Smooth Muscle and Contraction Mechanisms

1.         Most smooth muscle are found in the walls of hollow organs and tubes.

2.         Importance in the maintenance of Homeostasis

a.         Movement of materials:

i.          since smooth muscle are not attached to bones of the skeleton, the contraction of smooth muscle will generate force to move material through the lumen of hollow organ.

ii.          the contractions of smooth muscle around the oesophagus and intestinal tract generates waves of contraction that move ingested material from the oesophagus to the colon.

b.         Enable hollow organs to accommodate increased load in them:

i.          smooth muscle can sustain contractions for extended periods without fatiguing.

ii.          this ability allows the walls of organs to maintain tension with a continued load.

iii.         the filling of the urinary bladder with urine or the digestive tract with food will not be possible without this property of smooth muscle.

iv.         without this degree of flexibility conferred to the organ by the smooth muscle around it, the organ will not be able to hold excess load which may lead to obstruction in the urinary tract, as in the bladder or digestive tract, as in the rectum.

c.         Control of passage of materials through hollow organs:

i.          some smooth muscles are tonically contracted and maintain tension at most times.

ii.          the esophageal and urinary bladder sphincters are examples of tonically contracted muscles whose function is to close off the opening to a hollow organ.

iii.         these sphincters relax when it is necessary to allow material to enter or leave the organ.

iv.         a muscle that maintains a measurable level of tension is said to have a tone.

d.         Maintenance of homeostasis:

i.          there are smooth muscle around some vessels, like the arterioles, found in the dermis layer of the skin.

ii.          contraction and relaxation of these muscles will alter the diameter of the vessel.

iii.         the activity of these muscles are controlled by neuronal impulses in the central nervous system.

iv.         a decrease in body temperature will invariably cause a decrease in the diameter of the vessels, thereby reducing the blood supply from the skin.

v.         less heat is lost through the skin and in this way, the internal body temperature is maintained at a relatively constant level.

3.         Structure of Smooth Muscle

a.         Muscle fibers:

i.          they are small spindle-shaped cells with a single nucleus.

ii.          usually 2 to 5 microns in diameter and only 20 to 500 microns in length.

b.         Arrangement of fibers:

i.          the contractile fibers are not arranged in organized sarcomeres, so smooth muscle does not have distinct banding patterns as striated muscle does.

ii.          actin and myosin are arranged in long bundles that extend diagonally around the cell periphery, forming a lattice around the central nucleus.

iii.         the oblique arrangement of contractile elements beneath the cell membrane causes smooth muscle fibers to become globular when they contract.

c.         Regulatory proteins: smooth muscle lacks troponin.

d.         Myosin:

i.          the myosin found in smooth muscles is in a different isoform than that of skeletal muscle.

ii.          its ATPase activity is much slower, so the rate of crossbridge cycling is slower and the contraction phase of the twitch is longer.

iii.         one of the smaller protein chains in the myosin head, called myosin light chains, serves in a regulatory role to control contraction and relaxation.

e.         Actin:

i.          the long actin filaments attach to dense bodies of protein in the cytoplasm, structures that are analogous to the Z disks of the sarcomere.

ii.          the ends of the actin filaments terminate at protein plaques in the cell membrane.

iii.         some of the dense bodies are attached to the cell membrane.

iv.         others are dispersed inside the cell and held in place by a scaffold of structural proteins linking one dense body to another.

v.         some of the membrane dense bodies of adjacent cells are also bonded together by intercellular protein bridges.

vi.         it is mainly through these bonds that the force of contraction is transmitted from one cell to the next.

f.          Association of myosin with actin:

i.          the ratio of actin to myosin in smooth muscles is 10-12:1.

ii.          the less numerous myosin filaments lie bundled between the long actin fibers and are arranged so that their entire surface is covered by myosin heads.

g.         Sarcoplasmic reticulum: smooth muscle has relatively little sarcoplasmic reticulum.

h.         Sarcolemma:

i.          the calcium-storage function of the sarcoplasmic reticulum is supplemented by caveolae, small vesicles that cluster close to the cell membrane.

ii.          the membranes of caveolae contain gated Ca²⁺ channels that open in response to either a change in membrane potential or the binding of a ligand, allowing Ca²⁺ concentrated inside the caveolae to enter the cell.

4.         Similarity between smooth and skeletal muscle

a.         Both have actin and myosin filaments.

b.         Contraction is caused by the same attractive forces between myosin and actin.

c.         The contractile process is both muscles is activated by Ca²⁺ ions.

d.         The Ca²⁺ ions are stored in the sarcoplasmic reticulum of both muscles.

5.         Structural differences between smooth and skeletal muscle

6.         Types of Smooth Muscle: there are two types, single unit and multi-unit:

7.         Characteristics of Smooth Muscle Contraction

a.         Contraction of smooth muscle leads to a change in shape and this is due to the oblique arrangement of the contractile elements in smooth muscle.

b.         Ability of smooth muscle to maintain tension over a wide range of fiber lengths:

i.          smooth muscles have longer actin and myosin filaments.

ii.          the extra length allows the fibers to be stretched more, yet still maintain enough overlap to create optimum tension.

iii.         once contraction begins, the actin filaments can slide along myosin for longer distances before the end is reached.

iv.         this property is important for organs whose luminal volume may vary, such as the stomach, intestine, bladder and uterus.

v.         filling of bladder with urine: the smooth muscle must stretch as the lumen fills yet still maintain the ability to contract in order to expel the contents of the lumen.

c.         Variable force of contraction:

i.          in multi-unit smooth muscle, increasing the force of contraction requires the recruitment of additional fibers.

ii.          in single-unit smooth muscle, the firing of one fiber will lead to the firing of the rest as they are electrically connected – graded contractions vary with the amount of Ca²⁺ that enters the cell.

d.         Slow-cycling of cross-bridges:

i.          the recycling of cross-bridges in smooth muscle – that is, their attachment to actin and release from actin, and reattachment for the next cycle – is slower in smooth muscle.

ii.          this may be due to the deceased ATPase activity in the cross-bridge heads so that degradation of the ATP that energizes the movements of the heads is greatly reduced.

iii.         the fraction of time that the cross-bridges remain attached to the actin filament is greatly increased in smooth muscle.

e.         Energy Usage:

i.          requires little energy to maintain contraction.

ii.          may be due to the slow attachment of cycling of the cross-bridges and because only one ATP molecule is required for each cycle, regardless of its duration.

iii.         this economy of energy utilization enables smooth muscle to maintain tonic muscle contraction indefinitely as in the bladder and gall bladder.

f.          Slowness on onset of contraction and relaxation of smooth muscle:

i.          a smooth muscle contract 50 to 100 milliseconds after it is excited.

ii.          it reaches full contraction 0.5 second later and declines in contractile force in another 1 to 2 seconds, giving a total contraction time of 1 to 3 seconds.

iii.         this is caused by the slowness of attachment and detachment of the cross-bridges.

g.         Force of contraction:

i.          maximum force of contraction of smooth muscle is greater than that of skeletal muscle.

ii.          this results from the prolonged period of attachment of the myosin cross-bridges to actin filaments.