Neurons (Cahill Stuff) Flashcards
What is the Davson- Danielli model
That cell membranes had a 3 layered structure: protein, bilayer, protein
Based on electron micrograph a
What happened in 1966
Branton used freeze-fracturing to split cell membranes and proteins were seen through the fracture plain
Experimental evidence for fluid mosaic model
Distinctly Labelled mouse and human proteins were combined in a hybrid cell, the surface proteins became intermingled
(Frye and Edidin, 1970)
What are the two main kinds of non covalent interactions in the fluid mosaic model
Hydrophobic and hydrophilic
What does the value of the potential difference across a membrane reflect
The concentration of unbalanced charge
Is the inside of a cell more or less negative
Inside is more negative
How does a cell membrane act as a variable resistor
It contains channels that allow a flux of ions under specific conditions
How is electrical potential across a membrane measured
A micro electrode is inserted within the neuron (usually in the soma) and a second is placed in the extra cellular fluid
By Whom and when was the ionic basis of resting potentials put forward
What was he aware of
Bernstein, 1902
That intracellular K> extra cellular
And that the membrane was more permeable to K than any other ion
What is the equilibrium potential
A balance between concentration gradient and charge gradient
What can’t electrodes do that means the Nernst equation is needed
Can’t tell us which ions account for the charge recorded
What are the assumptions in the Nernst equation
For a single ion
Completely permeable membrane to that ion
Ion in equilibrium
What is the Nernst equation
E=RT
-— x ln([ref]/[test])
zF
Does reference= out or in
Out
What is Nernst equation simplified to at room temperature and at body temperature
58xlog([out]/[in])
Body: 61.5log([out]/[in])
How did Hodgkin and Horowicz realise other ions contribute to the potential
At high [K] the results agreed with the Nernst equation but at lower concentrations, the results deviated
Give the Goldman Hodgkin Katz equation
RT Pk[out]+PNa[out]+PCl[in]. — x ln ———————————
F Pk[in]+PNa[in]+PCl[out]
What is the pump ratio for K and Na ions In the K/Na pump
3 Na ions are pumped out for every 2 K ions pumped in
What does the Na/K pump require and what can it be describe as
Hydrolysis Of ATP
Electrogenic
What is the equivalent of a battery in a neuronal circuit
Na/k pump
Stored potential that is maintained
What is the equivalent of a resistor in a neuronal circuit
Ion channels
Prevent or allow flow of ions in and out of a cell
What is the equivalent of a capacitor in a neuronal circuit
Cell membrane
The charge separation across it
Hyperpolarising current pulses only ______ changes in membrane potential
What does this mean
Passive
They decay over time
Small depolarising pulses elicit only _____ responses
Passive
When does a depolarising current produce an action potential
If it meets or exceeds a threshold, hence why they are all or nothing events
What does increasing the strength of a current do for an action potential
No effect on amplitude
Number of spikes increase
What happens if you apply a voltage to a capacitor
What happens when the voltage is removed
It begins to store energy until it reaches maximum capacity and stays there
The capacitor discharges it’s voltage across the resistor until it has been fully discharged
What happens if a circuit has a resistor but no capacitor
When power is applied the voltage but change to its steady state value and when removed it would then drop to zero immediately
How does the capacitor resist drastic changes
It takes time to charge and discharge
What does τ (tau) measure
The time constant
How quickly the neuron’s potential decays back to the resting state
How long is τ exactly
How long it takes a potential to get to 1/e of its original value
(63% of original value)
What equation describes the rise of voltage
V= Vmax(1-e^(-t/τ))
What equation describes the decay of voltage
V= Vmax x e^-t/τ
τ=?
Rm x Cm
What are the 2 major sources of resistance ions need to overcome in the axoplasm
Fibre length
Lack of membrane resistance (too much leaking)
What is the length constant
λ
An index of how well a sub threshold potential will spread along an axon as a function of distance
What does it mean to have a length constant of 1mm
1mm away from the axon cell body, 37% of the voltage remains
What is the actual equation for the decay of the potential with distance
V=Vmax(e^-x/λ)
What actually does λ=
Why can Ro be ignored
λ= sqrt (Rm )
(———- )
(Ro + Ri )
Ro«
What ion is the membrane of a resting neuron most permeable to ?
