Smooth muscle Flashcards
What is the shape and position of the nucleus of smooth muscle cells?
Spindle shaped with a single nucleus in the middle
What are the two types of forms
Single-unit and multi-unit smooth muscle
What is the most common smooth muscle type? How does this differ from the other type of smooth muscle?
Single-unit smooth muscle
It is where the cell act in unison, often spontaneously active whereas multi-unit are made of discrete bundles of cells which can contract individually
Where would you find multi-unit smooth muscle?
In the iris, rising the individual hairs of the body etc.
Where would you find the single-unit smooth muscle?
Inside cavities (i.e. stomach, blood vessels…)
What is a structural difference between multi and single unit smooth muscle? How does this affect their function?
Single-unit have gap junctions which mean that they contract as a whole while multi don’t have gap junctions
How can multi-unit smooth muscle be individually excited?
the nerve cells run through the tissue and have individual contact to each cell
How are the single uni smooth muscle arranged in hollow organs? What does this allow?
The cells are arranged horizontally (i.e. wrapping around in a circle) and longitudinally (i.e. along the length of the tube)
Allows for controlled manipulation of the tube
Does the smooth muscle have T-tubules? explain
No, but it does have caveolae which act to increase the surface area which performs a similar function (allow communication however in a different mechanism FYI)
What are the equivalent of z-lines in smooth muscle? What do they do?
Dense bodies anchor actin to sarcolemma
Do smooth muscle have z-lines? What does this allow?
No
It allows the filaments to slide past each other further in compression so the smooth muscle can be squashed more
What is used to connect the smooth muscle cells together? What kind of smooth muscle has this? What does this allow?
Gap junctions
Single - unit
Allow for electrical conduction of signals between cells
What is the function of intermediate filaments? Where are they found?
it is the cytoskeleton element
How much sarcoplasmic reticulum is in smooth muscle relative to cardiac and skeletal muscle?
Very little
How are the thick and thin filaments arranged within the smooth muscle? What is this arrangement called? Why in this arrangement?
They are running across the diagonals of the smooth muscle
It is less organised
Allows for greater shortening
What is the regulatory protein in smooth muscle? What does this replace compared to skeletal and cardiac?
Calmodulin replaces troponin
How much can smooth muscles shorten by?
60-75%
How is smooth muscle initiated? What needs to be initiated to cause contraction?
By neural, hormonal or spontaneous
Ca2+ is released
What causes spontaneous contraction?
Slow increase in RMP up to threshold initiation
Where can spontaneous / myogenic contraction occur?
In the gut
Where can neurally induced contraction occur?
ciliary an diris muscles
Where does the increase in Ca2+ occur in muscle tissue to cause contraction?
In the cytosol for ALL types of muscle types
In most cases, where is the calcium coming from to initiate smooth muscle contraction? Why is this?
From outside the cell (i.e. extracellular fluid)
The SR has very low amounts of Ca2+ in smooth muscle
How does the Ca2+ get into the cell?
Through voltage gated or hormonal gated channels
What does the Ca2+ bind to inside smooth muscle?
Calmodulin
What happens when calmodulin bonds with Ca2+?
It becomes activated and activates a kinase protein into myosin light chain kinase (MLCK)
What does the myosin light chain kinase (MLCK) do?
Phosphorylates myosin which then exposes the actin binding site on the myosin
How does the binding site differ between smooth muscle and skeletal/cardiac muscle?
Smooth has the binding site on the myosin which gets exposed, in skeletal/cardiac the binding site is on the actin which gets exposed
What can’t myosin do unless it is phosphorylated in smooth muscle?
It cannot break down ATP
How is the cross-bridge cycle similar and different from smooth muscle and skeletal/cardiac tissue?
It has all the same step (i.e. power stroke, detachment etc.)
however it occurs much slower and the myosin heads stay attached to the actin for much longer
Why does the myosin stay attached to the actin?
It enables the smooth muscle to maintain tone (i.e. not become completely limp)
How can the smooth muscle be constantly tensed while skeletal muscle is normally in a relaxed state?
Smooth muscle is enzyme activated which uses less energy than skeletal muscle which is ATPase based, this means that smooth muscle can be constantly toned without using too much energy
What makes smooth muscle a lower energy method of contraction than skeletal muscle?
Smooth muscle CONTRACTION is stimulated by phosphorylation of the light myosin chains, in contrast to skeletal muscle being neurogenic and generating contraction through the depolarisation of the T-tubules
Also RELAXATION occurs when myosin light chain phosphatase dephosphorylates myosin while in skeletal muscle, ATPase causes it to relax as it decreases the concentration of calcium
How are smooth muscle relaxed?
Myosin light chain phosphatase (MLCP) removes the phosphate from myosin deactivating it
If the Ca2+ concentration decreases but the myosin is still activated on smooth muscle, will it still contract? Why? What does this allow?
Yes because unless the myosin is deactivated there will still be Ca2+ activating contractions
Means that there is always some kind of tone in the smooth muscle
What does smooth muscle being enzyme regulated make it?
Slow but energy efficient
What are the three ways which smooth muscle contraction can be regulated?
Cytosol concentration of Ca2+, myosin light chain kinase or phosphatase activity
How does increasing Ca2+ impact smooth muscle contraction?
It increases the muscle tone (i.e. more contraction)
How does increasing myosin light chain kinase activity impact smooth muscle contraction?
It increases the muscle tone (i.e. more contraction)
How does increasing myosin light chain phosphatase activity impact smooth muscle contraction?
It decreases the muscle tone (i.e. relaxes muscle)
How are smooth muscle innervated?
Automatic nerve fibres branch over smooth muscle cells into wide synaptic clefts called ‘diffuse junctions’ and secrete neurotransmitters onto the matrix coating which diffuses into muscle cells
What do and where are varicosities?
Located in the terminal axons they contain the neurotransmitters for automatic nerve fibres for smooth muscle
What happens when you stretch smooth muscle?
Initially it contacts to resist the stretch, then is slowly relaxes
How does the smooth muscle contract to resist stretch?
Stretch activated calcium channel allows Ca2+ into the cytosol causing muscle contraction
How does the smooth muscle relax after being stretched?
Calcium dependent K+ channels are opened as the Ca2+ enter due to the stretch activated calcium causing Ca2+ to leave the cell in K+ co-symporter causing relaxation
How is the Ca2+ in SR in smooth muscle released?
By IP3 signalling protein
What are the contractive filaments of smooth muscle?
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