Muscle Morphology

Question 1

Three types of muscle

Answer 1

Skeletal, cardiac and smooth

Question 2

7 muscle proteins (in spec)

Answer 2

Actin, myosin, tropomyosin, troponin, dystrophin, alpha-actinin, titin

Question 3

Myosin structure

Answer 3

Large complex with two heavy chains and two pairs of light chains. The two thick chains are twisted together as myosin tails. Globular projections are present on each of the heavy chains forming the myosin heads.

Question 4

Actin structure

Answer 4

Thin helical filaments formed of two chains of actin wrapped around each other, with a G actin monomer containing a binding site for myosin.

Question 5

Tropomyosin structure

Answer 5

Long coil of two polypeptide chains located in the groove between the two twisted actin strands

Question 6

Troponin structure

Answer 6

A complex of three subunits, TnT which attaches to tropomyosin, TnC which attaches to calcium ions and TnI which regulates the myosin- actin interactions

Question 7

Explain the striated appearance of skeletal muscle fibres

Answer 7

The dark parts of myofibrils are called A bands and contain myosin. The I bands are lighter and contain only actin.

Question 8

Connective tissue of the muscle (3) + location

Answer 8

Each muscle fibre is invested by the endomysium.

A bundle of 10-100 muscle fibres and their associated endomysium form fascicles, which are each covered in perimysium.

The individual muscles are then surrounded by the epimysium.

Question 9

Structures of the three connective tissues

Answer 9

Endomysium- thin, reticular fibres and scattered fibroblasts

Perimysium- nerves, blood supply and lymphatic vessels

Epimysium- dense, irregular connective tissue. Septa extend inwards to allow large nerves and blood vessels to enter the muscle.

Question 10

Function of connective tissues

Answer 10

Blood and nerve supply and continuity of connective tissues ensure that the force generated at the ends of the fibres are transmitted to the tendon at the end of the whole muscle- continuous with myotendinous junctions.

Question 11

Stages of muscle name

Answer 11

Progenitor cells give rise to myoblasts which fuse to form multinucleate cells called myotubes.

Myotubes develop into a mature multinucleate syncytium

For muscle cells- myocytes, muscle fibres, myofibres

Question 12

What are found within myofibrils?

Answer 12

Myofibrils composed of my-filaments, such as actin and myosin.

Question 13

Sarcomere definition

Answer 13

Repetitive functional subunit of the contractile apparatus, extending from Z disc to Z disc.

Question 14

Sarcomere structure

Answer 14

2.5 micrometers long, between the Z discs

Central A and Peripheral I zones due to actin and myosin respectively. A band also includes some actin.

H zone in the centre, only contains myosin.

M line in the centre of the sarcomere.

Question 15

A-actinin function

Answer 15

Actin binding protein that attaches actin to the Z discs

Question 16

Titin function

Answer 16

Links myosin to Z line and maintains sarcomere architecture, allowing molecular spring for passive elasticity of muscle, whilst promoting passive stiffness. (largest known protein).

Question 17

Dystrophin associated glycoprotein complex function

Answer 17

Connects cytoskeleton of muscle fibre to surrounding extracellular matrix through the cell membrane. Helps scaffold various signalling and channel proteins.

Question 18

Dystrophin structure and location

Answer 18

Rod shaped cytoplasmic protein. Located between the sarcolemma and outermost layer of the myofilaments in the muscle cell.

Question 19

Structure of skeletal muscle fibres

Answer 19

Sarcoplasmic reticulum, sarcolemma with T tubules

Question 20

Sarcoplasmic reticulum function

Answer 20

Contains pumps and other proteins for calcium sequestration. Calcium release from cistern triggered by membrane depolarisation.

Question 21

Transverse tubule structure and function

Answer 21

Infoldings of the sarcolemma that penetrate deep into the cell that allows an action potential to travel through into the cell, enabling simultaneous contraction.

Question 22

What is the triad?

Answer 22

Two terminal cistern adjacent to a T tubule.

Question 23

Motor unit definition

Answer 23

All the muscle cells supplied by one motor neurone

Question 24

Satellite cell function

Answer 24

Precursors to skeletal muscle cells that can give rise to differentiated skeletal muscle cells and or myonuclei. Key function is muscle repair.

