Cellular Basis of Muscle Contraction

Ch. 16

Myosin moves towards:

The barbed (+) end of an actin filament

All myosins share a characteristic motor (head) domain.

The head domain contains two sites:

• A site that binds an actin filament

• A site that binds and hydrolyzes ATP to drive the myosin motor

The head domains of various myosins are similar, whereas the tail domains are:

Highly divergent

The fibrous tail portion of myosin II plays a structural role so it can form:

Bipolar filaments

Because they are bipolar, the myosin heads at the opposite ends of a myosin filament can:

Pull actin filaments toward one another

A skeletal muscle fiber is a:

Multinucleate cell

Each muscle fiber contains hundreds of cylindrical strands called:

[actual part of the muscle that contracts]

Myofibrils

Each myofibril consists of a repeating linear array of contractile units called:

[smallest unit of a muscle cell that can contract]

Sarcomeres

A sarcomere:

Extends from one Z line to the next Z line

Has a densely staining line that lies in the center of the H zone

The I band contains only:

Thin filaments [Actin]

The H zone contains only:

Thick filaments [Myosin]

The part of the A band on either side of the H zone represents the region of overlap and contains:

Both thin & thick filaments

Thin filaments of a skeletal muscle contain:

Actin, tropomyosin, and troponin

An elongated molecule that fits securely into the grooves within the thin filament and its associated actin subunits

Tropomyosin

A globular protein complex composed of three subunits that contact both actin and tropomyosin

Troponin

The thick filament is composed of several hundred ___ molecules together with small amounts of other proteins

Myosin II

The center of the thick filament is composed of the ___ of the myosin molecules and is ___ of heads

Opposing tail regions; devoid

The third most abundant protein of skeletal muscles

Titin

Titin Fxn

Elastic & prevent the sarcomere from being pulled apart during muscle stretching

Maintains myosin filaments in their proper position within the center of the sarcomere during muscle contraction

The ___ molecules act as “molecular ruler” by regulating the number of actin monomers that are allowed to assemble into a ___ filament

Nebulin; thin

Shortening of sarcomeres results from:

Thin & thick filaments sliding over one another

As a sarcomere shortens:

[A] band remains constant in length.

[H] band and [I] bands decrease in width and then disappears altogether.

[Z] lines on both end of the sarcomere move inward until they contact the outer edges of the [A] band

• Therefore, the distance between the Z-lines decreases

Neuromuscular junction

Contact b/w nerve & muscle

Excitation contraction coupling

The linking of the nerve impulse to the shortening of the sarcomere

Action potential in muscles is propagated into the cell interior by

Transverse (T) tubules

The T tubules terminate in proximity to the ___ which store calcium until needed.

Sarcoplasmic reticulum

In a relaxed sarcomere, tropomyosin blocks the ___ on the actin molecules

Myosin-binding sites

To initiate contraction of the sarcomere:

1. Ca2+ binds to tropomyosin causing a conformational change

2. The movement of troponin is transmitted to the adjacent tropomyosin

3. This shift in position of the tropomyosin exposes the myosin-binding sites on the adjacent actin molecules, allowing the myosin heads to attach to the thin filaments

During a contraction:

• Each myosin head extends outward and binds tightly to a thin filament, forming across-bridge between the two types of filaments

• When bound to the actin filament, myosin head undergoes a conformational change that moves the thin actin filament

• Since each thin filament is contacted by a team of a hundred or so myosin heads that beat out of synchrony with one another; it undergoes continuous motion during each contractile cycle

Energy is provided by ___ in the myosin head

ATP

Activated myosin attaches to actin, initiating:

The power stroke

Release of bound ___ is followed by binding of another ___.

ADP; ATP

Absence of ATP prevents dissociation of cross bridges causing ___.

Rigor mortis

Contractile Cycle

1. binding of ATP to a cleft in the myosin head, detaching the head from the actin filament.

2. hydrolysis of ATP to Pi and ADP which energizes the head

3. head binds weakly to the actin filament

4. release of Pi, which causes a tighter attachment of the myosin head to the thin filament, and the power stroke that moves the thin filament toward the center of the sarcomere.

5. release of the ADP which sets the stage for another cycle