Contractile Mechanisms Flashcards
What are the 3 types of muscle in the human body?
- Smooth
- Skeletal
- Cardiac
Outline how smooth muscle contraction occurs
- IP₃ binds to its receptor
- Induces Ca2+ release
- Ca2+ channels also open bringing in even more Ca2+
- Calmodulin activates myosin light chain kinase
- myosin phosphorylated at light chain (regulatory area)
Ser19 - Increases ATPase activity of myosin head
- Allows actin-myosin binding
List features that all 3 muscle type contractions share
- All 3 involve the Sarcoplasmic reticulum (some more
than others) - All require ATP hydrolysis to activate myosin head
- All 3 involve the binding of actin and myosin filaments
- All 3 have Ca2+ sensors involved (troponin in cardiac /
skeletal, calmodulin in smooth)
Explain how muscle contraction is initiated
- Depolarisation initiated at nAchR creating kink in
membrane - Maintained by Na+ channels in synapse
- Spreads into t tubules via sarcolemma (DHSR)
- Voltage sensing ryanodine on SR Ca2+ channels open
Compare and contrast Ca2+ channels and DHPs
Both are blocked by Dihydropyridines
Ca2+ allow ions through, DHP can’t conduct ions
How is the secondary messenger IP₃ formed?
Inositol triophosphate produced via cleavage of PIP₂ by Phospholipase C
Which receptors enable contraction in skeletal muscle?
The physical interaction between DHP and RyR - create Ca2+ influx in skeletal muscle
What is calmodulin?
Ca2+ sensor that moderates calcium (instead of troponin) activates myosin light chain kinase
Explain what happens when Cardiac muscles are activated
- Depolarisation causes voltage gated Ca2+ channels to
open - Influx of Ca2+ down t tubules into cardiac cells
- Calcium induced Calcium release occurs (CIRC)
- Ryanodine receptors opened due to increase in Ca2+
on their external surface - causes contraction of cardiac muscles
How is muscle contraction in smooth muscle deactivated?
Myosin light chain phosphatase dephosphorylates the myosin
What is meant by a contraction?
The interaction of actn and myosin fuelled by ATP and driven by a rise in [Ca2+]
How do cardiac muscles enable a Ca2+ influx for contraction?
CICR (calcium induced calcium release) promoted by the opening of Ca2+ channels
How are actin filaments arranged in skeletal muscle?
Tropomyosin forms long thin threads wound around actin filaments
Describe how smooth muscle contraction differs from cardiac and skeletal muscle contraction
Smooth muscle differs because
- contractile proteins not in regular array
- not a tubule system
- has a less developed Sarcoplasmic reticulum (SR)
- Contractions sustained for long periods
- myofilaments connect with dense bodies
- caldesmon and calponin instead of troponin
- Different form of myosin (MYHII)
- Lower ATPase activity - lower ATP affinity
Where in the body is skeletal muscle found?
- bicep
- calf
- thigh
- diaphragm
How does smooth muscle relaxation occur?
- [Ca2+] drops
- Myosin light chain kinase becomes inactive
- Myosing light chain Phosphatase dephosphorylates
Ser19 - lowers ATP affinity of myosin head
- Acto-myosin complex remains for latch phase before
relaxing
Explain what happens when skeletal muscle contracts
Contraction in skeletal muscle is the removal of a supressant protein tropomyosin
What are DSHR and DHP?
Dyhydropyridine proteins are volatge sensing receptors that physically interact with Ca2+
channels
They are ion channels that don’t conduct ions
They are blocked by dihydropyridines that are well known calcium channel blockers
How often does a crossbridge cycle occur during contraction?
Each myosin head cycles approximately 5 times per second
How is skeletal muscle organised?
In bands with the M (myosin) line in the middle
and Z disks at the end of each sarcomere
The A band represents the entire myosin length with remains unchanged during contraction
The H band consists of myosin only
The I band is actin filaments only
How does depolarisation cause tropomyosin to unblock actin-myosin binding sites?
Depolarisation -> Na+ influx -> DHSR -> Ryanodine cause Ca2+ influx
Ca2+ diffuses along T tubules
Binds to troponin -> pulling away tropomyosin
Unblocking crossbridge binding site
Compare and contrast skeletal and smooth muscle contraction
Smooth Muscle
- Has greater shortening of muscle
- Slower contraction speed
- Greater force generation
- Sustained contraction (latch phase)
Skeletal muscle requires more energy than smooth muscle
Where is smooth muscle in the body?
- arteries
- gut
- reproductive organs
- bladder
Describe the components of the structure of skeletal muscle
A muscle fillacious is made of muscle fibres composed of myofibrils that are made up out of myofilaments
Explain the role of Ca2+ in muscle contraction
When [Ca2+] increased due to influx, Ca2+ binds to troponin C
- causes removal of suppressive tropomyosin
- allows actin to interact with primed myosin
- ATP dependent crossbridge cycling occurs
Describe the structure of Actin filaments
Actin is the thinner globular protein, arranged into long chains that are twisted around one another, forming a helical strand
Which muscle contractions require an influx of Ca2+?
Cardiac and Smooth muscle
Name the 2 muscle types that contract using a t tubule network
Skeletal muscle and Cardiac Muscle
Describe the structures of skeletal and cardiac muscles
Transverse tubules interact with the sarcoplasmic reticulum of muscle cells
Outline how actin and myosin interact to cause muscle contraction in skeletal muscle
- ATP hydrolysed by ATPase in myosin head => ADP +
myosin - Causes myosin head to move and bind to actin
filament - Actin pulled along by myosin head, releasing ADP
- New ATP molecule hydrolysed
- Myosin head detaches to form new cross bridge
- > repeat cycle
Describe how the structure of myosin enables contraction to occur
Myosin head is complementary to both ATP and actin filaments
What are transverse tubules?
aka T tubules are cell membranes that extend deep into the cell interiors
What is rigormortis?
Permanent cell contraction due to lack of new ATP molecule production => myosin head never released from actin filaments; muscles permanently contracted til ATP runs out
Describe the structure of myosin filaments
Myosin filaments line up in a thick bundle appearing as the darker filaments in striated muscles
