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.