Lecture 7: striated muscle excitation - contraction coupling Flashcards
What are the 3 types of muscle?
- skeletal muscle- attaches to bone and is involved in moving skeleton
- cardiac muscle
- smooth muscle
Describe the **structure **of skeletal muscle
- has striated appearance - due to alternating dark and light bands
- multiple nuclei
- elongated shape
- each skeletal muscle cell is called a muscle fiber
**What is a myofibril?
- skeletal cells are composed of many long tubular cells called muscle fibers
- these muscle fibers contain many chains of myofibrils (rod like organelle of muscle fiber)
What is the striated appearance of skeletal muscle due to?
- the** repeated pattern of dark and light bands** due to the arrangement of the thin (actin) and thick (myosin) filaments
How are the filaments in a skeletal muscle fiber organised?
- Actin containing filaments/ thin filaments are anchored to the Z lines at each end of the sarcomere
- the actin filaments free ends wrap around the myosin / thick filaments in the A band in the centre of the sarcomere
What is a sarcomere?
- Many sarcomeres located in 1 myofibril
- Sarcomere - the contractile/structural unit of a myofibril consisting of a dark A band and a light I band
What is a sarcomere composed of?
- 2 filaments - thick (myosin) and thin (actin)
- A band (dark) - myosin, I band (light) - actin , H zone (part between 2 actins) etc
Describe the **structure of the thin filament **in a sarcomere
- Actin
*in sarcomere, contains 2 other proteins called troponin and tropomysosin which regulate contraction
Describe the structure of the thick filament
- Myosin
- Composed of heavy chain & light chain
- These** chains** combine to form molecule with 2 globular heads -cross bridges
- contains actin binding site and** ATP binding site**
What happens to the** I band** in sarcomere when a skeletal muscle contracts?
- the I band/ actin filaments gets smaller (towards centre) when contracted
Describe the events of the cross bridge cycle/ sliding filament mechanism
1.** binding of Ca2+** to troponin on myosin, this activates the cross bridge cycle, tropomyosin moves out of way from binding site &myosin binds to actin
2. cross bridge moves/ the power stroke , **ADP and Pi are released
3. another ATP** molecule comes and **breaks the cross bridge link **and binds to ATP binding site on myosin
4. ATP is then hydrolysed by myosin ATPase which refroms the energised state of myosin
5. cycle can begin again
What is the role of ATP in the cross bridge cycle?
- ATP binds to myosin and breaks cross bridge link between actin and myosin- allowing another cycle to begin
- ** ATP hydrolysis releases energy** which is then** used as energy for the cycle**
What is contraction in skeletal muscle initiated by?
- an increase in cytosolic Ca2+ concentration
What is the sarcoplasmic reticulum?
- same as the endoplasmic reticulum in most cells
- it functions to store Ca2+ ions
What structure is present at the end of each segment of the SR?
- terminal cisternae / lateral sacs
- they contain the Ca2+ binding protein calsequestrin which allows the storage of a large amount of Ca2+
What are **Transverse / T tubules **?
- they are tubular structures that surround the myofibrils
What is the NMJ?
- the neuro muscular junction- a synapse between an alpha motor neuron and a muscle cell/ muscle fiber
what is the function of the NMJ?
- it allows a motor neuron to transmit an AP to a muscle fiber, which later causes a contraction
Describe the sequence of events of the NMJ
- AP reaches axon terminal causing **depolarisation **
- depolarisation leads to opening of voltage gated Ca2+ channels
- Influx of Ca2+
- Ca2+ binds to Synaptotagmin
- This causes Ach vesicles to fuse with membrane and be** released** into synaptic cleft
- Ach binds to nicotinic receptors
- binding allows Na+ and K+ entry into muscle fiber - EPP
- depolarisation exceeds threshold and all voltage gated Na+ channels open in fiber
- AP propagates through muscle fiber
How is the AP propagated along the muscle fiber ?
- it is propagated into the** interior of the muscle fiber** along the transverse tubules to the region of the sarcoplasmic reticulum, where it produces a release of Ca2+ ions from the reticulum
how does the relaxation of a contracting muscle fiber occur?
- relaxation occurs as a result of active transport of cytosolic Ca+ bacj into the sarcoplasmic reticulum
- calcium is removed from troponin which allows tropomysoin to move back to its original place and preveent actin-myosin binding
what is the **triad junction **in the skeletal muscle?
- 2 ** terminal cisterns** (at the end of the sarcoplasmic reticulum segments) in close association to the T tubule
- occurs at every Z line
what type of membrane protein is present on the T tubule membrane ?
- **DHPR **- dihydropyridine receptor - which is a modified version of L type Ca2+ channel in T tubule
- functions as an voltage sensor (ie senses the AP) - allows T tubules to bring action potentials into the interior of muscle fiber
What protein is embedded in the SR membrane?
- the** ryanodine receptor** (RYR1)
- contains foot processes that connect to the DHP receptor
- forms a** Ca2+ channel**
Describe the mechansim of the cytosolic increase in Ca2+ performed by T tubules/ SR
- during a T tubule AP, charged amino acid residues within the DHP receptor protein** induce a conformational change**
- this acts as the foot process to open the ryanodine receptor channel
- Ca2+ is then released from the terminal cisternae of the SR into the cytosol where it can bind to troponin
What is the SERCA pump?
- Sarcoplasmic Endoplasmic ATPase
- Ca2+ ATPase pump
- transfers Ca2+ from cytosol to sarcoplasm
What are the 4 main proteins responsible for Ca2+ cycling and storage within the SR?
- DHPR
- RYR1
- SERCA pump
- calsequestrin
what is calsequestrin?
- high capacity Ca2+ binding protein found in the terminal cisternae of the SR