K
What did Bernstein suggest about an action potential
What did the peak of the AP represent here then
Who noticed he was wrong
Due to a loss of membrane selectivity
The point of maximum conductance so would approach 0 mV
Cole and Curtis (1939)
How does capacitance change in an AP
It doesn’t
How did Hudgkin and Huxley experiment with APs originally
What did they observe
Modified the concentration of sodium by adjusting the concentration of salt water
The AP and overshoot became slower the more they reduced the Na
What did Huxley’s calculations about Na predict
The peak AP amplitude would track the Na equilibrium potential
How does improving insulation improve passive current flow
Reduced leakage
How does myelin affect the long motor neuron in the leg, speed wise
Without: speed= 1m/s
With: speed= 10-100m/s
What do the Glia do in nodes of Ranvier
Secrete specialised proteins that recruit Na channels to the node
What were Rushton’s predictions relating myelin and diameter
Myelinated: velocity is proportional to diameter
Unmyelinated: velocity is proportional to the square root of the diameter
What was the general rule of thumb that Erlanger and Gassers discovered in the 1920-30s
Larger diameter neurons are heavily myelinated, have low thresholds and are rapidly conducting
Unmyelinated have high thresholds and conduct slowly
Name some symptoms of MS
Dizziness, visual problems, headaches, neurological problems that persist for days then remittance
What happens due to demyelination in MS
Conduction is slowed and Na channels spread outward from nodes
How does a current clamp work
Experimenter injects a current in a ‘step’ fashion and measures how it impacts the membrane potential using a voltmeter and amplifier
Can be used to look at subthreshold stimulation forms of passive membrane currents
What was Hodgkin’s hypothesis concerning conductance of Na
Why Na
At a certain membrane potential, conductance (g) of Na increases
At resting potential thete is a high driving force acting on it
What does the voltage clamp amplifier do
Clamps the membrane potential at a set level and can detects the difference in current that needs to be injected to stabilise that level
(This required current is proportional to the conductance according to Ohm’s law)
What did H and H record from voltage clamp experiment
As the membrane was depolarised there was an inward [Na] dependant current
K conductance was found to lead to a time dependent outward current
How can the change in current with PD by shown
What drugs will change this graph
Am I-V curve
Any treatment that alters ionic conductance will alter its shape and slope
What can the slope of the I-V curve be used to calculate
Conductance
When does the IV curve pass through the voltage axis for a channel specific to ion X
At X’s Nernst equilibrium potential
What did the toxin TTX demonstrate
Why
Slow outward current was mediated by K
It blocks Na voltage gated channels
When are leak and gated channels important
Leak- to set resting potential
Gated- in active changes
In a channel how many helices are in each subunit
Which form the pore
Which segment is charged and how do prevalent models suggest this moves upon depolarisation
6 transmembrane domains
S5&6
S4 - translocated from inside to outside the cell
Describe the helical screw model
Depolarisation causes S4 to rotate and twist up through membrane, changing the exposure of charges from the intra- to extra cellular solution
Describe the paddle model
S4 makes a large translation such that the most extra cellular charge goes from exposed to the extra cellular medium in the open state to completely buried in the bilateral in the closed state
What happens when the channels open upon depolarisation
Initially a few Na VGC open, increasing the amount of Na entering the cell, continuing to depolarise the membrane, triggering more Na VGC to open
When do Na VGC become inactive
It is time dependent- they are inactive after a set time from the VGC opening
The cell is now impermeable to Na, contributing to the refractory period
When are the Na VGC closed
When the membrane repolarises
Why is an action potential self supporting
Positive feedback loop of Na VGC opening. This also contributes to the all or nothing quality
What is the delayed rectifier
The Na VGC take 1ms to open upon depolarisation. This delay and the fact that K conductance serves to rectify the membrane potential.
