Musculoskeletal System, Nervous System & Bioelectricity MNB.6 Muscle Ultrastructure and Biochemistry of Movement

Detailed flashcards covering muscle ultrastructure, the sliding filament theory, the ratchet mechanism, and excitation-contraction coupling.

What is the basic repeating unit of muscle tissue?

The sarcomere

What is the name of the special cell membrane that encloses each muscle fiber?

The sarcolemma

What is the cytoplasm of a muscle fiber called?

The sarcoplasm

What specific substances is the sarcoplasm (cytosol) rich in?

Glycogen, ATP, Creatine Phosphate, and glycolytic enzymes

What are the series of membranous folds extending from the plasma membrane that transmit electrical signals?

Transverse tubules (T-tubules)

What is the primary function of the Sarcoplasmic Reticulum (SR)?

To sequester (collect) calcium

What structures are embedded in the sarcoplasm and have a diameter of $1\,\mu m$?

Myofibrils

What is the name of the light band in a myofibril?

The I-band (isotropic)

What is the name of the dark band in a myofibril?

The A-band (anisotropic)

What dark line is located in the middle of the I-band?

The Z-line (or Z disc)

What is the structural unit defined as the segment between two Z-lines?

The sarcomere

Which two main protein filaments are responsible for muscular contraction within a sarcomere?

Actin and myosin

What is the common name for actin filaments?

Thin filaments

What is the common name for myosin filaments?

Thick filaments

What are the three components that make up the thin filament?

Double stranded F-actin, tropomyosin, and troponin

What is the structural form of F-actin?

A double stranded protein molecule wound into a double helix

Where is tropomyosin situated in the thin filament?

In the groove of the F-actin helix

What are the three globular proteins that constitute troponin?

TnC, TnI, and TnT

What is the role of tropomyosin in a resting muscle?

It covers the myosin binding sites on actin filaments

To which structure does the troponin protein TnT bind?

Tropomyosin

What does the troponin protein TnI bind to?

F-actin

What is the binding target of the troponin protein TnC?

$Ca^{2+}$ ions

How does the release of $Ca^{2+}$ from the SR initiate binding between myosin and actin?

$Ca^{2+}$ binds to TnC, moving tropomyosin away from the myosin binding site

How many myosin polypeptides typically make up a myosin filament?

About 400 polypeptides

What part of the myosin filament forms cross bridges with actin?

The compact ‘head’ regions

What name is given to the majority of the long tail region of myosin?

Light meromyosin

What components make up heavy meromyosin?

The myosin head and a portion of the tail

What is the chain composition of a myosin molecule?

Two heavy chains and four light chains

What is the length of the coiled-coil α-helical rod-shaped tail of myosin?

$$1500\,Å$$

Which enzyme-like activity is performed by the myosin head?

Binding to and hydrolysing ATP

In what form do myosin heads store energy prior to contraction?

ADP and inorganic phosphate-$P_i$

Who proposed the sliding filament theory in 1950?

A. F. Huxley and H. E. Huxley

What causes the force of contraction according to the sliding filament theory?

The movement of cross-bridges

During the ratchet mechanism, what event occurs as inorganic phosphate is released?

The myosin heads bind to the actin proteins to form a cross-bridge

What event initiates the power stroke during the contraction cycle?

The release of ADP

What causes the actin-myosin cross-bridge to separate?

The binding of a new ATP molecule to the myosin head

What occurs when the ATP molecule on the myosin head is hydrolyzed to ADP and $P_i$?

The energy causes the head to pull back like the trigger of a gun (cocking)

How far does the power stroke drag the actin filament towards the M-line?

$$5-12\,nm$$

What happens to the length of the A-band during muscle contraction?

The A-band remains the same length

What happens to the I-band during muscle contraction?

The I-band shortens

What happens to the H-zone (or H-band) when a muscle contracts?

It is reduced or disappears

What is the biochemical purpose of the ratchet mechanism?

Converting the chemical energy of ATP into the physical work of filament displacement

In resting muscle, why is most myosin in the Myosin-ADP-$P_i$ form?

Because the release of ADP and $P_i$ is slow without the presence of actin

What does Excitation-Contraction Coupling (ECC) describe?

The communication between electrical signals in the plasma membrane and calcium release from the SR

Through which system does the wave of depolarization pass into the fiber to reach the SR?

The T-tubule system

From which specific area of the SR is $Ca^{2+}$ released into the sarcoplasm?

The terminal cisternae

Which chemical is the initial signal for muscle contraction at the neuromuscular junction (NMJ)?

Acetylcholine (ACh)

How does the disengagement of TnI from actin lead to contraction?

It causes a conformational change in tropomyosin that uncovers the active sites on F-actin

What is the effect of depolarizing muscle relaxants like Suxamethonium?

They cause an initial contraction but prevent further contractions

What is the effect of non-depolarizing muscle relaxants such as Tubocurarine?

They prevent the muscle from contracting