13. Smooth muscle, excitation and contraction Flashcards
Where is smooth muscle found
Walls of hollow organs eg gastrointestinal tract, bladder, ureters, uterus, blood vessels, respiratory tract, around hair follicles, iris and cilliary body
2 types of contraction
Why is contraction slower than skeletal
Tonic= sustained, slow contraction
Phasic= rapid contraction and relaxation
Because myosin heads breakdown atp much slower, low ATP ase activity
Morphology of the smooth muscle cells
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
What increases the surface area of smooth muscles
small invaginations called caveolae
(involved in regulation of intracellular Ca2+ conc)
Do they have Z lines?
No
Actin filaments bind to DENSE BODIES
They have actin, myosin, tropomyosin
What are the regulatory proteins (eqv to troponin)
Calmodulin and caldesmon
Differences btw the 2 types of smooth muscle
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
Where can you find unitary smooth muscle
gastrointestinal tract, bladder, uterus, and ureter
Where can you find multi unit smooth muscle
Large arteries and airways
Cillilary body and iris
Vas deferns
pilomotor muscles (hair)
Which type has pacemaker potential
Single unit
The frequency of slow waves sets a characteristic pattern of
action potentials within an organ, which then determines the frequency of contractions
Physiological properties of smooth muscle
Excitability
Conductivity
Contractility
Elongation
Plasticity= smoothly change length while maintain steady force of contraction
What is the membrane potential of smooth muscle
-45 to -60mV
Where is the major sources of Ca2+
Extracellular fluid
Sarcoplasmic reticulum
What is the role of calcium ions in the regukation of smooth muscle contraction
- 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
How is the cytoskeleton of smooth muscles organized and what are their properties
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
Force velocity relationship compared to skeletal muscle
Maximal force is similar with both
But max shortening velocity of smooth muscles is 100 times lower
Passive tension in smooth muscles
They exhibit more resustance to stetch due to network of connective tissue that reisst overextension
Influence of contraction of smooth muscle compared to skeletal
skeletal= impulses
smooth= various stimuli
Regulation of smooth muscle contraction (stimuli)
- Automaticity phenomenon in single-unit type of smooth muscles.
- Stimuli coming from the autonomic nervous system.
- Hormones that reach smooth muscles through the blood:
epinephrin, vaposresin, oxytocin. - Local humoral factors like: serotonin, endothelins, angotensin II,
prostaglandins, NO, or changes in pO2, pCO2, or concentration of H+, K+ - Mechanical stretching in single-unite type of smooth muscles.
- Changes in the temperature.
How does the conc of Ca2+ in cytoplasm increase
- Enters from extracellular space through voltage gated/ ligand gated channels
- Mechanically when sudden muscle stretching causes ca2+ channels open
- Release from SR mediated by IP3
- By calcium channels that open when Ca 2+ ions are depleted in SR.
How is calcium ions released from SR
Pharmaco mechanical coupling
Mediated by second messenger, IP3
- IP3 generated by breakdown of membrane phospholipid (PIP2) by the PLC.
- PLC is activated by Gq receptor when neurotransmitter/hormone such as Epinephrine,
Norepinephrine, Angiotensin II, or Endothelin binds to it - IP3 activation leads to opening of specific
Ca2+ channels expressed of the surface of SR
How is calcium ions released from SR
Calcium induced calcium release
Induced by entry of Ca 2+ ions
through Ca 2+ channels on the cell membrane.
How does smooth muscle relax
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
is contraction/relaxation slow or fast in smooth muscles
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.
WHat is cross bridge latching
When myosin heads are already dephosphorylated BUT still attached to actin = can do long tonic contractions
No energy is used = no ATP breaksown