This delay regulates the relative refractory period
What does the refractory period do
Limits number of APs in a given nerve per unit time and prevents back propagation of APs
What do TTX and TEA stand for and what do they do
TTX- tetrodotoxin
- binds to and blocks the Nav channels
TEA- Tetraethylammonium
- targets Kv channels
What is a channelopathy
Disease caused by disturbed function of ion channel subunits or the proteins that regulate them
What is myokymia
Reduced activity of K channels which delays AP repolarisation and lowers threshold, producing abnormal spontaneous AP generation
What is myotonia
Mutations in Na channels cause abnormal tendency for channels to open later or more persistently after depolarisation
Residual Na entry reinitiates the cycle of depolarisation
What is a Gap Junction
They are formed when 2 cells in close proximity that have channels allowing the propagation of an impulse almost directly
How are gal junctions structured
Built from 6 connexin subunits that comprise one connexon, then two connexons comprise one gap junction
2 advantages of electrical transmissions
Speed
They favour synchrony when firing
Advantage of chemical transmission
Allows for more specificity
When and how did Dale shoe transmission could be chemical
1913
Injecting acetylcholine into a cat heart, inhibiting the heartbeat
A purified substance produced the same effect as parasympathetic nerve stimulation
Describe Ringer’s heart experiments
2 beating frog hearts isolated in containers of Ringer’s Solution
The vagus nerve (PNS) was stimulated in heart A, slowing its beat. Fluid from A moves to B, slowing the beat of heart B
What did Ringer’s experiments show
A soluble substance released by the vagus nerve modulated heart rate, which he called Vagusstoff but is actually ACh
Where are NT synthesised and stored
In the axon cytosol
Stored in synaptic vesicles
What happens when an impulse reaches a presynaptic terminal
Depolarisation causes opening of VG Ca channels
Then vesicles can fuse with the presynaptic membrane and release their load
How and why are intercellular [Ca] levels kept low in neurons
So there is a strong driving force for it to enter once channels open
Transporters and pumps are recruited
Eg Na/Ca exchanger and Ca pumps
How is the VG Ca channel deactivated
Ca dependant inactivation site providing negative feedback to close the channel when [Ca] increase
What do SNARE proteins do
They’re on the synaptic vesicles and plasma membrane, and form a complex. This acts like a zip, bringing together the two membranes.
Ca then binds to synaptotagmin causing the cytoplasmic region of this protein to insert into the plasma membrane, bind to SNAREs and catalyse fusion and exocytosis
Do neurons need to recover vesicles
Yes
They are recovered via endocytosis and the recycled vesicles is refilled with NT
What is Dale’s principle
A given neuron produces only one type of neurotransmitter (now disproved as co-transmitters, nerve terminal with multiple NTs, have be shown to exist)
For ACh in a NMJ, give the following:
a) synthesis
b) vesicles
c) Release
d) action
e) inacitvation
a) ChAT
b) Vesicles
c) Ca Dependant
d) nAChR
e) AChE
What are ionotrophic receptors
Ligand gated channels
Give an example of metabotrophic receptors
GPCRs
Give an important specialisation of the NMJ
It’s size- it is one of the largest synapses in the body
How does the motor end plate increase surface area
It has many folds
Give the resting potential and threshold of the motor end plate
- 100mV
- 65mV
Give the experiment that shows that the presynaptic signal for full EPPs is Ca dependant
If NMJ is bathed in a solution of low [Ca], stimulating the neuron evokes EPPs reduced to the size of MEPPS
How could you show post synaptic signals are dependent on the nicotinic subtype of AChR
The antagonist, Curare, can reduce the EPP
What type of receptor are NAChR in the NMJ
What about in the heart
Ionotropic
Muscarinic AChR
When is the membrane potential ~0mV
When it is equally permeable to Na and K ie when many channels are open
What happens if membrane potential is <0mV before ACh was applied
What do you call the critical value when the flow direction changes
The direction of flow through ACh-gated ion channels is inward and vice versa
Reversal potential (which is 0 in this case)
What is driving force
How different the membrane potential is
Vm-Erev
Are nAChR permeable to K or Na
What does conductance depend on
Both
How many channels are open which depends on [ACh]
What is the Quantal Transmission Hypothesis
Give its two main observations
