Muscles Flashcards
What makes up a motor unit
The presynaptic neutron and collection of muscle fibres which it innervates
What is the diameter of a muscle fibre generally
1-2 micrometer
How wide is a synaptic cleft
50-70 nm
What opens when an AP arrives at a presynaptic terminal?
What does this result in?
The motor axon opens voltage gates Ca channels
Causes vesicles to fuse with plasma membrane, releasing their contents into the cleft
How has vesicles fusion bee visualised
Using rapid freezing of the muscle during neuromuscular transmission followed by freeze fracture EM
What is the conc of ACh in vesicles
100-200mM
Roughly 10^4 molecules/vesicle
What is the is the release of ACh vesicles proportional to?
[Ca2+]^4
What is the mean synaptic delay?
Where does most of the delay occur?
1ms
Within the presynaptic terminal
What is the timing of ACh diffusion and post synaptic response
Diffusion: 10 microseconds
Response: <100 microseconds
ACh is an anion
True or false
False
It is a cation
What forms ACh
Choline and acetyl CoA by the enzyme choline acetyltransferase
What happens to ACh chemically when it is released?
What does it yield
It is hydrolysed by ACh-ase
Acetic acid and choline
How many subunits does a AChR channel have
How are they structured
5
The hydrophilic, negative side chains point inwards creating a selectivity filter which rejects anions but accepts Monovalent cations
Lipophilic point out into lipid bilayer, stabilising the pentamer
The alpha subunits each contain an extra cellular binding site for ACh
How was the current through a single AChR investigated?
Neher and Sakmann (and others) used a patch clamp
How long do AChR open for
What experiment was done to show this
<1ms
Bathed in a maintained concentration of ACh - opened in random durations
What is required for an AChR to open
Why is this
2 ACh molecules must bind to both alpha subunits
Suppressed responses to small quantities of ACh but produces a sharp response to higher concentrations
Should you still reread the handouts despite learning these flash cards
Absolutely. I have skipped entire paragraphs that Michaelmas term Joe thinks irrelevant but may later be important
What dictates the decay time of an end plate potential
Fibre diameter: the larger the fibre, the faster the decay
What is the reversal potential for the ESPC
0mV
Is the AChR exclusively a sodium Channel
No
It is a monovalent cation channel permeable to both K and Na
It is also slightly permeable to Ca
Do ESPCs speed up with depolarisation
Why does this occur
Yes due to some voltage dependence of AChR gating
It is energy efficient: if the membrane is already depolarised, the conductance switches off faster when it is no longer needed to produce depolarisation
What is the active ingredient in curare
D-tubocurarine
Why is curare toxic
D-tubocurarine competes with ACh for binding on AChR without opening the channel
This reduces the post synaptic response leading to separation of EPP and AP
How can the amount of ACh releases in the cleft be studied
Eserine blocks ACh-ase
What effect does eserine have
Prolongs EPSP and EPSCs
State nicotine’s relation to ACh
Agonist
It acts specifically on ion channel ACh receptors without activating muscarinic ACh receptors which mediate slow responses in other types of cell
How is snake venom deadly
Contains α-Bungarotoxin which binds tightly and blocks AChRs
How are black widows deadly
Their venom contains alpha-latrotoxin which produces a release of the pool of available presynaptic transmitter, resulting in a block from ACh depletion
How is Botulinum toxin toxic
Prevents release of ACh
What is a muscle fibre
How do they connect to bone
A multinucleate cell with a diameter between 50-150 microns
They are continuous with connective muscle tissue tendons which connect to bone
What structures directly reflect a contractile function
Myofibrils (made of contractile proteins that run along the fibre axis and are between 1-2 microns in diameter)
Are T tubules continuous with extra Elul are fluid?
Yep
What is the sarcoplasmic reticulum
An intracellular membrane network isolated from the extra cellular space which stores Ca ions
What muscles are striated
Cardiac and skeletal
In striation what is dark and what is light
Dark - A band containing thick filaments mainly containing myosin
Light - I band with only thin filaments, containing actin, troponin, and tropomyosin
Where is the Z band
In the centre of the I bands and provide attachment sites
The Z to Z units of myofibril form the sarcomere
Structure of myosin
1.6micrometers in length
Consists of a tail of 2 long light meromyosin strands twisted together which are both connected to globular heads
What does myosin bind to and what does this trigger
Actin
ATPase activity in the S1 fraction
When is the reaction between the myosin and actin permitted
If the tropomyosin molecule is moved deeper into the groove that is formed by the thin filaments caused when Ca binds to the troponin
Generally what does a cross bridge cycle do
Pulls the thin filaments towards the centre of the sarcomere
What produces sarcomere shortening
The relative sliding of the interdigitated thick and thin filaments
What does increasing myofilament overlap do?
Increases force generation
Why is there a constant isometric force at sarcomere lengths below 2.2 to 2.0 microns?
