Exam 2 Flashcards
how do we test the proposed functions of domains of channel proteins?
sequence homologies– determine which portions of primary sequences of ion channels are same/very similar (we look at the same channel from several different species, and what is conserved must be important)
sequence homologies in the ACh gated channel
ACh receptor portion of the channel is highly conserved
sequence homologies in Vg channels
they all have a membrane spanning region with charged amino acids at each third position; the voltage sensor is found only in voltage gated channels
structure of voltage gated channels
six transmembrane motifs, voltage sensor on S4, pore between S5 and S6; it takes four motifs to make a functional channel
what amino acid is highly common in voltage sensors
arginine
each ion has a different
radius, number of water molecules, and diameter of first hydration shield
carbonyls
replace shell of hydration in K+ but not Na+ in the K+ channel
procedure to transplant neurotransmitter receptors from brain to oocytes
injecting cell membranes or brain mrna
homogenize brain sample, centrifuge, collect and inject vesicles into oocyte
you can also homogenize then separate polyA+ RNA, inject it and wait 6 days to record
advantages of xenopus oocytes for studying ion channels
big cells so easy to do electrophysiology, will express large amounts of channel of interest, can introduce exogenous channel from native sources or recombinant DNA
patch clamp technique
record a small portion of the cell and current in that cell: can be cell-attached, whole-cell, or outside-out recording
patch clamp can be used to study
currents through individual ion channels
Na and K+ channels ______ by depolarization
are opened
VgNA+C at depolarization
quickly open, inactivate, returned to closed state
VgK+C at depolarization
don’t inactivate, delayed/slower opening
alpha subunits
are the pore forming subunits
K+ channels are a
family of voltage gated channels with greatest diversity
function of K channels
regulating excitability in many tissues, in axons and somatodendritic compartments; also help set resting potential
protein name format for voltage gated channels
ion, subscript family name (v for voltage), family number, position or individual number of protein
Nav1.7
Kv2.1
don’t inactivate, contribute to the falling phase of the AP, called delayed rectifiers
Kv4
an A type channel that rapidly inactivates, regulating AP frequency in areas like the hippocampus
HERG channels (Kv11.1)
delayed current after depolarization, contribute to duration of hyperpolarization after depolarization
Inward rectifying channels
more active at hyperpolarized potentials, increasing threshold for AP and including KATP to produce K+ when ATP is low
Kca
protect neurons from excess depolarization when Ca2+ is high (more conductance with more Ca2=)
2-p family
K+ leak channels that set resting potential
functions of K+ channels
resting membrane potential
mediate AP downstroke
regulate length of hyperpolarization after AP
regulate excitability in dendrites
decrease excitability during metabolic stress like high ca and low atp intracellularly
channelopathies
diseases caused by ion channel dysfunction, can be caused by genetic mutation or can be acquired via an autoimmune attack, can have developmental or episodic manifestations and affects many tissues,
episodic disorders
affect excitable tissues like brain, muscles, heart, characterized by attacks
causes of episodic disorders
mutations to channels or to excitability, autoimmune diseases, brain lesions/tumors
what triggers an attack?
