Smooth Muscle Contraction Flashcards
Why is there no banding pattern in smooth muscle ?
Not organised into myofibrils, and there are NO sarcomeres
What is the name of the contraction mechanism used in smooth muscle ?
Sliding-filament mechanism
Where is smooth muscle found ?
Smooth muscle surrounds hollow structures and organs that undergo changes in volume with accompanying changes in the lengths of the smooth muscle fibers in their walls
What is the shape of smooth muscle cells ?
Spindle-shaped
What is the average height and length of smooth muscle cells ?
Height: 2-20 μ
Length: 20-200 μ
What is the amount of myosin and actin smooth muscle contains, relative to the amount of myosin and actin found in striated muscle ?
10%
What is the actin:myosin ratio in smooth muscle cells ?
15:1
What are the steps of smooth muscle contraction ?
- Initiated by calcium from Extracellular Fluid or SR.
- Calcium binds to calmodulin (instead of troponin as in skeletal muscle)
- Ca-calmodulin-MLCK (Myosin light chain kinase) complex leads to phosphorylation of MLC (requires 1 ATP)
- MLC (Myosin light chain) is part of myosin head
- Phosphorylated myosin head binds to actin and power stroke occurs automatically
- A second ATP is required to release myosin head from ac/n
MLC = Myosin light chain
MLCK = Myosin light chain kinase
Why is myosin phosphorylated as part of smooth muscle contraction ?
Because only the phosphorylated form of smooth muscle myosin can bind to actin and undergo cross-bridge cycling.
What are the steps of smooth muscle relaxation ?
- Calcium concentration dips below a critical level due to being pumped out of the cell or into the SR
- Calcium is released from calmodulin
- MLCP (Myosin light chain phosphatase) removes phosphate from the MLC, causing detachment of the myosin head from the ac/n filament, causing relaxation.
What are the sources of Calcium contributing to the rise in systolic concentration of calcium in smooth muscle, thus initiating smooth muscle contraction ?
-SR Ca2+
-Extracellular Ca2+ entering the cell through plasma-
membrane Ca2+ channels
When smooth muscle relaxes, where does the Calcium go (to reduce cystolic Calcium concentrations) ?
To the SR or to the Extracellular fluid (ECF)
What are some inputs which influence smooth muscle contractile activity ?
- Spontaneous electrical activity in the plasma membrane of the muscle cell.
- Neurotransmitter released by the autonomic neurons.
- Hormones.
- Locally induced changes in the chemical composition (paracrine factors, acidity, oxygen, osmolarity, ion concentrations) of the extracellular fluid surrounding the cell.
- Stretch
What is the membrane potential change occurring during the spontaneous depolarization to threshold ?
Pacemaker Potential
What are the main types of spontaneous electrical activity found in smooth muscle cells ?
- Some types of smooth muscle cells generate action potentials spontaneously in the absence of any neural or hormonal input (membrane potential change occurring during the spontaneous depolarisation to threshold is known as a pacemaker potential.)
- Other smooth muscle pacemaker cells have a slightly different pattern of activity. The membrane potential drifts up and down due to regular variation in ion flux across the membrane. These periodic fluctuations are called slow waves.
What is the advantage in having smooth muscle pacemaker cells throughout the GI tract ?
Gut smooth muscle tends to contract rhythmically even in the absence of neural input
Where are the neurotransmitters affecting smooth muscle found ? When are they released ? By what structure ?
In vesicles contained by varicosities.
They are sometimes released when an AP passes the varicosity, by the autonomic neuron ending
What is the equivalent of motor end plate region for smooth muscle cells ?
Varicosities
Explain how the following is possible:
- A number of smooth muscle cells are influenced by the neurotransmitters released by a single neuron
- A single smooth muscle cell may be influenced by neurotransmitters from more than one neuron.
Because varicosities from a single axon may be located along several muscle cells, and a single muscle cell may be located near varicosities belonging to postganglionic fibers of both sympathetic and parasympathetic neurons.
What are possible effects of neurotransmitters on smooth muscle ?
Increase contractile activity
Reduce contractile activity
True or False: a given neurotransmitter may produce opposite effects in different smooth muscle tissues
True
Give an example of a neurotransmitters producing opposite effects in different smooth muscle tissues.
Norepinephrine (released from most postganglionic sympathetic neurons), enhances contraction of most vascular smooth muscle by acting on alpha-adrenergic receptors, but produces relaxation of airway (bronchiolar) smooth muscle by acting on beta-2 adrenergic receptors.
What does the type of response of a neurotransmitter (excitatory or inhibitory) depend on ?
On the receptors the chemical messenger binds to in the membrane and on the intracellular signalling mechanisms those receptors activate.
What are examples of local factors which can alter smooth muscle tension ?
Paracrine signals, acidity, O2 and CO2 levels, osmolarity, the ion composition of the extracellular fluid, stretch
What is the effect of Nitric oxide on smooth muscle ? Which kind of local factor does Nitric Oxide act as ?
Smooth muscle relaxa/on
Paracrine Signal
How can some smooth muscle react to stretch ? How so ?
By contracting
Stretching opens mechanosensitive ion channels, leading to membrane depolarization. The resulting contraction opposes the forces acting to stretch the muscle.
What are the main types of smooth muscle ?
- Unitary
2. Multiunitary
What is the difference between unitary smooth muscle and multiunitary smooth muscle ?
Unitary: sheets of electrically coupled cells which acts in unison - often spontaneously active
Multiunitary: Tissue made of discrete bundles of cells which are densely innervated and contract only in response to its innervation
Give examples of unitary smooth muscles.
A ‘syncytium’ e.g., gut and blood vessels
Give examples of multiunitary smooth muscles.
Vas deferens, iris, piloerectors
What are other names for unitary smooth muscle ?
Single unit, visceral
What are other names for multiunitary smooth muscle ?
Multi unit
