Smooth Muscle Contraction Flashcards
What are the 7 Places smooth muscle is found?
1) Blood vessels
2) Digestive tract
3) Bladder wall/urinary tracts
4) Respiratory tract
5) Reproductive tract
6) Eyes
7) Skin
How large are smooth muscle cells?
Smooth muscle cells are small – about 100-200µm in length
How are smooth muscles cells organised?
Smooth muscle cells are small – about 100-200µm in length
How are smooth muscle cells organised?
They are spindle shaped cells arranged in sheets that are perpendicular to each other, and are less regularly organised
Do smooth muscle cells have striations?
How many nuclei do they have?
Smooth muscle cells do not have striations
They have a single nucleus
Are smooth muscle cells under voluntary or involuntary control?
Involuntary
What 3 things controls the smooth muscle cells?
1) Autonomic Nervous System
2) Hormones
3) Stretch
How is smooth muscle organised in the intestinal tract?
Smooth muscle in the intestinal tract is arranged in longitudinal and circular layers
What are the two types of smooth muscle?
- Multi unit
- Unitary
What does multi unit smooth muscle cells consist of?
Multi -unit smooth muscle consists of discrete/separate cells
What do each of the multi unit cells receive?
Each of these cells receive their own branch of an autonomic neuron
How do the multi unit cells contract and what is an advantage of this?
Independent from one another
How are multi unit cells stimulated to contract?
By neurons
What are 4 examples of where multi-unit smooth muscle is located?
1) Ciliary muscle of the eye
2) The Iris
3) Piloerector muscles
4) Vas deferens
What does unitary visceral consist of?
Unitary/visceral smooth muscle consists of sheets of electrically coupled cells
How do unitary cells contract?
These cells contract in unison
How are unitary cells stimulated to contract?
They are all stimulated to contract by a single neuron, resulting in contraction in unison, so that who whole organ synchronises its contraction
What is the most common smooth muscle fibre?
Unitary muscle
What are 5 example of where unitary smooth muscle is found?
1) GI tract
2) Bile ducts
3) Ureters
4) Uterus
5) Blood Vessels
How are the actin and myosin filaments arranged in smooth muscle?
Actin and myosin filaments are arranged diagonally along fibres, and are less regularly organised
What do dense bodies correspond to?
Dense bodies of smooth muscle correspond to Z discs of skeletal and cardiac
What is the structure of dense bodies?
Lattice like structures
What are the roles of dense bodies?
- They anchor actin within the fibre and tether contractile proteins to the sarcolemma
- They are also responsible for transmitting forces of contraction within and between cells, which allows for contraction in unison
What are dense bodies composed of?
- Dense bodies are composed of intermediate filaments such as :
1) Α-actinin
2) Desmin
3) Vimentin
What is the role of gap junctions in smooth muscle?
- Gap junctions electrically couple cells in unitary smooth muscle
- Focal adhesions connect cells together mechanically
What are 3 key differences in smooth muscle structure compared to skeletal and cardiac muscles?
1) No troponin in smooth muscle and no tropomyosin covering myosin binding sites – regulation of smooth muscle contraction differs to both skeletal and cardiac muscle
2) No t-tubules in smooth muscle – no invagination of the cell membrane to pass the electrical current into the cell
3) The SR is much less developed in smooth muscle
How is the SR arranged in smooth muscle cells?
In smooth muscle cells, the SR is largely associated with the sarcolemma and not clearly aligned with myofilaments
How does the arrangement of SR in smooth muscle cells mean for the amount of calcium held?
With far less SR, this means far less ability of smooth muscle cells to hold calcium in the cell compared to cardiac and skeletal muscle cells
What is released form the SR?
Calcium is released from the SR, but it is not the main source of contraction
What is the main trigger for contraction?
Extracellular calcium is the amin trigger from contraction, which is similar to cardiac muscle
What are caveolae?
Caveolae are pouch like infoldings of the sarcolemma of smooth muscle cells
What do caveolae contain?
Caveolae contain large numbers of Ca2+ channels
How are caveolae linked to the SR?
They can be considered as rudimentary t-tubules
What do SR and caveolae do together?
