Sarcomere Structure Flashcards
What are the 5 defining characteristics of a skeletal muscle cell (fibre)?
- Multinucleated
- Contains many mitochondria
- Has special structures called Transverse (T) tubules
- Has myofibrils and sarcomeres
- Has specific terms for some intracellular structures
What is the specific skeletal muscle cell term for plasma membrane?
Sarcolemma
What is the specific skeletal muscle cell term for cytoplasm?
Sarcoplasm
What is the specific skeletal muscle cell term for smooth ER?
Sarcoplasmic reticulum
What are muscle fibres arranged in?
Fasciculi
What is the name of the structure that is a dense sheath that lies on the surface of the muscle?
Epimysium
What structure lies between fasciculi?
Perimysium
What structure separates the muscle fibres?
Endomysium
What is the width of a muscle fibre (muscle cell)?
50 - 100 micrometers
Define sarcolemma
True cell membrane and encloses the muscle fibre
Define sarcoplasm
Intracellular fluid that fills spaces between myofibrils
What are myofibrils predominantly composed of?
Actin and myosin filaments
What is a sarcomere?
The portion of a myofibril that lies between two successive Z disks
What is the width of a sarcomere?
1 - 2 micrometers
What are the different areas/sections of a sarcomere?
\+ A band \+ M line \+ I band \+ H zone \+ Z disk
What is the THICK filament that myofibrils are predominantly composed of?
Myosin
What is the THIN filament that myofibrils are predominantly composed of?
Actin
What structures make up myosin?
+ Tail
+ Two globular heads
+ Chains (head and tail)
What does the tail of myosin consist of?
Two intertwined heavy chains
What are features of the globular heads of myosin?
+ Extend out forming cross-bridges
+ Contain heavy and light chains
+ Contain binding sites fro actin and ATP (myosin-ATPase)
What size is myosin?
1.6 micrometers
What size is actin?
1 micrometer
What are features of actin?
+ Contractile protein
+ Each G actin has a binding site for myosin
What are the proteins associated with actin?
Tropomyosin and troponin
What are features of tropomyosin?
+ Regulatory protein
+ Overlaps binding sites on actin for myosin
+ Inhibits interaction when in the relaxed state
What are features of troponin?
+ Regulatory protein
+ Forms a complex with the other proteins of the thin filament (actin and tropomyosin)
+ Binds Ca2+ reversibly
What happens when troponin binds Ca2+?
Once bound, it changes conformation to pull tropomyosin away from the myosin interaction sites
What is the effect of Ca2+ binding to troponin?
It regulates skeletal muscle contraction
How does Ca2+ binding to troponin regulate skeletal cardiac muscle?
It moves tropomyosin away and allow myosin to interact with the actin
What are the structural proteins in a sarcomere?
+ Titin (connectin)
+ Z-line (alpha-actinin)
+ M-line (myomesin, M-protein)
+ Nebulin
What are the contractile proteins in a sarcomere?
+ Actin
+ Myosin
What properties does titin provide?
+ Elasticity
+ Stabilises myosin
What is the role of nebulin?
It helps align actin
What are myofibrils?
The structures that give skeletal (and cardiac) muscle their characteristic striated appearance
What are striations the result of?
The orderly arrangements of thick and thin filaments
What type of filaments form I-bands in a cross section through a sarcomere
Thin filaments only
What type of filaments form H-zones in a cross section through a sarcomere
Thick filaments only
What type of filaments form M-lines in a cross section through a sarcomere
Thick filaments linked with accessory proteins
What type of filaments form the outer edges of A-bands in a cross section through a sarcomere?
Thick and thin filament overlap
What is the ultrastructural basis of sarcomere banding for A-bands?
Primarily myosin filaments in region of overlap
What is the ultrastructural basis of sarcomere banding for I-bands?
Actin filaments bisected by Z-line
What is the ultrastructural basis of sarcomere banding for H-zones?
Region of A-band which contains only myosin filaments
What happens to the zones of a sarcomere when it shortens (muscle is contracted)?
+ I-bands and H-zones shorten
+ A-band remains constant
What are features of T-tubules?
+ Walls are continuous with the sarcolemma
+ Fluid within is continuous with external environment
What is sarcoplasmic reticulum?
In muscle, it is homologous to the endoplasmic reticulum found in most cells - Ca2+ is stored and released following membrane excitation
How are T-tubules and SR connected?
With junctions - they involve two integral membrane proteins, one in the T-tubule membrane and the other in the membrane of the sarcoplasmic reticulum
What are features of the T-tubule protein?
It is a modified voltage-sensitive Ca2+ channel known as the dihydropyridine (DHP) receptor, which acts as a voltage sensor
What is the protein embedded in the SR membrane known as?
Ryanodine receptor
What does the ryanodine receptor form?
Ca2+ channels
What does sarcoplasmic reticulum store?
Ca2+
Where do T-tubules bring action potentials?
Into anterior of muscle fibre
