Tissues 7- Muscles

Question 1

What are muscles

Answer 1

Specialised cells that are responsible for movement through the generation of force.

Question 2

What is skeletal muscle

Answer 2

Attached to bone and produces movement of the body relative to the external environment. Connect to bones in the arms, legs and spine. Used in complex coordinated activities.

Question 3

What is the role of cardiac muscle

Answer 3

To pump blood around the body through the blood vessels.

Question 4

Where is smooth muscle found and what is its function

Answer 4

Exists within the lining of hollow organs (blood vessels, gastrointestinal tract)- provided propulsion to move substances within the body.

Question 5

Is the mechanism through which force is generated similar or different in each of the three different types of muscle

Answer 5

Similar

Question 6

What do antagonistic muscle pairs consist of

Answer 6

A flexor and extensor

Question 7

What are the two types of muscle contraction

Answer 7

Isotonic contraction: muscle changes length  tension remains the same
Concentric: shortening
Eccentric: lengthening
Isometric contraction: tension develops  muscle does NOT change length

Question 8

What is an example of isometric contraction

Answer 8

Holding shopping bags- arm stays straight- but the muscle cells are still contracting.

Question 9

What does skeletal muscle consist of

Answer 9

Skeletal muscle  bundle of muscle cells known as myofibres

Question 10

Describe the characteristics of myofibres

Answer 10

Large & Cylindrical
Multinucleate
Packed with myofibrils

Question 11

Describe the myofibrils

Answer 11

Extend across the length of the cell, they can be further divided into light and dark bands giving them a striated appearance. Sarcomeres are also present within the myofibril.

Question 12

What is the sarcomere

Answer 12

The functional unit of the muscle, lies between two Z lines. It has a particular arrangement of myosin and actin. During contraction, it can become 30% shorter than its original length.

Question 13

What is the Z-line

Answer 13

Defines the lateral boundaries of the sarcomere

Question 14

Describe the arrangement of actin within the sarcomere

Answer 14

Polymeric thin filament composed of two twisted -helices - displays polarity
Along with another rope-like protein called tropomyosin, forms a chain around the actin filament, also associated with troponin. Actin is not found in middle

Question 15

What is nebulin

Answer 15

Large filaments associated with actin

Question 16

Describe the arrangement of myosin in the sarcomere

Answer 16

Found in the centre of the sarcomere

Thick filaments  ‘motor proteins’. Contain numerous ‘globular heads’ that interact with actin

Question 17

What is the role of titin within the sarcomere

Answer 17

Very large ‘spring-like’ filaments anchoring myosin to the Z-line.

Question 18

What is the role of CapZ and tropomodulin

Answer 18

Cap actin at certain points
CapZ at the Z-line
Tropomodulin- at centre

Question 19

Describe two properties of myofibres that play a major role in excitation-contraction coupling in skeletal cells

Answer 19

T-tubules: Membrane invaginations that contact the extracellular fluid
Sarcoplasmic reticulum (SR): extensive network of Ca2+-stores surrounding each myofibril

Question 20

Ultimately, what is muscle contraction caused by

Answer 20

An increase in cytosolic Ca2+ concentration. Skeletal muscle cells maintain a low cytosolic Ca2+ concentration due to the actions of Ca2+ ATPase, that continually pumps Ca2+ from the cytosol into the sarcoplasmic reticulum.

Question 21

Describe the excitation of a myofibril

Answer 21

Action potential (AP) propagates along myofibre membrane (sarcolemma) & T-tubules
Depolarisation activates dihydropyridine  receptors (DHPR)  conformational change in DHPR- contacts RyR.
This change is transmitted to ryanodine receptors (RyR) on SR  opening of RyR & Ca2+ release from intracellular stores 
Thus depolarisation  Increase in intracellular Ca2+

Question 22

Describe the sliding-filament model of muscle contraction

Answer 22

In the presence of Ca2+ movement of troponin from tropomyosin chain
Movement exposes myosin binding site on surface of actin chain
‘Charged’ myosin heads bind to exposed site on actin filament
This binding & discharge of ADP causes myosin head to pivot (the ‘power stroke’)  pulling actin filament towards centre of sarcomere- inward movement
ATP binding  releases myosin head from actin chain
ATP hydrolysis  provides energy to ‘recharge’ the myosin head- allows myosin head to bind to a different point on the actin filament.