The middle of the thick filament lacks myosin heads, restricting opportunities for cross bridge formation, which shortens filament overlap
What does excessive myofilament overlap result in
What does this do
Thin filaments collide with one another
OR
thick filaments colliding with the Z lines
Diminish force production
How are cross bridges broken
A process that requires hydrolysis of ATP
this thus allows further cross bridge formation
What is rigor Mortis
A progressive recruitment of cross bridges that fail to dissociate
What 2 factors affect production of force from cross bridge cycles
Number of interacting sites
Force generated by each site
Why do muscles usually act at a disadvantage
What does this mean
The perpendicular distance from the line of action of the muscle to the joint is normally considerably smaller than the distance of the load to the joint
What does repetitive activation of of a muscle result in
What about for higher stimulation frequencies
A sequence of twitches with no increase in peak tensions
Muscles may be reactivated befor the previous twitch has fully recovered to result in a twitch that rises above that of a single twitch
What happens during tetanus
High activation frequencies results in tensions summing
Describe the length tension relationship
Gradual increase in tension with small degrees of stretch but beyond a certain degree of elongation there is a significantly non linear increase in tension.
After a certain length, tension decreases as length increases
Why does tension eventually decreases as length increases
Thick filaments fail to overlap so active tension cannot be developed
Equation for force velocity relationship
(F+a)(v+b)=(Fmax + a)b
What are smooth muscles
What is the load they work against
Muscles that line walls of hollow organs
The pressure within the tube
Shape of smooth muscle
Elongated and spindle shaped
NOT striated (no sarcomere)
Thick and thin are NOT transversely aligned but attached in dense bodies in the cytoplasm and to attachment plaques at the membrane
No T Tubules
What systems control smooth muscle
Autonomic (both para and sympathetic) and hormonal
Can generate active tension without nerve activity - nerves often modulate
What does caldesmon do
Binds to actin-tropomyosin to inhibit cross bridge cycling
Ca causes caldesmon to dissociate from actin by binding to the regulatory calcium binding protein calmodulin
How does PKC cause dissociation
Phosphorylates caldesmon
What does binding of Ca to myosin might chain do
Increases cross bridge formation
Role of myosin light chain kinase
What is this process called
Phosphorylation of myosin light chain causing increased cross bridge cycling which lasts until dephosphorylation by myosin phosphatase
Covalent regulation (covalent bind formed)
What happens if the cross bridge is dephosphorylated
Can maintain tension without cycling or ATP consumption
Hence why smooth muscle is >300x more efficient than skeletal in maintained contraction
What happens when the gut is stretched
Stretch activated ion channels lead to mechanically induced depolarisation and contract.
This assisted peristalsis
Where does an action potential travel in muscles
Over surface membrane and T tubule which triggers release of intracellularly stored Ca from SR
What is an inward rectifier and what do it do
A channel that Allows K to pass more easily into than out of a cell, minimising leaks and the amount of inward current required for the plateau
What are Ca channels responsible for in muscle
Plateau phase
What does the ryanodine receptor do
Acts as a calcium channel in SR
How many axon branches innervate a muscle fibre
Where do they attach on the muscle fibre
Each branch attaches to a single muscle fibre to form a motor end plate
The centre
How big are muscle fibres
1-2μm in diameter
How frequent are junction folds
Every 1-2μm
What are muscular active zones
Specialised thickenings immediately above the junction fold in the Presynaptic axon membrane
How big are the vesicles in muscle
Where are they found
50nm in diameter
Clustered around the active zone
What is in the synaptic cleft
Mucopolysaccharide glue
What is the basal lamina
A layer of extracellular material visible in EM
What happens as the AP reaches the motor axon
VG Ca channels open in the presynaptic terminal. This rise in [Ca] causes vesicles to fuse with the plasma membrane releasing their contents
What is the minimum synaptic delay in a frog at 17 degrees C
What is the mean
500μs
1ms
Where does most of the synaptic delay occur
How fast does ACh diffuse across the cleft and how fast is the response
In the presynaptic terminal
In about 10μs
<100μs
How is ACh synthesised
What happens after release
From choline and acetyl CoA
By choline acetyltransferase
It is rapidly hydrolysed by AChase which is localised to the basal lamina
What are the products of ACh hydrolysis
Acetic acid and choline
What is the structure of an AChR
4 types of gylcoprotein subunit (α,β,γ,δ) but 5 subunits total arranged ααβγδ
Hydrophilic negative side chains point into the pore creating a selectivity filter which rejects anions
The lipophilic side chains point out and stabilise the pentamer
Each α subunit contains an ACh binding site on the extracellular domain
What was the conductance of a single AChR
30pS or 30/Ω
What happens if AChRs are bathed in ACh
Openings are short <1ms and random in duration
Openings tend to be clustered into bursts with long intervals
What happens after a long period of maintained [ACh]?
Why does this happen?
The AChR stops opening or is desensitised
Protective mechanism in the case that limits the dissipation of the ion gradient in the event of catastrophic release of transmitter
How did Katz eliminate the AP
By partially blocking synaptic transmission with curare
What is an EPP
A residual subthreshold potential which rise every few miliseconds then decay (the larger the fibre diameter the larger the decay)
Why are EPSPs slower than EPSCs
The time taken to charge and discharge the membrane capacitance
What happens to the EPSC at more and more depolarised holding potentials
EPSC gets smaller and disappears at 0mV then gets larger again in the opposite direction
How do we know the AChR is simply a monovalent cation channel
The current varies linearly so the summed current varies linearly and the reversible potential is near 0mV
Do ESPCs change speed with depolarisation
How is this beneficial
Yes they speed up due to some voltage dependance of the AChR
Saves energy: if the membrane is already depolarised the conductance switches off faster when it is no longer needed to produce depolarisation
What does Eserine do
What is it also called
Why is it useful
Blocks AChase
Physostigmine
Measurement of how much ACh is released
What is nicotine in muscles
An agonist for AChRs
Works specifically on ion channels receptors without activating muscarinic AChRs which mediate slow responses in other cells
What does snake venom contain
Why are they bad
How have they been used
α-bungarotoxin
Blocks AChRs
Radioactive ones have been used to count the number of AChRs in a NMJ
How does black widow venom work
Produces a massive release of ACh in pre synaptic terminal depleting ACh stores
What does botulinum A do
Acts as an enzyme, cleaving SNAP 25 to prevent vesicle fusion with the presynaptic terminal, resulting in paralysis of the muscle.
