BMS 10.9, 10.10

myosin light-chain kinase

(MLCK) -an enzyme that is activated by the Ca2+ calmodulin complex to phosphorylate the smooth muscle myosin head

single-unit smooth muscle cells

(i.e. visceral smooth muscle cells) -are the most abundant and are stimulated to contract in unison found in several places: -walls of the digestive, urinary , and reproductive tracts -small portions of the respiratory tract -most blood vessels

calmodulin

-a protein that binds Ca2+ to form a Ca2+ calmodulin complex

intermediate filaments

-are linked with dense bodies at points where they interact within the sarcoplasm of the smooth muscle cell -are linked by dense plaques at points where they attach on the inner surface of the sarcolemma

fatigue resistant

-b/c of low ATP requirement -necessary for maintaining tonic (i.e. long duration contractions)

similarities between cardiac and skeletal muscle

-cardiac muscle cells are striated like skeletal muscle -cardiac muscle cells contain sarcomeres like skeletal muscle cells -cardiac muscle cells contain a large number of mitochondria like skeletal muscle cells

differences between cardiac and skeletal muscle

-cardiac muscle cells branch unlike skeletal muscles -cardiac are both shorter and thicker than skeletal muscle -cardiac muscle cells have intercalated discs unlike skeletal muscle cells -cardiac muscle cells can have only one nucleus (but may have two) unlike skeletal muscle cells which are always multinucleate -cardiac muscle cells use aerobic respiration almost exclusively to generate the ATP required for their unceasing work -cardiac muscle cells are stimulated by a specialized authorhythmic pacemaker

locations of smooth muscle tissue

-cardiovascular system: blood vessels -respiratory system: bronchioles (air passages) -digestive system: stomach, small intestine, and lg. intestine -urinary system: ureters -female reproductive system: uterus -others: iris of eye, ciliary body of the eye, arrestor pili

phosphorylation of smooth muscle myosin head

-causes activation of its ATPase activity

sarcolemma of smooth muscle

-contains various types of Ca2+ channels ( e.g. voltage-gated, chemically gated, modality gated) that allow these cells to respond to different types of stimuli

autonomic nervous system

-controls smooth muscle contraction (control cardiac muscle, smooth muscle, and glands involuntarily)

thick filaments in smooth muscle

-have myosin heads along their entire length, rather than only at the ends as in skeletal muscle

caveolae

-invaginations in smooth muscle cells -voltage gated Ca2+ channels here

Myosin light-chain phosphatase

-is an enzyme that dephosphorylates the myosin head, resulting in the activation of the ATPase activity (inactivation requires relaxation of smooth muscle)

smooth muscle relaxation

-is more complex than skeletal muscle -requires, in addition to both cessation of stimulation and the removal of Ca2+ from the sarcoplasm, the dephospho rylation of myosin by myosin light-chain phosphate

smooth muscle

-is stimulated to contract by various hormones, a decrease in pH, low oxygen concentrations, increased carbon dioxide levels, certain drugs, and pacemaker cells

initiation and duration of contraction

-low developing contractions -relatively long latent period (b/c of phosphorylation the myosin head via MLCK and ATPase activity) -long duration contractions (b/c of dephospho rylation of the myosin head via MLCP and myosin locking to actin, i.e. latch bridge mechanism)

myosin heads of smooth muscle

-more of these can form additional cross bridges with actin to produce a powerful muscle contraction -have modifications that allow them to "latch" on to the actin of thin filaments and remain attached without using additional ATP (i.e. latch bridge mechanism)

length-tension curve

-no optimal length for smooth muscle contraction as is seen in skeletal muscle contraction

multi-unit smooth muscle cells

-receive stimulation to contract individually -found in several places and are arranged in motor units: -iris and ciliary muscle of the eye -arrector pili muscles in the skin -wall of lg. air passageways within the respiratory system -walls of lg. arteries

smooth muscle

-small and fusiform-shaped -have only one centrally located nucleus -much smaller than skeletal muscle -small tapered ends overlap the larger middle area of adjacent cells to provide for close packing of cells -sarcolemma contains various types of Ca2+ channels (e.g. voltage-gated, chemically gated, modality gated) that allow these cells to respond to different types of stimuli -lack t-tubules but have invaginations called caveolae

stress-relaxation response

-smooth muscle is "stressed" by being stretched and it responds by contracting, but after a given period of time, it relaxes

autorhythmic pacemaker

-what cardiac muscle cells are stimulated by

Steps in smooth muscle contraction

1) in response to stimulation, Ca2+ enters the sarcoplasm from both the interstitial fluid and sarcoplasmic reticulum 2) Ca2+ binds to calmodulin to form a Ca2+ calmodulin complex that then binds to MLCK, resulting in its activation 3) the activated kinase (MLCK) phosphorylates the myosin head to both activate the myosin ATPase activity of the myosin head and to allow the myosin head to bind to actin forming a cross bridge 4) Myosin ATPase hydrolyzes ATP to produce the power stroke 5) They myosin head releases and reattaches to the actin repetitively, causing the thin filament to slide past the thick filament -this sliding results in a pull on the attached dense bodies anchored to the intermediate filaments of the cytoskeleton and the dense plaques attached to the sarcolemma -the anchoring intermediate filaments move inward and the entire smooth muscle cell shortens

Three characteristics of smooth muscle contraction

1) initiation and duration of contraction 2) fatigue resistant 3) Length-tension curve