Cellular Electrophysiology & Nerve Flashcards
what are the roles of the cell membrane?
compartmentalisation, control of fluxes, attachment of enzymes/cytoskeleton/secondary messenger models, sensory receptors, binding sites for hormones, secretion by exocytosis, uptake by pinocytosis, endocytosis and phagocytosis
what are the variables controlled by homeostasis?
concentration of ions, control of chemicals, physical characteristics of blood, concentration of blood gases, number of RBCs and WBCs, metabolic rate, body weight, bone density, muscle mass, temperature, heart rate
what is gain?
amount of output signal per unit error
what is a high gain?
large output from a small error
what is a low gain?
small output from a large error
what is feed-forward control?
predicted change in output necessary to maintain a constant level
what is the cephalic phase of insulin release?
feed-forward stage, when food smelt/enters mouth insulin released- minimises rise in blood glucose
what are the properties of hormones?
released into blood from endocrine glands, low concentrations so small changes have large effect, high specificity of binding, signal from 1 cell type to another
what are the types of hormone receptor?
GPCRs and RTKs
what type of receptor is the glucagon receptor?
a GPCR
what type of receptor is an insulin receptor?
a RTK
what is an ionotropic receptor?
coupled to ion channels, opens ion channel, used by fast neurotransmitters
what is an autocrine signalling molecule?
signalling molecule released by same cell type it acts on
what is a paracrine signalling molecule?
signalling molecule that acts locally via extracellular space on different cell type
what is the adenohypophysis?
anterior pituitary
what does the hypothalamus release in the cortisol release system?
CRH (corticotrophin releasing hormone)
what does the pituitary release in the cortisol release system?
ACTH (adrenocorticotropic hormone)
what does ACTH do?
inhibits CRH release, stimulates cortisol release and MSH release
what causes CTH release from the hypothalamus?
stress, starvation
what does CRH do?
stimulates ACTH release
what releases cortisol?
the adrenal cortex
what does the adrenal cortex do in cortisol release?
releases cortisol
what does cortisol do?
increases blood glucose, reduces inflammation, inhibits CRH and ACTH release
what happens in Addison’s disease?
adrenal gland can’t produce enough cortisol - leads to weakness, fatigue and hypotension, less inhibition so CRH and ACTH levels rise
what does MSH lead to?
more melanocytes so more pigmentation
what cause veterinary Cushing’s
excess ACTH from pituitary gland causes high cortisol levels
what are the properties of water? (6)
high boiling point, density decreases when freezes, high specific heat capacity and latent heat of vaporisation, electrical dipole, dissociates spontaneously and reversible, can act as solvent for ions
what is charge?
quantity of electricity (measure in coulombs) due to atoms/molecules that have gained or lost electrons
what is voltage?
difference in charge relative to another region (in volts)
what is capacitance?
amount of charge stored per volt
what is current?
number of charged particles flowing past a point per unit time
what is resistance?
restriction of flow of current
what is Ohm’s law?
voltage= current x resistance
what is resting potential?
potential across cell membrane at rest
what are excitable cells?
cells that can change their state from resting to excited and back again using bioelectricity
what is the cell membrane of nerve cells and muscle fibres composed of?
lipophilic phospholipid bilayer, transmembrane proteins that form ion channels and pumps and allow charged ions to move across the bilayer
what is a capacitor?
2 conductors separated by a non-conductor with different numbers of ions
what does membrane potential arise from?
different numbers of positive/negative charges across the membrane
what is a high resistance environment in terms of cell membranes?
no/closed ion channels
what are the 2 forces that determine movement of ions in solutions?
charge gradient and concentration gradient
when is ion flux at equilibrium?
when charge gradient and concentration gradient are balanced
what is the equilibrium potential of an ion?
the membrane potential at which the ion is at equilibrium
what is the Nernst equation used for?
to find the equilibrium potential of an ion
what is needed for the Nernst equation?
ideal gas constant (R), temperature (T in K), charge of the ion (Z), Faraday’s constant (F), [C] outside (ref) and inside (rest) (C is ion)
what are the assumptions of the Nernst equation?
only 1 ion at a time, membrane completely permeable to ion, ion at equilibrium
what is the Nernst equation at room temperature (20 degrees)?
E= 58 x log([ion]out/[ion]in)
what is the Nernst equation at body temperature?
61 x log([ion]out/[ion]in)
what are the typical ion concentration of mammalian neurons inside vs outside?
high [K+], low [Na+], [Cl-], [Ca2+] inside vs outside
in resting state what ion is primarily moving across neuron membranes?
K+
in active state what ion is primarily moving across neuron membranes?
Na+
what provides the driving force for ions to move across membrane?
difference between potential and equilibrium potential
what are microelectrodes?
recording devices attached to a voltmeter
what is resting membrane potential usually between?
