Contractile Mechanism Flashcards
What are the 3 structures that muscles are broken down into?
- Structure
- Physically
- Physiologically
What is skeletal, cardiac and smooth muscle?
Skeletal muscle - voluntary motor muscle, in pairs, movement of bones e.g thigh and calf
Cardiac - located in the heart
Smooth Muscle - involuntary action of the ANS e.g the gut, digestive system
What are the visible structural features of these 3 muscle groups?
Skeletal - Striated, Multinucleate, uniform cells
Cardiac - Striated, Mononucleate, Branched structure, Intercaluated discs between cells
Smooth Muscle - non-striated, Mononculeate, actin and myosin are arranged in a regular way.
Define contraction
interaction of actin & myosin fuelled by ATP and driven by a rise in [Ca2+]
GIve an overview of muscle cells (A-level)
• Muscle cells are arranged in fibres, which can contract (becoming shorter) to produce a force.
• Cannot elongate without an antagonist: opposing muscle, elastic recoil, hydrostatic pressure.
• Involuntary smooth muscle consists of individual cells, tapered at both ends → spindle shaped.
Each cell ≈ 500μm long, 5μm wide, containing a nucleus and bundles of actin and myosin.
Contracts regularly and slowly → does not tire easily → controlled by ANS.
Arranged in longitudinal and circular layers → found in walls of tubular structures.
• Cardiac muscle forms the muscular part of the heart. Long, branched fibres → cross bridges.
Cross bridges → electrical stimulation spreads evenly and contraction is a squeezing action.
Cells joined by intercalated discs (specialised plasma membranes) → free diffusion of ions.
Contracts powerfully and continuously → does not tire easily → controlled by ANS.
• Voluntary (skeletal/striated) muscle occurs at joints in the skeleton → contraction moves skeleton.
Each fibre is multinucleate, surrounded by sarcolemma (membrane).
Actin and myosin arranged in a particular banded pattern → striped appearance.
Voluntary muscle contracts quickly/powerfully → fatigues easily → controlled by SNS.
• Muscle cell cytoplasm → sarcoplasm → many mitochondria and extensive sarcoplasmic reticulum
• The neuromuscular junction occurs between the nervous system and muscle. Similar to synapse
1. Action potentials arrive at end of axon → open calcium ion channels → calcium ions flood into axon.
2. Calcium causes vesicles of acetylcholine to move towards and fuse with the membrane.
3. Acetylcholine diffuses across junction and fuses with receptors in sarcolemma.
4. Wave of depolarisation spreads along sarcolemma → down transverse tubules → into muscle fibre.
• Motor unit: motor neurones divide → connect to several muscle fibres → contract together.
GIve an overview of smooth muscle
Smooth Muscle
• No troponin
• Actin and myosin
• Tropomyosin does not interact with the myosin bindings sites
• Myosin in smooth muscle is a different isoform than the skeletal muscle (MYH11 vs MYH1).
• Lower ATPase activity
• Lower affinity for ATP
• Contractile not arranged in regular arrays – not striated
• No T-tubule system
• Less developed SR
• Able to sustain contraction for long periods of time
• Myofilaments with dense bodies
• No troponin but Caldesmon and Calponin
How does smooth muscle contract (briefly)
Membrane events -> Rise in calcium -> Binding to sensor -> Contractile mechanism and ATP hydrolysis by myosin -> actin – myosin interaction -> cell shortens
What happens at a skeletal cell?
- Here we have a t-tubule and the sarcoplasmic reticulum – an important store of calcium.
- The action potential passes down the t-tubule depolarising the cell, deep into the cell.
- This depolarisation will modify any proteins which are able to sense voltage
- The channel (DR receptor - coffee bean) does not allow calcium ions through, it has a molecular change.
- This channel is in physical contact with the ryanodine receptor, which causes the ryanodine receptors to open causing calcium to enter.
- The DHP (dihydropyridine) protein is an ion channel that does not conduct ions.
- It is so called because it is blocked by dihydropyridines that are well known calcium channel blockers
What happens in a cardiac cell?
- This is a calcium channel that is VERY similar to the DHP protein in skeletal muscle. But it does allow ions through.
- Also blocked by dihydropyridine drugs.
- Here we have a t-tubule and the sarcoplasmic reticulum – an important store of calcium.
- The action potential passes down the t-tubule depolarising the cell, deep into the cell.
- The ryanodine receptor will open in response in to a local rise in calcium.
- Calcium induced calcium release.
What happens in smooth muscle?
In adrenergic pharmacology
How does calcium produce contraction?
In order for muscles to contract we need a calcium receptor.
• In skeletal and cardiac muscle, the receptor is troponin. Here you have the removal of a suppressive element.
• In smooth muscle it is calmodulin. Here you have the activation of myosin.
What is the mechanism of action in skeletal and cardiac muscle?
• Muscle stimulated → action potential passes along sarcolemma down the transverse tubules into the muscle fibre → carried to sarcoplasmic reticulum → calcium ions released into sarcoplasm (from sarcoplasmic reticulum) → calcium ions bind to troponin → alters shape of troponin → pulling the tropomyosin aside → exposes binding sites on actin → ATP hydrolysis occurs, cocking myosin head in position for binding to actin producing ADP and Pi → the myosin head is now in high energy configuration → myosin heads bind to the actin → cross bridges between the filaments form → myosin heads move → actin pulled past myosin filament towards sarcomere, power stroke, ADP is released myosin undergoes a conformational change → ATP binds to myosin causes it to detach, myosin heads detach from actin → ATP is then hydrolysed again so it can bind further up actin filament.
What is the mechanism of action in smooth muscle?
On image
