contractia musculara.ppt

7/27/2019 contractia musculara.ppt

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Muscle Contraction


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Muscle Tissue

Muscle is a tissue built of specialized contractile cells, called muscle

cells ormuscle fibers ormyocytes ormyofibers.

There are two main categories of muscle:

1) Striated muscle tissue has alternating light and dark bands (which

come from the organization of the contractile proteins), giving it a

striated appearance. In vertebrates, the striated muscle makes up the

skeletal (most) and cardiac muscle.

2)Smooth muscle tissue also uses contractile proteins, but they are

not organized in the same fashion, so it doesnt have a striated

appearance. In vertebrates, smooth muscle lines the gut, respiratory

tract, blood vessels, etc.

Tissue built to generate contractile force


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Neuromuscular Junction

Where neurons and muscles meet


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Fast chemical transmission at the NMJ

Step-by-step:

1) AP depolarizes terminal

2) Voltage-gated Ca++ channels

open

3) ACh is released into the synapse

4) ACh binds to nictotinic receptors

5) Nicotinic recepors allow Na+ and

K+ ion flow, depolarizing the muscle

cell


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Fast chemical transmission at the NMJ

6) The muscular AP propagates to all

parts of the muscle, perhapsresulting in contraction

7) Acetylcholinesterase hydrolyzes

ACh into acetate and choline

8) Choline is actively transported

back into the motor neuron terminal.


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Muscle Contraction

When the current at the muscle is

sufficiently large, the muscle cell

contracts. Exocytosis of each vesicle of

ACh releases a quantum of ACh

(analogous to an EPSP). Approx. 100

quanta are required to initiate an AP in

the muscle. That number can be

released in response to a single AP

reaching the NMJ.

Although each fiber is only innervated by one nerve, there may be

multiple synapses from that nerve, which promotes more synchronous

depolarization of the fiber (which can be 15 cm long in humans).


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Skeletal Muscle Organization

Skeletal Muscles are Organized Hierarchically

Skeletal muscle consists of multiple bundles

of muscle fibers

Myofibers are long, multinucleate, cylindrical

cells, organized in parallel

Each myofiber consists of many parallel

myofibril subunits

Each myofibril is composed of repeatedsarcomere subunits

The sarcomere is the fundemental unit of

contraction, which consists ofactin and

myosin filaments


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Skeletal muscle consists of multiple bundles of muscle fibers


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Myofibers are long, multinucleate, cylindrical cells, organized in parallel


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Each myofiber consists of many parallel myofibril subunits


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Each myofibril is composed of repeated sarcomere subunits


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The sarcomere is the fundemental unit of contraction, which consists of

actin and myosin filaments


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Sarcomeres

Skeletal muscle fibers are subdivided into repeated 2 m long striatedsarcomeres with a z disk at each end.

Actin (thin) filaments: ~2000 per disk. Attached at their midpoint to z-

disks and project to either side

Myosin (thick) filaments: ~1000 per sarcomere.At the center of each

sarcomere and attach to actin at each end.

Titin filaments: 10% of total muscle mass. Function as elastic bands.


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Actin FilamentsThe thin filaments

Each actin filament is

composed of:

1) Two twisted, bended

polymer chains of globularactin molecules

2) Two strands oftropomyosin molecules that lie from end to end in

the grooves formed by the actin chains

3)Troponin molecules attached at intervals to tropomyosin strands

Tropomyosin and troponin act to control whether myosin cross-bridges

can interact with the thin filaments


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Myosin FilamentsThe thick filaments

Each myosin filament has ~200

myosin heavy chain molecules

arranged in helical pairs.

Each myosin molecule has aglobular head with an actin binding

domain (actin attachment site), a

nucleotide pocket (ATP hydrolysis

site), and a flexible neck with two

light chains of myosin.

The head-neck segment tilts to a

smaller angle when the molecule

interacts with actin.


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Muscle Contraction

The Sliding Filament Theory


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Muscle Contraction

The Sliding Filament Theory


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Muscle Contraction

Molecular interactions of contraction


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Muscle Contraction

1) The rigor state


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Muscle Contraction

2) ATP binding


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Muscle Contraction

3) ATP hydolysis


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Muscle Contraction

4) Rebinding to actin


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Muscle Contraction

5) Phosphate release and filament sliding(power stroke)


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Muscle Contraction

6) ADP release (back to rigor)


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Muscle ContractionMolecular interactions of contraction


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Muscle ContractionMolecular movement requires ATP and Ca2+ ions

No Calcium Calcium ions present

Ca2+ bound to

troponin

In the absence of Ca2+, tropomyosin blocks the myosin binding

sites on the actin filaments


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Muscle ContractionMolecular movement requires ATP and Ca2+ ions


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T-tubules and the SRTransverse Tubules and the Sarcoplasmic Rectulum membrane systems

The sarcolemma (the myocyte cell

membrane) formsdeep transverse

invaginations into the myocyte, called

transverse tubules (orT-tubules).

The T-tubule system is continuous with

the ECF and thus contains extremely

high Ca2+ concentrations.

The sarcoplasmic recticulum orSR is

a network of longitudinal membrane-bound tubules contained entirely within

the myocyte between two T-tubules.

Protein receptor channels join the T-

tubules and SR.

T-tubu

le

The SR

NMJ

Muscle cell

Axon

terminal


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Excitation-Contraction CouplingSarcolemma Depolarization

1) ACh release at the NMJ

2) Opening of ACh gated ion

channels

3) AP propagates down the T-tubules


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Excitation-Contraction CouplingCa2+ efflux from the SR

4) Depolarization reaches the DHPR

receptors, opening the associated

RyR receptors and releasing Ca2+

from the SR

5) Ca2+ ions diffuse into the cytosol

and bind to troponin, enablingfilament sliding

6) Cross bridges go through several

cycles of movement while the Ca2+

is present.


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Excitation-Contraction CouplingAP termination

7) Meanwhile at the NMJ,

acetylcholinesterase (AChE)

hydrolizes ACh to terminate the AP

8) Depolarization ceases at the T-

tubules and RyR channels close

9) Ca2+ ATPases actively transport

Ca2+ ions back into the SR


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Excitation-Contraction CouplingCa2+ efflux mediates filament sliding

Relaxed Contracted

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