Lecture 7 Flashcards
What is a Sarcomere?
Basic unit of striated muscle
Hundreds of thousands of sarcomeres are arranged in a
Repeated pattern over and over again
Z-disks function to
Provide plate for actin filaments to attach to
The Sliding filament model is the
Sequence of events in actin-myosin based muscle contraction
Myosin molecule extends by
Straightening its neck
A cross bridge is formed when
Myosin head forms a bond with actin filaments
What happens when Myosin pulls on the Actin filaments?
The two Z-disks are pulled together, shortening the sarcomere
A power stroke is when
The myosin head pulls on the filament
The arrangement of actin and myosin before contraction determines the
Contraction force
What happens if the arrangement of actin and myosin is disrupted?
The force generated by each sarcomere in the muscle will decrease
How is the force if Z-disks are originally too close?
Force will decrease because actin filaments will collide and there is no room for shortening
How is the force if Z-disks are originally too far?
Force will decrease because myosin will not overlap with thin filaments and fewer cross-bridges will form
The relationship between the length of the sarcomere before contraction and resulting force generated is
Bell shaped
If the sarcomere is lengthened or shortened from this optimal length, the force of contraction will
Decrease
What is a Myofibril?
Single continuous stretch of interconnected sarcomeres
Myofibril are covered in a plasma membrane called a
Sarcolemma
What are considered as muscle cells?
Myofibril
What is the first step of muscle contraction?
AP travels down the motor neuron and reaches the axon terminal, triggering the release of Acetylcholine that has a receptor on the sarcolemma
The binding of Acetylcholine to its receptor on the sarcolemma results in
A graded potential to form in the myofibril
What happens if enough sodium enters the muscle cell?
The membrane potential will cross the threshold potential of -50 mV
AP is formed in the muscle cell and consists of
Depolarization, repolarization and hyperpolarization
Newly created AP spreads along the
Sarcolemma, down the T-tubules
What happens when AP goes down the T-tubules?
This causes the release of Ca2+ from the Sarcoplasmic Reticulum through voltage-gated calcium channels on the SR
What stimulates the sarcomeres to contract with the help of Troponin and Tropomyosin?
Released Ca2+
What are Troponin’s three subunits?
- TnC: Ca2+ sensor that can bind to Ca2+
- TnI: blocks myosin binding site on actin
- TnT: Binds to tropomyosin, which keeps troponin associated with actin
In a relaxed muscle, IC Ca2+ is
Low, with Ca2+ binding sites on TnC open
Low Ca2+ results in
TnI blocking myosin binding sites on actin
When Ca2+ leaves SR and enters muscle, it binds to
TnC
What happens when Ca2+ binds to TnC?
This binding induces a conformational change where TnI moves away from the myosin binding site on actin by sliding down tropomyosin, exposing the myosin binding site on the actin
Muscle contraction is stopped via the enzyme
Acetylcholinesterase
Acetylcholinesterase functions to
Break the acetylcholine bound to the acetylcholine receptor
With acetylcholine broken down, ligand-gated Na+ channels close, stopping
Graded potentials from forming, further stopping AP from being created
Without AP, Ca2+ moves back into
The SR
Without AP, Ca2+ levels inside the muscle
Lower, causing Ca2+ binding sites on TnC to be available
Myosin binding sites on actin are blocked by TnI when
Ca2+ levels are low
