Chapter 8- Smooth muscle

Question 1

What are multi unit smooth muscles?

Answer 1

Multi unit smooth muscle consists of independent nerve fibers each innervated by one nerve for that specific fiber.

Multi unit is important as nervous system signals control contraction of the fibers, which are found in the ciliary muscle, iris muscle, and pilo-erector muscles for body hairs.

Basically the smooth muscles that are not in the gut are multi unit smooth muscle.

Question 2

What are unitary smooth muscles?

Answer 2

• Unitary smooth muscle means that a lot of fibers work together and contract as one, or a unitary contraction. There are a lot of gap junctions as the flow of ions can perpetuate action potentials without jumping gaps. Another name for the smooth muscle is syncytial smooth muscle, which is defined as a large tissue mass that does not clearly define when one cell ends and the next begins. The gut, bile ducts, ureters, uterus, and many blood vessels all use unitary smooth muscle.

Question 3

What molecule in smooth muscle fills the role of troponin in SkM?

Answer 3

calmodulin

Question 4

What in smooth muscle fills the role of Z discs in SkM?

Answer 4

dense bodies.

Actin filaments are arranged so that they surround the myosin filaments in the middle of the smooth muscle fiber, and these pull on the Dense bodies, which serve as anchors for the filaments.

Question 5

Is smooth muscle striated?

Answer 5

nope

Question 6

What are the differences between SM and SkM as far as speed of crossbridges?

Answer 6

SM- much slower

SkM- fast, quick contractions, more ATPase activity

Question 7

What are the differences between SM and SkM as far as energy requirements?

Answer 7

SM-slower cycling=lower energy consumption

SkM-spiked when in use

Question 8

What are the differences between SM and SkM as far as latent period?

Answer 8

SM-much longer due to latch mechanism

SkM-much quicker

Question 9

What are the differences between SM and SkM as far as force generated?

Answer 9

SM-longer attachement periods = greater force

SkM- lower force per area

Question 10

What are the differences between SM and SkM as far as ability to recover from stretch?

Answer 10

SM- great ability to recover

SkM- less able to return from stretch, deforms easier

Question 11

How does myosin get activated by an increase of Ca?

Answer 11

Ca binds to CAM –> Ca-CAM activate MLCK –> phosphorylates myosin using ATP –> Myosin-P can bind to actin and cause muscle contraction

Question 12

What enzyme inactivates myosin-P?

Answer 12

MLCP

myosin light chain phosphorylase

Question 13

What is the “latched state”?

Answer 13

• During a contraction, myosin kinases and phosphatases are working strongly, and myosin heads are cycling, but allows myosin heads to stay attached to the actin filament like a latch. Unlike skeletal, latch allows tonic contraction over a long period of time, and much less energy is needed to maintain contraction.

The only ATP used is to detach a head, since the head rarely detaches, only a small amount of ATP is used.

Question 14

How is Ca released for SM contraction?

Answer 14

In contraction, calcium first enters through the sarcolemma through calcium channels, which then excites the Sarcoplasmic reticulum to release its stores of calcium, raising the concentration of calcium ions.

The initial influx of Calcium can be produced from nerve stimulation, hormonal stimulation, stretch, or change of chemical environment. Calcium binds with calmodulin, which then binds to myosin kinase and activates that enzyme.

Question 15

What are the mechanisms for SM relaxation?

Answer 15

• Relaxation in smooth muscle is mainly controlled by myosin phosphatase. Dephosphorylation of the myosin light chains causes the myosin heads to uncouple from actin fibers, and not allows them to contract.

Generally speaking, increased myosin phosphatase activity, decreased calcium ion concentration, and hyperpolarization of the smooth muscle all can play a role in relaxation of the smooth muscle.

Question 16

How is SM controlled by ANS innervation?:

Answer 16

• Skeletal muscle is controlled by somatic nerve fibers, while smooth muscle is controlled by autonomic nerve fibers. Smooth muscle is different as it uses both acetylcholine and norepinephrine as its transmitters, and one will antagonize the other. Their function changes with the different organ smooth muscles that they innervate – acetylcholine will not cause contraction in all cases, and may do the opposite in another smooth muscle location.

Question 17

What are spike potentials?

Answer 17

• Spike potentials are the common action potentials, which feature a characteristic spike in the potential of the membrane. These are also seen in skeletal muscle. This is the shortest action potential out of the three.

Question 18

What are plateau ptotentials?

Answer 18

• Plateau potentials can last up to 1 second as opposed to the short 10 to 50 ms spike potentials. These occur in the ureter and uterus.

In plateau potentials, Calcium channels are responsible for the plateau phase, as calcium channels that open more slowly than the sodium channels remain open longer after the initial depolarization. This creates the characteristic plateau effect seen on the following figure.

Question 19

Whatg are slow waves?

Answer 19

• Slow waves feature a slow slide into threshold range, and are characteristically self-excitatory. The cause is not known, but the slow wave come in beats, and cause the cell membrane to go from -60 to -35 mV over the slow wave.

The slow wave is not the actual action potential, but a change that brings the smooth muscle to threshold (around -35 mV). These are very useful in rhythmical contraction of the gut

Question 20

How is vasodilation/constriction regualted by O₂ demand?

Answer 20

• Firstly, remember that when more blood flow is needed, it generally means that the area affected needs more oxygen, or needs to take away CO2. Vasodilation occurs when increased flow is needed to bring much needed nutrients to that area.

Since vasodilation is relaxing of the blood vessel, that means smooth muscle is relaxing in response to control factors, such as calcium decrease, mentioned earlier, will cause local vasodilation, or relaxation of the smooth muscle.

Again, generally speaking, vasodilation is required when an area has too much of a bad thing (CO2), or needs more of a good thing (O2).

Question 21

What hormones will cause SM contraction?

Answer 21

norepinephrine, epinephrine, acetyocholine, angiotensin, endotehlin, vapopressin, oxytocin, serotonin, and histamine.

These usually attach to specific receptors and are either excitatory or inhibitory depending on what the hormone’s role is.