Tissues 7- Muscle

Question 1

What does Isotonic Contraction mean?

Answer 1

muscle length changes but tension remains the same

Tension > Load

Muscle contracts and the fibres shorten

Question 2

What does Isometric Contraction mean?

Answer 2

muscle length stays the same but tension changes
Tension = Load

You get actin being pulled in and out

You still use ATP because it is needed to return myosin heads to the charged state

Question 3

Describe myofibres

Answer 3

Large and cylindrical
Multi-nucleated
Packed with myofibrils

Question 4

Describe the bands visible in skeletal muscle

Answer 4

A- band: Dark band intersected by darker region -> H zone

I- Band: Light band intersected by dark Z disc

Z disc: made up of alpha actin and capz

Question 5

Describe the sacromere

Answer 5

Functional unit of muscle-lies between two z discs

Contains
Actin-
Thin filament
consists of two twisted a-helices

TITIN -
VERY LARGE spring like filament - keeps the myosin in place

Nebulin -
large filament associated with actin - doesn’t really do anything

CapZ and Tropomodulin -
associate with the ends of actin

Myosin-
Thick filaments which are motor protein
Numerous globular heads that interact with actin

Question 6

Describe the excitation of myofibril bundles

Answer 6

1. AP propagates along the myofibril membrane (sarcolemma) and T-tubules.
2. Depolarisation activates dihydropiridine receptors (DHPR) causing a conformational change in DHPR.
3. The change in shape allows DHPR to make physical contact with Ryanodine Receptors (RyR) on the SR.

4,This leads to a conformational change in RyR which opens the RyR channel and causes Ca2+ release from the SR.

Question 7

What is the sliding filament theory?

Answer 7

1. Presence of Ca2+ causes movement of tropomyosin chains which expose myosin binding sites on the actin chain
2. Charged myosin heads bind to the sites
3. Binding and discharge of ADP cause the myosin heads to pivot pulling the actin filaments close to the centre of the sacromere
4. ATP binds which cause the myosin to release from the binding sites on the actin chains
5. Hydrolysis of the ATP cause the myosin heads to reset

Question 8

Describe cardiac muscle

Answer 8

Cardiomyocytes are STRIATED muscle

Intercalated Disc:
Specialised discs connecting individual cardiomyocytes
They have desmosomes and gap junctions
Desmosomes - hold the membrane structures together
Gap Junctions - allow electrical communication between cells

Mechanism of contraction is THE SAME as skeletal muscle

Question 9

Describe Cardiac Excitation

Answer 9

AP in the heart muscle is generated by pacemaker cells in the nodes

The AP moves down T tubules and comes into contact with VGCCs

This causes the VGCC to open allowing Ca2+ influx

The Ca2+ then binds to the RyR and causes CALCIUM INDUCED CALCIUM RELEASE (CICR)

Question 10

Why is smooth muscle ‘smooth’

Answer 10

because you DO NOT get the striated pattern of actin and myosin that you get in skeletal and cardiac muscle - though it does still contain actin and myosin

Question 11

Describe contraction in smooth muscle

Answer 11

Depolarisation activates and open the VGCC (slightly different from the VGCC in cardiomyocytes) causing Ca2+ influx

The calcium moves into the cell and binds to Calmodulin (CaM) forming a Ca2+-CaM complex

Ca2+-CaM complex activates Myosin Light Chain Kinase (MLCK)

MLCK phosphorylates myosin light chains (MLC20)

This changes the appearance of smooth muscle cells from elongated to contracted

This leads to vasoconstriction