How is the frequency of MEPPs affected as observed by Fatt and Katz
Decreased by decreasing external Ca or increasing Mg
It was increased by increasing external [K+] or osmotic pressure
These allow affect the release of vesicles
What did Katz suggest about vesicle release
1 quantum = release of 1 vesicle
Release is probabilistic: the great increase in release during an EPP would correspond to a transient increase in probability of release for the vesicle population
How should EPSPs fluctuate
Probabilistically in size but be comprised of single integral numbers of quanta
How to find probability of a peak
The area of the peak
Pk =?
(e^-m) x (m^k)/k!
What is m in the Poisson equation
Mean number of quanta released
ie quantal content
What can be seen if the AP is recorded near the end plate
The upstroke also shows the initial EPP as the membrane charges up to the threshold
What can you see of EPPs when curare is added to the end plate
What does this mean
EPPs are subthreshold and the peak size is seen to decay exponentially with distance from the end plate
The region over which a pre synaptically evoked muscle AP is limited to the area near the end plate
Discuss the uses of Botulinum A
By preventing ACh release it can be used to relax the muscles causing wrinkles in cosmetics.
However it is also the most lethal toxin known to man, causing widespread paralysis
Describe the action of α latrotoxin
forms pores to allow Ca entry resulting in vesicle release and initial stimulation, followed by depletion as continual release occurs
Can muscles depolarise in the absence of presynaptic activity
Yes - MEPP
At what intervals for MEPPs occur
Random intervals
What produces a MEPP
A quantum of about 10,000 molecules of ACh
Can you find depolarisation if tetrodotoxin is used to block the presynaptic APs
Yes: MEPPs occur despite this
What caused the frequency of MEPPs to change
Frequency decreased by a decrease in external Ca2+
Decreased with an increase in Mg
Increased by increasing external [K+] or osmotic pressure
What causes the frequency of MEPPs to decrease
Decreasing external calcium
Increasing external Magnesium
What causes the frequency of MEPPs to increase
Increasing external [K+] or osmotic pressure
What does a single quantum correspond with
Who showed this
A single presynaptic vesicle containing ACh
Katz et al
What dictates when an ACh vesicle is released
Probability
The transient increased release during an EPP is due to a transient increase in probability of release of the population of presynaptic vesicles
The hypothesis that quantum release is probabilistic leads to a number of predictions. Name one.
EPSPs should fluctuate probabilistically in size but be composed of integral numbers of quanta
How did Katz show the probabilistic nature of vesicle release
What did he see
Used low Ca2+ conditions to reduce probability of release
Distinct peaks that can be seen, reflecting the simultaneous release of 1,2,3 or 4 quanta
What should the area of peaks of vesicles release follow
The Poisson distribution
What is assumed when we say vesicle release follows the Poisson distribution
Vesicles are identical and independent
Probability of release is low
Give the Poisson distribution equation
Pk =?
(e^-m) x (m^k)/K!
In the Poisson distribution equation, what do each of the following stand for:
a) k
b) m
c) Pk
a) number of quanta
b) mean number of quanta released during an EPP
c) probability that the EPSP comprises k quanta
What is quanta content
Mean number of quanta released during an EPP
If we recorded the voltage near the end plate and an AP, what would we see?
