L17 Synaptic Transmission Flashcards
what is a homomeric connexon?
consists of 1 type of connexin
what is a heteromeric connexon?
formed by a mix of different connexin isoforms
what are hemichannels?
connexins + pannexins
what are gap junctions?
only connexins
connexons may dock with an identical connexon to form a homotypic or a different connexon to form a heterotypic channel
what is the speed of transmission through gap junctions?
FAST (under 0.1ms)
unidireciton transmission
bidirectional transmission
what type of transmission is seen in electrical synapses?
gap junctions
what are gap junctions made of?
2 hemichannels (connexons) which are composed of 6 units of 4 membrane spanning regions (connexins)
where are gap junctions found (structures)?
CNS - spinal cord, hippocampus, mesensephalic nucleus and retina between horizontal cells
between astrocytes which allows them to function in sync when buffering K+ ions
are synapses most chemical or electrical?
chemical
what is a human disease with connexin mutation?
Charcot-Marie tooth syndrome
the postsynaptic membrane has extensive ___ regions?
dense - where NT is received in the postsynaptic membrane
what is an axodendritic synapse and what is an example?
excitatory on firing rate
cortical pyramidal cell’s dendrites
what is an axosomatic synapse and what is an example?
inhibitory on firing rate
cortical pyramidal cell’s soma
what is an axoaxonic synapse and what is an example?
inhibitory on NT release
cortical pyramidal neurons
what is a tripartite synapse?
between the presynaptic neuron
postsynaptic neuron
astrocyte endfeet - take in NT released by neurons and release others
define ionotropic receptor and give example?
ion channel proteins that open (or close) when they are hit by the NT which leads to a change in gm and Vm
consist of multimeric different proteins with 5 subunits and varying number of membrane spanning domain
nicotinic receptor
define metabotropic receptor and give example
mediate action via G protein influencing the activity of adjacent enzymes to induce changes in the 2nd messenger in the postsynaptic cell
muscarinic receptor
what is synaptic delay?
Delay between arrival of impulse and the onset of a response
from arrival of an impulse in the presynaptic nerve ending till the onset of the postsynaptic cell’s response after binding to a postsynaptic receptor and production and breakdown of 2nd messengers
what are the mechanisms for removing NT from the synaptic cleft?
- diffusion - neuropeptide, all transmitters
- extracellular enzymatic degradation Ach via acetylcholinesterase
- uptake into nerve endings or glial - glutamate, GABA, catecholamines
- intracellular enzymatic degradation
what are the 3 most important ionitropic glutamate receptors>?
NMDA
AMPA
kainate receptors
what is the role of the glutamate ionotrophic receptors?
cation channels that allow an influx of Na+ ions (NMDAR also Ca2+) and efflux of K+ ions
influx of Na+ ions exceeds efflux of K+ ions because the Na+ electrochemical gradient is larger than the K+ gradient
what does the net entry of cations evoke?
depolarization = excitatory postsynaptic potential EPSP
what does the amplitude of the EPSP reflect?
the amount of glutamate released by exocytosis in response to a presynaptic impulse
DEPOLARIZATIONS CAN BE GRADED IN SIZE DEPENDING ON THE APPLIED LICAL GLUTAMATE CONCENTRATION AND THE NUMBER OF OPEN GLUTAMATE-GATED CHANNELS IN POSTSYNAPTIC MEMBRANES
what is the reversal potential for the excitatory response?
E cation
define reversal potential?
when the potential is set close to 0, the AMPA channel opening does not cause current to flow because the inlfux of Na+ ions is counterbalanced by the efflux of K+ ions. the potential at which no current flows!
local interneurons release inhibitory transmitters onto _____
motor neurons
for examples - the afferents not only excite motor neurons via AMPA receptors but they also excite local interneurons via AMPA receptors
what type of influence do interneurons have on motor neurons of antagonist muscle and why?
inhibitory action on the motor neurons of the antagonist muscles because they release glycine on to the motor neurons for the antagonist muscle
an impulse in the interneuron evokes what?
an inhibitory postsynaptic potential IPSP
what type of channels are ionotropic glycine or GABA receptors?
anion channels - mostly Cl-
describe the IPSP
transient hyperpolarization with rapid falling phase and a slower repolarizing phase
what is the reversal potential for this inhibitory response?
-65 mV
where are EPSP generated and where are they found?
synapses
dendrites, soma and axon hillock
do EPSPs spread with decrement?
Yes - the induced depolarization spread BUT decline in amplitude according to their distance from the sites
what type of summation is seen in EPSP?
spatial and temporal
what does the firing rate of the neuron depend on?
the combined effect of the spatial and temporal summation of EPSPs and IPSPs occurring at many sites in its soma and dendrites
what does cocaine do?
block Na+ channels from inside - also blocks the transporters for reuptake of dopamine, noradrenaline and serotonin and thereby prolongs their presence in cleft
what does morphine do?
acts as an agonist for the receptors mediating the presynaptic inhibition of C fiber terminals - which relay pain info to second order neurons in the spinal cord
where does action of morphine take place?
nociceptors
spinal cord
supraspinal sites
limbic system
transmitter action is ____ within the synaptic celft
brief
define active zone
where exocytosis occurs in presynaptic membrane
which receptor will have a longer delay?
metabotropic - >100 ms delay due to 2nd messenger system = SLOWER = MORE CONTROL!
ionotropic - 0.5 ms delay due to mech of NT release
how many motor neurons does an 1a-afferent sensory neuron innervate?
about 100 motor neurons for the muscle it monitors
if a muscle is stretched, how will the 1a afferents be excited?
GLUTAMATE
the 1a afferent sensory fibers will use glutamate to make synapses with what 2 things?
each motor neuron
interneurons –> release glycine onto antagonist muscle –IPSP!
define spatial summation
the sum of the excitatory currents will add to = a larger EPSP than either of them alone
define temporal summation
multiple impulses arriving so close together – they add to one another because the effect of the previous impulse has no relaxed back to the resting state - add on top of eachother!
what are examples of ionotripic receptors?
nicotinic AChR, GABAa, NMDA and AMPA receptors for glutamate
what are examples of meabotropic receptors?
muscarinic AChR and NE receptors
what does cocaine do?
local anesthetic
blocks sodium channels from inside
blocks the transport for reuptake of dopamine, seratonin, NE and prolongs their presence in hte clefts
what does morphine do
analgesic
binds to the receptors for neuropeptide (mu-enkephalin) that are located on the terminals of C fibers (nociceptors) in the substantia gelatinosa in the dorsal horn
agonist for receptors mediated the inhibition of C fibers terminals which relay pain info to second order neurons in spinal cord
