Smooth Muscle Flashcards
Smooth muscle is under _____ control
involuntary
Smooth muscle are nonstriated b/c…
actin and myosin aren’t arranged in regular sarcomeres
Smooth muscle responds to what types of stimuli?
- electrical signal
- hormones
Contraction of smooth muscle leads to…
alteration in dimension of organ
- propels substance w/in organ
- increase resistance to flow of substance
T/F: smooth muscles are more variable than skeletal muscles
T, b/c they operate over a wide range of lengths
Smooth muscles are arranged in…
- circular
- longitudinal
- oblique
- bundles
- sheets
Differences between smooth and skeletal muscles:
- contract and relax slower
- lose less E
- have less localized receptor sites
Smooth muscle is different based on…
- location
- contractions
- arrangements
Visceral muscle:
lines walls of hollow internal organs
- organs part of gastrointestinal, urinary, respiratory, and reproductive systems
Vascular muscles are located in…
blood vessels
Types of smooth muscle that differ in location:
- visceral
- vascular
- ocular
Types of smooth muscle that differ in contractions:
- phasic
- tonic
Phasic contractions depend on…
the demands of the situation
Tonic contractions:
change diameters slowly
Tonic contractions can be…
maintained for long periods without fatigue
Tonic contractions use high/low ATP due to…
low, latch-bridge state
Smooth muscle cross-bridges cycle at…
slower rate than skeletal muscle
Different types of arrangements:
- multiunit
- unitary
Multiunit arrangement:
individual cells and small groups of cells
- not electrically coupled
Multiunits act as…
many separate units b/c fibers operate independently
Multiunits are densely innervated by…
ANS fibers
Lots of nerves provide…
separate signals, which allows for greater control
Examples of multiunit smooth muscles:
- very small blood vessels
- muscles of skin
- iris
- ciliary body of eye
Unitary (single unit):
large # of cells that function together as a single unit
- not as strictly controlled by ANS
- electronically coupled
- can be initiated by pacemaker cell
Unitary arrangements make up the bulk of…
visceral organs
Unitary arrangements control…
peristalsis (aka wave contraction)
Single unit and multiunits are both mostly…
combination of cell-cell coupling and locally produced activators/inhibitors
Innervations of smooth muscle are under…
involuntary NS control
- doesn’t need NS innervation
Smooth muscles are innervated by…
motor neurons of ANS
Motor neurons of ANS:
- have no motor end plate
- supply smooth muscle flow across tissue
- NT released at varicosities
- has electromechanical coupling like skeletal muscles
T/F: smooth muscles can also contract without innervation
T
- many have intrinsic activity
If smooth muscles can contract without innervation, what are the influenced by then?
- hormonal factors
- environmental factors
Examples of hormonal factors:
- uterine muscle response to oxytocin
- vasoactive substances (paracrine)
Environmental factors:
- under myogenic control
- contract when stretched
- ex: distention of stomach
Smooth muscles that can contract without innervation acts via…
pharmacomechanical coupling
Smooth muscle cells are generally arranged in what kind of layers:
- circular
- longitudinal
- helical (oblique)
T/F: smooth muscle cells are in antagonistic pairs
F
Smooth muscle cells around large blood vessels are arranged in…
helical/circular layers
Smooth muscle cells in arterioles are arranged in…
singular circular shape
When circular muscle is contracted…
- diameter is decreased
- stops movement
When longitudinal muscle is contracted…
- organ is shortened
- moves substance through
Varicosities:
- periodical swellings of ANS neurons
- has vesicles w/ NT
- affected cells will depolarize
- whole sheets contract as functional syncytium
APs in varicosities have…
long durations
- spread to neighboring cells via gap junctions
Smooth muscle cells don’t have:
- fast VGC Na+
- T-tubules
- troponin
- visible striated sarcomeres
Smooth muscle cells have:
- slow L type VGC Ca2+
- caveolae
- calmodulin as regulatory protein