Caveolae and SR will both initiate the flow of calcium into the cytosol to trigger the next step in the events of contraction
What is the role of calcium in smooth muscle cells?
Calcium is still the key molecule involved in couple the electric signal in the form of an action potential into a mechanical contraction
What are the two sources of calcium that contribute to this?
1) The SR
2) Extracellular Ca2+ influx
What are the 3 mechanisms that lead to an increase in cytoplasmic calcium concentration?
1) Voltage gated L-type Ca2+ channels
* Leading to calcium induced calcium released (CICR) via ryanodine receptors on the SR
2) Receptor operated Ca2+ channels
* (RRCC) leading to IP3 receptor activation and CIRC
3) Store operated Ca2+ channels (SOC)
What is triggered when calcium concentration in cytoplasm increases?
As calcium concentration in the cytoplasm increases, it causes a space, which has a knock-on effect to receptors of the cell membrane, allowing or more extracellular calcium to flow into the cell
What may enhance the release of calcium?
Agonists may also enhance the release of calcium e.g chemical messengers binding to G-protein couple receptors
How is myosin in smooth and cardiac/skeletal muscle similar?
The tertiary structure of myosin in smooth muscle is similar to that of skeletal/cardiac muscle
How does the myosin in smooth and skeletal/cardiac muscles differ?
1) Amino acid sequence
2) Arrangement of myosin heads
* Along the entire length of the muscle
* Head hinges opposing direction on the same filament
* This results in pulling in opposite directions, increasing shortening
* Up to 80% in smooth muscle, compared to 30% in cardiac/skeletal
3) Myosin in smooth muscle needs to be phosphorylated before it can interact and form cross-bridges with actin
What does vascular/GI tract consist of?
1) α-SMA – vascular
2) γ-SMA – GI tract
What is the role of calmodulin?
Calmodulin is the key regulatory protein enabling myosin to interact with actin
What is not present in smooth muscle?
Troponin
Describe the 6 steps of smooth muscle contraction
1) Smooth muscle contraction is initiated by calcium influx from extracellular and SR
2) 4 calcium bind to calmodulin (instead do troponin as in skeletal muscle), activating it
3) Calmodulin can now become an activator of a myosin light chain kinase (MLCK)
4) Ca-calmodulin-MLCK complex leads to phosphorylation of the hinge region of the myosin head (Myosin light chain – MLC)
5) Phosphorylated myosin head binds to actin, and the power stroke occurs automatically
6) A second ATP is required to release myosin head from actin
Describe the 4 steps of smooth muscle relaxation
1) Wen the stimulus ends, calcium is pumped out of the cell or into the SR
2) When calcium drops below a critical level, calcium dissociates from calmodulin, which inactivates MLCK
3) Myosin phosphatase removes phosphate from the MLC, causing detachment of the myosin head from the actin filament
4) This causes relaxation
What determines the times for relaxation in smooth muscle?
The time for relaxation in smooth muscle is determined by the amount of active myosin phosphatase in cells
How does the time for relaxation compare in smooth muscle cells to skeletal/cardiac muscle cells?
Typically, smooth muscle contracts at far slower rates than skeletal/cardiac cells
Where is calcium transported to after contraction?
After contraction, calcium is transported out of the sarcoplasm and into the extracellular fluid or into the SR
What are the 3 ways calcium is transported?
1) Membrane Ca2+ ATPase (active)
2) Ca2+ ATPase (SERCA) (active)
3) Na+ - Ca2+ exchangers (passive)
Why is calcium transported in these ways?
This is done to ensure sufficient calcium is returned to the SR
What senses the calcium leaves in the SR?
Stim1 senses calcium levels in the SR
What does Stim1 activate?
It can activate store-operated Ca2+ channels (SOCs) for the influx of calcium back into the SR which enables the SR to refill
This occurs at specialised regions where SR encounters the sarcolemma
What do smooth muscle cell membranes contain that skeletal muscle cell membranes?
Unlike skeletal muscle cells, smooth muscle cell membranes contain receptors which can initiate or inhibit contraction
What does smooth muscle lack that is seen in skeletal muscle?
