Lecture 6 Smooth Muscle, Cardiac Muscle, and Control of Blood Flow Flashcards
Describe the two types of organization of smooth muscle.
- Unitary (visceral): electrically coupled cells acting in unison, often spontaneously due to syncitium (gut and blood vessels)
- Multiunitary: discrete bundles of cells densely innervated which contract in response to stimulation (vas deferens, iris, piloerectors)
Describe smooth muscle junctions, whether there is a difference between unitary and multiunitary smooth muscle junctions, and compare them to skeletal muscle junctions.
Simpler than skeletal muscle. Neurons of ANS have varicosities instead of synaptic terminals, which secrete neurotransmitter. Visceral smooth muscle contains “diffuse junctions,” and multiunitary smooth muscle contains contact junctions. Excitation is produced by Ca2+ action potential or by electrical coupling via gap junctions
Compare and contrast smooth muscle contraction with skeletal muscle contraction.
Smooth muscle contracts by a sliding filament mechanism, like skeletal muscle, but unlike skeletal muscle, it lacks troponin complex and is triggered by calmodulin activation. Ca2+ is an indirect trigger, as it activates calmodulin; the Ca2+-calmodulin complex activates myosin light chain kinase (MLCK), and the contraction is regulated by MYOSIN not actin.
MLCK phosphorylates the myosin light chain of each myosin head.
Phosphorylated myosin head binds to actin filament causing contraction. It remains latched until it is dephosphorylated by myosin light chain phosphatase. Contraction events are much slower.
Describe the phases of the ventricular myocyte action potential.
- Fast (VG) Na+ channels open
- Fast VG channels close, fast VG K+ channels open
- VG Ca2+ channels open, fast VG K+ channels close-causes AP plateau
- slow VG K+ channels open
- relaxation to RMP at about -90 mV
Describe the theories involved in smooth muscle regulation of blood flow.
The vasodilator theory describes acute regulation of blood flow. Increased metabolism by-products (CO2, lactic acid, ADP/adenosine, histamine, K+, H+) cause dilation.
The oxygen demand theory describes long-term regulation of blood flow, and states that a decreased in oxygen availability increases blood flow.
Pupil dilation is a good example of which type of smooth muscle organization?
Multiunitary– discrete bundles of cells densely innervated which contract in response to stimulation
Unitary organization of smooth muscle is when electrically coupled cells act in unison. What is it called when this occurs spontaneously? Where might it occur in the body?
Syncytium
in the gut and blood vessels
How much O2 does smooth muscle use in relation to skeletal muscles?
~1%– energy efficient
What is a myogenic activity and what are examples?
Myogenic activity is spontaneously active
Hormones, neurotransmitters, etc.
Neurons of the ANS have variscosities that contain neurotransmitter. What are variscosities?
Swelling of the nerve where the neurotransmitter is then secreted
The sarcoplasmic reticulum is virtually absent in smooth muscle. Where does Ca2+ come from?
Most Ca2+ entry is from the extracellular fluid. ~50-300 ms is required before contraction
What 3 tissues receive the most ml of blood per minute?
Kidney (360), Thyroid gland (160) and Adrenal glands (300)
What type of effect would lactic acid have on blood flow?
Metabolic by-products cause dilation, which means increased blood flow.
What blood borne hormones play a role in vasoconstriction?
Norepinephrine and epinephrine
Sidenafil (Viagra) inhibits what?
PDE
