Smooth and cardiac muscle Flashcards
Where is smooth (visceral) muscle found?
In walls of hollow organs
What does smooth muscle produce? (contractions)
Continuous contractions of relatively low force
What is involuntary control influenced by?
By nervous system, hormones and local metabolites
How much muscle mass is contracted?
Contraction of whole muscle mass
Characteristics of smooth muscle cells
Smaller, spindle-shaped cells with a single nucleus, lack striations
Characteristics of smooth muscle fibres
- Bound together
- Irregular
- Varying according to the organ
What is the type of nucleus in smooth muscle?
Elongated and centrally located nucleus
What is the activity of smooth muscle modulated by?
A.N.S input
What controls cross-bridging ?
Controlled via effect of alterations in [Ca2+] on regulatory myosin light chain rather than troponin
How are contractile proteins arranged?
- Not in myofibrils
- Do not appear striated
Shape of smooth muscle cells
Spindle-shaped cells that taper at the end
What is the dense body anchored by?
- Intermediate filaments (desmin)
- Thin filaments (actin)
What is the purpose of the thick and thin filaments?
Both stabilise and link the dense body to the plasma membrane
How are thin and thick filaments linked?
Thin filaments cross link to myosin thick filament
What does the dense body allow for?
Allows for shortening of cells in characteristic bulging pattern
Where is the dense body located? (line)
Analogous to Z-line
What is the shape of the lattice of smooth muscle?
Elongated-diamond shaped lattice
How many filaments are in the arrangement of smooth muscle?
10-15 actin filaments/myosin
filament
What do contractions do to cell?
Shorten it
What happens where bulging is present?
The filaments attaches to plasma membrane
What does the contraction require (and not)
- Requires Ca2+
- Not troponin
How is actin pulled?
Along longer distances and in opposite directions
What is smooth muscle contraction induced by?
Calcium
What can be graded?
- Calcium released
- Stretch
- Hormones
- ANS input
Where does extracellular fluid-electrochemical gradient enter?
Via voltage gated and receptor regulated channels in plasma membrane
What is the extracellular fluid-electrochemical gradient dependent on?
- Depolarisation
- Second messenger
Calcium release from internal stores (direct or indirect and via what?)
Mainly indirect
- via activation of phospholipase C,
- Production of inositol-3-phosphate (IP3)
Calcium release from internal stores
- via activation of phospholipase C,
- Production of inositol-3-phosphate (IP3)
Is the calcium release from internal organs direct or indirect and why?
Mainly indirect as a result of receptor activation - can be modulated
How is the internal store calcium channel opened?
Via specific receptor
What is the process of smooth muscle contraction?
- Muscle excitation
- Rise in cytosolic Ca2+
- Series of biochemical events
- Phosphorylation of myosin cross bridges in thick filament
- Bind of actin and myosin at cross bridges
- Contraction
What is the process of skeletal muscle contraction?
- Muscle excitation
- Rise in cytosolic Ca2+
- Physical repositioning of troponin and tropomyosin
- Uncovering of cross-bridge binding sites on actin in thin filament
- Binding of actin and myosin at cross bridges
- Contraction
Where is the Ca2+ from in smooth muscle?
Mostly from extracellular fluid
Where is the Ca2+ from in skeletal muscle?
All from intracellular sarcoplasmic reticulum
How does phasic smooth muscle contraction
Contracts in bursts driven by action potentials
An example of where phasic smooth muscle present?
Digestive tract
What is the resting membrane potential of tonic smooth muscle?
-55mV to -40mV
How is tonic smooth muscle always contracted?
Partially contracted at all times
Give an example of where tonic smooth muscle is present
Walls of arterioles
Why is Ca2+ always present in cytosol in tonic smooth muscle?
As VDCCs are open at these potentials
Where is most Ca2+ from in phasic SM
From extracellular fliud
What does Dihydropyridine (DHP) receptors function as in phasic SM?
As Ca2+ channels
What does Ca2+ trigger in phasic SM?
Triggers opening of Ca2+ channels in the SR
open
How can IP3 be generated?
By G-protein linked receptors that engage noradrenaline
How can Ca2+ be boosted in tonic SM?
By sarcoplasmic reticulum
What does sarcoplasmic reticulum have?
IP3 receptors
What is the function of IP3 receptors?
They conduct Ca2+ release channels
What tubules & sarcoplasmic reticulum are in smooth muscle and sarcoplasmic reticulum?
No T-tubules and poorly developed sarcoplasmic reticulum
Is there sufficient Ca2+ to enable cross-bridging throughout cell and why?
Yes as SM are small cells
How do multiunits of smooth muscle function?
Independently
What type of smooth muscle is multiunit?
- Phasic
- Neurogenic
Where is multiunit present?
- walls of large blood vessels
- small airways of lung
- muscles of eye
- base of hair follicles
What type is a single unit?
Tonic or phasic and myogenic
How does a single unit function?
Syncytia
What does syncytia mean?
Linked by gap junctions
Where is a single unit present?
- Digestive
- Reproductive
- Urinary tracts
- Small blood vessels
What type of activation is multi-unit involved in?
A.N.S neurogenic activation
What type of activation is single unit involved in?
Myogenic activation (ANS modulation)
What are the two major categories of smooth muscle based on type of excitation?
- Multi-unit
- Single unit
How can depolarisation and extent of calcium elevation be modified?
Can be neurally and hormonally modified to grade frequency and force of activity
Why is there depolarisation in pacemaker potential?