-60 and -80mV
what dis Hodgkin and Keynes find when comparing resting membrane potential to the Nernst equation for K+?
fit very well at high [K+], deviated at low [K+]
what did Hodgkin and Keynes suggest as an equation for membrane potential?
Goldman equation, taking into account Na+
what does the Goldman-Hodgkin-Katz equation account for?
K+, Na+, Cl-, relative permeability of the membrane to the ions
which is the most permeant ion across the cell membrane at rest?
K+
why do K+ ions tend to move out of the cell?
high conc in cell at rest, driving force to reach equilibrium potential which is around -90mV
what is the equilibrium potential of K+?
around -90mV
what is the equilibrium potential of Na+?
around +50mV
what are the changes in membrane potential during an AP
starts at around -70mV, increase to -55mV, rapid increase to around +40mV, decrease to almost -90mV, return to around -70mV
what experiment can be used to record voltage signals during an AP?
current clamp
what does axonal malfunction arise from?
injury, degeneration, demyelination
what is the time constant of a membrane?
how long it will take for the membrane to get to 1/e of its original charge/time to charge capacitance to 63%
what does the time constant depend on?
membrane resistance and membrane capacitance
how is the time constant calculated?
membrane resistance x membrane capacitance
what does a larger time constant mean?
changes in membrane potential will be slower
what is length constant?
distance over which voltage decays to 37% of its initial value
what is length constant dependent on?
membrane resistance, extracellular resistance (assumed to be negligible), intracellular resistance
what does a smaller length constant mean?
voltage signal decays over shorter distance
how can length constant be improved?
myelination
how does myelination improve the length constant?
increases membrane resistance
what is myelination in the PNS?
Schwann cells which wrap membrane around axon to form 20-160 concentric layers of membrane
what is myelination in the CNS?
oligodendrocytes wrap membrane around axon
what is between the segments of myelination?
unmyelinated nodes of Ranvier
how does myelination increase membrane resistance?
ions less likely to leak out of the membrane
why are action potentials ‘all or none’ events?
if threshold potential is reached, an action potential will occur, if not then it won’t, size of action potential not changed by size of stimulus
what is the absolute refractory period?
state when sodium channels cannot be opened as they are in inactive state, so new AP can’t be generated
what does the absolute refractory period do?
prevents transmission of impulse in both directions
what causes the relative refractory period?
K+ channels are delayed in closing so hyperpolarisation of membrane occurs- greater depolarisation required for new AP as further from threshold value
what does increasing size of stimulus current do?
increases frequency of action potentials
where are voltage gated ion channels concentrated in myelinated axons?
at Nodes of Ranvier
what is saltatory conduction?
rapid jumping of current from node to node
how are axons classified?
speed of conduction, myelination, diameter of axon, function
what is the general rule for characteristics of large diameter neurons?
heavily myelinated, low thresholds for electrical stimulation, rapid conduction
what did Cole and Curtis observe in 1939?
AP clearly over-shot 0mV and change in membrane resistance during AP
what did Hodgkin and Huxley find about the AP using squid giant axon?
AP overshot to around +40mV, sodium movement responsible for AP
what are examples of conditions involving damage to axons?
prolapsed intervertebral disc, poliomyelitis, motor neurone disease, hypo-myelinating neuropathy
what is the C fibre afferent?
type of sensory afferent fibre that is important for alerting brain to harmful stimuli
what are local anaesthetics?
class of drugs that can cross membrane and block VGNaCs from inside- shut down APs in pain fibres
why do voltage gated ion channels have selective permeability?
due to amino acids
what are the reversal potentials for closing VG ion channels?
the Nernst equilibrium potential for the permeant ion
what is capacitive current? (voltage clamp experiment)
current needed to be injected
what is the command voltage? (voltage clamp experiment)
the membrane potential decided by the experimenter
how is the command voltage reached in the membrane? (voltage clamp experiment)
microelectrodes record actual membrane potential, compare it with command voltage, then inject current into axon to change it to command voltage
what is the recording taken in the voltage clamp experiment?
how much current had to be injected to reach command voltage- and if positive or negative- shows current passing through the open membrane channels at the command voltage
what is the current injected equivalent to when command voltage is made more positive in voltage clamp experiment?
the current moving across the membrane during the depolarisation phase of an AP
what is seen in the voltage clamp experiment?
rapid capacitative current (current injection), rapid transient inward current, delayed outward current
what is seen in the control experiment for voltage clamp experiment (membrane potential changed in opposite direction by same amount)?
only capacitative current and no ionic current
what does voltage clamp experiment prove?
currents ‘gated’ by specific voltage change
what suggested that the rapid transient inward current in the voltage clamp experiment may be due to Na+ ion flux?