- actin and myosin wrapped in spirals between dense bodies
T/F: smooth muscle is not myosin driven
F
Single units are connected by…
gap junctions
- forms low-resistance pathway
- allows for electrical and chemical communication
Single units are…
- small
- mononucleated
- spindle shaped
Dense bodies in single units act as…
z line because it connects actin and myosin bundles
T/F: there are the same amount of actin in skeletal muscle as smooth muscle
F, there are x2 the amount of actin in smooth muscle
Actin:
- has no troponin, so not bound to tropomyosin
- myosin binding site is exposed
Myosin:
- not in ready state
- not cocked
- 1/4 of the amount in skeletal muscle
Myosin has myosin linked regulatory proteins, which are:
- myosin light chain (MLC)
- myosin light chain kinase (MLCK)
- myosin light chain phosphatase (MLCP)
Myosin light chain (MLC) is at the…
base of myosin head
Function of myosin light chain kinase (MLCK):
phosphorylates MLC, which will activate myosin ATPase
Function of myosin light chain phosphatase (MLCP):
dephosphorylates MLC, which decreases myosin ATPase activity
Ratio of MLCK and MLCP determine…
the ability of smooth muscles to respond to Ca2+
Sarcoplasmic reticulum (SR):
storage reservoir for Ca2+
Release of Ca2+ from SR is controlled by…
- dihydropyridine (DHPR) by electromechanical signal
- IP3 gated Ca2+ channel by pharmacomechanical Gq mechanism
Function of SERCA primary transport pump on SR:
return Ca2+ to SR
Single units are rich in…
- mitochondria
- myoglobin (stores O2)
Sarcolemma:
Specialized to regulate intracellular Ca2+
Ca2+ channels on sarcolemma can be…
VG, LG, or MG
T/F: VG and MG Ca2+ channels are store independent
T
- Ca2+ source is ECF
- decrease in Ca2+ leads to decrease in contractions
Ca2+ enters cell via…
- store operated Ca2+ channels (SOCC)
SOCC is also known as…
receptor operated Ca2+ channel (ROCC)
SOCC is opened by…
intracellular Ca2+ and gets released from storage in SR
All the ways to stimulate contraction are…
- myosin linked
- work by increasing intracellular Ca2+
Contraction can be stimulated by…
- hormones
- nerves
- pacemaker cells
- drugs
T/F: AP is usually needed for contraction
F, usually not needed
RMP of smooth muscles can be…
variable b/c of depolarizing and repolarizing cycles (slow wave potentials)
Pacemaker potentials occur via…
AP made from threshold being reached
Pacemaker potentials are responsible for…
rhythmic activity of GI tract
For all mechanisms, the amount of contraction is proportional to…
levels of intracellular Ca2+
3 ways to initiate Ca2+ influx:
- electromechanical coupling
- pharmacomechanical coupling
- mechanomechanical coupling
Electromechanical coupling is less common b/c…
more energy expensive
Electromechanical coupling is stimulated by…
- norepi
- serotonin
- histamine
- some drugs
Electromechanical coupling acts on…
nonCa2+ related channels to generate AP
- opens L-type VGC Ca2+ in sarcolemma
- not a lot of VGC Na+
Ca2+ for electromechanical coupling comes from…
ECF
Pharmacomechanical coupling:
- no significant effect on membrane potential
- requires turning on phospholipase C (PLC), which differs depending on the receptor
What is the predominant coupling mechanism in vascular smooth muscle?
pharmacomechanical coupling
How does PLC get activated w/ norepi in vascular smooth muscle?
- norepi binds to alpha 1 receptor
- Gq mechanism activated
- alpha q subunit activates PLC
How does PLC get activated w/ ADH in vascular smooth muscle?
- ADH binds to V1 receptors
- Gq mechanism activated
- alpha q subunit activates PLC
How does PLC get activated in GI smooth muscle?
- Ach binds to M3 receptor
2. beta gamma subunit activates PLC
How does PLC get activated in bronchial smooth muscle?
paracrine histamine action via Gq
How does PLC get activated on uterine smooth muscle?
oxytocin binds to class 1 G protein coupled receptor
Activated PLC will…
cleave PiP2, which turns into DAG and IP3
Diacylglycerol (DAG):
- activates PKC
- inhibits MLCP
Inositol triphosphate (IP3):
- opens LGC Ca2+ on SER
- liberates Ca2+ from SR
What happens to smooth muscle at rest?
