Non-striated Muscle I

Locations of non-striated muscle in vertebrate body

• GI tract (including internal, but not external sphincters)

• Blood vessels and lymphatics (except capillaries)

• Pulmonary airway

• Genitourinary tract

• Piloerectors attached to hairs

• Iris of eye

Questions to consider

• Structure/function of the contractile machinery

• Control of sliding filaments

• Excitation-contraction coupling

• Control systems involving hormones and neurotransmitters

Dense plaque or dense band attaches actin to sarcolemma

Organization of the contractile machinery in smooth muscle

Dense body made of alpha actinin attaches actin to actin

Calciosomes

Dense plaque

This is rat ileal smooth muscle. The SR takes the form of small vesicles (calciosomes) near the sarcolemma.

Dense body

On-off regulation of cross-bridges in smooth muscle is primarily based on the thick

filament, with secondary force control on the thin filament.

Control of myofilaments: thin filament vs thick filament based control systems

• The classical thin filament based system: troponin and tropomyosin present and bound to thin filament, as in heart and skeletal muscle. The control by Ca++ cannot be removed by washing the filaments.

Thick filament based: troponin is absent; control by Ca++ is lost when the filaments are washed and can be restored by adding back soluble factors, which are now recognized as calmodulin (CM) and myosin light chain kinase (MLCK)

The sequence of events in thick filament control

• Ca++ binds to calmodulin

• Ca++-calmodulin binds to MLCK

• MLCK-CM-Ca++ phosphorylates myosin regulatory light chain

• Head associates with actin and begins to cycle

• Cycling continues until Pi is removed by myosin light chain phosphatase (MLCP)

Thick-filament-based regulation through myosin light-chain kinase (MLCK)

MLCK phosphorylates the myosin regulatory light chain at site x - this allows the myosin head to cycle

A Ca++-independent mechanism of relaxation -

Protein Kinase C phosphorylates myosin light chain at site y - this antagonizes activation by phosphorylation at site x.

Single Unit vs Multiunit

• Multiunit muscle is not connected by gap junctions - each cell must receive innervation. Multiunit muscle shows little spontaneous activity and is controlled by its dual autonomic innervation..

• Single Unit (or unitary) muscle is joined by extensive gap junctions so multiple fibers act as a unit. Spontaneous activity is typical of such muscle; innervation modulates this activity.