FR-Muscle physiology

Question 1

What does controlled contraction of muscles allow?

Answer 1

(1) purposeful movement of the whole body or parts of the body (such as walking or waving your hand),
(2) manipulation of external objects (such as driving a car or moving a piece of furniture),
(3) propulsion of contents through hollow internal organs (such as circulation of blood or movement of a meal through the digestive tract), and
(4) emptying the contents of certain organs to the external environment (such as urination or giving birth)

Question 2

Explain the structure, features and organisation of skeletal muscle

How does skeletal muscle form during embryonic development?

Answer 2

• Relatively large, elongated, and cylinder shaped, measuring from 10 to 100 micrometers (mm) in diameter and up to 750,000 mm, or 2.5 feet, in length
• A skeletal muscle consists of a number of muscle fibers lying parallel to one another and bundled together by connective tissue
• The fibers usually extend the entire length of the muscle. During embryonic development, the huge skeletal muscle fibers are formed by the fusion of many smaller cells called myoblasts
• Presence of multiple nuclei dispersed just beneath the plasma membrane in a single muscle cell
• Abundance of mitochondria, the energy-generating organelles, as would be expected with the high energy demands

Question 3

What are myofibrils?

Answer 3

Cylindrical intracellular structures 1 mm in diameter that extend the entire length of the muscle fiber

Myofibrils are specialized contractile elements that constitute 80% of the volume of the muscle fiber.

Each myofibril consists of a regular arrangement of highly organized cytoskeletal microfilaments

Question 4

Describe a cross section of an A band

Answer 4

Thick and thin filaments overlap that each thick filament is surrounded by six thin filaments and each thin filament is surrounded by three thick filaments

Question 6

What is the size of thick filaments?

What is the size of thin filaments?

Answer 6

Thick filaments: are 12 to 18 nm in diameter and 1.6 mm in length

Thin filaments: are 5 to 8 nm in diameter and 1.0 mm long

Question 7

What are the levels of organisation in skeletal muscle?

Answer 7

Question 8

What are A and I bands and what do they look like under a microscope?

How is the striated appearance produced?

Answer 8

• Viewed with an electron microscope, a myofibril displays alternating dark bands (the A bands) and light bands (the I bands)
• The bands of all the myofibrils lined up parallel to one another collectively produce the striated appearance of a skeletal muscle fiber visible under a light microscope.
• Alternate stacked sets of thick and thin filaments that slightly overlap one another are responsible for the A and I bands

Question 9

What is an A band made up of?

What is the H zone?

What is the M line?

What is an I band made up of?

What is a Z line?

What is a sarcomere?

Answer 9

• An A band is made up of a stacked set of thick filaments along with the portions of the thin filaments that overlap on both ends of the thick filaments.
• The thick filaments lie only within the A band and extend its entire width.
• The lighter area within the middle of the A band, where the thin filaments do not reach, is the H zone.
• Only the central portions of the thick filaments are found in this region.
• Supporting proteins that hold the thick filaments together vertically within each stack can be seen as the M line, which extends vertically down the middle of the A band within the center of the H zone.
• An I band consists of the remaining portion of the thin filaments that do not project into the A band. Visible in the middleof each I band is a dense, vertical Z line. The area between two Z lines (a flat, cytoskeletal disc that connects the thin filaments of two adjoining sarcomeres) is called a sarcomere, which is the functional unit of skeletal muscle

Question 10

What is the functional unit of an organ?

What is a skeletal muscle functional unit?

Answer 10

A functional unit of any organ is the smallest component that can perform all functions of that organ

Accordingly, a sarcomere is the smallest component of a muscle fiber that can contract

Question 11

An I band contains only thin filaments from two adjacent sarcomeres but does it contain the entire length of these filaments?

Answer 11

An I band contains only thin filaments from two adjacent sarcomeres but not the entire length of these filaments

Question 12

How does muscle increase in length during growth?

