Smooth muscle - Structure, function and control

Question 1

Compare smooth muscle and striated muscle

Answer 1

• Smooth muscle not striated
• No troponin in smooth muscle: uses calmodulin for modulation instead
• Less mitochondria
• Metabolism mostly glycolytic
• Autonomic innervation

Question 2

Smooth unit: Single unit vs multi-unit

Answer 2

Single unit: E.g. in the GIT. Has gap junctions to allow the action potential to spread throughout the cells. The cells contract as one.
Multi-unit: E.g. piloerector muscles. Cells contract independently, NOT electrically linked

Question 3

Smooth muscle: Excitation-contraction coupling

Answer 3

Calcium released from the SR
Calcium binds calmodulin, calmodulin releases P.
Calmodulin-Ca activates MLCK
MLCK phosphorylates myosin and upregulates myosin kinase.
Leads to increased muscle tone

Question 4

Second messenger systems in smooth muscle contraction

Answer 4

RhoGDP –> RhoGTP –> Rho kinase phosphorylates myosin phosphatase
This inactivates the phosphatase and prevents the dephosphorylation of myosin kinase.
This removes an inhibitory signal BUT requires a stimulatory signal for contraction to actually occur.

Question 5

Describe the exocrine functions of the pancreas

Answer 5

Acinar cells: Secrete metabolic enzymes

Duct cells: Secrete bicarbonate to neutralise stomach acid

Question 6

Pancreatic acinar: Structure and function

Answer 6

Acinars are formed from clusters of cells which surround a duct
They function to synthesise, store (in zymogen granules) and secrete metabolic enzymes