smooth muscle

Question 1

Single unit vs Multi unit Smooth muscle

Answer 1

Single Unit: Functional syncytium, spontaneously active, sxcitation spreads from cell to cell (ie intestinal muscle)

Multiunit: Each cell is an individual unit (muscle is a multiple of single units, no spontaneous activity, requires excitation, (ie aorta)

Question 2

role of low resistance pathways and gap junction is coordination of smooth muscle

Answer 2

maintain active tone and coordinate excitation and relaxation, rhythmic contraction of spontaneous smooth muscle, autonomic nerve fibers play an important role in viscera, there isnt a specialized neuromuscular junction (like in skeletal muscle) nerve fibers form diffuse junctions with different distances between nerve fibers and their smooth muscle targets. Terminal axons form varicosities in the tissue (high in sympathetic)

Question 3

smooth muscle structure

Answer 3

no striations or sarcomeres
have actin, myosin, and tropomyosin (but no Troponin), elevated calcium mechanism for cross bridging is different,
no z lines, but thin filaments attach to dense bodies (made up of desmin and vimentin) linked via intermediate filaments
Gap junctions in single unit smooth muscle (not in multi unit)

thin filaments crisscross and are very long-> so smooth muscle can shorten a lot and causes blebs

Smooth muscle has a lot more thin filaments

Question 4

Role of Calcium in smooth muscle

Answer 4

Contraction of Smooth muscle is due to a modified sliding filament mechanism

Phosphorylating the Myosin light chain (via myosin light chain kinase) determines contractile force (via cross bridging)

MLCK is activated by calcium calmodulin
MLCP (phosphatase) decreases phosphorylation of light chain and can cause relaxation

Velocity of contraction of smooth muscle is directly proportional to the degree of phosphorylation of myosin light chain, and myosin sensitivity to Ca can be regulated so that even though theres a lot of phosphorylation the force can be weak (force maintained but rate of cross bridging is slow)

Question 5

Myosin in smooth muscle

Answer 5

phosphorylated Myosin light chain-> ATP hydrolysis activity of heavy chain and begin cross bridging

the more ATPase activity, the faster the rate of force is generated

high levels of force with low levels of Ca can be generated if MLCPase is active (but the velocity of bridging is slow)

Question 6

Smooth muscle Electrophysiology

Answer 6

Electrophysiological responses vary widely among smooth muscle from various sources. These can take the form of graded potetials, slow oscillations , pacemaker, or inhibitory

Question 7

Smooth muscle Transmembrane potential

Answer 7

Em: much less negative than skeletal muscle, much lower gK at rest. can be steady or show spontaneous oscillations, AP may or may not occur. Action potentials are due to the influx of Ca rather than Na

Membrane depolarization happens when outside K is elevated (lowering the gradient)

Question 8

Ion Channels regulating Em in smooth muscle

Answer 8

several different ion channels affect the transmembrane potentials in smooth muscle, Restin Em is a product of excitatory and inhibitory inputs. Membrane potential is highly determined by contractile force so small changes in EM give large changes in the contractile force

CA and K mostly
Cl can also be activated by ca

Question 9

Membrane potential and contractile force in smooth muscle

Answer 9

Depolarization-> contraction , hyperpolarization-> relaxation

Tight relationship allows for precise control of active tone of smooth muscle,
changes in contractile force can also be due to changes in AP frequency or slow wave amplitude or (pharphacomechanical coupling due to MLCP altering Ca sensitivity)

Question 10

Excitation-contraction coupling in smooth muscle

Answer 10

control of Ca influx by coltage gated channels in membrane, and ca release from the sarcoplasmic reticulum

2 types of Ca channels in the SR (Ryanodine and IP3)

IP3 activated: open in response to IP3 (via phospholipase C activity)
Ryanodine sensitive Ca channels (in response to Ca induced Ca release)

Question 11

Extracellular Ca entry vs SR Ca release

Answer 11

No T tubules, because smooth muscle al ready has a very large surfave area. SR activity is limited in smooth muscle.

Most of the time SR is on the membrane and regulates Ca lelvels

Question 12

Cell mechanism to increase Cytoplasmic Ca levels in Smooth muscle

Answer 12

Ca entry:

For regulating tone- Membrane voltage gated Ca channel (TRP and stretch also help)

Question 13

Removal of Ca in smooth muscle

Answer 13

SERCA (ATPase)
Plasma CA (ATPase) membrane (SERCA)
Na/Ca exchanger (major mechanism for Ca extrusion in cardiac muscle)