ion imbalance, chemical triggers, stress, etc
seizure
abnornal/excessive excitation or synchronization of a population of neurons
epilepsy
spontaneous recurrent seizures unprovoked by any systemic or acute neurologic insults
epileptogenesis
sequence of events that converts a normal neuronal network into an epileptic network
epilepsies
many but not all caused by channelopathies with excess Na/Ca currents or reduced K/Cl currents
caused by periodic excess excitation in brain, GOF in depolarizing, loss of function in hyperpolarizing
hyper and hypokalemic periodic paralysis
muscle weakness triggered by exercise or stress, causing loss of muscle tone; categorized by levels of potassium associated with attacks and caused by mutations in ion channels in skeletal muscle
(hypo: low serum K_
in hypokalemia, low serum K+ is accompanied by
prolonged inactivation/reduced function of VgNaCs
prolonged inactivation/reduced function of VgNaCs means that
muscle excitability is insufficient to complete movements
migraines and seizures occur after
excess excitation in the brain; GOF in depolarizing or LOF in hyperpolarizing
familial hemiplegic migraine
disorder characterized by headaches and weakeness on one side of the body, many known genetic causes including more activity of Cav2.1
GEFS
generalized epilepsy with febrile seizure, seizures accompanied by high body temp, mutations cause slowed inactivation of VgNAcs, so excess depolarization when AP is triggered
list some diseases caused by altered ion channels
gefs, myotonia, paralysis
episodic ataxia type 1
uncoordinated movement provoked by stress, startle, exercise caused by LOF mutations in K channels in cerebellum, causing inadequate repolarization
Nav1.7
mediator of aps in nociceptive neurons
congenital insensitivity to pain
lack of pain perception, characterized often by injury or loss of extremities; loss function of Nav1.7 causing less output by DRG neurons
paroxysmal extreme pain disorder
more activity of Nav1.7, reduced inactivation, episodes of extreme pain in jaw, eyes, rectum
Nav1.7 associated with
congenital insensitivity to pain and paroxysmal extreme pain disorder
myotonia
delayed relaxation of muscle after contraction/movement due to loss of hyperpolarizing Cl- current in muscle prolonging contraction
function of electrical synapses
synchronization of electrical activity of large neuron populations
why is synchronization in electrical synapses important
important for functions requiring fast responses like reflexes and pacemakers
electrical vs chemical synapses
chemical involves neurotransmitter across a membrane, electrical involves direct connection via gap junctions where ions flow directly between cells
distance between membranes in gap junctions
wider part is 20 nm, closer part is 3.5 nm
connexin
a subunit of connexonm 6 connexins make a connexon in pre or post synaptic membrane, and two hemi channels come together to make one gap junction channel
connexin topology and structure
4 alpha helices connected by extracellular loops
how large are connexon pores
large enough to pass ions and small molecules, size cutoff is 500 Da for vertebrates and 1000 for invertebrates
do connexons have a negatively charged path
yes, in the edges of the membrane facing cytoplasm
function of gap junctions
rapid signalling between neurons due to coupling, so postsynaptic response is a fraction of a millisecond after presynaptic
gap junctions in the brain
allow for synchronous activity in hippocampus, thalamus
in hypothalamus, neurons secreting peptide hormones are connected by gap junctions, allowing for a burst of hormone
majority of synapses in our nervous system are
chemical synapses, which are slower but more diverse in signalling
characteristics of chemical synapses
- slower than electrical, diverse in signaling
- fast, local (msec, tens of nm)
- postsynaptic cell contains receptors for neurotransmitter
presynaptic terminals are distinguished by
synaptic vesicles
postsynaptic side is distinguished by
postsynaptic density
how long is the response delayed at the postsynaptic terminal
1 msec
vesicular neurotransmitter release is
quantized, meaning the vesicles empty their entire contents and discrete amounts of NT are released (as seen in mini postsynaptic potentials)
spontaneous miniature EPPs
occur in postsynaptic cell even without presynaptic stimulation
vesicles vary in appearance based on
neurotransmitter they contain
what contributes to differential release probability?
differential positioning of vesicle types
vesicles close to active zone
readily releasable, high release probability, moderate Ca influx
vesicles far from active zone
lower release probability, larger sustained Ca influx
low frequency stimulation
preferential release of small-molecule nt
high frequency stimulation
release of both types of nt
vesicular NT release can be altered
to change synaptic strength
axo-axonic synapses
promote of prevent neurotransmitter release
release probability
intrinsic likelihood a vesicle will fuse with plasma membrane
when release probability is high
presynaptic terminal releases more nt
low release probability
less nt is released
can the presynaptic release probability be altered
yes, changes synaptic strength; brief increase in release after high frequency APs (tetanus)
post tetanic potentiation
brief increase in release probability following high frequency short train of action potentials, partially due to higher calcium levels in cell after tetanus
post tetanic depression
longer term decrease in release probability following medium frequency longer train of action potentials (tetanus), usually due to depletion of NT vesicles
NMJ is useful for study
due to accessibility, importance in function
electrically excitable tissues
neurons, cardiac, skeletal
process of muscle contraction
muscle cell plasma membrane depolarizes, travels down T tubules, which interact with ER, causing Ca release into cytoplasm, myosin moves against actin, causing contraction
acetylcholine binds
nicotinic ach receptors (cation channels, so permeable to na and K)
sufficient depolarization triggers muscle cell
action potential, which requires Na+ channels in muscle cell membrane (sarcolemma)
nAch receptor is made of
5 subunits
one nach4 binds ___ to open
2 Ach molecules
the reversal potential for nAchRs is
0 mV, reflecting equal permeability to Na and K
reversal potential for an ion channel
membrane potential at which there is no net current, so electrochemical forces balance out
gaba lets in what ion
Cl-
reversal potential for GABA receptor
-50 to 60 mv, same as equilibrium for Cl
how does lower external Na affect reversal potential
shifts it to the left
how does higher external K+ affect reversal potential
shifts it to the right
what determines current amplitude?
