Lecture 3: Striated Muscle Flashcards
What is muscle?
complex tissue composed of myocytes (muscle cells) and many other types of cells
- myocytes are contractile – contractile apparatus uses actin and myosin
How are muscle types classified?
mostly based on microscopic appearance, but can also be categorized using other characteristics
What are the two main types of muscle?
- striated muscle: skeletal and cardiac
- smooth muscle
Which muscle types are voluntary? Which muscle types are involuntary?
- voluntary muscle: skeletal
- involuntary muscle: cardiac and smooth
What are thick filaments?
myosin
- polymers of approximately 300 myosin II hexamers (2 myosin II motor and 4 light chains)
- typical vertebrate myosin thick filament is 1.6 μm with an axial helical repeat of 429 Å
How do thick filaments assemble?
myosin molecules assemble in a manner that creates a bipolar filament with a central bare zone
What are thin filaments?
actin
- thin filaments are polymers of alpha-actin (while microfilaments are polymers of beta-actin)
- capped by special proteins at the ends to stabilize structure
- associated with several structural proteins (ie. troponin and tropomyosin)
Which filaments are troponin and tropomyosin associated with? What do they do?
- associated with thin filaments
- regulate interaction between actin and myosin in striated muscle
What is tropomyosin?
long, thin, double-stranded protein that extends over about 7 actin monomers
What is troponin?
trimer of TnC, TnI, and TnT that binds to every 7th actin on the thin filament
- TnC: binds Ca2+ (calcium sensor)
- TnI: inhibitory, binds troponin to actin
- TnT: binds troponin to tropomyosin
What are sarcomeres?
functional unit of a muscle fibre
- thick filament surrounded by an array of thin filaments, in association with structural proteins
- in vertebrate striated muscle, thick filament is surrounded by 6 thin filaments on either end
What is the Z-disk?
protein plates at the end of the sarcomere where the (+) end of actin thin filaments are attached
- composed of actin, titin, and other proteins
What is the M-line?
centre of the sarcomere between (-) ends of actin
- region where thick filaments do not overlap with thin filaments
What is the A-band?
region where thick filaments occur
What is the I-band?
portion of thin filaments that does not overlap with thick filaments
- spans the Z-disk
What is the H-zone?
portion of thick filaments that does not overlap with thin filaments
What is titin?
protein that anchors thick filament to Z-disk and provides elasticity
What is nebulin?
protein that twists around thin filament to establish the appropriate length for the filament
What is tropomodulin?
protein that caps the (-) end of thin filament
What is capZ?
protein that anchors (+) end of thin filament to Z-disk
What are some similarities and differences in flight muscle sarcomeres of highly divergent groups?
similarities:
- sarcomeres from insect (invertebrate) and vertebrate flight have the same basic structure
differences:
- some unique proteins
- insect sarcomeres are longer
- myosin arrangement is different
Describe sarcomere contraction.
- during its power stroke, each myosin head pulls a thin filament towards M-line
- combined effort of all these power strokes pulls Z-disks closer together, which decreases sarcomere length
Length-Force Relationship
When can cross-bridges form?
can only form when myosin heads of a thick filament can interact with actin units of a thin filament
Length-Force Relationship
What increases/decrease the amount of force that a sarcomere can produce during contraction?
- force increases as the number of myosin heads that can contact a thin filament increases
- force decreases as the overlap between thin filaments of adjacent Z-disks increases – at the shortest sarcomere length, thick filaments will collide with Z-disks and no further contraction is possible
