Neuromuscular junction and muscular contraction Flashcards
Summary of the ionic events in skeletal muscle contraction.
- AcH binds to nicotinic acetylcholine receptors on the sarcolemma. This causes a small depolarisation.
- The opens Na+ and K+ channels. Triggers a small amount of Na+ into the muscle cell which triggers an action potential. Na+ enters myocyte whereas K+ leaves the cell.
- This triggers the release of Ca2+ from the sarcoplasmic reticulum.
- Ca2+ binds to troponin and moves tropomyosin allowing myosin to bind to actin at the crossbridge.
- ATP is hydrolysed which causes contraction and the shortening of the sarcomere.
Myofilament
Protein filament in muscles that are either thick (myosin) or thin (actin).
Each thick filament can bind with 6 thin filaments.
Each thin filament can bind to 3 thick filaments
Describe the striations in skeletal muscles.
A band: Composed of myosin filament and some overlapping of actin filaments.
I band: Composed of only actin filaments.
H band: composed of only myosin filaments
M line: composed of only myosin tails
What happens to the sarcomere during contraction.
A band: does not change in length. The myosin fibres are not pulled so do not change position.
M line: Disappears as actin filaments are pulled closer.
H band: shortens.
I band: shortens.
Myosin
Forms thick protein filaments in skeletal muscles.
Composed of a straight double helical tail with two heads at one end. Aggregation of tails form thick filaments
There is a hinge between the head and neck.
Function: the protein that pulls actin via a power stroke.
Cross bridge cycling
- Myosin releases actin when ATP binds to the myosin head. Resets head angle to 90 degrees.
- Myosin head contains ATPase which cleaves ATP to ADP and Pi. This activates myosin head into a high energy state.
- Myosin head binds to actin when Ca2+ is present.
- Myosin head performs a power stroke and changes to 45 degrees. ADP is released after.
Thin filaments
Anchored to Z lines in the sarcomere and forms the I band.
Composed of:
Actin
Tropomyosin
Tropinin
F-actin helix has a groove on each side occupied by tropomyosin. Troponin is attached to tropomyosin and actin.
Actin
Protein that is the main component of the thin filament.
Composed of G-actin monomers with form F-actin double helical strand.
Every 7 G-actin monomers produce a helical turn.
Troponin
Protein in the thin filament that is made from a complex.
Complex;
Troponin-I: Binds to actin. The main inhibitory component that blocks troponin.
Troponin-C: Calcium binding site.
Troponin-T: Tropomyosin binding site.
Excitation-contraction coupling
- Membrane depolarisation causes the voltage gated Ca2+ channels in plasma membrane to undergo a conformational change.
- Triggers sarcoplasmic reticulum channels channels to undergo conformational change and open. This is enabled by ryanodine receptors.
- Ca2+ flows down its concentration gradient from sarcoplasmic reticulum into the cytosol.
Difference between twitch and tetany
Twitch- Very slow contraction of a muscle. Occurs when contraction is stimulated but the interval between AP allow complete relaxation of muscles.
Ca2+ can be pumped back into SR.
Tetany- A state of maximal contractile force and shortening of the muscle.
Acetylcholine receptors
Nicotonic
Muscarinic
Nicotinic receptors
Receptor protein that responds to acetylcholine and the agonist nicotine.
It is an ionotropic receptor (ligand-gated).
When activated by an ion, the voltage inside increases. This causes Na+ to flow out and K+ to flow in.
It is a non-selective cation channel.
Carbachol
Nicotinic acetylcholine receptor agonist
Treats glaucoma- miosis
Curare
Nicotinic acetylcholine receptor antagonist
Poisonous compound that causes muscle paralysis.
Competitive inhibitor that causes asphyxiation due to diaphragm paralysis.
