PHIL 1000X Study Guide - Final Guide: Myofilament, Cytosol, Unit
Course CodePHIL 1000X
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MODULE X7 - MUSCLE PHYSIOLOGY
Smooth & Cardiac Muscle
When you have finished this unit, you should be able to:
1. Distinguish between single-unit and multi-unit smooth muscle tissue and give examples
of where each is found in the body.
a. Single-unit: cells are electrically connected by gap junctions, and they contract as
a coordinated unit
i. Most smooth muscle is single-unit; called visceral smooth muscle b/c it
forms the walls of internal organs
b. Multi-unit: the cells are not linked electrically and each muscle cell functions
i. Found in iris and ciliary muscles
2. Distinguish between myofilament organization in smooth and skeletal muscle cells.
a. Have less myosin and the myosin filaments are longer
b. Enables smooth muscle to stretch more while still maintaining optimal tension
3. Describe the events in smooth muscle contraction and relaxation.
a. Increase in cytosolic calcium (from ECF and sarcoplasmic reticulum) INITIATES
b. Calcium binds to CALMODULIN (protein found in cytosol)
c. The binding is the first step in a cascade that ends in phosphorylation of myosin
d. This phosphorylation enhances myosin ATPase activity and RESULTS IN
e. DEPHOSPHORYLATION OF MYOSIN DOES NOT AUTOMATICALLY CAUSE
i. Ratio of MLCK AND MLCP (myosin light chain phosphatase)
ii. MLCP is always active so it depends on MLCK (which depends on
4. STEPS OF RELAXATION
a. Removal of calcium in cytosol
b. Calcium unbinds from calmodulin -> MLCK activity decreases
c. MLCP removes phosphate from myosin light chains -> myosin ATPase activity
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