How would it be different to if it were >5mm from the end plate
An upstroke showing the initial EPP as the membrane charges up to the AP threshold
The initial EPP disappears
How happens to subthreshold EPPs in a curare treated muscle
What is this due to
Peak size of EPPs decays exponentially with distance from the end plate
The cable properties of the muscle
What do the cable properties of the muscle mean
Without regenerative inward current through VG Na+ channels, the depolarisation fades with distance as more and more of the end plate current means out of the muscle fibre membrane
Why do postganglionic cells often offer dual innervation
They often have opposite actions
What do post ganglionic cells of the sympathetic nervous system release
What is its effect on cardiac muscle
Noradrenaline
Excitatory
What do post ganglionic cells release
ACh
What are the 4 ways skeletal muscle structure reflect their specialised function
1) myofibrils
2) internal membrane systems to regulate muscle contraction
3) specific organelles
4) abundance of myoglobin and creatine phosphate, as well as dystrophin
How do myofibrils directly reflect the contractile function of muscle
Made up of contractile proteins that run along the fibre axis
What are the 2 internal membrane systems in muscles that are specialised to regulate muscle contraction
Transverse tubules
Sarcoplasmic reticulum
What are transverse tubules
A fund network of tubes whose lamina are continuous with the ECF
Their extensive networks are placed regularly along the fibre length and transversely across the fibre axis
Describe the sarcoplasmic reticulum
What is the purpose
Forms a network of tubes and sacs whose lamina are isolated from the ECF
Intracellular storage of Ca2+
What organelles are particularly important in skeletal muscle cells (5)
Ribosomes Lysosomes Lipid granules Mitochondria Glycogen granules
Why is dystrophin important
Abnormalities in dystrophin are associated with the pathogenesis of muscle dystrophy
What is the A band
Anisotropic
Contains thick fibres
What is the I band
The lighter isotrophic band
Contains only thin fibres
Where are the Z lines
At the centre of the I bands
These provide attachment sites
What is the M line in muscle
The cross connections that alight the thick filaments
What forms a sarcomere
The repeating anatomical unit of a myofibril that extends from Z line to Z line
What are the thick proteins mostly made up of
Myosin
What is a myosin molecule made of
A tail of 2 long light meromyosin strands that are twisted together and each tail is connected to a globular head
The head is made of a heavy meromyosin with 2 fractions (S1 and S2)
How long is a myosin molecule
1.6μm
What are the functions of the 2 fractions of the myosin head
S1 - contains globular heads; where ATPase activity happens
S2 - contains the necks which connect the heads to the tails
The myosin heads binds to actin, which in turn triggers ATPase activity in the S1 fraction
What are thin filaments made of
Units of actin with troponin and tropomyosin
How long are thin filaments in amphibians
2.05μm
How long is the periodicity of a thin filament
5nm
How are thin filaments formed
Each actin unit is polymerised in vivo to form thin filaments
These thin filaments are organised into paired chains that are twisted around each other with a periodicity of 36.5 nm
Can actin bind to myosin in vitro
Yes
Actin is capable of binding with myosin in vitro and in vivo
Describe the structure of tropomyosin
Rod shaped about 40nm long
Forms α helical subunits that become packed into the depth of the groove formed by intertwined helical actin chains
How does a tropomyosin molecule compare to actin size wise
Why
One tropomyosin molecule spans 7 actin units
Prevents the binding of these actin units to myosin in resting skeletal muscles
Physically what must happen on a molecular level to allow actin to bind to myosin
What controls this configuration change
Only allowed if tropomyosin is moved deeper into the groove formed by thin filaments
Troponin
Describe the structure of troponin
Consists of 3 subunits: TnC, TnT and TnI
What does the TnT subunit do
It is associated with the tropomyosin ribbon at 40nm intervals
What happens to troponin when Ca2+ binds
Triggers a conformational changes in TnC subunit to pull tropomyosin into the actin groove
This exposes the myosin binding sites present on the actin molecule
What is the purpose of TnI subunit
It is uncertain
How does increasing filament affect force produced
Increases force generation
Do myofilament lengths change
No
Below what sarcomere lengths is isometric force constant
Why
2.2 to 2.0 microns
There are no myosin heads in the middle of the thick filaments
How long does a single muscle twitch last
How many twitches are produced by activation from a single nerve
> 200ms
1
Is there a change in peak tension if there is repetitive activation of muscle at a low frequency
No it simply elicits a sequence of twitches with no increase in peak tension
Why may peak tension in muscles increase if they are repeatedly activated at a high frequency
The muscle may be reactivated before the previous twitch has fully recovered to result in a tension that rises above that of a single twitch
How can tension be increased passively in muscles
Passively stretching a quiescent muscle
How does tension vary as stretch increases
As stretch increases, tension increases gradually to a point, beyond which it increases at a faster rate
However if stretched beyond the point where thick and thin filaments don’t overlap, tension will drastically decrease
What did AV Hill do
Demonstrated the inverse relationship between velocity of muscle contraction and the force generated
What is the formula for the force velocity relationship
(F+a)(v+b) = (Fmax + a)b
Where a and b are constants
When is maximum force generated
At isometric tension
How much ATP is stored in cells for muscles
Enough for 8 twitches
If there is only enough ATP in the cell for 8 twitches, how do we increase the amount of ATP
ATP is regenerated by reacting ADP and phosphocreatine (CP)
Give the equation for the reaction of ADP and phosphocreatine
ADP + CP —> ATP + creatine
CP provides an immediate backup energy supply. How much ATP can it provide for muscles
Enough for 100 twitches
What is the substrate mainly utilised in muscles during intense exercise to produce ATP
Glucose and fatty acid (via the Krebs cycle)
How is glucose utilised for ATP in exercise when O2 levels are low
Glucose is anaerobically converged to lactic acid with a limited conversion of ADP to ATP
How can proteins contribute to ATP levels for muscles
Proteins can be deaminated to produce ketoacids which can act as intermediates in The Krebs cycle
Type 1 muscle fibres have relatively high levels of aerobic enzymes. True or false?
True
How do different types of muscle fibres vary in concentrations of aerobic or anaerobic enzymes?
