Download Muscle

Muscle
contraction
Students participating in the presentation:
1- naif aljabri 430101612
2- yousif alessa 430105885
3- faris abalkheel 430101692
4- Ali Moshaba AL-Ahmary 430103532
5-abdulelah bin numay 430104473
S KELETAL
MUSCLE
S KELETAL
MUSCLE
S KELETAL
MUSCLE
S KELETAL MUSCLE
&
M USCLE FILAMENTS

Skeletal muscle

Contraction of skeletal muscle is under voluntary control.

each skeletal muscle cell is innervated by a branch of a
motoneurons .

Muscle filaments

each muscle fiber behaves as a single unit

is multinucleate and contains myofibrils

the myofibrils are surround by sarcoplasmic reticulum are
invaginated by transverse tubules ( t tubules)

each myofibril contains interdigitating thich and thin filaments.
T HICH FILAMENTS
&
T HIN FILAMENTS

Thich filaments

are comprised of a large molecular weight protein called myosin

Thin filaments

are composed of 3 proteins: 1- actin.2-tropomyosin.3-troponin.
Arrangement of thick and
thin filaments in sarcomeres
CYTOSKELTAL
PROTEINS
T RANSVERSE TUBULE AND THE
SARCOPLASMIC RETICULUM

Transverse tubule and the sarcoplasmic
reticulum

are continuous with the sarcolemmal membrane and invaginated
deep into the muscle fiber, making contact with terminalcisternae
of the sarcoplsmic reticulum.
E XCITATION - CONTRACTION COUPLING
Excitation-contraction coupling

In skeletal muscle the method of excitation contraction coupling
relies on the ryanodine receptor being activated by a domain
spanning the space between the T tubules and the sarcoplasmic
reticulum to produce the calcium transient responsible for
allowing contraction.

The motor neuron produces an action potential that propagates
down its axon to the neuromuscular junction.

The action potential is sensed by a voltage-dependent calcium
channel which causes an influx of Ca2+ ions which causes
exocytosis of synaptic vesicles containing acetylcholine.

Acetylcholine diffuses across the synapse and binds to nicotinic
acetylcholine receptors on the myocyte, which causes an influx of
Na+ and an efflux of K+ and generation of an end-plate potential.
E XCITATION - CONTRACTION COUPLING

The end-plate potential propagates throughout the myocyte's
sarcolemma and into the T-tubule system.

The T-tubule contains dihydropyridine receptors which are
voltage-dependent calcium channels and are activated by the
action potential.

Opening of the Ryanodine receptors causes and flow of Ca2+ from
the sarcoplasmic reticulum into the cytoplasm.

Ca2+ released from the sarcoplasmic reticulum binds to Troponin C
on actin filaments, which subsequently leads to the troponin
complex being physically moved aside to uncover cross-bridge
binding sites on the actin filament.
E XCITATION - CONTRACTION COUPLING

By hydrolyzing ATP, myosin forms a cross bridges with the actin
filaments, and pulls the actin toward the center of the sarcomere
resulting in contraction of the sarcomere.

Simultaneously, the sarco/endoplasmic reticulum Ca2+-ATPase
actively pumps Ca2+ back into the sarcoplasmic reticulum where
Ca2+ rebinds to calsequestrin.

With Ca2+ no longer bound to troponin C, the troponin complex
slips back to its blocking position over the binding sites on actin.

Since cross-bridge cycling is ceasing then the load on the muscle
causes the inactive sarcomeres to lengthen.
M ECHANISM OF TETANUS
Mechanism of tetanus

A single action potential result in the release of a fixed amount of
ca+ from the sarcoplasmic reticulum which produce a single twist
is terminated ( relaxation occurs ) when the sarcoplasmic

Step 1: At the end of the previous round of movement and the
start of the next cycle, the myosin head lacks a bound ATP and it is
attached to the actin filament in a very short-lived conformation
known as the 'rigor conformation'.

Step 2: ATP-binding to the myosin head domain induces a small
conformational shift in the actin-binding site that reduces its
affinity for actin and causes the myosin head to release the actin
filament.
M ECHANISM OF TETANUS

Step 3: ATP-binding also causes a large conformational shift in the
'lever arm' of myosin that 'cocks' the head into a position further
along the filament. ATP is then hydrolysed, but the inorganic
phosphate and ADP remain bound to myosin.

Step 4: The myosin head makes weak contact with the actin
filament and a slight conformational change occurs on myosin
that promotes the release of inorganic phosphate.

Step 5: The release of inorganic phosphate reinforces the binding
interaction between myosin and actin and subsequently triggers
the 'power stroke'. The power stroke is the key force-generating
step used by myosin motor proteins; forces are generated on the
actin filament as the myosin protein reverts back to its original
conformation.

Step 6: As myosin regains its original conformation, the ADP is
released, but the myosin head remains tightly bound to the
filament at a new position from where it started, thereby bringing
the cycle back to the beginning.
M ECHANISM OF TETANUS