9 - Smooth Muscle Flashcards
There are two types of smooth muscle. What are they?
1 - Multi-unit
2 - Unitary (syncytial or visceral)
What is multi-unit smooth muscle?
Discrete smooth muscle fibers that contract independently of each other
Where would you find multi-unit smooth muscle?
Ciliary and iris muscles of the eye as well as piloerector muscles
Do multi-unit smooth muscles communicate through gap junctions?
No - they do not communicate with each other
What are multi-unit smooth muscle fibers innervated by?
Autonomic nervous system
What is unitary smooth muscle?
A group of smooth muscle fibers that contract together as a single unit
How are unitary smooth muscle fibers usually arranged?
In sheets or bundles - these arrangements usually work together in concert
What are unitary smooth muscle fibers innervated by?
The Autonomic nervous system as well as non-neuronal regulators such as hormones or metabolites
Is each fiber of a unitary muscle innervated?
No - this is different from multi-unit smooth muscle.
Each bundle or sheet of unitary smooth muscle is innervated at specific points and the signal is then communicated between fibers via gap jucntions
What does this system of innervation and communication allow for?
Unitary smooth muscles are able to exhibit synchronized contractions for the entire group of fibers
Where would you find unitary smooth muscle fibers?
- GI tract
- Bile duct
- Ureters
- Uterus
- Blood vessels
What contractile elements will you find in smooth muscle?
Actin
Myosin
Tropomyosin
What are you missing in smooth muscle contraction that you utilize in skeletal muscle contraction?
Troponin
How is myosin different in smooth muscle than it is in skeletal muscle?
In smooth muscle, all the myosin heads are not arranged in the same direction
What does the irregular arrangement of myosin in smooth muscle allow for?
Multi-directional contraction or force generation in multiple planes
What are dense bodies similar to in skeletal muscle?
Dense bodies are functionally similar to Z discs of skeletal muscle
What do dense bodies consist of?
Structural proteins that are disbursed throughout the cell
What is the function of dense bodies?
THey serve to anchor adjacent cell to each other
What does the cellular connection via dense bodies allow for?
The force of contraction can be transmitted from one cell to the next
What else do dense bodies serve to do?
They are the anchoring point for actin thin filaments
Do actin and myosin have similar or different functions in smooth muscle and skeletal muscle?
Very similar
What does actin attach to in smooth muscle?
Dense bodies
Where do you find myosin in smooth muslce?
Interspersed between the actin filaments
During smooth muscle contraction, do actin and myosin undergo the same sliding overlap?
It is similar but it has a different contractile structure than skeletal muscle due to the arrangement of mysoin heads
What occurs structurally at the molecular level during smooth muscle contraction?
Due to the multidirectional arrangements of myosin heads and the arrangement of structural proteins in all planes of the cell, the contraction allows for SHRINKING and BULGING of the cell
How is the shrinking and bulging of the cell in smooth muscle different from the muscle contraction in skeletal muscle?
In skeletal muscle, the muscle fiber simply shortens in length rather than getting pulled in multiple directions
How is the regulation of myosin different in smooth muscle than it is in skeletal muscle?
The source of calcium is different
What is the primary source of Ca++ in skeletal muscle?
Sarcoplasmic reticulum
What is the primary source of Ca++ in smooth muscle?
The extracellular fluid
What accounts for this difference?
The sarcoplasmic reticulum in smooth muscle is not well developed and is a weak source of calcium
What does the Ca++ of smooth muscles do once it enters the cell?
Binds to a protein called calmodulin
What does the calcium-calmodulin complex activate?
Myosin light-chain kinase (MLCK)
What does the MLCK enzyme do?
Phosphorylates the regulatory chain of myosin - this ultimately regulates the activity of the myosin ATPase
Is the attachment and detachment cycle of myosin and actin using ATP similar or different to the cycle seen with skeletal muscle?
Similar
What does a decrease of calcium lead to?
Dephosphorylation of the regulatory domain of myosin
What happens when the regulatory domain of myosin is not phosphorylated?
Relaxation
What is the latch state?