Type 1: high levels of aerobic enzymes
Type 2: intermediate
Type 2b: high levels of anaerobic enzymes
Where is smooth muscle found
What is the load on smooth muscle
Lining the walls of hollow organs
The pressure exerted on the walls of the hollow organ by its contents
Do smooth muscles always have tonic activation
No
Some muscle like in blood vessels require tonic contraction to maintain pressure
However in the gut, phasic contraction is required to propel contents through the tube
Which is faster at contracting: skeletal or smooth muscle
Skeletal
But smooth is better for sustained contraction
Are skeletal muscles the same size as smooth muscle
No smooth is much smaller
What is the shape and size of skeletal muscle
Spindle shaped often
Typically 3-4μm in diameter and several hundred μm long
Why are smooth muscles smooth
They have no visible striations or sarcomeres in their cytoplasm
Are thick and thin filaments arranged longitudinally in the cytoplasm it smooth muscle
Yes but they are not aligned transversely
Where do filaments attach in smooth muscle
Dense bodies in the cytoplasm and to attachment plaques at the membrane
Where can α actin be found
Attachment plaques
Describe the T tubular system in smooth muscle
There is none
Where is the SR in smooth muscles
There is a vesicular sarcoplasmic reticulum near the membrane
What do gap junctions do in smooth muscle
Allow propagation of waves of electrical excitation between cells or transmission of intracellular messengers through the tissue
Can smooth muscles generate active tension without nerve stimulation
Yes
What is often the role of neural input for smooth muscle
Modulatory
Do nerves act specifically on smooth muscule
No the NT released acts over a wide area of smooth muscle
All smooth muscle is innervated by sympathetic fibres alone. True or false?
False
There is often dual innervation by sympathetic and parasympathetic fibres with reciprocal functions
Is there troponin in smooth muscle
No
What does Caldesmon do
Bonds to actin-tropomyosin thin filaments and inhibits cross bridge cycling
How does Ca2+ affect caldesmon
Ca2+ causes caldesmon to dissociate from actin, promoting contraction, by binding to the regulatory calcium binding protein calmodulin
What is the Ca. CaM and what does it do
The Calcium calmodulin complex
Binds to caldesmon causing it to dissociate from the thin filaments
How else can the dissociation of caldesmon from actin be caused (other than by Ca. CaM)
Direct phosphorylation of PKC (protein kinase C)
What regulates PKC
Diacylglycerol (DAG)
What is DAG a product of
Phospholipase C activation
How does Ca2+ directly affect myosin
Calcium binds directly to the cross bridge to increase cross bridge cycling
What does the myosin light chain kinase do
How long does this effect last
Phosphorylates the myosin light chain to increase cross bridge cycling
This lasts until dephosphorylation by myosin phosphatase
What activates myosin light chain kinase
The calcium calmodulin complex
What kind of regulation is myosin light chain kinase phosphorylation
Why?
Covalent regulation
Phosphorylation forms a covalent bond
What happens if the myosin light chain is dephosphorylated when attached to the thin filament
What is this called
It remains bound with high affinity
A cross bridge in this state is called a latch bridge
What do latch bridges allow
The maintenance of tension without cross bridge cycling and ATP consumption
Which is more efficient: smooth muscle or skeletal muscle?
Why is this?
Smooth muscle is 300x more efficient than skeletal in maintained contractions due to latch bridge formation
What accounts for the slowness it smooth muscle contraction
The control of cross bridge cycling by biochemical cascades
What indicates that the sliding filament mechanism is present in smooth muscle
Smooth muscle shoes a similar force-length relationship to skeletal muscle for isometric contractions
Is contraction velocity with maximal stimulation higher in skeletal or smooth muscle
Lower velocity in smooth muscle
How can velocity of shortening be increased in smooth muscle
Increased levels of cross bridge phosphorylation
Is lots of phosphorylation required for maximum isometric force generation in smooth muscle
No
What happens to calcium and the rate of cross bridge phosphorylation in a sustained contraction
Calcium and the rate of cross bridge phosphorylation increase to a peak initially to produce rapid shortening and the subsidise to lower levels while tension is maintained
Name 3 organs which have smooth muscle that contacts phasically in response to stretch
Bladder
Uterus
Gut
What causes smooth muscle phasic contraction in response to stress in organs like the bladder
Mechanically induced depolarisation due to stretch activated ion channels in the muscle membrane
What does tonic stretch induced contraction allow
Compensative adjustment of tension to keep length constant
In cardiac and skeletal muscle how is calcium mediated
Through its binding to the TnC subunit of troponin
How many calcium binding sites are on a skeletal TnC subunit
Which binding sites are special
4
2 of the sites have a much higher affinity for Ca2+ than Mg2+ and are critical sites for regulation of cross bridge activation
Is the T tubule extracellular space
What does this mean
Yes
The total surface area is 6-10x larger than that of a sarcolemmal cylinder alone
Where are the T tubule networks found
Between the A and I band in mammalian skeletal muscle
In the Z band in amphibian skeletal muscle and mammalian cardiac muscle
What is the terminal cisternae
Where the sarcoplasmic reticulum comes into a close relationship with the T tubules
How are the SR and terminal cisternae arranged
The membranes or 2 membrane systems come into close proximity where 2 terminal cisternae sandwich a T tubule
This gives rise to a triad arrangement
How much of the tubular surface is triadic regions
70% (in frog muscle)
What happens when the tubular membrane in muscle is depolarised
It triggers release of intracellular calcium from the SR, elevating free cytosolic calcium concentration
This activates contractile proteins and initiates mechanical activity
What is the stimulation frequency required for tetanus is
a) slow muscle
b) fast muscle
a) 40 per second
b) 300 per second
Which ion channel is particularly common in muscles cell membranes
Are they equally popular on the tubular surface
Sodium
No they are less common there
How many types of potassium channels are there in nerve and muscle
1 type predominates in nerve
There are at least 3 types of K channel in muscle
Are inward rectifying K channels found in muscle
Yes
What are the 3 types of K channel in muscle
One is activated by depolarisation over a time course similar to that in nerve membranes
One is activated over a considerably longer time course of hundreds of ms
The final type is an inward rectifying K channel
What is the purpose of the muscle’s inward rectifying K channel
Minimise leak currents to reduce the amount of inward current required to sustain the plateau phase of cardiac APs
How does the resting skeletal muscle cell differ from a nerve cell in terms of conductance
Resting skeletal muscle shows a significant chloride conductance
What is the point of the Cl- conductance I’m resting skeletal muscle
Important in stabilising membrane potential between episodes of electrical activity
What is myotonia congenita
A deficiency of functioning Cl- channels in skeletal muscle which leads to unwanted repetitive AP firing
What are responsible for the inward currents that give rise to the regenerative activity in invertebrate muscle
Ca2+ channels
What phase of the cardiac AP is Ca2+ responsible for
The plateau phase
What may abnormalities associated with calcium permeability lead to?
