Smooth Muscle

smooth muscle cells

spindle shaped, uninucleate cells with a smooth appearance

lack troponin and T tubules

intermediate filaments

form cytoskeleton

non contractile cells in the smooth muscle cell

dense bodies

form cytoskeleton, actin attaches to these

Vascular category

blood vessel walls, except capillaries

gastrointestinal category

walls of digestive tract and associated organs

urinary category

wall of bladder and ureters

respiratory category

airway passages, changes diameter in airways

reproductive category

uterus in females and other reproductive structures in males and females

ocular category

iris and ciliary body, not for physically moving the eyes

contraction pattern type 1

a phasic smooth muscle that is normally relaxed and only contracts when needed

Ex. esophagus

contraction pattern type 2

a phasic smooth muscle that cycles between contraction and relaxation with low amounts of contraction

contraction pattern type 3

a tonic smooth muscle that is usually contracted, remains contracted unless to let something pass

contraction pattern type 4

a tonic smooth muscle whos contraction is varied when needed. always some level of contraction

unitary smooth muscle

contains gap junctions, allows coordinated contraction of many cells causing muscle to contract as a single unit

multiunit smooth muscle

not electrically couplesd

smooth muscles in eye, male reproductive tract and in the uterus

smooth muscle innervation

usually innervated by the autonomic nervous system and can be innervated by multiple neurons capable of releasing different neurotransmiiters

Smooth muscle ap types

brief spikes, and plateaus

contraction without action potentials

autonomic neurons create a local depolarization that spreads electronically throughout the muscle fibre triggering Ca2+ entry

single unit action potentials

autonomic AP initiation (spikes or plateaus)

spontaneous AP (slow wave or pacemaker)

electromechanical coupling

contraction due to a change in the membrane potential

Ca entry through voltage gated channels

smooth muscle cells respond to graded stimulation or action potentials, both of which produce an influx of Ca2+ through voltage gated L type Ca channels

Calcium release from the SR

Occurs via Ca2+ induced Ca release and the IP3 pathway

IP3 pathway can cause contraction with minimal depolarization and negligible extracellular Ca2+ influx

Calcium entry through voltage independent channels

depletion of Ca2+ in the SR activates store operated channels which cause a Ca2+ influx across the cell membrane. Allows calcium to remain elevated and replenishes SR

pharmacomechanical coupling

Ca2+ release from SR via IP3 pathway and entry of Ca2+ via store operated channels that are voltage dependent

stretch activated contraction

there are stretch activated ion channels in the cell membrane of some smooth muscle that when activated lead to depolarization

Calcium signal in myosin head

phosphorylates the regulatory lite chain which turns on the myosin ATPase

calmodulin

a Ca2+ binding protein similar to troponin C of striated muscle

myosin light chain kinase

phosphorylates the regulatory light chain near the myosin head which alters the conformation of the head, increasing its ATPase activity and allows it to interact with actin

Cross bridge cycling

the same as skeletal muscle but it is much slower and cell crumples up instead of shortening in a uniform fashion

relaxation of smooth muscle

the regulatory light chain must be dephosphorylated by myosin light chain phosphatase

latch state

after dephosphorylation of regulatory light chain some smooth muscle can maintain force for an extended period of time with little ATP use

increasing contractile force through calcium

MLC phosphorylation is regulated by the Ca-CaM complex, which depends on levels of intracellular Ca

contractile control by regulating Ca

sensitivity of proteins to Ca regulates contraction

inhibiting MLCP or activated MLCK leading to greater contraction at lower Ca levels