Pathophysiology of Acontractile Smooth Muscle Syndrome

Presentation (Elaboration), 2018
9 Pages, Grade: 1.2


Due to extensive epidemiological studies and technological advancements in the field of
medicine, new diseases are being regularly discovered. Understanding human physiology and
pathophysiology has enhanced the search for cures and therapeutic remedies to most diseases.
However, some health conditions are accompanied by unprecedented controversy owing to the
absence of known etiological causes. In most cases, knowing the aetiology of a certain disease
helps in developing medicines and treatment therapies, in order to cure the disease or manage the
disease symptoms, as it is the case with chronic illnesses. In this case study, the pathophysiology
of Acontractile Smooth Muscle Syndrome (ASMS) seems easy to understand because its cause is
known. The fact that ASMS is caused by a genetic mutation of recessive genes implies that it
affects tissues and organs which have smooth muscles (Webb, 2003). Therefore, this paper will
present a number of hypotheses on the possible pathophysiology of ASMS.
Mechanism of Smooth Muscles
It is arguable that ASMS causes changes in smooth muscle physiology in the affected
body tissue and organs. As such, the mechanism of smooth muscles in the affected organs is
impaired and this is manifested by the characteristic symptoms which are associated with the
smooth muscle disorder. ASMS is characterised by lymphedema, cyanosis and increased
incidence of urinary tract infections. In addition, it is reported that ASMS causes systolic heart
failure, the leading cause of death in people with the disorder although ASMS is not known to
have a direct effect on the contractility of cardiac muscles. However, the pathophysiology of
ASMS is not known.
ASMS is reported to have reduced ability of smooth muscle to either relax or contract,
leaving it rigid or acontractile. Therefore, it is apparent that this disorder disrupts the physiology

of smooth muscles in the body. As such, it is worth discerning the general mechanism of smooth
muscles, although different types of smooth muscles in different organs exhibit differences in
their functioning. These changes are attributable to the differences in the activation of the
respective muscle cells. Nevertheless, the outcome of muscle cell activation leads to the
contraction and relaxation of all smooth muscles regardless of their types and location in the
body (Webb, 2003).
In the body, the contractile mechanism of smooth muscles involves the mechanical
activation of myosin and actin which are the principal contractile proteins. In most cases,
contraction of the smooth muscles is triggered by the activation of stretch-dependent Ca
channels which are located on the plasma membrane owing to changes in membrane potential.
During smooth muscle contraction, contractile proteins, myosin and actin, interact under the
influence of myosin light chain (MLC) kinase which controls the phosphorylation of myosin end
plates. On the other hand, ATPase is involved in the process of energy generation for the binding
of myosin and actin proteins. This enzyme catalyses the release of ATP energy which is utilised
in the binding of myosin and actin. This results in a contraction due to the cycling of myosin
cross-bridges with actin end-plates. Therefore, it is worth noting that contractility of smooth
muscles is a highly regulated process which is determined by the activity of myosin light chain
kinase and ATPase (Webb, 2003).
In smooth muscle contractions, changes in calcium ions across the plasma membrane
control the contractile activity of smooth muscles. Ordinarily, Ca
-mediated changes in the thick
filaments are responsible for contraction. This phenomenon is different from the one occurring in
striated muscles in skeletal tissues in which Ca
-mediated changes in the thin filaments causes
muscle contractility. Calcium ion-dependent contraction of smooth muscles is triggered by

specific stimuli which cause an increase in calcium ions in the intracellular space of the smooth
muscle cell. As a result, the acid protein, calmodulin, combines with calcium ions to form an
activator complex. In turn, the complex formed by the combination of calcium ions and
calmodulin activates myosin light chain kinase which is responsible for phosphorylating the light
chain of myosin. On the other hand, the increase of cytosolic Ca
which is caused by the release
of Ca
from the sarcoplasmic reticulum, an intracellular store for calcium, and entry of Ca
from the extracellular space through receptor-operated Ca
channels, leads to the stimulation of
phospholipase C activity. This enzyme catalyses the formation of diacylglycerol (DG) and
inositol triphosphate (IP3) from a lipid known as phosphatidylinositol 4, 5-bisphosphate, a
principal membrane lipid. These two compounds act as messengers in which IP3 triggers the
release of Ca
by the sarcoplasmic reticulum, whereas diacylglycerol activates protein kinase C
under the influence of high Ca
concentration. As a result, protein kinase C phosphorylates the
light chain of myosin; thus, causing cross-bridge cycling (Webb, 2003).
Smooth muscles in lymphatic, airway and vascular system have an endogenous
pacemaker mechanism which is controlled by a cytosolic Ca
oscillator to generate rhythmical
contractions which are known to last for an extended period (Berridge, 2008). However, the
mechanism of contraction is the same in all smooth muscles despite the differences involved in
the activation process.
The second phase of smooth muscle activity is the relaxation phase which occurs after
contraction. As such, relaxation of the smooth muscles occurs after the removal of the contractile
stimuli. Therefore, this phase occurs at low cytosolic Ca
concentration coupled to the increased
activity of myosin light chain phosphatase which is responsible for the dephosphorylation of
myosin cross-bridges. Evidence shows that the relaxation of smooth muscles is caused by a

decrease in the intracellular concentration of Ca
and this occurs through a number of
physiological mechanisms. First, Ca
-binding proteins in smooth muscles such as calreticulin
and calsequestrin prevent the release of Ca2+ from the sarcoplasmic reticulum which leads to
decreased intracellular Ca
concentration. Second, the entry of Ca
from the extracellular space
is blocked by the activity of Ca/Mg-ATPases which stimulate plasma membrane Ca
pump by
binding with calmodulin. As a result, the stimulated plasma membrane Ca
pumps calcium ions
from the cytosolic space of smooth muscle cells leading to a decrease in intracellular Ca
concentration. This process is enhanced by the activity of Na+/Ca
exchangers located on the
plasma membrane (Berridge, 2008).
During the relaxation of smooth muscles, myosin light chain cross-bridges with actin
breaks up because of the effect of phosphorylation of myosin light chain by myosin light chain
phosphatase. As a result, the contractile smooth muscle returns to a relaxed state.
Organs with Smooth Muscles
In the body, smooth muscles are found in different tissues and organs. Reproductive
systems, especially the uterus and vas deferens, have smooth muscle cells which play different
roles. In vas deferens, smooth muscles are responsible for the rapid peristaltic contractions
during ejaculation of the sperms. On the other hand, smooth muscles in the uterus are
responsible for the weak twitches experienced during early pregnancy and the onset of labour.
Smooth muscles are also found in the bladder and the ureter. The ureter smooth muscles aid in
the transfer of urine from the kidney to the bladder and the urethral smooth muscles are
responsible for the passage of urine. The bladder is also surrounded by layers of destrusor
smooth muscle cells which are involved in the bladder contractions during the expulsion of
urine. It is also worth noting that smooth muscles are found in the gastrointestinal tract, airways,
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Pathophysiology of Acontractile Smooth Muscle Syndrome
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pathophysiology, acontractile, smooth, muscle, syndrome
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Patrick Kimuyu (Author), 2018, Pathophysiology of Acontractile Smooth Muscle Syndrome, Munich, GRIN Verlag,


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