Smooth Muscle Contraction Flashcards
State the approximate length of smooth muscle cells
30-200 micrometres in length
Where is smooth muscle found ?
Blood vessels
Digestive tract
Reproductive tract
Respiratory tract
Skin
Eyes
etc
Briefly describe smooth muscle cells
Small
Spindle shaped cells
Arranged in sheets
Less regularly organised
NO striations
Single nucleus
Involuntary - ANS, hormones and stretch
State the types of smooth muscle
Multi-unit
Unitary
Describe multi-unit smooth muscle
Discrete/separate fibres each with its own nerve ending
Independent contraction
Mainly innervated by nerve signals
Where can you find multi-unit smooth muscle ?
Ciliary muscle of the eye
Iris
Piloerector muscles
Vas deferens
Describe unitary smooth muscle
Sheets of electrically coupled cells - syncytium/visceral smooth muscle
Contract in unison
Connected by gap junctions
Where can you find unitary smooth muscle ?
GI tract
Bile ducts
Ureters
Uterus
Blood vessels
State a difference between multi-unit and unitary smooth muscle
Multi-unit : Independent contraction
Unitary : Contract in unison
Describe the structure of smooth muscle
No striations
Contain :
- Dense bodies
- GAP junctions
- Focal adhesions
Do NOT contain :
- Troponin
- T Tubules
What does smooth muscle contain for structure ?
GAP junctions which electrically couple cells in UNITARY smooth muscle.
Focal adhesions which connect cells together mechanically.
What is a ‘Dense Body’ ?
Dense bodies correspond to Z discs, which are lattice like structures anchoring actin within the fibre and tethers contractile proteins to the sarcolemma.
Function of dense bodies
Transmit force of contraction within and between cells.
Difference between GAP junctions and focal adhesions
GAP junctions couple adjacent cells electrically
Focal adhesions couple adjacent cells mechanically
State the difference in sarcoplasmic reticulum of smooth muscle and skeletal/cardiac muscle
The SR in smooth muscle is much less developed.
The actin and myosin filaments are less regularly organised.
What are caveolae ?
Pouchlike infoldinfgs of the sarcolemma.
They contain large numbers of calcium channels.
What is the main trigger for smooth muscle contraction ?
Extracellular calcium is the main trigger for contraction.
State the 3 mechanisms which lead to an increase in intracellular calcium concentration
Voltage gated L type calcium receptors
Receptor operated calcium channels
Store operated calcium channels
Function of voltage gated L type calcium channels in excitation contraction coupling
They lead to calcium induced calcium release via the ryanodine receptor activation
Function of receptor operated calcium channels in excitation contraction coupling
They lead to IP3 receptor activation and CIRC
Describe excitation contraction coupling
Following stimulation, an influx of calcium couples the excitation into mechanical contraction.
State the differences between smooth muscle myosin and skeletal/cardiac muscle myosin
They differ in :
- Amino acid sequence
- Arrangement of myosin heads
Describe how myosin in smooth muscle differs from skeletal/cardiac muscle
In smooth muscle,
- Myosin heads are arranged along the entire length of the molecule
- Head hinges opposing direction on the same filament - pulls in opposite directions, increasing shortening
State the types of smooth muscle actin
Alpha-SMA : vascular
Gamma-SMA : GI tract
No troponin
What is calmodulin ?
A key regulatory protein enabling myosin to interact with actin.
Briefly describe activation of myosin
Phosphorylation of myosin needed for cross-bride formation with actin.
Via calmodulin / myosin light chain kinase (MLCK)
Briefly describe relaxation of myosin
Myosin must be de-phosphorylated for relaxation to occur.
Whilst phosphorylated, it will continue to form cross-bridges with actin.
Explain the process of contraction in smooth muscle
Contraction is initiated by calcium influx from extracellular fluid in the SR.
Calcium binds to calmodulin (instead of troponin as in skeletal muscle)
Ca-Calmodulin-MLCK complex lead to phosphorylation of MLC (requires ATP)
Phosphorylated myosin head binds to actin and power stroke occurs automatically.
Explain the process of relaxation in smooth muscle
When stimulus ends, calcium is pumped out of the cell or into the SR.
When calcium drops below a critical level, calcium dissociates from calmodulin (inactivates MLCK)
Myosin phosphate removes phosphate from the MLC, causing detachment of the myosin head from the actin filament, causing relaxation.
What does the time of relaxation depend on ?
Determined by the amount of active myosin phosphate in cells.
How is calcium transported out of the sarcoplasm ?
Active transport via :
- Membrane Ca2+ ATPase
- Ca2+ ATPase (SERCA)
Passively via :
- Na+ / Ca2+ exchangers
What monitors calcium levels in the SR ?
Stim1 senses calcium levels in the SR and activates STORE OPERATED CALCIUM CHANNELS for influx of calcium back into the cell.
Function of store-operated calcium channels (SOCs)
Enables the SR to refill
Influx of Ca2+ back into the cell
What does smooth muscle lack ?
Highly specialised neuromuscular junctions
Discuss the innervation of smooth muscle
Autonomic nerve fibres branch diffusely creating synaptic clefts.
Unitary muscles have wide diffuse junctions
Multi-unit muscle cell junctions are closer in proximity - contact junctions
What is a varicosity ?
Varicosities are swellings which release neurotransmitter in the general area of smooth muscle cells.
Where do varicosities originate from ?
Postganglionic fibres of both sympathetic and parasympathetic neurons.
Name some neurotransmitters in smooth muscle
Acetylcholine
Noradrenaline
What do smooth muscle cell membranes contain ?
Receptors which can initiate or inhibit contraction.
Function of neurotransmitters
They can either stimulate or inhibit contractile activity.
Describe a neurotransmitter that may produce opposite effects in different smooth muscle tissues
Noradrenaline stimulates contraction of most vascular smooth muscle by acting on alpha-adrenergic receptors
BUT
produces relaxation of airway smooth muscle by acting on beta2-adrenergic receptors.
Describe how smooth muscle contraction is regulated
ANS stimulation
- Spontaneous electrical activity
- Stretch
- Hormones
- Local chemicals within extracellular fluid
State some local chemicals within extracellular fluid
Oxygen
Carbon dioxide
Acidity
Ion concentration
Nitric oxide
What is smooth muscle resting membrane potential ?
-50 to -60 mV
How can unitary smooth muscle spike potentials be generated by ?
Electrical stimulation
Hormones
Stretch
Spontaneous depolarisation from pace-maker cells of the intestinal wall
Why is smooth muscle contraction prolonged ?
Due to slower ATPase activity / cross-bridge formation
AND
Slower response to influx of Ca2+ ions
Why is the force of contraction in smooth muscle greater than that of skeletal muscle ?
Due to the longer cross-bridge attachments between actin and myosin.
Function of the latch mechanism
Maintains prolonged contraction, with minimal ATP use - only 1 ATP required for each cycle
When does the latch mechanism occur ?
When myosin is de-phosphorylated while still attached to actin.
(only if [Ca2+]i remains elevated above background levels)
