13. Smooth muscle, excitation and contraction

Question 1

Where is smooth muscle found

Answer 1

Walls of hollow organs eg gastrointestinal tract, bladder, ureters, uterus, blood vessels, respiratory tract, around hair follicles, iris and cilliary body

Question 2

2 types of contraction

Answer 2

Tonic= sustained, slow contraction

Phasic= rapid contraction and relaxation

Question 3

Morphology of the smooth muscle cells

Answer 3

Spindle/ fusiform shape
Smaller than skeletal muscle
Actin and myosin filaments BUT NO STRAITIONS
NOT organised into sarcomeres
Central 1 nucleus
Less developed SR
No myofibrils

Question 4

What increases the surface area of smooth muscles

Answer 4

small invaginations called caveolae

(involved in regulation of intracellular Ca2+ conc)

Question 5

Do they have Z lines?

Answer 5

No
Actin filaments bind to DENSE BODIES

Question 6

They have actin, myosin, tropomyosin

What are the regulatory proteins (eqv to troponin)

Answer 6

Calmodulin and caldesmon

Question 7

Differences btw the 2 types of smooth muscle

Answer 7

Single unit:Not all cells are innervated by ANS- only modulating effect. They can self excite spontaneously.
Linked by GAP JUNCTIONS

Multiunit: Each cell is innervated by the ANS

Question 8

Where can you find unitary smooth muscle

Answer 8

gastrointestinal tract, bladder, uterus, and ureter

Question 9

Where can you find multi unit smooth muscle

Answer 9

Large arteries and airways
Cillilary body and iris
Vas deferns
pilomotor muscles (hair)

Question 10

Which type has pacemaker potential

Answer 10

Single unit

The frequency of slow waves sets a characteristic pattern of
action potentials within an organ, which then determines the frequency of contractions

Question 11

Physiological properties of smooth muscle

Answer 11

Excitability

Conductivity

Contractility

Elongation

Plasticity= smoothly change length while maintain steady force of contraction

Question 12

What is the membrane potential of smooth muscle

Answer 12

-45 to -60mV

Question 13

Where is the major sources of Ca2+

Answer 13

Extracellular fluid

Sarcoplasmic reticulum

Question 14

What is the role of calcium ions in the regukation of smooth muscle contraction

Answer 14

• C\2+ binds to calmodulin- complex results in activation of myosin light chain kinase
• MLCK phosphorylates light chains of myosin leading to activation of myosin ATPase
• Myosin ATPase breaks down ATP and release energy for contraction and increase affinity between actin and myosin
• Ca2+/ calmodulin complex also increases affinity of acitn and myosin therefore promotes CROSS BRIDGES
• Caldesmon = functional analogue to troponin

Question 15

How is the cytoskeleton of smooth muscles organized and what are their properties

Answer 15

Dense bodies and intermediate filaments where actin and myosin attach

It has multidirectional spatial arrangement of cytoskeleton filaments so contraction is transmitted in all directions

Question 16

Force velocity relationship compared to skeletal muscle

Answer 16

Maximal force is similar with both

But max shortening velocity of smooth muscles is 100 times lower

Question 17

Passive tension in smooth muscles

Answer 17

They exhibit more resustance to stetch due to network of connective tissue that reisst overextension

Question 18

Influence of contraction of smooth muscle compared to skeletal

Answer 18

skeletal= impulses

smooth= various stimuli

Question 19

Regulation of smooth muscle contraction (stimuli)

Answer 19

1. Automaticity phenomenon in single-unit type of smooth muscles.
2. Stimuli coming from the autonomic nervous system.
3. Hormones that reach smooth muscles through the blood:
   epinephrin, vaposresin, oxytocin.
4. Local humoral factors like: serotonin, endothelins, angotensin II,
   prostaglandins, NO, or changes in pO2, pCO2, or concentration of H+, K+
5. Mechanical stretching in single-unite type of smooth muscles.
6. Changes in the temperature.

Question 20

How does the conc of Ca2+ in cytoplasm increase

Answer 20

1. Enters from extracellular space through voltage gated/ ligand gated channels
2. Mechanically when sudden muscle stretching causes ca2+ channels open
3. Release from SR mediated by IP3
4. By calcium channels that open when Ca 2+ ions are depleted in SR.

Question 21

How is calcium ions released from SR

Pharmaco mechanical coupling

Answer 21

Mediated by second messenger, IP3

1. IP3 generated by breakdown of membrane phospholipid (PIP2) by the PLC.
2. PLC is activated by Gq receptor when neurotransmitter/hormone such as Epinephrine,
   Norepinephrine, Angiotensin II, or Endothelin binds to it
3. IP3 activation leads to opening of specific
   Ca2+ channels expressed of the surface of SR

Question 22

How is calcium ions released from SR

Calcium induced calcium release

Answer 22

Induced by entry of Ca 2+ ions
through Ca 2+ channels on the cell membrane.

Question 23

How does smooth muscle relax

Answer 23

Ca2+ conc decreases by calcium pumps pumping it out into extracellular fluid or SR
SLOWER ACTING THAN SKELETAL so lasts a couple of seconds

Myosin phosphatase activated which dephosphorylates light chains of myosin head separating myosin from actin

THEREFORE TIME FOR RELAXATION DEPENDS ON CONC OF MYOSIN PHOSPHATASE

Question 24

is contraction/relaxation slow or fast in smooth muscles

Answer 24

Contraction and
relaxation are very slow processes,
because the myosin heads break down
ATP very slowly.

Due to the slow rate of ATP breakdown
in smooth muscle, no signs of fatigue
are observed during prolonged
contraction.

Moreover, they can perform very long
(in duration) tonic contractions with
high tension while very low energy is
used.

Question 25

WHat is cross bridge latching

Answer 25

When myosin heads are already dephosphorylated BUT still attached to actin = can do long tonic contractions

No energy is used = no ATP breaksown