Smooth Muscles

Question 1

are smooth muscles striated?

Answer 1

no

-no sarcomeres

Question 2

general characteristics

Answer 2

thick and thin filaments more loosely organized

• thin filaments are attached to dense bodies and desmosomes
• small spindle shaped cells are connected by desmosomes and often by gap junctions
• do not fatigue
• do not twitch, contact slowly and maintain tension
• variable tension (not all or nothing)
• can maintain tension even when stretched (longer filaments w/o sarcomeres)
• can maintain tension for longer periods of time without crossbridge cycling and ATP utilization by formation of the latchbridge structure with caldesmon
• tension development is under multiple controls (paracrine, endocrine, neurocrine, local factors)

Question 3

single unit smooth muscle

Answer 3

connected by gap junctions

Question 4

multi unit smooth muscle

Answer 4

not connected by gap junctions

-finely controlled

Question 5

caldesmon

Answer 5

forms latchbridge structure and helps maintain tension for long periods of time

Question 6

dense bodies

Answer 6

similar to z-lines in sarcomeres

-get closer as contraction occurs

Question 7

desmosomes

Answer 7

• adhesive junctions

- mechanical attachment to smooth muscle neighbor

Question 8

gap junctions

Answer 8

allows depolarization of smooth muscle cells
-all contract at same time
“single unit”

Question 9

how are the opposite to skeletal in terms of force

Answer 9

100% recruitment, but variable force

Question 10

main pathways of Excitation-contraction coupling

Answer 10

1. Ca++ enters the cell from the extracellular fluid
   - facilitated diffusion through voltage gated or ligand gated channels
   - Ca++action potentials
2. increased [Ca++] in cytoplasm
3. calmodulin
4. Ca/CAM is a positive allosteric modulator for myosin light chain Kinase (MLCK)
5. MLCK phosphorylates myosin light chains, which activates the myosin ATPase cycle
6. increased myosin ATPase activity causes increased tension
   - in smooth muscle, the myosin binding sites of actin are not covered by topomyosin, so the myosin can bind to the actin as soon as the light chains are phosphorylated
7. myosin light chain phophatase dephosphorylates myosin and inactivates it

Question 11

Ryanodine receptor

Answer 11

enables calcium induced calcium release
-calcium binds and more calcium releases from SR
similar to IP3 receptor

Question 12

what does smooth muscle have to bind to calcium

Answer 12

calmodulin

-unlike skeletal, which has troponin

Question 13

what factors impact smooth muscle

Answer 13

• has receptors for a variety of surrounding factors

- ph, temperature, calcium, etc.

Question 14

control of cAMP verse Ca++

Answer 14

• more Ca++=more tension

- more cAMP=less tension

Question 15

what happens if you increase IP3

Answer 15

activates Gp protein and phospholipase C, causes increase in [Ca++}, increases tension

Question 16

Guantylate cyclase in smooth muscle tension regulation

Answer 16

part of nitric oxide pathway

• cGMP prevents opening of Ca++ channels
• cGMP causes opening of K+ channels
• cGMP activates cGMP dependent kinase which activates myosin phophatase

Question 17

pharmacomedical coupling

Answer 17

type of muscle tension regulation in smooth muscle in response to hormones

• constant tension, add X, increases tension for a period
• add Y, decreases tension for a period