Pathological changes in dystrophic muscle
What is important about fact that the T tubule lamina constitute a restricted extracellular space
Ions can accumulate or be depleted from here as diffusion with the rest of the ECF is v slow
What is excitation contraction coupling
The series of events that connect membrane depolarisation with myofilament activation
The T tubules are capable of only passive electrical conduction. True or false?
False
They themselves can also propagate action potentials
Are t tubules important in excitation - contraction coupling?
Yes they play a vital role in initiating and synchronising contractile activity through the entire cross section of muscle fibre
What does excitation contraction coupling begin with
The detection of the T tubular membrane depolarising by a voltage sensor
What is a voltage sensor
A modified calcium channel protein located in the tubular membrane of muscle
Its configuration varies steeply with membrane potential
What causes the voltage sensor to change conformation
What does this change lead to
Charge movements
Release of Ca2+ into the cytosol from the SR
In skeletal muscle, what does elevated cytosolic calcium reflect?
It almost entirely reflects the release from the SR (there is v little influx from the ECF)
In skeletal muscles where does the transduction of Ca2+ from the SR to cytoplasm occur
Why do we think this
At the triad
EM studies show foot processes joining the cisternae and tubular membranes in the triad complex.
What are the foot processes in the triad thought to be
Cytoplasmic components of the ryanodine receptor
Where are the intracellular portions of the ryanodine receptor found
What does this portion receptor act as?
Within the SR membrane
A calcium channel,
the cytoplasmic portion makes up most of the foot process
What are the foot processes on the SR close to
Structures embedded in the T tubule membrane, thought to be voltage sensors
How are changes in polarisation in the T tubule connected to the SR
Direct mechanical coupling between the ryanodine receptors and voltage sensors
What is the result of the coupling of the ryanodine receptors and the voltage sensors on the T tubule
The release of intracellular Calcium ultimately initiates mechanical activity through their binding to troponin
What is malignant hyperthermia
A genetic defect of the ryanodine receptor resulting in muscle spasms and excessive heat generation
What are the clinical manifestations of malignant hyperthermia usually triggered by
Halothane (a common general anaesthetic)
How is Ca re-sequestered after activation
Active transport of Ca2+ into the SR by a membrane Ca-ATPase
Where is the Ca-ATPase found
In the longitudinal regions of the SR membrane, remote from the terminal cisternae
Describe the Ca-ATPase molecule
Molecular weight of 100kD
Transports 2 Ca2+ for each molecule of ATP hydrolysed
Pumps calcium from the cytosol to SR, building up a thousand fold concentration gradient across the membrane
Is the Ca2+ just floating around in the SR
NO
a number of intra-luminal proteins sequester the luminal Ca2+
Eg calsequestrin
Name a luminal protein that sequesters luminal Ca2+
Describe it
Calsequestrin
Has a 1:45 binding ratio for Calcium
Occurs most abundantly in the terminal cisternae lumina
Why is sequestering required in the SR
To return cytosolic calcium concentration to levels below those required for significant troponin binding thereby ending the twitch
How big are cardiac muscle cells compared to skeletal muscle cells
Cardiac muscle cells are significantly shorter (~10 microns in diameter and 200 microns in length)
How are cardiac muscle cells linked
They are linked in a branched and end to end fashion by intercalated disks, producing a syncytium
Does the syncytium in cardiac muscle cells conduct electrical or mechanism a forces between component cells?
Both
At the ultrastructural level, cardiac muscle resembles which other muscle type
How are they similar and different
Skeletal
Both have a SR and T tubule system
Cardiac SR is less developed
The SR - T tubule complex is a dyad in cardiac but a triad in skeletal
What are the different functions of different types of cardiac muscle
Contractility
Or
Impulse conduction/
Impulse generation
Where are the cardiac muscle cells specialised for impulse conduction/ generation found?
