Smooth Muscle Flashcards
Two major types
1) Multi-unit smooth muscle
2) Unitary Smooth muscle
Arrangements
Circumferential
Circumferential and longitudinal
Varied
Multi-unit smooth muscle
Fibers operate individually
Innervated by single nerve
Ex) ciliary muscles of eye, iris, piloerector muscles
Unitary smooth muscle
Visceral smooth muscle or syncytial
Works together as a unit
Cell membranes adhere and contain gap junctions
Ex) GI tract, bile duct, uterus
Contraction of Smooth muscle
No true sarcomere structures.
Actin attaches to dense bodies/adherens junctions
Myosin heads have bi-directional arrangement
Smooth muscle contraction timing and force
Cycling of myosin cross-bridges is slower than sk muscle
Time myosin and actin are attached is grater=greater force
ATP demand is lower
Excitation slows, contration remains–known as latch mechanism
Calcium in Smooth muscle
Increased calcium levels in cytosol caused by nerve, but also hormones, stretch, or environmental ques
Key molecule is calmodulin
Mechanisms of Contraction in smooth muscle
1) Calcium enters cytosol thru plasma membrane calcium channels
2) Calcium binds reversible to CaM (Calmodulin)
3) Calmodulin-calcium complex activates myosin light chain kinase
4) Phosphorylated myosin light chain
5) Relaxation
Nerve Stimulation of Smooth muscle
Varicosities serve as NT release sites (Sometimes called diffuse junctions)
Release not just ACh
Distance btwn varicosities and fibers differ
Control of Smooth Muscle
Norepinephrine/epinephrine–Excitatory or inhibitory depending on organ
ACh–Excitatory or inhibitory depending on organ
Angiotensin 2, vasopressin, endothelin–contraction
Adenosine–Relaxation
NO–Inhibitory=relaxation
Environmental Control of Smooth Muscle
Cardiovascular arterioles, meta-arterioles, pre-capillaries all have smooth muscle and respond to: Hypoxia, excess CO2, Increased H+, Adenosine, LA, increased K+, etc
Smooth Muscle AP
Resting potential is -50mv to -60 mv
Both inhibitory and excitatory signals
Graded potentials are common and can control contractile activity
Usually occur in unitary smooth muscle
Two forms: AP with plateaus and spike potentials
Control of Smooth Muscle activity
Spike Potentials–Stimulated by hormones, NT. stretch, spontaneous
Slow waves–Oscillating Ca2+ influx and K+ efflux, Pacemaker capabilities
Plateaus–stimulated by NT and stretch
Calcium’s role in smooth muscle AP
Calcium is responsible for generating action potential, rather than sodium
Fewer voltage-gated sodium channels
More voltage gated calcium channels–open slowly, stay open longer
Latch Mechanism
Smooth muscle contraction: way to increase tension while decreasing ATP usage
Dephosphorylation of light chain
Cycle proceeds slowly
Attached cross bridges are still generating tension