Due to shifts in passive ionic flux
How is pacemaker potential initiated?
By non-contractile pacemaker cells
How does the AP spread in pacemaker potential?
AP spreads via gap junctions
Where is slow-wave potential present?
Only in smooth muscle of digestive tract (nonmuscle
pacemaker cells
Slow-wave potential polaristaion
Alternating depolarisation and hyperpolarisation
Is threshold always reached by depolarisation in slow-wave potential?
No
What is the threshold of depolarisation of slow-wave potential influenced by?
By neural and local factors
What is always present in slow-wave potential and why?
Always some Ca2+ present to maintain some level of tone
What is the type of excitable tissue involved in end-plate potential?
Skeletal muscle
What is the type of excitable tissue involved in receptor potential?
Afferent neurons
What is the type of excitable tissue involved in summation of excitatory post-synaptic potentials?
Efferent neurons, interneurons
What are the types of excitable tissue involved in pacemaker potential?
- Smooth muscle
- Cardiac muscle
What are the types of excitable tissue involved in slow-wave potential?
Smooth muscle (in digestive tract only)
Give a description of the pacemaker potential event
Gradual depolarisation of the membrane on its own because of shifts in passive ionic fluxed accompanying automatic changes in channel permeability
Give a description of the slow-wave potential event
Gradual alternating hyperpolarizing and depolarising swings in potential brought about by unknown mechanism; depolarizing swimg may or may not reach threshold.
How is the portion of cross-bridges activated varied by ?
By varying cytosolic Ca2+
In the graduation of single-unit smooth muscle contraction does ANS initiate contraction?
No but can modify rate
In the graduation of single-unit smooth muscle contraction, what varies / remains the same regarding fibres?
Number of fibres contracting remains the same but amount of tension varies
What does ANS express in single-unit smooth muscle?
- Cholinergic
- Adrenergic receptors
Which type of muscle, smooth or skeletal has a wider range of lengths at which active tension can be generated?
Smooth muscle
Is there a loss of cross-bridge potential in smooth muscle in stretched state and if not why?
No as actin-myosin overlap persists even in stretched state - no bare-zone of myosin
Give an example of why the wider range of lengths at which active tension in smooth muscle can be generated important or it’s function?
Detrusor muscle adapts to bladder filling (stress -
relaxation response) and retains the ability to generate force for bladder emptying.
How is the resting length for smooth muscle?
It’s below optimal length- more stretchable
Speed of smooth muscle
Slow and economical
What is the latch phenomenon?
Actin and myosin engagement and detachment takes a longer time
What is an advantage of the smooth muscle length–tension relationship?
It inherently relaxes when stretched
What is O2 delivery in smooth muscle sufficient for?
For low rate of oxidative phosphorylation
What are the types of cells in cardiac muscle?
Striated cardiomyocytes
What does excitation contraction coupling in cardiac muscle involve?
Troponin - same as skeletal muscle
What is the calcium source from in cardiac muscles?
From ECF
What type of modulation is in cardiac muscle?
A.N.S modulation
What does ECF store?
Pacemaker
What drives the syncitia in cardiac muscle?
Gap junction driven
What is visible in cardiac muscle cells?
- Abundant mitochondria
- Lipid droplets and glycogen granules
Does skeletal or cardiac muscle have a more defined T tubule system and sarcoplasmuc Reticulum?
Skeletal
What are cardiac muscle fibres interconnected by?
Intercalated discs
What are the two types of membrane junctions that intercalated discs contain?
- Mechanically important desmosomes
- Electrically important gap junctions
How are bundles of cardiac muscles arranged?
Spirally around the ventricle.
What happens when cardiac muscle fibres contract?
They ‘wring’ blood from the apex to the base where the major arteries exit.
What do desmosomes provide and contribute to?
- Provide strong adhesion between cells
- Contribute to mechanical strength
Where is cardiac muscle excitation pacemaker potential controlled at?
At nodes in heart (SA and AV)
What is the sympathetic nervous system’s effect on the cardiac muscle excitation curve?
- Increases potential slope
gradient - Decreases maximum
- Negative potential
What is the parasympathetic nervous system’s effect on the cardiac muscle excitation curve?
- Flattens potential slope
- Increases maximum
- Negative potential.
What is the heart innervated by?
The autonomic nervous system
What is the cardiac muscle interconnected by?
By gap junctions found in intercalated discs that join
cells together
What are the characteristics of cardiac muscle fibres?
- Highly organised, striated, slender, and short fibres
What is the classification of skeletal muscle?
Striated muscle, voluntary muscle
Give a description of skeletal muscle?
Bundles of long, thick, cylindrical, striated, contractile, multinucleate cells that extend the length of the muscle
What is a typical location of skeletal muscle?
Attached to bones of skeleton
What is the function of skeletal muscle?
Movement of body in relation to external environment
What is the classification of cardiac muscle?
Striated muscle, involuntary muscle
Give a description of cardiac muscle
Interlinked net-work of short, slender, cylindrical,
striated, branched, contractile cells connected cell to cell by intercalated discs
Give a location of cardiac muscle
Wall of heart
Give a function of cardiac muscle
Pumping of blood out of heart
What is the classification of smooth muscle?
Unstriated muscle, involuntary muscle
Give a description of smooth muscle
Loose network of short, slender, spindle- shaped,
unstriated, contractile cells that are arranged in sheets
Give typical locations of smooth muscle
Walls of hollow organs and tubes, such as stomach and blood vessels
Give a function of smooth muscle
Movement of contents within hollow organs