initially got larger with more depolarisation, as membrane potential approached +50mV (equilibrium potential of Na+) started to get smaller
what suggested the delayed outward current was due to K+ in the voltage clamp experiment?
increased with more depolarisation suggesting equilibrium potential of ion moving was more negative
how was the importance of Na+ for the transient inward current proved?
by removing the Na+ in the seawater bathing the axon or by blocking the VGNaC conductance using tetrodotoxin
how was the importance of K+ for the delayed outward current proved?
by using TEA to block VGKCs- can’t change intracellular K+ concentration
what are channelopathies?
clinical conditions associated with non-functional mutated ion channels
what is the structure of the VGNaC?
single α-subunit polypeptide with associated β-subunits
what key functional features are present in the α-subunit of VGNaCs?
sensitivity to voltage, selectivity to Na+ ions
how many repeating domains do VGNaCs have?
4
how many membrane spanning regions do the domains of VGNaCs contain?
6 (S1-S6)
how are the 4 domains of VGNaCs arranged?
around a central ion channel pore
where is the voltage sensor located in VGNaCs?
around S4
what is the ion channel pore formed from in VGNaCs?
the linker sequence between S5 to S6
how long after opening will the VGNaC inactivate?
1ms
what region of the VGNaC determines selectivity?
loop between S5 and S6
how is the intracellular pore of VGNaCs opened?
when S4 detects voltage S5 moves, opening the pore
what happens in the inactivated state of the VGNaC?
intracellular loop blocks channel
what causes a positive feedback cycle of VGNaCs opening?
if nearby VGNaCs are close enough for the resulting depolarisation from 1 opening to be at threshold value they will also be opened
what is the difference between the structure of VGNaCs and VGKCs?
the α-subunit in VGKCs is equivalent to a single domain of the VGNaC so 4 α-subunits are needed to form VGKCs
what were the 2 theories of transmission between neurons and targets?
electrical and chemical
what did Dale and colleagues show in 1913?
IV injection of ACh inhibits heartbeat of cat, as does parasympathetic nerve stimulation, suggesting parasympathetic nerve stimulation could be chemical
what did Otto Loewi find in 1921?
put 2 beating hearts in 2 containers filled with Ringer’s solution. stimulated vagus nerve in one, heart rate slowed. took solution from this container, put in other container- heart rate of other heart slowed- seemed a substance released by vagus nerve slowed heart rate- chemical
who used microelectrodes to directly record chemical synaptic transmission at the NMJ?
Katz
where are neurotransmitters stored?
in vesicles in the presynaptic axon terminals
what is an advantage of chemical over electrical transmission?
diversity of neurotransmitters and their receptors in the post-synaptic membrane produces different types of response
what does an AP cause when it reaches the axon terminal?
activation of VGCaCs so rapid influx of Ca2+
what is the structure of VGCaCs similar to?
VGNaCs
what is intracellular concentration of Ca2+ like and why?
kept very low by buffers/transporters/pumps
what are examples of proteins that keep intracellular Ca2+ concentration low?
the Na+/Ca2+ exchanger, the calcium pump (pumps 1 Ca, 2 H ion out, uses 1 ATP)
what does influx of Ca2+ cause?
the neurotransmitter containing vesicles to fuse with the presynaptic membrane and release the neurotransmitter by exocytosis
how is the vesicle membrane recovered after neurotransmitter release?
endocytosis
what are electrical synapses called?
gap junctions
how are gap junctions formed?
a connexon on the pre and post-synaptic membrane form a gap junction channel, conducts ions if there is depolarisation in 1 of the neurons to the 2nd neuron
what forms a connexon?
6 connexins
what enzyme is used to synthesis ACh?
choline acetyltransferase (ChAT)
what are end plate potentials?
depolarisations when ACh binds to receptors at the post-synaptic region of the muscle cell in the NMJ
what do EPPs cause?
depolarise the muscle membrane enough to open VGNaCs in the muscle cell membrane causing a muscle AP
why will an AP in the motor neuron reliably produce an AP in the muscle cell?
AP in the motor neuron causes the release of sufficient ACh to open VGNaCs in the muscle cell
what enzyme inactivates ACh in the synaptic cleft?
acetylcholinesterase (AChE)
what are MEPPS?
miniature EPPs- represent the smallest unit of chemical synaptic transmission- spontaneous small depolarisations at endplate when motor neuron not being stimulated
what is the quantal theory of synaptic transmission based on?
neurotransmitter is released in parcels (quanta), giving rise to MEPPs; EPP amplitudes correspond to the sum of MEPPs
what is 1 quantum (MEPP) equivalent to?
