraction of skeletal muscle

Excitation:

Resting membrane potential - -90mv

Stimulation of motor neuron results in setting up of action potential in the motor end plate.

Excitation contraction coupling

1. Action potential reaches the ‘T’ tubules.

2. Activation of dihydropyridine receptors present on the ‘T’ tubule membrane.

3. Opening of calcium release channels (ryanodine receptors) in the sarcoplasmic reticulum.

4. Calcium ions diffuse into the cytoplasm. The conc. increases 2000 times.

5. The calcium ions get attached to Troponin-C , start chain of events which produce contraction.

1. Initiation of cross-bridge cycling

Binding Site Tropomyosin

Troponin Calcium

2. Formation of Actin- Myosin complex

Energised myosin head is at 90° to the thick filament- cocked up position

Myosin

3.Power stroke

Power stroke ADP and Pi are released Conformational change in myosin head occurs causing it to flex towards the arm of the cross- bridge. Flexion of head from high energy conformation to 45° conformation generates mechanical force. This causes actin filaments to slide over myosin filaments producing muscle shortening.

4. Cross- bridge detachment

ATP molecule attaches to myosin head and causes dissociation of myosin head from actin filament

Cocking of Myosin head

Energised head attaches to next actin molecule and the whole cycle is repeated. Thus with each cross – bridge cycle, there is movement of actin filament towards centre of myosin to a small degree. Such repeated cross- bridge cycling results in muscle shortening. This cycle continues till enough level of calcium is maintained in the cytoplasm.

From: http://www.3dotstudio.com/zz.html

SARCOMERE RELAXED

SARCOMERE PARTIALLY

CONTRACTED

SARCOMERE COMPLETELY CONTRACTED

Actin myosin contraction

http://www.sci.sdsu.edu/movies/actin_myosin.html

Muscle Relaxation

Within few seconds of A.P. calcium pump in the sarcoplasmic reticulum (Ca ATPase) transports calcium ions from sarcoplasm into longitudinal portion of sarcoplasmic reticulum. Removal of calcium from troponin restores the blocking action of troponin- tropomyosin complex. Myosin cross- bridge cycle ceases and muscle relaxes.

Role of ATP Binding of ATP is essential for detachment of myosin head from actin. Hydrolysis of ATP by Ca ATPase provides energy for removal of calcium ions from sarcoplasm. Applied: Rigor Mortis, which is shortening and rigidity of muscle after death due exhaustion of ATP molecules.