Lecture 4 - Excitation-Contraction Coupling Flashcards
Characteristics of skeletal muscle
Striated, multiple nuclei and an abundance of mitochondria
Characteristics of a skeletal muscle cell
Relatively large, elongated and cylindrical in shape
Why are multiple mitochondria required in skeletal muscle
As the muscle has high energy demands
What are the muscle fibres formed by during embryonic development
Fused myoblasts
What are the specialised contractile elements within skeletal muscles
Myofibrils
What does each myofibril consist of
Thick and thin filaments
What protein makes up the thick filaments
Myosin
What protein makes up the thin filaments
Actin
What are the different areas a myofibril can be split into
A band, I band, Z line, M line and the H zone
What is the A band composed of
A stacked set of thick filaments and the thin filaments that overlap
In what area(s) of the myofibril are the thick filaments found
The A band
What is the lighter area within the A band known as
The H zone
What causes the lighter portion within the A band
It is the point where the thin filaments do not overlap with the thick
What holds the thick filaments together
A system of supporting proteins
The proteins that hold the thick filaments together form the
M line
What is the I band composed of
The portions of the thin filaments that don’t overlap with the thick filaments
What is the dense vertical line present in the I band called
The Z line
What is the area between two Z lines called
A sarcomere
What is the functional unit of skeletal muscle
The sarcomere
What is the Z line
Flat, cytoskeletal discs that connect the thin filaments
What is the smallest functional unit that can undergo contraction
A sarcomere
If a muscle is to grow, how does it do this
By adding new sarcomeres onto the ends of the myofibrils
What is excitation-contraction coupling
How the muscle converts an electrical stimulus, action potential, into a mechanical response, contraction
What cycle brings about contraction
The cross-bridge cycle
What are the two main proteins involved in the cross bridge cycle
Actin and myosin
The cross bridge interactions cause
The sliding filament mechanism
Why is calcium so important
As it is the link between excitation and contraction
The release of what stimulates skeletal muscle to contract
Acetylcholine
Where is acetylcholine released
The neuromuscular junction
What does the binding of ACh cause
A change in the membrane permeability which results in an action potential being fired
What are the two important membranous structures that help to ensure conduction of the AP over the whole muscle
The transverse (T) tubules and the sarcoplasmic reticulum (SR)
Within the muscle where are T tubules found
At the junction between the A band and the I band
Where do the T tubules run from
The muscle cell membrane into the central portion on the muscle fibre
What does the presence of an AP in the T tubules cause
A change in permeability of the sarcoplasmic reticulum
What is the sarcoplasmic reticulum (SR)
A modified endoplasmic reticulum that consists of a fine network of interconnected membrane enclosed compartments surrounding each myofibril
What are the sac like regions at the ends of the SR known as
The lateral sacs (terminal cisternae)
What is stored within the lateral sacs
Ca2+
What does the spread of an AP down a T tubule trigger
The release of Ca2+ from the SR (lateral sacs) into the cytosol
What serves as Ca2+ release channels
Foot proteins
What are foot proteins also known as
Ryanodine receptors
WHat are the foot proteins made up of
Four subunits
What is ryanodine
A plant chemical that holds open the calcium release channels
What are the T tubule receptors made up of
Four subunits
What is another name for T tubule receptors
Dihydropyridine receptors
What effect does the drug dihydropyridine have on T tubule receptors
It blocks them
What type of sensors are the dihydropyridine receptors
Voltage-gated
What occurs to the ryanodine receptors when an AP is propagated down the T tubule
The local depolarisation causes the receptors to open
Opening of the Ca2+ channel in direct contact with the T tubule causes what
The other half of the receptors that aren’t connected to the T tubule to open
Once all the receptors are opened what happens
Calcium is released into the cytosol
During the cross-brigde cycle what does the released Ca2+ cause
For the binding sites on actin to be exposed
When the binding sites of actin are exposed, what does this signal the beginning of
The cross bridge cycle
What is unique about the APs of cardiac muscle
The exhibit a prolonged positive phase accompanied by a prolonged period of contraction
The AP of cardiac muscle has a characteristic shape due to
The plateau phase
What happens during the plateau phase of a cardiac muscle AP
Calcium is constantly entering the muscle cells
Where does the calcium, required for contraction, in cardiac muscle come form
The external environment
What is activated to allow calcium to move from the external environment into the cardiac muscle cells
The activation of L-type Ca2+ channels
What does the entry of the Ca2+ from the external environment cause
The release of more Ca2+ from the sarcoplasmic reticulum
What leads to cross bridge cycling and contraction within cardiac muscle
Ca2+ induced Ca2+ release
What causes variation in cross bridge activity
The amount of cystolic Ca2+ present
