Physiology of Skeletal Physiology of Skeletal Muscle ContractionMuscle Contraction

The Muscle Action Potential The Muscle Action Potential ( AP )( AP )

Muscle RMP = -90 mV ( same as in Muscle RMP = -90 mV ( same as in nerves ) .nerves ) .

Duration of AP = 1-5 ms ( longer duration Duration of AP = 1-5 ms ( longer duration than nerve AP , which is usually about 1 than nerve AP , which is usually about 1 ms ) . ms ) .

Conduction Velocity = 3-5 m/s ( slower Conduction Velocity = 3-5 m/s ( slower than big nerves ) .than big nerves ) .

Each muscle Each muscle cell (fiber) is cell (fiber) is covered by a covered by a cell-membrane cell-membrane called called Sarcolemma.Sarcolemma.

Each cell Each cell contains contains between a few between a few hundreds to a hundreds to a few thousands few thousands Myofibrils.Myofibrils.

Each Myofibril contains Actin Each Myofibril contains Actin filaments (thin) & Myosin (thick) filaments (thin) & Myosin (thick) filaments .filaments .

Each myofibril is striated: consisting Each myofibril is striated: consisting of dark bands (called A-bands) and of dark bands (called A-bands) and

light (I-bands).light (I-bands).

SarcoplasmSarcoplasm==

matrix inside muscle fiber in which matrix inside muscle fiber in which myofilaments susbendedmyofilaments susbended

Sarcoplasmic reticulumSarcoplasmic reticulum==

it is endoplasmic reticulum inside it is endoplasmic reticulum inside sarcoplasm full of Casarcoplasm full of Ca..

T- tubulesT- tubules-:-:

extend from one side of muscle to extend from one side of muscle to other (function?)other (function?)..

SarcomereSarcomere = =

contractile unit of muscle, it is the contractile unit of muscle, it is the zone between two Z lines zone between two Z lines ( discs)=2 micrometer in length ( discs)=2 micrometer in length in resting statein resting state..

Z discs (lines)Z discs (lines) = lines extend all = lines extend all way across myofibrilsway across myofibrils

The functional unit of a myofibril is the Sarcomere

Inside each sarcomere there Inside each sarcomere there are 3 are 3 bandsbands::--

I band I band = of actin only= of actin only--

- -H bandH band= of myosin only= of myosin only

- -A bandA band= formed of actin & = formed of actin & myosin filamentsmyosin filaments

When contraction When contraction

takes place Actin & takes place Actin & Myosin slide upon Myosin slide upon each other , & the each other , & the distance between two distance between two z-discs decreases : z-discs decreases : This is calledThis is called

Sliding Filament Sliding Filament Mechanism Mechanism

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

EM Evidence for Sliding Filaments

Muscle ContractionMuscle Contraction muscle proteins :muscle proteins :

a. Thick filament: a. Thick filament: MyosinMyosin

b. Thin filament :b. Thin filament :

1.1.ActinActin

2.2. TroponinTroponin

3. 3. TropomyosinTropomyosin

Sarcomere filamentous proteins

From: Alberts et al.. 1994 Molecular Biology of the Cell.

Thick filament:

Myosin filament

it has it has head + tailhead + tail

cross bridges (?)cross bridges (?)

- Head has ATP site- Head has ATP site-?-?

Thin filament

Two F-Actin strands

Groove between the 2 F-actin strands

MOLECULAR MECHANISM OF MOLECULAR MECHANISM OF MUSCLE CONTRACTIONMUSCLE CONTRACTION

Excitation –contraction Excitation –contraction couplingcoupling

Events of muscle contractionEvents of muscle contraction : :*** *** Acetylcholine released by motor nerve Acetylcholine released by motor nerve »»»»» EPP »»»»» »»»»» EPP »»»»»

depolarization of CM (muscle AP) »»»»» depolarization of CM (muscle AP) »»»»» Spread of AP into T tubule »»»»»release of Ca from Spread of AP into T tubule »»»»»release of Ca from

sarcoplasmic reticulum into the cytoplasm sarcoplasmic reticulum into the cytoplasm »»»»» Ca combines with troponin »»»»» troponin pull »»»»» Ca combines with troponin »»»»» troponin pull

tropomyosin sideway »»»»» exposing the active site on tropomyosin sideway »»»»» exposing the active site on actin »»»»» myosin heads with ATP on them, attached to actin »»»»» myosin heads with ATP on them, attached to actin active site actin active site

»»»»» myosin cross bridges bend pulling actin toward »»»»» myosin cross bridges bend pulling actin toward center of sarcomere (Power stroke) using energy of center of sarcomere (Power stroke) using energy of ATP»»»»»ADP & P released »»»»» Linkage between actin ATP»»»»»ADP & P released »»»»» Linkage between actin & myosin broken as new ATP binds to myosin cross bridge & myosin broken as new ATP binds to myosin cross bridge >>> ATP hydrolyzed and cross bridge go back to its >>> ATP hydrolyzed and cross bridge go back to its original conformation. original conformation.

Events of muscle contractionEvents of muscle contraction::

When a new ATP occupies the vacant When a new ATP occupies the vacant site on the myosin head, this triggers site on the myosin head, this triggers detachment of myosin from actindetachment of myosin from actin

The free myosin swings back to its The free myosin swings back to its original position, & attached to original position, & attached to another actin, & the cycle repeat its another actin, & the cycle repeat its selfself

Events of muscle relaxationEvents of muscle relaxation::When ca is pumped back into When ca is pumped back into

sarcoplasmic reticulumsarcoplasmic reticulum

»»»»»»»»»» ca detached from troponin ca detached from troponin »»»»»»»»»» tropomyosin return to its original position tropomyosin return to its original position

»»»»»»»»»» covering active sit on actin covering active sit on actin »»»»»»»»»» prevent formation of cross bridge prevent formation of cross bridge »»»»»»»»»» relaxationrelaxation

Therefore , on order to release the head of Therefore , on order to release the head of Myosin from Actin , a new ATP is needed to Myosin from Actin , a new ATP is needed to come and combine with the head of Myosin .come and combine with the head of Myosin .

Q: What is Rigor Mortis ?Q: What is Rigor Mortis ? Q: ATP is needed for 3 things : what are they Q: ATP is needed for 3 things : what are they

?? ATP is needed for 3 things :ATP is needed for 3 things : (1) Power stroke .(1) Power stroke . (2) Detachment of myosin from actin active sites (2) Detachment of myosin from actin active sites (3) Pumping C++ (3) Pumping C++ back into the Sarcoplasmic reticulum . back into the Sarcoplasmic reticulum .

Q: Is muscle relaxation a passive or active Q: Is muscle relaxation a passive or active process ? process ?

A : it is active ; Why ? Because it needs ATP A : it is active ; Why ? Because it needs ATP ..

Q: Q: What happens to A-band and I-band What happens to A-band and I-band during contraction ?during contraction ?

Q: Ca++ is needed in nerve & muscle : Q: Ca++ is needed in nerve & muscle : when and where ?when and where ?

A : In nerve A : In nerve needed for exocytosis ( & needed for exocytosis ( & release of Ach)release of Ach)

In Muscle In Muscle needed for contraction . needed for contraction .

ThanksThanks