EPPs are made of individual units, each equivalent of an MEPP
- NT is released in multimolecular packets
- Response EPPs fluctuate in steps of unit size corresponding to the size of mEPP
What is a simplistic way to determine the possible quantal content of an EPP
If you divide an EPP by an mEPP
How did Katz analyse the frequency of occurrence of MEPPs with respect to amplitude
Poisson Statistics
What is myasthenia Gravis
How are MEPPs affected
A chronic autoimmune neuromuscular disease that targets the nAChR, disrupting how the cells respond to synaptic transmission
Their amplitude is Greatly reduced
How do you treat myasthenia gravis
AChE inhibitors to prolong ACh activity
Are post synaptic potentials bigger in the brain or in the NMJ
Why
Brain PSPs are much smaller
In the CNS, PSPs can be fast acting and short-lived, triggered by iontropic receptors or slow and long lasting driven by metabotropic receptors
How can you measure the current produced by an EPP
Patch Clamp which measures discrete patches of membrane and the channels there-in
How do patch clamps work
The transmembrane potential is controlled by the patch clamp amplifier, the extra cellular fluid is considered the “ground/reference”
What is the outside-out patch
What does this allow
A configuration of the patch clamp where the extra cellular surface is on the external side of the pipette tip
Study of drug action on receptor surface
What can single channel recordings with the patch clamp allow
Study of unitary conductance and kinetic behaviour of ionic channels
This has shown that individual channel opening and closing are sudden, all or nothing events, triggered by ligand binding causing a conformational change to another
What muscle is contracted in the knee jerk reflex and what causes it
What kind of reflex is it
Quadriceps femoris
Hitting the patellar tendon
Protective reflex
How does hitting the patellar tendon cause a reflex action
The pressure stretches the muscles spindles causing contraction
What does an intact knee jerk indicate
What is the reflex arc
Integrity of the L2-L4 vertebral segments
Sensory efferent neurons project through dorsal root ganglion and synapse directly with the effector extensor motor neuron that projects back to the quadriceps muscle
What muscles are indirectly affected by the knee jerk reflex
The hamstring is inhibited by inhibitory interneurons which is necessary to avoid both muscles contracting simultaneously against each other
Are post synaptic potentials active
No they decay exponentially
What does the magnitude of a PSP depend on
The number of open ion channels which in turn depends on amount of NT released
How do the EPSP and ISPS act
EPSP -brings a membrane potential closer to threshold
IPSP - hyper-polarises the cell away from threshold
What did Brock notice about IPSP’s in the 1950s
Reversal only occurred when intracellular microelectromechanical contained KCl, suggesting Cl- is necessary for reversal
What is released from the presynaptic terminal of the sensory neurons in the extensor motor neuron of the patella reflex
What does it bind to
Glutamate
AMPAR
What does AMPAR activation lead to
Depolarisation of post synaptic cell via conduction of cations like a ligand gated ionotropic receptor
What do glycine receptors conduct?
Are they voltage gated?
Conduct Cl- and therefore lead to hyperpolarisation
No they are ligand gated ionotropic receptors
Is temporal summation active
It is strictly passive
What is the spatial summation influenced by
The space constant
What is equilibrium potential of an ion
The voltage that would occur if the membrane were fully permeable to that ion
What determines the threshold for an action potential
The sodium current exceeding the outward potassium current
What is the greatest delay at the NMJ
Ca2+ accumulation
Which band shorten during contraction
A stayed the same length H and I bands shorten
What are the effects and noradrenaline on the heart
Phosphorylation of phospholamban, troponin I and phosphorylation of calcium channels in the p.m.
What is the most important factor influencing brain bloodflow
Metabolic autoregulation
What determines maximum expiratory flow rate primarily
Elastic recoil of the lungs
What regulates alveolar ventilation when breathing normal air at sea level
PCO2
Contraction of the diaphragm and intercostal muscles does what to intrapleural pressure
Intrapleural pressure becomes more negative
How do glucose, galactose, and fructose cross the luminal border of the small intestine
Glucose and galactose: SGLT1
Fructose: GLUT5
All exit via GLUT2
