Smooth Muscle Contraction Flashcards
1
Q
Smooth muscle filament types
A
- smooth muscle has thick and thin filaments but no sarcomere
- dense body anchoring points for actin filaments (condensed Z-line)
- actin filaments bundled to create a spindle
- myosin filament interspersed between actin filaments
- since there is no regular organization of the thin and thick filament complexes, smooth muscle lacks A and I bands and is non-striated
2
Q
Smooth muscle contractions and relaxation speed
A
- a smooth muscle twitch is characterized by slow contraction velocity and slow relaxation
- smooth muscle can contract to less than 1/3 of initial resting length
3
Q
Smooth muscle contraction
A
- sarcolemma has microdomains called caveolae that are enriched for cell receptors and ion channels, they contain receptors (muscarinic ACh receptors, adrenergic receptors) and ion channels (L type Ca channels, ATP sensitive K channels, Ca sensitive K channels)
- temporal relationship- Ca2+ and tension and the pCa-tension relationships are similar to those of skeletal and cardiac muscle already described, except that the onset of contraction is slower and the duration of tension is usually longer in smooth muscle, smooth muscle action potential is Ca2+ dependent, inward depolarizing current is carried by calcium ions
4
Q
Speed and force of contraction of smooth muscle
A
- smooth muscle usually exhibits prolonged tonic contractions, sometimes lasting hours or even days
- cross-bridge cycling is much slower in smooth muscle than in skeletal or cardiac muscle, but the proportion of time spent in the tension generating phase of the cross-bridge cycle is longer thus resulting in a greater force of smooth muscle contraction with less energy expenditure (ATP hydrolysis)
5
Q
Percentage shortening in smooth muscle
A
- skeletal muscle usually contracts only about 1/4 to 1/3 of its stretched length
- smooth muscle can often contract to less than 1/3 of its stretched length, enabling the gut, bladder, blood vessels, and other hollow organs to reduce their luminal diameters from very large to almost zero
6
Q
Unitary Smooth Muscle
A
- electrically coupled via gap junctions and can be spontaneously active (peristalsis)
- the term unitary refers to smooth muscle in which millions of smooth muscle cells organized in sheets or bundles contract in a coordinated fashion as a single unit
- little innervation
- function in syncytium
- response to stretch
- little response to SNS
- local control of contraction
- small blood vessels, GI tract, uterus, most arteriolar muscle
7
Q
Multiunit smooth muscle
A
- composed of discrete smooth muscle fibers, each of which is innervated by a single nerve ending as in skeletal muscle
- their contraction is seldom spontaneous
- examples include the smooth muscle fibers of the ciliary muscle and the iris of the eye, and the piloerector muscles that cause erection of the hairs when stimulated by the sympathetic nervous system
- function as individual units
- innervated
- few gap junctions
- little response to stretch
- response to SNS
- neural factors control contraction
- airway smooth muscle, piloerector muscle, ciliary muscle of eye, some arteriolar muscle
8
Q
SM contraction driven by calcium influx
A
- use two sources for Ca: the extracellular fluid and the SR
- in most cases an AP depolarizes the sarcolemma, resulting in an influx of Ca2+ primarily through L type
- Ca2+ can also be released after agonist binding to G-protein coupled receptors- those receptors activate phospholipase C which generates IP3
- Ca2+ is then released into the sarcoplasm from rudimentary SR by IP3 and possibly some Ca induced Ca
- contraction is caused by increased concentration of intracellular calcium
- Ca is removed from sarcoplasm by Ca2+ pumps in the SR membrane and in the sarcolemma and by 3Na/Ca exchange across the sarcolemma
9
Q
Smooth muscle contraction regulatory proteins
A
- the key regulatory protein in smooth muscle is myosin light chain kinase (MLCK)
- Ca2+ binds to a calmodulin moiety on myosin light chain kinase (MLCK) resulting in phosphorylation of the regulatory light chain (RLC) of myosin
- a conformational change in the regulatory light chain (RLC) then permits the myosin to interact with actin
10
Q
Smooth muscle relaxation regulatory proteins
A
- when MLCP a soluble phosphatase in the sarcoplasm, dephosphorylates the regulatory light chain of myosin, then interaction between actin and myosin is blocked and the muscle relaxes
- reduction of the concentration of intracellular calcium by calcium ion pumps in the sarcolemma and in the sarcoplasmic reticulum membrane also cause relaxation
11
Q
Categories of smooth muscle contraction
A
- electromechanical- action potential or stretch- in this case depolarization opens L type calcium channels leading to an increase in cytosolic calcium, activation of MLCK, phosphoylation of myosin and contraction
- pharmacomechanical- ligand binding to cell surface receptor- G protein activation (Gq), PLC gamma activation, IP3 generation +Dag, IP3 receptor opening in SR
- increase in intracellular Ca2+- CICR (Ca2+ induced Ca2+ release), Ca2+ calmodulin activation of MLCK, activation of smooth muscle myosin, contraction
12
Q
Smooth muscle activation
A
- contractile activity of smooth muscle may be spontaneous as in the peristaltic waves of the intestine, stomach, colon, and during the uterine contractions of labor
- uterus at term develops synchronous contractions and pacemakers with diastolic depolarizations
- vascular smooth muscle usually does not exhibit spontaneous activity, but instead contracts in response to excitation by stretch, adrenergic neurons, endothelial cells or circulating chemical factors in the blood
13
Q
Membrane potential in smooth muscle
A
- resting potential of smooth muscle is about -50 to -60mV, some 30 mV less negative than skeletal muscle
- spike action potentials in unitary smooth muscle are of short duration (10-50 msec) relative to the contraction time. Depolarization is caused mainly by an inward current of Ca2+ followed by repolarization by an outward current of potassium. Most smooth muscle APs have no sodium current
- some visceral smooth muscle exhibits slow waves (or pacemaker waves) that initiate superimposed spike potentials; uterine smooth muscle APs have plateaus
14
Q
Basal electric rhythm
A
- refers to waves of rhythmic depolarization of intestinal smooth muscle cells, which originate at a specific point and then are propagated along the length of the GI tract
- pacemaker activity of the network of interstitial cells of Cajal are not of suffient magnitude to initiate APs in the smooth muscle
15
Q
Latch state
A
- during sustained smooth muscle contraction it is observed that Ca2+ concentrations in the sarcoplasm fall and myosin light chain becomes dephosphoylated
- caused by MLCP dephosphorylation of the myosin light chain while the myosin head is bound to actin
- in latch bridge state the crossbridge maintains tension and the subsequent dissociation of the myosin head from the actin filament is very slow
