Lecture 23

Internal structure of skeletal muscle Nucleus: Contain the genetic material of the cell Sarcolemma: Plasma membrane of muscle cell T Tubule: Invagination of the Sarcolemma project deep into

Muscle cells interior Terminal cisternae: Serve as specialized reservoirs for Ca

ions Myofibril: Bundle of contractile filaments

The myofibrils composed of individual contractile proteins called myofilaments

There are two types of myofilaments:

1. Thin filaments composed mainly of protein Actins

2. The thick filaments is composed mainly of protein myosin

Structure of Myofibrils

Organization levels of skeletal muscle Myofilaments: Smallest building blocks.

Compose of thick and thin Myofibril: Many myofilaments bundle

together making a single myofibril Muscle cell: Many myofibril are contained

within the muscle cell Fascicle: Many muscle cells are packed

into a fascicle Muscle: Many fascicle make up the muscle

The Sliding Filament

Sliding filament occurs as a thin filaments slide past the thick filament. This involve the activity of:

1. Myocin

2. Actin

3. Tropomyosin

4. Troponin

5. ATP

6. Ca ions

The Thick Filament (Myosin)

Consists of the protein called myosin.

A myosin molecule is shaped a bit like a golf club, but with 2 heads.

The heads stick out to form the cross bridge

Many of these myosin molecules stick together to form a thick filament

Each head contains two binding sites, one for actin and one for ATP.

one m yosin m olecule

m yosin heads(cross bridges)

m yosin tails

Thin Filament (Actin) The thin filament consists of a protein called actin. It

compose of actin subunit twisted into double helical chain. Actin has specific binding site to which the myosin head binds

The thin filament also contains tropomyosin. The position of tropomyosin cover the binding sites on the actin during unstimulated muscle

The third component is troponin. Attached along the tropomyosin strand. Which expose the binding site of actin to myosin

actin monom ers tropomyosin

Arrangement of Myofilament The arrangement of thick and thin myofilaments forms

light and dark alternating bands (striation). In the middle of the light band is the Z-line The repeating unit from one Z-line to the next is called

the sarcomere

I Band = actin filaments

Ca Ion

After action potential, Ca ions released from the T-Tubules and bind to troponin. This causes change in conformation of the troponin – tropomyosin complex and thus dragging tropomyosin strands off the binding site

Sliding-Filament Mechanism

Sliding-Filament Mechanism

Sliding-Filament Mechanism

Sliding-Filament Mechanism

Sliding-Filament Mechanism

Steps of cross bridge cycle1. The influx of Ca ion, triggering the exposure of

binding sites on actin

2. The binding of myosin to actin

3. The myosin cross bridge pulls the thin filament inward toward the centre of sarcomere

4. The binding of ATP to the myosin head disconnecting from actin

5. The hydrolysis of ATP leads to repositioning of the myosin head

6. The transport of Ca ion to the back into the sarcoplasmic reticulum