Answer 12

During growth, a muscle increases in length by adding new sarcomeres on the ends of the myofibrils, not by increasing the size of each sarcomere

Question 13

Single strands of a giant, highly elastic protein, extend in both directions from the M line along the length of the thick filament to the Z lines at opposite ends of the sarcomere.

What protein is this?

Answer 13

Titin

The largest protein in the body, being made up of nearly 30,000 amino acids

Question 14

Titin serves 3 important roles

Answer 14

1. Serving as scaffolding.

Along with the M-line proteins, titin helps stabilize the position of the thick filaments in relation to the thin filaments, thus contributing to sarcomere stability.

2. Acting as an elastic spring.

By acting like a spring, titin greatly augments a muscle’s elasticity. That is, titin helps a muscle stretched by an external force passively recoil to its resting length when the stretching force is removed, much like a stretched spring. Because it behaves like an elastic spring and lies parallel to the thick and thin filaments, titin (along with the elastic connective tissue surrounding the muscle fibers) constitutes the parallel-elastic component of muscle.

3. Participating in signal transduction.

Titin is also involved in diverse signaling pathways, such as the complex pathway involved in muscle enlargement in response to weight lifting

Question 15

What can be seen extending from each thick filament toward the surrounding thin filaments in the areas where the thick and thin filaments overlap?

How does this look three-dimentionally

Answer 15

cross bridges

Three-dimensionally, the thin filaments are arranged hexagonally around the thick filaments. Cross bridges project from each thick filament in all six directions toward the six surrounding thin filaments. Each thin filament, in turn, is surrounded by three thick filaments

Question 16

Explain the shape and structure of myosin

Answer 16

• The tail ends of the two subunits are intertwined around each other like golf-club shafts twisted together, with the two globular heads projecting out at one end.
• Myosin can bend at hinge points in two location:

one along the tail and the other at the “neck” or junction of the tail with each head.

• The two halves of each thick filament are mirror images made up of myosin molecules lying lengthwise in a regular, staggered array, with their tails oriented toward the center of the filament and their globular heads protruding outward at regular interval

Question 17

What part of myosin forms the cross bridges?

Answer 17

The heads form the cross bridges between the thick and thin filaments.

Each cross bridge has two sites crucial to the contractile process:

(1) an actin-binding site and
(2) a myosin ATPase (ATP-splitting) site

Question 18

Thin filaments consist of three proteins:

Answer 18

• actin
• tropomyosin
• troponin

Question 19

Describe the structure of actin and how it interacts with myosin

Answer 19

Actin molecules, the primary structural proteins of the thin filament, are spherical. The thin filament’s backbone is formed by actin molecules joined into two strands and twisted together, like two intertwined strings of pearls.

Each actin molecule has a binding site for attaching with a myosin cross bridge. Binding of myosin and actin at the cross bridges leads to contraction of the muscle fiber

Question 20

Describe the structure of tropomyosin molecules

Answer 20

threadlike proteins that lie end to end alongside the groove of the actin spiral.

In this position, tropomyosin covers the actin sites that bind with the cross bridges, blocking the interaction that leads to muscle contraction

Question 21

Describe the structure of troponin:

What happens when Ca²⁺ is bound?

What happens when Ca²⁺ is not bound?

Answer 21

• A protein complex made of three polypeptide units: one binds to tropomyosin, one binds to actin, and a third can bind with Ca²⁺.
• When troponin is not bound to Ca²⁺, this protein stabilizes tropomyosin in its blocking position over actin’s cross-bridge binding sites
• When Ca21 binds to troponin, the shape of this protein is changed in such a way that tropomyosin slips away from its blocking position
• With tropomyosin out of the way, actin and myosin can bind and interact at the cross bridges, resulting in muscle contraction

Question 22

Why are tropomyosin and troponin often called regulatory proteins?

Answer 22

because of their role in covering (preventing contraction) or exposing (permitting contraction) the binding sites for cross-bridge interaction between actin and myosin.

