Physiology Revision - Smooth Muscle

Question 1

Where is smooth muscle generally found?

Answer 1

Smooth muscle is a type of muscle that is generally found in organs; it commences involuntary contraction for longer periods of time without fatigue

Question 2

What are the main differences between smooth muscle and skeletal muscle? (excluding Ca2+)

Answer 2

NO SACROMERES/non-striated

Actin filaments attach to the cell through dense bodies

Smooth muscles cells are electrically joined through gap junctions on the membrane for whole unit contraction (but don’t always contract as a whole unit)

Smooth muscle is activated through neurotransmitters that are released from varicosities of axons (no neuromuscular junctions)

Very low rate of ATPase activity which means the muscle sustains a contraction for a much longer time but activation is at a slower rate

Question 3

What are the two kinds of activation of smooth muscle? How is smooth muscle activated?

Answer 3

Smooth muscle is activated through spontaneously occurring APs due to the unstable resting membrane potential alike the SA node in cardiac muscle. Smooth muscle has slow waves; graded potentials that are wavering below threshold as the muscle is influenced by excitatory and inhibitory inputs

Single unit smooth muscle (blood vessels, intestines, uterus)

• Cells are electrically coupled by gap junctions and undergo synchronous activity, the whole tissue responding to stimulation as a single unit

Multi unit smooth muscle (lung airways, large arteries)

• Cells are not electrically coupled and respond independently, behaving as multiple units

Question 4

What is the role of calcium in smooth muscle? How is this different from skeletal muscle?

Answer 4

X-bridge cycling is controlled by an Ca2+ enzyme that phosphorates myosin

Ca2+ is sourced intracellularly (SR) or extracellularly

Ca2+ acts on thick filaments as opposed to thin filaments in skeletal muscle

Troponin is absent so binding sites are always open for binding of myosin

Question 5

What is the sequence of events for x-bridge cycling?

Answer 5

Ca2+ binds to and activates calmodulin

Ca 2+ - calmodulin complex binds to myosin light-chain kinase

Myosin light-chain kinase uses ATP to phosphorylate myosin x-bridges

Phosphorylated x-bridges bind to actin filaments

x-bridge cycle produces tension and shortening