Smooth muscle contraction Flashcards
(43 cards)
-The contraction of the smooth muscles is regulated by ____________________________
the thick filaments (the contraction of the smooth muscles is thick filament dependent). In order to initiate contraction, the thick filaments change their conformation, following the increase of intracellular calcium.
We can distinguish two main classes of smooth muscles:
1) Multi-unit smooth muscles- ciliary muscle, the iris muscle, piloerector muscles.|
2) Unitary/single/syncytial/visceral smooth muscles- gastrointestinal tract, bile ducts, ureters, uterus, and many blood vessels.
Describe the Phasic SM
In some organs, the smooth muscles cells contract rhythmically (consecutive contraction-relaxation). This type of activity corresponds mainly to the single unit category, because in this case the smooth muscles cells contract in response to the action potential. The action potential reaches the muscles, the cells of this kind of smooth muscles are electrically coupled, so the action potential can travel along the muscles, and this enables the muscle fibers to contract ad relax all at the same time. E.g. the gastrointestinal tract can contract simultaneously because they are connected through gap junction. The activation is in an oscillatory pattern, so it’s not keeping the same state for a long time but it’s changing continuously.
Characteristics of single-unit muscles?
this type of smooth muscles contains cells that are electrically coupled, it means that the ionic flow can move from one side to another. This kind of organization allows that when the cell is electrically stimulated the stimulation can also affect the neighboring cells (gap junctions). This coupling results in a wave of contraction (peristalsis), requiring a strict coordination of the neighboring smooth muscles cells. Often innervated by sympathetic nervous system.
Characteristics of multi-units smooth muscles?
cells that compose this kind of smooth muscle are not electrically coupled. These muscles are composed of different, discrete smooth cell fibers, each of which is independent from the others. Each unit is independently innervated and commonly innervated by the sympathetic nervous system.
Activity pattern of smooth muscles- smooth muscle types can also be distinguished according to their activation patterns:
Phasic SM and Tonic SM
Several types of junctions have been identified in the smooth muscles which are ________. They serve as low resistance electrical coupling structures between the plasma membranes of two different cells
Gap junctions
Smooth muscles can also operate in the opposite direction- ____________ the size of an organ by relaxing the muscles
increasing, the smooth muscles covering the walls of the bladder allow to increase the size of the bladder when more liquid needs to be conserved in here.
Describe the Tonic SM
In other organs, some cells are tonically (continuously) activated. Their activation is not associated with action potentials , They can keep the tone (state) of contraction for a long time. E.g. vascular wall of the vessels changes (restricting or enlarging) its diameter in a way to affect the blood pressure. This modification has to be persistent over a long time period. This can be also seen in the sphincters or in the airways. The sphincter is normally contracted and can change its level of contraction but can also maintain the contraction for a long time period.
Electrically coupled cells are _____________ or ____________
syncytial cells, visceral cells.
SM cells lack the T-tubules, Instead the sarcolemma of the smooth muscles is provided with some tiny sack-like structures called _________
caveolae, located close to the sarcoplasmic reticulum. It’s believed that these caveolae, together with the sarcoplasmic reticulum, are involved in the regulation of the intracellular calcium concentration in these muscles
what are adherens junctions?
Another kind of junction is represented by the attachment plaque, and this is mainly a mechanical coupling. It is characterized by some gaps between the plasma membranes of two cells. These structures are mainly used as anchor points for the thin filaments, so they provide a mechanical linkage.
there are three ways that the SM decrease the amount of calcium:
1) Serca protein complex
2) Calcium ATPase channel
3) Sodium-Calcium antiporter
Morphological characteristics of smooth muscle cells
single nucleus cells. Smaller than skeletal muscle cells, but still quite large. They are specifically larger in the region of the nucleus and are conic shaped toward the ends (sweet potato), so they are spindle shaped.
the intracellular calcium is represented by ____________________, which can activate some calcium channels (IP3‐gated Ca2+ channel).
the inositol trisphosphate
The myosin content is_______ than that of the skeletal muscles
lower
Do Smooth muscles contain Troponin C?
No, instead they have Calmodulin with MLSK ( for myosin head phosphorylation)
Do smooth muscles have Z lines?
No, instead they have dense bodies and they also lack nebulin
The contractile activity of the smooth muscles can be controlled by different subjects, like
hormones, autonomic nerves (ANS), cells showing a pacemaker activity (auto-depolarization) or different drugs. All these agents allow to contract the smooth muscle by increasing the intracellular concentration of calcium
Neural regulation of SM
one way to control the contraction of the muscles is the neural regulation, that depends on the type of neurotransmitters that are released and the type of nerves that are acting on the smooth muscles. It also depends on the distance from the point where the neurotransmitters are released and the place where the smooth muscles receive them, and on the receptor expressed.
Then, this complex (called calcium-calmodulin complex) activates the______________________, which phosphorylates the regulatory light chains of myosin
MLCK (Myosin light-chain kinase), The phosphorylation step is crucial for the interaction of the smooth muscle myosin with the smooth muscle actin. In addition to the phosphorylation step, an ATP molecule is used to stimulate the myosin cross-bridge. After the activation (phosphorylation) of the myosin, now the head of the myosin has the ability to repeatedly bind the actin, which allows the protein to proceed to the cycling process.
Smooth muscles are innervated by the autonomic nervous system, that is composed of two parts:
the sympathetic and parasympathetic systems. They move the system in opposite direction:
The sympathetic system usually increases the arousal and activates more systems.
The parasympathetic system decreases the activity.
some smooth muscle cells that have no innervation, like the smooth muscle cells of the __________.
uterus
The ___________________ can be generated by interacting with guanosine-nucleotide-binding-G protein, that activates another enzyme, the phospholipase C (PLC).
inositol trisphosphate, Then, the inositol reaches the sarcoplasmic reticulum and opens some channels IP3-gated calcium channel. The opening of these channels promotes the release of calcium in the sarcoplasm. The membrane potential of the cell is not affected.
