The Crossbridge

The crossbridges are also known as the myosin heads and are those regions of the myosin molecule that span the gap between the thick and thin filaments in the sarcomere.  Essentially they contain both the motor and regulatory domains of myosin. 

Although it is known that each myosin molecule has two 'heads' relatively little is known about how these heads interact. For example, in smooth muscle myosin it is known that both heads are required in order for regulation to take place. It is also known that in some of the unconventional myosin's the molecules walk in a processive manner down their actin tracks. However the degree of co-operativity between the two heads of skeletal myosin (if indeed there is any) is unknown.

• There are currently known to be 13 different types of myosin molecules all of which serve a very specific cellular function. The muscle myosin's are known as group II or conventional myosin's as they were discovered first.
• A processive myosin is one that does not let go of the actin track, i.e. that one of the two heads is in contact with the actin at any given point in it's motion.

Two representations of the crossbridge structure The first a simple stick cartoon of a the head and lever arm (a.k.a. the regulatory domain). The second a ribbon diagram of chicken skeletal S1 from Rayment et al, 1993.

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An historical look at muscle contraction