Skeletal muscle vs smooth muscle contraction

Where is smooth muscle found?

along the walls of hollow organs (blood vessels, bronchioles, digestive tract, etc)

Which system controls smooth muscle?

autonomic nervous system (not somatic motor)

Smooth muscle cell contraction is “graded” or has a range of lengths

Not “all or nothing” like the contraction of skeletal muscle cells

• The more calcium that influxes, the more forceful the smooth muscle contraction

A twitch is smooth muscle is much (faster/slower) than skeletal muscles and requires less ATP

slower!

• so smooth muscle cells are largely fatigue resistant

Contractile proteins are not arranged into sarcomeres in smooth muscle, and these proteins are cued into action by electrical or chemical signals or both

Contraction is cues by a more complicated cascade of reactions (similar to a G-protein coupled receptor cascade)

Smooth muscle lacks specialized receptor regions like the motor endplates on skeletal muscle cells

Receptors are scattered all over smooth muscle cell membranes and receive neurotransmitter from nearby varicosities

Notice shape of smooth muscle cells, where on hollow organs they are positioned, receptors, all over surface, control by autonomic varicosities, etc

• smooth muscle cells can also be connected via gap junctions

Notice that myosin and actin are present, but there are no repeating sarcomeres

A muscle “twitch” is one contraction and relaxation cycle

• notice the difference in twitch speed for a skeletal and smooth muscle cell

The events that lead to contraction!

• Electrical and/or chemical signal allows Ca++ to enter from ECF, which stimulates Ca++ release from the SR

→ this is called “calcium-induced calcium release”

• Ca++ activates calmodulin

• Calmodulin activates myosin light chain kinase (MLCK)

• MLCK phosphorylates myosin, myosin ATPase activity then increases, and tension is produced via power stroking in much the same way as in skeletal muscle

→ the lumen of a hollow organ will now constrict (decrease in diameter)

Events that lead to contraction pt 2

• When stimulation of smooth muscle ceases, Ca++ is pumped back into ECF and also into the sarcoplasmic reticulum

• Activated calmodulin and MLCK levels fall

• Activity of myosin light chain phosphatase (MLCP) INCREASES

→ it will dephosphorylate myosin…

→ Myosin ATPase activity then decreases

• Smooth muscle relaxes due to less power stroking

→ the lumen of the hollow organ will now dilate (increase in diameter)

Skeletal muscle fiber types: they’re not all created equal

Which one contracts the slowest?

• Slow-twitch oxidative

• Fast-twitch oxidative-glycolytic

• Fast-twitch glycolytic

Which has the most glycogen stored in it?

HINT: what IS glycogen? A long chain of what monomers?

HINT: which energy apthways would be used to break that monomer down for ATP production?

• Slow-twitch oxidative

• Fast-twitch oxidative-glycolytic

• Fast-twitch glycolytic

Which one would be considered the most “aerobic”

HINT: what intracellular organelle must be plentiful to support that?

• Slow-twitch oxidative

• Fast-twitch oxidative-glycolytic

• Fast-twitch glycolytic

Which one produces the most lactic acid

HINT: do you remember the energy pathway associated with that molecule?

• Slow-twitch oxidative

• Fast-twitch oxidative-glycolytic

• Fast-twitch glycolytic

Which one has the most myoglobin?

• Slow-twitch oxidative

• Fast-twitch oxidative-glycolytic

• Fast-twitch glycolytic

Which one has the most capillaries around it?

• Slow-twitch oxidative

• Fast-twitch oxidative-glycolytic

• Fast-twitch glycolytic