SAN
AVN
Bundle of His
Give an overview of a cardiac action potential
There is an initial rapid depolarisation but then after the early overshoot, the ventricular membrane potential falls quickly to 0mV and remains here in a plateau
What are the 5 phases of ventricular action potential
Phase 0= very rapid depolarisation
Phase 1= initial Brief rapid repolarisation
2= the plateau
3= terminal repolarisation to Restore membrane potential to its resting potential
4= electrical diastole
What causes the initial rapid rising phase of an AP in cardiac muscle
The opening of Na channels
In cardiac muscle, what initiates excitation contraction coupling
What follows this
Prolonged inward Ca2+ currents
This produces a long lasting plateau phase
Where is the plateau phase in cardiac muscle particularly prominent
In the Purkinje and ventricular fibres
How long can a plateau phase be
Up to 500ms after the early upstroke
Does verapamil do
Name a similar drug
Blocks calcium channels, diminishing the amplitude of the plateau phase
Nifedipine
Discuss membrane resistance during the plateau phase
Membrane resistance is increased in the plateau phase as a result of inward rectification brought about by K channels at such voltages
What do inward rectifiers do during the plateau phase
Reduces inward current what would otherwise be required to hold the membrane potential at plateau level
This ultimately minimises the dissipation of calcium concentration gradients across the cell membrane
What does the repolarisation of cardiac APs result from
The gradual activation of further potassium channels
The outward current drives the membrane towards the resting level
How do APs adjust to heart rate
Why is this
The AP duration appears to adjust inversely to heart rate
To allow appropriate adjustment of the relative durations of systole and diastole in relation to changes in the interval between successive APs
What is the primary pacemaker
Why
SAN
These are the cells whose resting potential drifts towards the threshold at a faster rate
How often does the a) SAN b) AVN c) Purkinje fibres discharge
a) 60-80 times/min
b) 40-60
c) 30-40
When do the AVN or Purkinje fibres take over control of heart rate
These are secondary pacemakers so take over when there is severe SAN inhibition
Which contractile cardiac muscle cells produce pacemaker activity
cardiac muscle cells whose main function in contractile don’t usually produce pacemaker activity
What is the normal human heart beat frequency
How do the SAN cells maintain this
70 beats per minute
The membrane of SAN cells have a high background leak conductance that results in intrinsic firing
What does the SAN’s pacemaker role dictate
The rate and sequence of activation of different regions of the heart
When may the SAN function be abnormal
Sick sinus syndrome
What may cause congenital sick sinus syndrome
A mutation of the gene responsible for the formation of the α subunit of SCN5A (a sodium channel)
What does the AV ring do
Electrically isolates the ventricles from the atria - the AVN provides the only communication between them
What is the role of the AVN when the SAN works just fine
How does it perform this role
To synchronise the sequential atrial and ventricular contractions
Conduction through this node is slower (0.2 m/s) than through the remaining myocardium
Do cells in cardiac conducting tissues like the Bundle of His have myofilaments?
Yes but they have fewer, instead they show faster impulse generation and propagation than surrounding myocytes
What is the difference in conduction speed between His cells and normal cardiac myocytes
His: 2-5 m/s
Myocytes: 1m/s
What are the left and right bundle branches
Which is smaller
Two fans formed by the bundle of His immediately distal to the AVN
The right is smaller and the left divides into anterior/ superior and posterior/ inferior fascicles
What is the importance of the fascicles from the left bundle branch in the heart
Impulses from the AVN are conducted through the fascicles to the apex of the ventricles. Impulses are then propagated through surrounding myocardium at a slower rate.
What does the pattern of electrical conduction from the bundle branch fascicles result in
A ventricular contraction that optimises extrusion of blood as contraction begins at the apex and spreads to the base of the ventricles
When is cardiac muscle in the absolute refractory period
Why is this
I’m between the early rapid depolarisation to the point where the membrane potential is repolarised to ~-40mV
Inactivation of fast Na+ channels
When is the cardiac muscle in relative refractory period
From ~-40 to ~-80mV (complete repolarisation)
What happens to action potentials in cardiac muscle during the relative refractory period
Evoked APs have smaller amplitudes and rate of rise is conducted more slowly
How long is the relative refractory period
It is proportional to the duration of the action potential
order the following based on length of refractory period, from smallest to largest:
Ventricular, atrial, Purkinje fibre
Atrial< ventricular < Purkinje
Refractory period is directly proportional to heart rate. True or false?
False
The refractory period is inversely proportional to heart rate
a) Why do we have a longer refractory period on the heart
b) What would happen without this
a)To ensure the heart pumps rhythmically at appropriate intervals
The duration of the refractory period in a normal heart allows the impulse from the SAN to propagate throughout the entire myocardium just once
b) re-entry arrhythmia would occur
How can re entry arrhythmia be treated
By lengthening the refractory period by pharmacological means
Is tetanus possible in the heart?
Why is this?
