SMOOTH MUSCLE TISSUE

-        Cells are not striated

-        Fibers smaller than those in skeletal muscle

-        Spindle-shaped (about 50 to 200 microns long and only 2 to 10 microns in diameter); single, central nucleus

-        More actin than myosin

-        No sarcomeres

·         Not arranged as symmetrically as in skeletal muscle, thus NO striations.

-        Caveolae: indentations in sarcolemma;

·         May act like T tubules

-        Dense bodies instead of Z disks

·         Have noncontractile intermediate filaments

-        Is fatigue resistant

-        Activation is involuntary

•      Locations: Located in blood vessels, respiratory tract, iris of the eye, gastro-intestinal tract, urinary, & reproductive organs

-        Grouped into sheets in walls of hollow organs

·          An external longitudinal layer - muscle fibers that runs parallel to organ’s long axis. It shortens the tube and widens the lumen when contracted.

·          An internal circular layer - muscle fibers that runs around circumference of the organ. It lengthens the tube but narrows the lumen when contracted.

§   Both layers participate in peristalsis. Peristalsis is the  directional motion achieved from both of these sheets working together to alternate contraction

-        Is innervated by autonomic nervous system (ANS)

-        Visceral or unitary smooth muscle

·         Only a few muscle fibers innervated in each group

·         Impulse spreads through gap junctions

·         Whose sheet contracts as a unit

·         Often auto-rhythmic

-        Multiunit:

o   Cells or groups of cells act as independent units

o   Arrectorpili of skin and iris of eye

Smooth Muscle Contraction

As in skeletal-muscle cells, contraction in a smooth-muscle cell involves the forming of cross-bridges and thin filaments sliding past thick filaments. However, because smooth muscle is not as organized as skeletal muscle, shortening occurs in all directions. During contraction, the smooth-muscle cell's intermediate filaments help to draw the cell up, like closing a drawstring purse.

Calcium ions regulate contraction in smooth muscle, but they do it in a slightly different way than in skeletal muscle:

1.      Calcium ions come from outside of the cell.

2.      Calcium ions bind to calmodulin in the cytosol and then to an enzyme on myosin, called myosin light chain kinase.

3.      The enzyme uses ATP to transfer a phosphate group to myosin.

4.      This phosphate transfer activates myosin.

5.      Myosin forms cross-bridges with actin (as occurs in skeletal muscle).

6.      When calcium is pumped out of the cell, the phosphate gets removed from myosin by another enzyme.

7.      The myosin becomes inactive, and the muscle relaxes.