Smooth muscle lacks highly specialised neuromuscular junctions as seem in skeletal muscle
What do autonomic nerve fibres create?
Branch diffusely, creating wide synaptic clefts called diffuse junctions `
What are varicosities?
Varicosities are swellings that release neurotransmitters in the general area of smooth muscle cells, which diffuse across the diffuse junctions and hit receptors, initiating action potentials in the smooth muscle cell
How does the unitary muscle compare with the multi-unit muscle\?
In unitary muscle, it is not as individual as it is in the multi-unit, where they all receive their own individual stimulation from the autonomic neuron
What may happen to the layers in unitary smooth muscles?
Sometimes, the neurotransmitter may only associate with the top layer of cell, and relies on the gap junctions and excitation to spread through to the lower muscle cells
Where may varicosities from a single axon be located?
Varicosities from a single axon may be located along several muscle cells
Where do varicosities originate?
Varicosities originate from postganglionic fibres of both sympathetic and parasympathetic neurons
How many cells can be affected by how many neurons and vice versa?
Several smooth muscle cells are influenced by neurotransmitters released by a single neuron
How close are varicosities in multi unit smooth muscle?
When neurotransmitters are release in multi-unit smooth muscle, they will be targeting an individual cell they are intended for, which allows for finer control e.g focus in ciliary muscle of the eye
What are the 2 main neurotransmitters used in smooth muscle?
Neurotransmitter’s acetylcholine or noradrenalin are used in smooth muscle, which can be excitatory or inhibitory
What can neurotransmitters do to contractile activity?
Neurotransmitters can either stimulate or inhibit contractile activity
What affects does noradrenaline stimulate?
Noradrenaline stimulates contraction of most vascular smooth muscle by acting on α-adrenergic receptors
It also produces relaxation or airway (bronchiolar) smooth muscle by acting on β2-adrenergic receptors
What 5 things can initiate smooth muscle contraction?
1) ANS stimulation
2) Spontaneous electrical activity from the neurone
3) Stretch
4) Hormones
5) Local chemicals within the extracellular fluid:
* Oxygen
* Carbon dioxide
* Acidity
* Ion concentration
* Nitric acid
- This allows for fine turning of activity in response to the environment
How does smooth muscle resting membrane potential compare with that of skeletal muscle?
Smooth muscles resting membrane potential is -50 to -60mV, which is around 30mV less negative than skeletal muscle at around -80mV
This makes smooth muscle more excitable
What 3 things can an action potential in a smooth muscle cell be?
1) A simple spike
2) Spikes on top of slow waves
3) Spike followed by a plateau (similar to those seen in cardiac tissue)
What 4 things can unitary smooth muscle spikes be generated by?
1) Electrical stimulation
2) Hormones
3) Stretch
4) Spontaneous depolarisation from pacemaker cells (interstitial cells) of the intestinal wall
What is mainly responsible for the membrane potential in smooth muscle?
Calcium is mainly responsible for membrane potential in smooth muscle – slower to open and close than Na+ channels
What leads to elongation of contraction?
Binding of actin and myosin is slower, leading to elongation of contraction in smooth muscle
How long is smooth muscle?
Smooth muscle contraction is prolonged, lasting minutes, hours or even days (tonic contraction)
What 2 things is the length of smooth muscle contraction?
1) Slower ATPase activity from myosin, leading to slower cross-bridge formation between actin and myosin
2) Slower influx of Ca2+ ions (in comparison to skeletal and cardiac muscle)
How does force of contraction in smooth muscle compare with that of skeletal and cardiac muscle?
The force of contraction in smooth muscle is often greater than that of skeletal or cardiac muscle
This is due to the longer cross bridge attachments between actin and myosin
How is prolonged (tonic) contraction enabled?
Prolonged (tonic) contraction is made possible with the latch mechanism, which maintains prolonged contraction with minimal ATP use – only 1 ATP for each cycle
When does prolonged contraction ccur?
This occurs when myosin becomes dephosphorylated while still attached to the actin
What is needed for prolonged contraction to occur?
This only happens if cytoplasmic calcium concentration remains elevated among background levels