Question 23

What information lead to the development of the sliding-filament model of contraction

Answer 23

The fact that thick and thin filaments do not change in length when the sarcomere shortens.

Question 24

Describe the tension-load relationship in isotonic contraction

Answer 24

Muscle tension  force exerted by load
Muscle contracts  fibres shortens
Energy expenditure (ATP)  ‘recharging’ of myosin heads

Question 25

Describe the tension-load relationship in isometric contraction

Answer 25

``` Muscle tension = force exerted by the load Muscle DOES NOT contract  myosin heads reattach to the same point on actin chain Energy expenditure (ATP)  ‘recharging’ of myosin heads ```

Question 26

What does the contraction of the whole muscle result from

Answer 26

The activity of hundreds of myosin heads on a thick filament interacting with the actin filament. This is amplified by the hundreds of thick filaments in a sarcomere and thousands of sarcomeres in a muscle fibre.

Question 27

What does the myocardium consist of

Answer 27

The wall of the heart is primarily made up of cardiac muscle cells, but also contains pacemaker cells and conducting fibres.

Question 28

Describe the pacemaker cells

Answer 28

The pacemaker cells are stored within the SA and AVN and they are excitable cells that depolarise and generate action potentials in a regular rhythmic pattern. Their discharge rate determines the heart rate- the action potential spreads from the pacemaker cells to the cardiomyocytes,

Question 29

Describe the differences between the SAN and the AVN

Answer 29

Sinoatrial (SA) node: small, ‘empty’, spindle shaped cells, spontaneously active- found at the top of the right atrium. Atrioventricular (AV) node: spindle-shaped network of cells located at base of right atrium

Question 30

Describe the conducting fibres

Answer 30

Bundle of His: fast conducting cells adjoining the AV node & Purkinje fibres Purkinje fibres: large cells that rapidly conduct electrical impulses- travel to the apex to stimulate contraction of the heart.

Question 31

What is the appearance of cardiomyocytes

Answer 31

They are striated muscle cells. They are also unicellular

Question 32

How are individual cardiomyocytes connected to each other, and what is the importance of this

Answer 32

Individual cardiomyocytes are connected to each other at specialised regions called intercalated disk- where many gap junctions allow action potentials to spread rapidly from cell to cell- essential for synchronous contraction of the heart.

Question 33

What is meant by striated muscle

Answer 33

The muscle contains a regular arrangement of actin and myosin.

Question 34

Describe the excitation of cardiomyocytes

Answer 34

Action potential propagates along the cardiomyocyte membrane and the T-tubules. Depolarisation opens VGCCs- Ca2+ influx Ca2+ induced Ca2+ release by binding of Ca2+ ions to RyR receptors on the SR Further release of Ca2+ ions Binds to troponin Further depolarisation

Question 35

Why does smooth muscle appear smooth

Answer 35

No regular arrangement of actin and myosin.

Question 36

What is the consequence of smooth muscle not containing VGSCs on the speed of muscle contraction

Answer 36

Hence smooth muscle action potential is entirely dependent on depolarisation resulting from Ca2+ entering through VGCCs- hence the process of muscle contraction is a slower affair.

Question 37

Describe the process of excitation in smooth muscle

Answer 37

Depolarisation activates VGCCs- influx of calcium ions into cells Calcium ions bind to intracellular protein CaM- calmodulin- forming a complex. Ca2+-CaM complex activates the myosin light chain kinase MLCK phosphorylates the myosin light chain allowing them to form cross-bridges with actin filaments, resulting in contraction.