Smooth muscle

Question 1

What are the properties of multiunit smooth muscle?

Answer 1

1. Discrete fibers that contract independently 2. Each fiber is innervated by the ANS and does not communicate to neighboring fibers via gap junctions3. Found in ciliaris, iris, and piloerector muscles

Question 2

What are the properties of unitary smooth muscle?

Answer 2

1. Group of fibers that contract as a single unit2. Regulated by both ANS and non-neuronal regulators 3. Bundle or sheet is innervated at specific points and signal is relayed via gap junctions 4. Found in GI tract, bile ducts, ureters, uterus, blood vessels

Question 3

Does smooth muscle express troponin?

Answer 3

No

Question 4

How is myosin arranged in smooth muscle?

Answer 4

1. Multidirectional 2. Results in contraction in multiple planes

Question 5

What are dense bodies?

Answer 5

1. Have the same function as Z discs in skeletal muscle 2. Consist of structural proteins that are disbursed throughout cell and serve to anchor adjacent cells 3. Also serve as anchors for actin filaments

Question 6

What is the primary source of calcium in smooth muscle?

Answer 6

Extracellular fluid

Question 7

Describe calcium metabolism during smooth muscle cell contraction

Answer 7

1. Once inside cell, calcium binds to calmodulin (required) 2. Calcium-calmodulin complex activates myosin light chain kinase (MLCK)3. Activated MLCK phosphorylates regulatory chain of myosin 4. Phosphorylated myosin regulates activity or myosin ATPase

Question 8

Describe cross bridge formation during smooth muscle contraction

Answer 8

1. Cross bridge forms when myosin is phosphorylated and myosin head is bound to hydrolyzed ATP (ADP + Pi) 2. Cross bridge formation permits release of Pi and power stroke occurs3. ATP binds and releases myosin from actin 4. Cycle can continue as long as myosin is phosphorylated

Question 9

How many molecules of ATP are required for a smooth muscle contraction cycle?

Answer 9

1. Two 2. One for phosphorylation and one for cross bridge cycling

Question 10

Describe the relaxation phase in the smooth muscle contraction cycle

Answer 10

1. Occurs when there is a decrease in calcium leading to a decrease in MLCK activation and phosphorylation of myosin while it is bound to ADP + Pi 2. Myosin is not phosphorylated at regulatory site and there is a decrease in ATPase activity (cross bridge not formed) 3. Can also occur via dephosphorylation of myosin (myosin phosphatase)

Question 11

What is the latch state of smooth muscle contraction?

Answer 11

1. Allows for persistent contraction with low energy requirements2. Once actin myosin cross bridge forms, dephosphorylation of regulatory site can occur, resulting in reduced ATPase activity 3. Very slow power stroke occurs (tonic contraction)

Question 12

What are the key characteristics of regulation of smooth muscle contraction?

Answer 12

1. No structured NMJ - diffuse branches of nerve fibers overlie sheets (no end foot) 2. Primary NTs are NE and Ach 3. Chemical and physical factors and hormones regulate contraction 4. Electrochemical stimulation / inhibition5. Pharmacomechanical stimulation / inhibition

Question 13

What molecule is primarily responsible for the AP in smooth muscle contraction?

Answer 13

1. Calcium 2. Sodium - to a lesser extent

Question 14

What is the resting slow wave?

Answer 14

1. Continuous cycling of depolarization and repolarization without eliciting a spike potential that leads to contraction 2. Membrane is never at rest and continues to depolarize and repolarize

Question 15

During a slow wave, when can a spike potential / depolarization occur?

Answer 15

At the peak