Question 23

Describe the role of Ca²⁺ in turing in cross bridges

Answer 23

Question 24

Define power stroke

Answer 24

During contraction, with the tropomyosin and troponin “chaperones” pulled out of the way by Ca21, the myosin heads or cross bridges from a thick filament can bind with the actin molecules in the surrounding thin filaments

Myosin is a motor protein, similar to kinesin and dynein

Myosin cross bridges “walk” along an actin filament to pull it inward relative to the stationary thick filament

A single power stroke pulls the thin filament inward only a small percentage of the total shortening distance.

Question 25

Describe changes in banding pattern during shortening

Answer 25

• During muscle contraction, each sarcomere shortens as the thin filaments slide closer together between the thick filaments so that the Z lines are pulled closer together.
• The width of the A bands does not change as a muscle fiber shortens, but the I bands and H zones become shorter

Question 26

Do the cross bridges aligned with given thin filaments all stroke in unison?

Answer 26

No

• At any time during contraction, part of the cross bridges are attached to the thin filaments and are stroking, while others are returning to their original conformation in preparation for binding with another actin molecule.
• Thus, some cross bridges are “holding on” to the thin filaments, whereas others “let go” to bind with new actin.
• Were it not for this asynchronous cycling of the cross bridges, the thin filaments would slip back toward their resting position between strokes.

Question 27

Describe what happens in a single cross bridge interaction

Answer 27

• The two myosin heads of each myosin molecule act independently, with only one head attaching to actin at a given time.
• When the binding site on an actin molecule is exposed, the myosin molecule tilts at the hinge point on the tail, elevating the myosin head to facilitate the binding of this cross bridge to the nearest actin molecule.
• On binding, the myosin head tilts 45 degrees inward. Bending at this neck hinge point creates a “stroking” motion that pulls the thin filament toward the center of the sarcomere, like the stroking of a boat oar.
• This action is known as the power stroke of a cross bridge. A single power stroke pulls the thin filament inward only a small percentage of the total shortening distance.
• Repeated cycles of cross-bridge binding and bending complete the shortening

Question 28

What happens at the end of one cross-bridge cycle?

Answer 28

The link between the myosin cross bridge and actin molecule breaks. The cross bridge returns to its original angle and binds to an actin molecule behind its previous actin partner.

The cross bridge tilts inward again to pull the thin filament in farther, then detaches and repeats the cycle

Repeated cycles of cross-bridge power strokes successively pull in the thin filaments, much like pulling in a rope hand over hand.

Question 29

Define excitation–contraction coupling

Answer 29

The series of events linking muscle excitation (the presence of an action potential in a muscle fiber) to muscle contraction (cross-bridge activity that causes the thin filaments to slide closer together to produce sarcomere shortening)

Question 30

Skeletal muscles are stimulated to conract by the release of which neurotransmitter?

Answer 30

Skeletal muscles are stimulated to contract by release of acetylcholine (Ach) at NMJ’s between motor neuron terminal buttons and muscle fibers

Question 31

What is the plasma membrane in muscle cells sometimes called?

Why do AP’s spread down T tubules?

Answer 31

The sarcolemma

The T tubule membrane is continuous with the sarcolemma, an action potential on the surface membrane spreads down into the T tubule, rapidly transmitting the surface electrical activity into the interior of the fiber

Question 32

What is the sarcoplasmic reticulum?

Answer 32

• The sarcoplasmic reticulum (SR) is a modified endoplasmic reticulum that consists of a fine network of interconnected membrane-enclosed compartments surrounding each myofibril like a mesh sleeve
• This membranous network encircles the myofibril throughout its length but is not continuous. Separate segments of SR are wrapped around each A band and each I band. The ends of each segment expand to form saclike regions, the lateral sacs (alternatively known as terminal cisternae) , which are separated from the adjacent T tubules by a slight gap
• The lateral sacs store Ca21. Spread of an action potential down a T tubule triggers release of Ca21 from the SR into the cytosol.

Answer 33