A state that forms which allows persistent contraction with low energy requirements
What occurs that allows the latch state to be maintained
Once the actin-myosin bridge forms, the regulatory site can be dephosphorylated
There is reduced ATPase activity and therefore a “slow” power stroke
This causes the actin-myosin bridge to be maintained for a relatively long period of time in a tonic contraction
Are neuromuscular junctions found in smooth muscle like they are in skeletal muscle?
No
Instead of neuromuscular junctions, what type of innervation do we see from neurons?
DIffuse branches of nerve fibers that lay over sheets of smooth muscle
Is there an end foot at the end of a nerve terminal in smooth muscle?
No - instead there are varicosities, which release neurotransmitters
What types of neurotransmitters regulate smooth muscle contraction?
Norepinephrine
Acetylcholine
What chemicals regulate smooth muscle contraction?
Metabolic products, ions, gases
What hormones regulate smooth muscle contraction?
Endothelin
Angiotensin
Vasopressin
What is electromechanical stimulation?
A change in membrane permeability that results in a depolarization of smooth muscle
When would electromechanical stimulation occur?
If sodium or calcium channels were opened
What is electromechanical inhibition?
A change in membrane permeatiblity that results in the hyperpolarization of smooth muscle
When would electromechanical inhibition occur?
If membrane sodium or calcium channels were closed or if potassium channels were opened
What is pharmacomechanical stimualtion?
The stimulation of smooth muscle contraction by signaling molecules
When would pharmacomechanical stimulation occur?
Some hormones will activate the phospholipase C (PLC) pathway through their receptor, which then increases the intracellular concentration of the second messenger (IP3) which then allows intracellular stores of calcium to be released and smooth muscle to contract
What is pharmacomechanical inhibition?
The inhibition of smooth muscle contraction which is mediated by signaling molecules
When would pharmacomechanical inhibition occur?
Some hormones will activate the Protein Kinase A (PKA) pathway, which then phosphorylates MLCK, whcih prevents the calcium-calmodulin complex from activating MLCK, which blocks muscle contraction
Some hormones activate myosin phosphatase, which leads to an inhibition of the contractile process as well
How does calcium participate in electrical events in smooth muscles?
Depolarization due to movement of Ca++ into the cell
How does calcium participate in mechanical events in smooth muscle?
Calcium inside the cell binds to calmodulin, which is required to activate MLCK and contract the muscle
What is a spike potential of smooth muscle?
A type of smooth muscle action potential that resembles a normal action potential
What causes the depolarization (incline) of a spike potential?
Depolarization due to Ca++ and Na+ movement into the cell
What causes the hyperpolarization (decline) of a spike potential?
Repolarization due to K+ movement out of the cell
What is an action potential with a plateau in smooth muscle?
A type of smooth muscle action potential that depolarizes until it reaches its peak, begins to hyperpolarize and decline, but experiences a prolonged plateau before eventually completing the hyperpolarization back to resting potential
What causes the plateau in an action potential in smooth muscle?
Prolonged and slow opening of Ca++ channels
What is a resting slow wave of a smooth muscle membrane?
A continuous cycling of depolarization and repolarization without eliciting a spike potential that would lead to a contraction
Is the membrane of a resting slow wave ever at rest?
No - it continues to depolarize and repolarize
What occurs at the peak of depolarization of a slow wave?
Rapid depolarization or a spike potential can occur at this peak of depolarization - this can lead to contraction
Where would you find smooth muscle that exhibits a resting slow wave?
Smooth muscle of the gastrointestinal tract
What type of smooth muscle contains cells that are connected via gap junctions?
Unitary smooth muscle
What type of smooth muscle would you find in the small intestine?
Unitary smooth muscle
What is a contractile protein that is present in skeletal muscle but not smooth muscle?
Troponin
What protein can bind to calcium in smooth muscle?
Calmodulin
What protein can bind to calcium in skeletal muscle?
Troponin
Which type of regulator will stimulate smooth muscle contraction, acetylcholine or epinephrine?
CAN’T TELL - it is tissue specific
What is the primary source of calcium for smooth muscle contraction?
Extracellular fluid
What does the calcium from the extracellular fluid allow for?
Change in membrane potential and a contractile event
What ions are responsible for the depolarization of smooth muscle?
Sodium and calcium
True or false: a change in membrane potential is required to induce smooth muscle contraction
No - it CAN, but it is not required