Question 25

Satellite cell location

Answer 25

Located between the basement membrane and the sarcolemma of muscle fibres.

Question 26

Fast fibres adaptations to functions

Answer 26

specialised for rapid, short contractions, have few mitochondria and rely largely on anaerobic respiration. Rapid contractions lead to rapid fatigue as lactic acid builds up from glycolysis

Question 27

Slow fibres adaptations to function

Answer 27

specialised for slow, long contractions without fatigue. Large number of mitochondria and many surrounding capillaries

Question 28

Skeletal muscle repair forms

Answer 28

Satellite muscle cells become activated and re-enter the cell cycle, known as transit amplifying pool cells and form new myotubes. Fibroblasts may also deposit scar tissue which can impair muscle function.

Question 29

What characterises cardiomyocytes?

Answer 29

Branching mesh of centrally located- mononuclear striated cells, joined and electrically coupled by intercalated discs

Question 30

What are intercalated discs?

Answer 30

Presence of transverse lines that cross fibres at irregular intervals where myocardial cells join.

Question 31

What junctions are present and why?

Answer 31

Fascia adherens and desmosomes to provide strong intercellular adhesion

Gap junctions- serve as electrical synapses producing a electrical syncytium

Question 32

What connective tissue is present?

Answer 32

Endomysium around muscle cells, thicker layer of perimysium surrounds bundles and layers of the muscle fibres. Outer epicardium.

Question 33

Cardiac sarcomere structure

Answer 33

Have Z lines, A and I bands

Myofilaments form a continuous mass within the cell interrupted by extensions of sarcoplasm containing more mitochondria and SR.

Question 34

Dyad structure

Answer 34

One cisternae and one T-tubule form the dyad, where electrical excitation coupling takes place.

Larger T tubule lumen in ventral side compared to arterial.

Question 35

Purkinje fibers location and function

Answer 35

Located in ventricular walls of the heart, just beneath the endocardium. Specialised conducting fibres formed of electrically excitable cells with fewer myofibrils and may mitochondria compared to the cardiomyocytes. Allow the heart’s conduction system to occur in a synchronised manner,

Question 36

Regeneration of cardiac muscle explained

Answer 36

Cardiac muscle cells cannot proliferate, instead scar tissue is deposited if there is damage. They hypertrophy however, through the synthesis of extra myofibrils.

Question 37

Smooth muscle characteristics

Answer 37

Fusiform cells that lack striations, no sarcomeres and have slow, involuntary contractions

Question 38

Smooth muscle fibre structure

Answer 38

Long, elongated, spindle shaped each enclosed by a lamina and collagen fibres comprising the endomysium. Elongated nucleus in the broadest part.

Question 39

Smooth muscle location

Answer 39

contractile walls of hollow organs- respiratory tract, gut, genitourinary system

Question 40

Smooth muscle arrangement of cells

Answer 40

Single unit and multi unit

Question 41

Single unit appearance and function

Answer 41

Behave as one, enabled by electric coupling via gap junction. Allow the muscle to behave as a syncytium

Question 42

Multi-unit appearance

Answer 42

Each cell isolated and stimulated independently, allowing finer control of movement, such as the cilliary muscles.

Question 43

Sarcoplasm structure

Answer 43

Contains actin and myosin filaments and a cytoskeleton. Acting binding dense plaques present, linked to intermediate cytoskeletal filaments.

Question 44

Invaginations of the sarcolemma name and function

Answer 44

Caveoli- vesicular invaginations that increase the surface area. Possibly regulate calcium entry into the cell.

Question 45

Structure of actin and myosin

Answer 45

bundles of thick and thin filaments criss cross the sarcoplasm. Myosin filaments have a less regular arrangement . Actin filaments not associated with tropomyosin or troponin. - arranged as a lattice

Question 46

Dense bodies structure and function

Answer 46

anchoring cytoplasmic and plasmalemma associated dense Bodie, containing alpha actinin which anchors the actin my-filaments.

Submembranous dense bodies contain cadherins and desmosomes.

Serve as points for transmitting contractile force within cells and between cells

Question 47

Where are junctions located?

Answer 47

Focal densities

Question 48

Smooth muscle cell regeneration

Answer 48

Smooth muscle cells undergo mitosis and replace damaged tissue