No the refractory period is too long
It would be detrimental to heart function, in contrast to the situation in skeletal muscle
What is the resting potential of smooth muscle
-50 to -70mV
Name organs where you may get smooth muscle showing slow AP firing
Vas deferens
Uterus
Name organs where you may get smooth muscle showing plateau potentials
Stomach
Ureter
Name an organ where you may get smooth muscle showing rhythmic oscillations in membrane potential
Intestine
What is the effect or tetrodotoxin on smooth muscle
Tetrodotoxin does not block smooth muscle spiking activity as it is produced by VG Calcium channels
What is the important of the higher surface area to volume ratio in smooth muscle compared to skeletal muscle
For smooth muscle, it makes the entry of external calcium more important whereas Ca2+ release from the SR is more important in skeletal
What initiates mechanical activity in cardiac muscle
Increased [cytosokic Ca2+] following membrane depolarisation, resulting in calcium binding to troponin
How many calcium binding sites in cardiac troponin
3
What is the most important source of activator calcium in cardiac muscle
From the SR in response to T tubule depolarisation
Why is extracellular calcium still important in the heart
Calcium entry provides the stimulus from calcium release from the SR
Thus doubling extracellular calcium will nearly double maximum cardiac contractile force
Give 3 differences between the mechanical activation of skeletal and cardiac muscle
1) amount of calcium released
2) how they modulate the strength of contraction
3) the effect of calcium influx
How does the amount of calcium released on activation differ between skeletal and cardiac muscle
In skeletal the release of intracellularly stored calcium is well in excess of that required fro maximal contraction
In cardiac, the amount of intracellular calcium released is not supramaximal but is closely influenced by factors which influence inotropy
How does skeletal muscle modulate contractile strength
By varying recruitment of activated fibres
How does cardiac muscle vary strength of contraction
Intercalated disks link all muscle in a syncytium so all muscles cells must be activated. Therefore the strength of contraction is regulated by the amount of Calcium made available
How are cytosolic calcium levels maintained in cardiac muscle between APs
Both surface and SR membranes have calcium ATPase pumps which move Ca2+ into the ECF or SR lumina
Sodium - calcium exchange drives efflux of calcium across the surface membrane
How does the Ca - Na pump work in cardiac muscle
It utilises the energy from the influx of Na down the electrochemical gradient, which was established by a Na/K pump
How does digitalis and other cardiac glycosides work
Used in management in cardiac failure
They block the Na/K pump so intracellular sodium increases, decreasing its electrochemical gradient, making it harder for the Ca/Na exchanger to remove Ca
Intracellular calcium builds up, increasing contractile force
The passive tension-length relationship is much steeper in cardiac muscle than skeletal. True or false?
What does this mean
True
Even resting cardiac muscle is considerably more resistant to stretch than skeletal
Active shortening of cardiac muscle takes place on what part of the passive length tension curve
Almost entirely on the part where increased stretch leads to increased contractile force
What law may the steep passive length- active tension relationship of cardiac muscle explain
The Frank Starling Law
Which 2 ways make the heart a self regulating pump thanks to the Frank Starling Law
Both with respect to demands from the peripheral circulation and in balancing the pumping by the left and right sides of the heart
What is the Bowditch effect
When myocardial contractility increases with heart rate
What is the Anrep effect briefly
Myocardial contractility increases with afterload
Which neurotransmitter slows heart rate
How does it do this
ACh
Increases membrane K conductance which hyperpolarises the membrane of the SAN during diastole
Which neurotransmitter reduces cardiac contractility
How
ACh
By resting the low inward calcium current
Name 2 sympathetic transmitters of the heart
Noradrenaline
Adrenaline
How do noradrenaline and adrenaline increase heart rate
They increase the rate of pacemaker depolarisation during diastole and in hyperpolarisation early in diastole
Also increase inward calcium current that contributes to the inotropic effect exerted by catecholamines on cardiac function
What does the action potential look like after the effect of adrenaline
Shows a higher but shorter plateau
What does the ECG waveform record
The changes with time in potentials in the body surface caused by changes in the summated cardiac polarity brought about by electrical events in the myocardium
What does a positive deflection on an ECG generally denote
Generally when a depolarising impulse is conducted towards the electrode or if a repolarisation wave is propagated away from the electrode
Why are atrial deflections on an ECG smaller
They have smaller muscle mass
The P wave is equivalent to an atrial QRS complex. True or false?
True
How big should a P wave be
What if it is bigger than this
<0.12s wide and <0.3mV high
Suggests atrial enlargement
What should the PQ interval look like
What if it is not
Should be consistent and between 0.12 and 0.24s
If too short: accessory pathways present
If too long: diagnostic of first degree heart block
What should the QRS complex look like
What if it doesn’t
<0.12s
If too long: intraventricular conduction defect, including left and right bundle branch block
The sum of deepest QS and tallest R <3.5mV
If greater than this: left ventricular hypertrophy
What is the cardiac cycle length
RR or PP
How long should the QT interval be
But
Less than 0.45s
But it is frequency dependent
If too long: Long QT Syndrome (may be secondary to drug treatment)
The ST segment is usually isoelectric with normal QRS complex. True or false
True
What are the 3 major diagnostic categories for which the ECG is useful
Conduction disorders
Rhythm disorders
Disorders of Myocardial metabolism
Discuss disorders or rhythm
Arrhythmias May originate in the atria (better tolerated) or in the ventricles
Result in disrupted PQRST sequence
When does the muscle relax
When calcium influx stops
Other than VG calcium channels, what are other sources of intracellular calcium elevation ?
Receptor-operated calcium permeable channels mediate calcium influx (these are activated by hormones or NTs)
PLC catalysed formation of IP3 from PIP2. IP3 opens calcium channels in the SR