- active sites are exposed
- myosin heads aren’t in position to form cross-bridges
Steps of contraction:
- cells stimulated by electromechanical or pharmacomechanical signals
- influx of Ca2+
- 4 Ca2+ bind to calmodulin (CAM)
- MLCK activated
- MLCK phosphorylates MLC
- myosin heads move to cocked position
- cross-bridges form
CAM is a…
Ca2+ binding messenger protein
- acts as regulatory protein
In order for MLCK to phosphorylate MLC, it needs…
- terminal phosphate to be transferred to light chain
- phosphotase enzyme also activated
Function of phosphotase enzyme:
removes phosphate group
- slow so doesn’t affect contraction
Cross bridge cycle:
- similar to skeletal muscle
- cycles slower than skeletal muscle b/c detachment is slow
Steps of cross bridge cycle:
- attachment
- rotation of head
- detachment
- production of force and movement
Cross bridge cycle functions as latch bridge b/c…
spends most of their time attached
- b/c of this, has low ATP use (1/cycle)
Termination of contraction occurs when MLCK…
is inactivated by low Ca2+ levels
Ca2+ can be removed by…
- SERCA
- Ca2+ ATPase on membrane
- Na+/Ca2+ exchanger on plasma membrane
Cross-bridges will dephosphorylate by…
MLCP, which occurs at much slower rate than phosphorylation
During phasic contractions, muscles are most often…
at rest
Phasic contractions:
brief twitch-like contractions in response to single stimulus or brief bursts of stimulus
Phasic contractions are equivalent to what kind of contractions in skeletal muscles?
isotonic twitch-like contractions
Phasic contractions are often associated w/…
AP
Phasic contractions are usually in organs that…
propel/move substances within organ
Gut uses phasic contractions by…
slow wave AP, which moves food along in periodic contractions
The phasic contractions in the gut are activated by…
electromechanical activation by pacemaker cells
- opens L-type Ca2+ (DHP channel) on plasmalemma
AP in the gut is conducted via…
gap junctions
Afferent arterioles of the kidney utilizes what type of contraction?
phasic
Phasic contractions in the afferent arterioles of the kidney are activated by…
Ang 2, which in turn activates DHP L-type Ca2+ channels
Tonic contractions are…
- most often contracted
- sustained contractions in response to single/continued stimulation by nerves drugs, or hormones
Tonic contractions produce…
constant tension (aka tone)
Level of tone in tonic contractions are set by…
concentration of specific chemicals or hormones
- norepi and epi act on alpha 1, which leads to Gq mechanism being activated on sphincters of bladder and GI tract
Efferent arterioles of kidney utilizes what type of contractions?
Tonic
- Ang 2 stimulates Ca2+ entry via SOCC
Tonic contractions are in organs that…
have to stay contracted for long periods of time
Examples of organs that use tonic contractions:
- bladder
- sphincters
- many blood vessels
Relaxation of muscles that utilize tonic contraction leads to…
increased flow to the area
Contraction of muscles that utilize tonic contraction areas leads to…
diversion of blood elsewhere
Tonic contractions can develop…
high forces
Tonic contractions can also prevent…
movement of substances
- ex: sphincters
Relaxation is induced by…
- hormones
- NT
- drugs
4 ways of relaxation:
- norepi acts on beta 2 coupled to Gi
- norepi acts on alpha 2 coupled to Gi
- by-product of cellular metabolisms
- nitric acid
When norepi acts on beta 2, smooth muscle is relaxed during…
stress
Effects of norepi acting on beta 2 coupled to Gi:
- decreased gut motility
- delays micturition
- inhibits labor
- dilates small arteries to skeletal muscle and coronary arteries
- bronchodilates
When norepi acts on alpha 2 coupled to Gi, what is prevented?
sympathetic contraction of smooth muscle
Examples of beta 2 agonists that act via Gi:
- clonidine
- guanabenz
- guanfadine
- alpha methyldopa
Effects of norepi acting on alpha 2 coupled to Gi:
decrease in:
- cAMP in presynaptic neurons
- release of norepi
- contraction
By-product of cellular metabolism leads to…
decrease in [Ca2+]
Examples of by-products that lead to vasodilation:
- adenosine
- lactic acid
- increase in K+
- increase in CO2
- increase in body temp
- decrease in O2
Nitric acid gets released from…
endothelial cells of stressed blood vessels
Nitric acid acts on…
guanylyl cyclase, which catalyzes GTP and turns it into cGMP
Guanylyl cyclase:
- turns off IP3, which inhibits Ca2+ entry
- activates K+ channels, which leads to hyperpolarization and relaxation
- activates PKG -> activates MLCP -> dephosphorylates MLC -> relaxation
Sildenafil works by…
blocking phosphodiesterase removal of cGMP
- more cGMP present = more dilation of penile erectile blood vessels
Nitric acid can vasodilate…
- blood vessels directly
- indirectly by inhibiting Ang 2
Nitroglycerin gets converted to…
NO in blood vessels, which in turn dilates coronary blood vessels
Stress relaxation:
ability to return to resting force of contraction after stretch
Example of stress relaxation:
urinary bladder returns to resting force as bladder fills and is stretched
Reverse stress relaxation:
ability to return to resting force when force drops
