Excitation Contraction Coupling

Front 1

Describe skeletal muscle cell internal arrangement

Back 1

large, elongated and cylinder shaped

multiple nuclei

abundance of mitochondria

Front 2

What are myofibrils?

Back 2

specialised contractile elements that extend the entire length of the muscle fibre.

Front 3

Describe the structure of myofibrils

Back 3

consists of a regular arrangement of cytoskeletal elements- the thick and thin filaments.

Thick: special assemblies of the protein myosin

Thin: made up of the protein actin

Front 4

show the levels of organisation of skeletal muscle

Back 4

Front 5

What is an A band?

Back 5

A stacked set of thick filaments along with the portions of the think filaments that overlap on both ends of the thick filaments

The thick filaments ONLY lie within the A band and extend its entire width

Front 6

What is the H zone?

Back 6

The lighter area within the middle of the A band where the think filaments dont reach.

Front 7

What is the M line?

Back 7

A system of supporting proteins that hold the thick filaments together vertically within each stack. It extends vertically down the middle of the A band with the centre of the H zone

Front 8

What is the I band?

Back 8

Consists of the remaining portions of the thin filaments that do not project into the A band

Front 9

What is the Z line?

Back 9

Visible in the middle of each I band is a dense vertical line - z line. It is a flat, cytoskeletal disc that connects the thin filaments of 2 adjoining sacromeres.

Front 10

What is the area between two Z lines called and what is its function?

Back 10

a sacromere, which is the functional unit of skeletal muscle- the contractile unit. each relaxed sacromere is about 2µm in width

Front 11

relationship of whole muscle and muscle fiber

Back 11

Front 12

relationship of a muscle fiber and a myofibril

Back 12

Front 13

cytoskeletal components of a myofibril

Back 13

Front 14

What is excitation coupling?

Back 14

How the muscle converts an electrical stimulus (the action potential) into a mechanical response (contraction)

Front 15

What happens during contraction? How is it bought about?

Back 15

cycles of cross-bride interaction between actin and myosin brings about muscle contraction by means of the sliding filament mechanism.

Front 16

What is the link between excitation and contraction?

Back 16

Calcium

Front 17

What stimulates skeletal muscle to contract?

Back 17

The release of acetylcholine at the NMJ. The binding of ACh brings about membrane permeability changes in the muscle fibre, resulting in an action potential (AP) that is conducted over the ENTIRE surface of the muscle cell membrane.

Front 18

What 2 membranous structures play an important role in conduction of AP across cell memebrane?

Back 18

Transverse (t) tubules

Sacroplasmic reticulum

Front 19

How is the AP spread down the T tubules?

Back 19

At each junction of an A band and an I band, the surface membrane dips into the muscle fibre to form a transverse tubule. Each tubule runs from the surface of the muscle cell membrane into the central portions of the muscle fibre. Therefore, an AP on the surface membrane also spreads down the T tubule, rapidly transmitting the surface electrical activity into the central portions of the fibre. The presence of a local AP in the T tubules leads to permeability changes in the sacroplasmic reticulum.

Front 20

What is the sacroplasmic reticulum?

Back 20

A modified endoplasmic reticulum that consists of a fine network of interconnected membrane enclosed compartments surrounding each microfibril like a mesh sleeve.

Front 21

What are the terminal cisternae? (lateral sacs)

Back 21

The ends of each segment of SR (sacroplasmic reticulum) expand to form sac-like regions which are separated from the adjacent T tubules by a slight gap.

Front 22

What is the purpose of the lateral sacs?

Back 22

They store Ca2+, The spread of an action potential down a T tubule triggers the release of Ca2+ from the SR in the cytosol.

Front 23

What are foot proteins and their purpose?

Back 23

An orderly arrangement of foot proteins extends from the SR and spans the gap between the lateral sacs and the T tubule. These foot proteins not only bridge the gap but also serve as Ca2+ channels.

Front 24

What are ryanodine receptors?

Back 24

Another name for the foot protein Ca2+ channels.

Front 25

What are dihydropyridine receptors?

Back 25

Half of the SR’s foot proteins are“zipped together” with complementary receptors on the T tubule side of thejunction. •These T tubule receptors are known as dihydropyridinereceptors becausethey are blocked by the drug dihydropyridine. They are voltage gated receptors.

Front 26

How does a change in T tubule potential lead to the release of Ca2+ from lateral sacs?

Back 26

When an AP is propagated down the T tubule, the local depolarisation activates the V-gated dihydropyridine receptors. Activated T tubule receptors trigger the opening of the ryanodine channels in the adjacent lateral sacs of the SR. Opening of the Ca2+ release channels in direct contact with the dihydrophyridine receptors triggers the opening of the other half (that are not directly associated with the T tubule receptors)

Front 27

What happens after the calcium channels are opened?

Back 27

Calcium is released into the cytosol from the lateral sacs through all the open Ca2+ release channels. This release exposes the binding sites on the actin molecules so they can link with the myosin bridges at their complementary binding sites. This is the start of the cross bridge cycle.

Front 28

How dies electrical activity work with regards of contraction of the heart?

Back 28

Ca2+ entry through L-type channels in tubules triggers larger release of Ca2+ fromsarcoplasmic reticulum

Ca2+induced Ca2+ release leads to cross-bridgecycling and contraction– just like in skeletal muscle.

In skeletal muscle there is alwayssufficient Ca2+ released to turn on all crossbridges. In cardiac muscle cross bridge activity varies with amount of cystolic Ca2.

Front 29

diagram showing Excitation-Contraction Couplingin Cardiac Contractile Cells.

Back 29