Part 5: Smooth Muscle Flashcards
Visceral smooth muscle is what kind of smooth muscle?
- single unit smooth muscle
Single unit smooth muscle:
- sheets or bundles of smooth muscle fibers that function/contract as a single unit
- connected via gap junctions
- innervation of only a few muscle fibers results in excitation or inhibition of all muscle fibers in a given region
The two types of action potentials in visceral smooth muscle:
- spike potentials (slow wave followed by spike)
- responsible for basal rhythm and peristalsis
- plateau potentials
Spike potentials:
- slow wave induced by calcium channel opening
- threshold reached, spiked AP occurs
- potassium channels open, hyperpolarization
Visceral smooth muscle is innervated by ANS efferents. What type of synaptic junctions are formed between these efferents and visceral smooth muscle cells?
- diffuse
- neurotransmitter released and bathes all cells simulataneously
What motor neuron efferents cause contraction of visceral smooth muscle?
parasympathetic
What motor neuron efferents cause relaxation of visceral smooth muscle?
sympathetic
Dimensions of contraction in smooth muscle versus skeletal muscle:
- Smooth muscle contracts in 3D
- Skeletal muscle contracts in 2D
How does smooth muscle contract in 3D?
- very high actin:myosin ratio (15:1) when compared to skeletal muscle (2:1).
- actin filaments arranged obliquely instead of longitudinally.
Three ways to activate smooth muscle:
- action potentials: activate type-l calcium channels
- neurohormonal: activate voltage-independent calcium channels
- stretch: activate stretch activated calcium channels
What differentiates smooth muscle contraction from skeletal muscle contraction?
- Smooth muscle has:
- calmodulin dependent protein kinases (CaMKs)
- myosin light-chain kinases (MLKCs)
Steps in the activation of smooth muscle contraction:
- stimulus causes calcium influx
- calcium activates kinases and/or calcium pumps
- calcium binds to calmodulin (CaM)
- calmodulin-dependent kinases (CaMKs) activated
- CaMKs activate myosin-light chain kinase (MLCK)
- MLCK activates phosphomysin
- cross-bridge cycling and contraction
Steps in smooth muscle relaxation:
-
MLCK phosphatase inhibits phosphomyosin.
- primary cause of relaxation; interferes with CBC.
- pumps on the saroplasmic membrane pump out calcium from the sarcolemma.
The sympathetic nervous system causes vasoconstriction of arterioles (smooth muscle) via:
- epinephrine and norepinephrine.
- type alpha-1 adrenoreceptors.
Vascular smooth muscle controls:
blood pressure
What can effect vascular smooth muscle (and, therefore, blood pressure)?
- arterial tree
- sympathetic nervous system
- vasoactive metabolites
- hormones
What regulates capillary flow in vascular smooth muscle/arterioles?
precapillary sphincters
Steps in sympathetic nervous system causing vasoconstriction:
- norepinephrine/epinephrine binds to alpha-1 adrenoreceptors on vascular smooth muscle.
- G-protein cascade leads to influx of calcium and inhibition of calcium efflux via SERCA channels.
- sarcoplasmic calcium levels increase
- vascular smooth muscle contracts (vasoconstriction).
Vasodilatory drugs target:
- alpha adrogenic receptors or calcium channel blockers.
Steps in vascular smooth muscle relaxation:
- nitric oxide synthase/nitric oxide activate cGMP.
- cGMP activates kinases that activate membrane calcium pumps and SERCA pumps.
- sarcoplasmic calcium levels decrease
- vascular smooth muscle relaxes
Nitric oxide is a potent:
- Vasodilator
- Activates cGMP, which activates SERCA channels and causes calcium efflux from sarcoplasm of smooth muscle.
What enzymes block vasodilation?
- phosphodiesterases via inhibition of cGMP
- cGMP activates kinases that cause SERCA channel opening, calcium efflux, and vascular smooth muscle relaxation
Phosphodiesterase (PDE) inhibitors:
- Viagra
- promotes vasodilation
- inhibits cGMP, which causes activation of SERCA channels
Blood pressure =
BP = CO + TPR
- CO = cardiac output
- TPR = total peripheral resistance
Law of Laplace:
P = T/r
- increase radius, decrease pressure
Smooth muscle with a high stress relaxation:
- bladder smooth muscle
- expands to lower pressure
- less resistance to renal urine output
Smooth muscle with a low stress relaxation:
- vascular smooth muscle
- high pressure enables propulsion of blood/perfusion