drive/size of electrochemical gradient: closer membrane is to equilibrium potential, smaller net current
ion channels are not binary (T/F)
F
if reversal potential is more positive than threshold
excitation occurs
reversal potential more negative than threshold
inhibition occurs
IPSPs can depolarize the postsynaptic cell if
reversal potential is between resting and action potential threshold
synaptic contact can occur
on cell body, dendrites, or axon
spatial summation
synaptic potentials summed from multiple synapses across neuron
temporal symmation
one synapse elicits psps that are summed
AP frequency is shaped by
inhibitory input
synapses near where will have the greatest weight in summation?
axon hillock
threshold is lower for AP where
at hillock, enriched with NaCs, allowing distal dendritic potentials to elicit an AP
formal criteria for neurotransmitter
- chemical must be synthesized in neuron or otherwise present in it
- when the neuron is active, the chemical must be released and produce a response in some target
- the same response must be obtained when the chemical is experimentally placed on the target
- a mechanism must exist for removing the chemical from its site of activation after its work is done
what is a neurotransmitter?
molecules that
- carry messages between neurons via influence on postsynaptic membrane
- have little or no effect on membrane voltage but have a common carrying function like changing synapse structure
- communicate by sending reverse-direction messages that have an impact on the release or reuptake of transmitters (retrograde signaling)
nearly ubiquitous NT
glutamate (excitatory)
GABA/glycine (inhibitory)
neurotransmitters associated with specific neural circuits/functions
- ach
- dopamine
- norepinephrine
- serotonin
- peptide NT
- atypical NT
small molecule neurotransmitters
ach, amino acid neurotransmitters, amine neurotransmitters, purines
acetylcholine
excitatory via nicotinic ach receptors (ionotropic)
excitatory or inhibitory via muscarinic ach receptors (metabotropic)
amino acid NTs
glutamate, aspartate, GABA, glycine
glutamate
major excitatory nt of cns (ionotropic; metabotropic may be excitatory or inhibitory)
gaba/glycine
inhibitory cns, metabotropic gaba are inhibitory
amine nt
gpcr signalling (metabotropic), dopamine, norepinephrine, epinephrine, serotonin, histamine
small molecule nt
ATP usually copackaged with another small molecule transmitter
peptide hormones
also act as neurotransmitters, examples are enkephalins
how are nt made
cannnot cross bbb, so made locally from precursors, packaged into vesicles, signal terminated by degradation or reuptake
peptide nt synthesis
translation of protein in RER, precursor peptide copackaged with enzymes to make mature peptide transmitter, vesicle transported to axon terminal via active transport, after release, nt signal terminated by diffusion away from the cleft
proteolytic processing of pre-propeptides
pre-propeptide has signal sequence cleaved off to become a propeptide, becomes active peptides
neuronal signalling depends on
type of nt, neuron, brain region, receptor
ionotropic receptor diversity
within one receptor, subunit variants like in nAchRs, families of receptors may pass different ions, like glutamate ampa vs nmda- calcium
structure of gaba
pentameric cl channels
ampa type glutamate receptor
4 subunits
ioniotropic nt receptors
same endogenous ligand, different agonists, pass na, k, ca, cl, may be desensitized or voltage gated
ampa receptors
large current, quick desensitization
kainate and nmda
smaller peak current, slower desensitization
nmda type glutamate receptors
ligand and voltage gated so mg blocks at negative potentials, but at higher potentials mg is kicked out
