Sarcomeres
Sarcomere
-The smallest contractile unit of a muscle -The region of myofibril between two successive Z discs -Composed of microfilaments made up of contractile proteins
Ultrastructure of Myofilaments: Thick Filaments
-Thick filaments are composed of the protein myosin -Each myosin molecule has a rod-like tail and two globular heads
Ultrastructure of Myofilaments: Thin Filaments
-Thin filaments are chiefly composed of the protein actin -Each actin molecule is a helical polymer of globular subunits -The subunits contain active sites to which myosin heads attach during contraction
I band
A
I Bands
Actin only
A band
B
I band
C
Myosin Thick Filaments
D
M Line
Dark line that bisects the H zone
Z line
E
Region of overlap
F
H Zone
G
Sarcomere
H
Myosin Crossbridge
I
Z line
J
Actin Thin Filaments
K
H Zone
Mysosin only; thin filaments do not overlap thick filaments in the lighter H zone
2 Types of mycrofilaments are:
Thick (Myosin) and Thin (Actin)
Thin Filaments:
extended across the I band and partway into the A band
Thick Filaments:
extended the entire length of an A band
Z Bands
sheet of proteins (connectins) at the ends of the sarcomere that anchors the thin filaments and connects myofibrils to one another.