1 vesicle of neurotransmitter
what is the model for number of MEPPs?
number of quanta x probability of release
what happens if the NMJ is bathed in low Ca2+ solution?
EPPs are much smaller in amplitude- fewer quanta released
what is the post-synaptic receptor for ACh on the NMJ?
nicotinic ACh receptor
what is curare?
antagonist of the nAChR- reduces amplitude of EPPs
where are nAChRs found in the post-synaptic membrane?
junctional folds
what is the function of junctional folds?
increase the SA available for synaptic transmission at the NMJ
what sort of protein is the nAChR?
ligand-gated ion channel
what does ACh binding to the nAChR cause?
opening of an integral ion channel pore in the receptor protein
what is the pore of the nAChR permeable to?
Na+ ions and K+ ions
what is the sodium pump coupling ratio?
3Na pumped out for 2K pumped in
what is the resting MP of the endplate of a muscle cell?
around -100mV
what is the threshold for an AP in skeletal muscle?
around -65mV
where are VGNaCs located in the endplate?
at the bottom of junctional folds
what is myasthenia?
muscle weakness
what is an autoimmune cause of myasthenia gravis?
immune system attacking the nAChR
what is the main treatment for autoimmune myasthenia gravis?
prolonging duration of ACh activity in the synapse
what is a large difference between electrical and chemical synapses?
in electrical membrane of the 2 neurons very close together, in chemical larger synaptic cleft
what is a motor unit?
1 motor neuron plus all the muscle fibres it stimulates
what is a neuromuscular junction?
synapse between motor neuron and skeletal muscle end plate
what is the electron microscopy evidence for the quantal theory of synaptic transmission?
vesicles shown near membrane at rest and fusing with membrane after stimulation
what does postsynaptic response amplitude depend on?
amount of NT released and number of NT receptors
in a reflex response where is the ‘decision’ to contract a muscle made?
at the sensory afferent synapses onto the motor neuron
in voluntary movements where in the ‘decision’ to contract a muscle made?
at the level of cortical brain neurons
difference between synapses in the CNS vs NMJ?
many different NTs in CNS, just ACh in NMJ; synapses can be excitatory or inhibitory in CNS, just excitatory in NMJ; axons can contact different parts of neurons in CNS; neurons contact other neurons in CNS; an EPSP in excitatory neurons in the CNS doesn’t always cause an AP- an AP depends on input of all the different neurons; voltage changes produced at most synapses in CNS smaller than EPPs at NMJ
why are voltage changes produced at most synapses in the CNS smaller than EPPs at the NMJ?
NMJ is a larger synapse with more vesicles of ACh in the presynaptic terminal of the motor neuron, more nAChRs in junctional folds, than CNS synapses
what is an EPSP?
an excitatory postsynaptic potential
why is it beneficial for strong signals to be required to evoke a response in CNS post-synaptic neurons?
ensure energy isn’t wasted generating non-essential signals in brain and spinal cord, keeps good levels of response specificity
what neurotransmitter are EPSPs due to at most CNS synapses?
glutamate
what are the receptors for glutamate on post-synaptic receptors?
AMPA receptors, NMDA receptors, Kainate receptors
what are IPSPs in CNS synapses in the brain mediated by?
GABA
what are IPSPs in CNS synapses in the spinal cord mediated by?
glycine
how can glutamate cause excitation and inhibition?
causes excitation via AMPA receptor, and inhibition via GPCR
how does the knee jerk reflex work?
sensory afferent fibres from the muscle synapse with efferent extensor motor neuron, projects back to quadriceps muscle. glutamate released from pre-synaptic terminal of sensory neuron, binds to AMPA receptors in dendrites of the motor neuron. AMPA receptor activation leads to a depolarisation of the motor neuron dendrites. glutamate also binds to AMPA receptors present on inhibitory interneurons in spinal cord, excite the interneuron, which releases glycine to bind to glycine receptors on the motor neuron to the antagonist (flexor) muscle causing this muscle to be inhibited so both muscles don’t contract simultaneously against each other
what is synaptic integration?
combining of all the synaptic inputs in the dendrites of a CNS neuron
what is temporal summation?
when a single presynaptic neuron is activated several times to cause multiple rounds of neurotransmitter released- if response to the rounds of release occurs before 1st response fully decayed they add together to generate a larger EPSP
what is spatial summation?
when more than 1 presynaptic neuron is activated near-simultaneously causing neurotransmitter release at multiple sites, each EPSP adds together to generate a larger EPSP
what is shunting inhibition?
when an IPSP happens in the path of an EPSP- inward Cl- movement effectively pulls positive current out of the dendrite reducing the EPSP
what is primary active transport?
energy provided directly by ATP
what is secondary active transport?
energy provided from the Na+ or H+ gradients set up by ATP pumps
