Muscle

Question 1

Skeletal muscle

Answer 1

Voluntary control
Sarcomere arranged end to end to form a myofibril
Multiple peripheral nuclei, striations
Tendons
Numerous large mitochondria for ATP production
SR releases CA2+for contraction
No gap junctions or desmosomes

Question 2

Smooth muscle

Answer 2

Elongated muscle fibres (cells)
Pointed ends and single large, oval nucleus
Found in hollow organs
No straitions
Dense bodies
Few mitochondria

Question 3

The sarcomere

Answer 3

Functional unit of skeletal and cardiac muscle
Between z lines
A band only region containing myosin
H zone contains myosin
M line, centre of sarcomere, myosin attachment site; creatinine kinase, myomesin and c-protein

Question 4

Nebulin

Answer 4

Present in sarcomere in skeletal muscle
Determines exact length of actin filaments

Question 5

Nebulette

Answer 5

Present in sarcomere in cardiac muscle
Can bind both actin and alpha-actinin so it is believed to be involved in anchoring sarcomeric actin to the Z-disc

Question 6

Titin (connectin)

Answer 6

Titin is a template protein - it positions myosin between two z-discs
Largest known protein
N-terminal links with z-disc, overlaps with titin from the next sarcomere; capped by protein called T cap
Titin interacts with actin in the Z-disc through 45AA domains called z-repeats which bind with N-terminal of alpha-actinin

Question 7

Intercalated disc

Answer 7

Contain desmosomes and gap junctions
Strong cell-cell cohesion
low resistance pathways
To allow spread of excitation from one cell to another

Question 8

Three junction types in the ICD

Answer 8

Fasciae adherens(adherens junctions): transmit mechanical force from cell to cell
Gap junctions: mediate chemical and electrical coupling between myocytes
Desmosomes: form attachment sites for IF

Question 9

Gap junctions

Answer 9

Water filled channels linking cytoplasm of neighbouring cells
Allow exchange of inorganic ions and other small water solvable molecules CA2+

Question 10

Connexin

Answer 10

4x hydrophobic transmembrane segments

Question 11

Connexon

Answer 11

Hemichannel
6x connexins
Multiple connexons => gap junction

Question 12

Cardiomyocytes express gap junctions composed of…

Answer 12

Cx40
Cx43
Cx45

Question 13

Gap junctions function:

Answer 13

Transfer of metabolites to cells incapable of their synthesis
Coordination of cellular metabolism and development via intracellular secondary messengers

Question 14

Phosphorylation of connexin subunits leads to what structural changes of a connexon channel?

Answer 14

Closes it

Question 15

reversible conformational changes in gap junctions in response to (3)

Answer 15

Cytosolic pH change
Increased in cytoplasmic CA2+
Increases in neurotransmitters

Question 16

Costameres

Answer 16

Sub-sarcolemmal protein assembles that connect sarcomere to sarcolemma in striated muscle cells
Anchor myofibrils to sarcolemma by Z-lines
Maintain spatial organisation of myofibrils
Also play a role in cellsignalling

Question 17

Explain the role of costameres in mechanotransduction

Answer 17

Laterally transmit contractile forces from sarcomere across the sarcolemma to ECM and neighbouring muscle cells
Mechanical fortification of sites of lateral force transmission across the sarcolemma to minimise stress on the relatively labile lipid layer

Question 18

Functions of the Z disc

Answer 18

Continuity between sarcomere
Link actin filaments of one sarcomere to actin filaments in the adjacent sarcomere
Cytoskeleton anchor to actin and titin
Roles in signalling and muscle haemostasis
Act as mechanical stress sensors in cardiomyocytes

Question 19

Protein network in the Z disc

Answer 19

Desmin (IF) encircles the z-disc and cross-links to plasma membrane desmin -> forms links between neighbouring z discs -> maintains integrity

Question 20

Complexes in muscle

Answer 20

Dystrophin - glycoprotein complex
Integrin - metavinculin complex
Focal adhesion
(Plectin 1)

Question 21

Dystrophin-glycoprotein complex

Answer 21

Specialised adhesion complex linking ECM to cytoskeleton

Question 22

Muscular dystrophy occurs when….

Answer 22

Gene encoding dystrophin is mutated

Question 23

Duchenne muscular dystrophy key facts

Answer 23

X-chromosome 21;2
X linked recessive
Rapid muscle degeneration => death

Question 24

Pathogenesis of DMD

Answer 24

Loss of DGC=> increase CA2+ concentration in muscle fibres => increase muscle degeneration and necrosis(due to activation of CA2+ dependent proteases)
Changes in MAPK and GTPase signalling => increase damage and increase CA2+ influx

Question 25

Cardiac muscle

Answer 25

Heart
Length: 50-100um; width 10-20um
Short branching cylinders; single central nucleus; striations
ICD junctions join cells end-to-end
Intrinsic rhythm; involuntary autonomic modulation
Lifelong rhythm; variable force

Question 26

M line

Answer 26

Centre of sarcomere
Myosin attachment site
Creatinine kinase, myomesin and c-protein