synaptic transmission Flashcards
what are the components of a neuron?
dendrite, cell soma, axon (with or without a myelin sheath0 and presynaptic terminals
what is a dendrite and what does it do?
a dendrite are processes coming out of the cell body that increase the receptive surface of the cell. they receive transmitters and amplify the synaptic signal with voltage gated ion channels.
what is the function of the neuronal cell body?
synthesizes protein, process it and degrades material. also has receptors that can bind chemical transmiters from affarent neurons
where does the axon arise and what does it do?
arises from the cell body at the axon hillock. transmits action potential down to the terminals from transmitter mediated changes from the dendrites.
what can axons be surrounded by and what does it do?
glial cells forming myelin sheath that increases the speed of impulse propagation
what happens at the presynaptic terminals?
electrical signals from the axon are converted into chemical signals that are released into the synapse
what is axoplasm?
parallel arrays of microtubules that provide structural stability and transport along the axon bidirectionally
why do cancer patients receiving microtubule disrupting agents sometimes develop peripheral neuropathies?
because there is vesicle depletion of transmitters in the presynaptic terminal (especially in axons innervating distal muscles)
which motors facilitate anterograde transport and which promote retrograde transport and what do they carry?
kinesin mediates anterograde transport of mitochondria and vesicles from the soma.
dynein mediates retrograde transport of degraded vesicular membranes and absorbed toxins, viruses and growth factors
what connects cells at electrical synapses and what is facilitated?
gap junctions allow the direct passive flow of electronic current between cells to allow fast and bidirectional transmission (can cause action potential in the next cell)
what are gap junctions made up of?
each membrane contains a connexon that is made up of six connexins (intramembrane channels) that combine to form a gap junction
are gap junctions selective?
no, they are much larger than ion channels so they can transport ions and larger substances.
are electrical or chemical synapses more common in the nervous system? which types of signals are more likely to have electrical synapse?
chemical synapses
electrical synapses control more synchronous activities such as breathing and smooth/cardiac muscle contraction.
what are contained in synaptic vessels?
neurotransmitters
what are the 4 criteria needed to be a chemical neurotransmitter?
1) located in presynaptic terminal
2) released in a voltage and Ca dependent manner
3) specific receptors are present in target cell
4) there is a mean to inactivate the transmitter
what are the stops of synaptic transmission?
1) synthesis and packaging of transmitters into vesicles
2) action potential arrives and depolarization of terminal opens voltage gated Ca channels
3) increased calcium triggers vesicle fusion
4) transmitter diffuses across cleft and binds to postsynaptic receptors
5) a postsynaptic response occurs and transmitter is cleared
how are transmitters cleared from synaptic clefts?
enzymatic degradation, uptake and diffusion
what is contained within presynaptic terminals?
mitochondria and synaptic vesicles containing transmitter
what are active zones?
a specialized site where some vesicles are docked. these vesicles are the ones that will be released when an action potential and influx of calcium occurs in the presynaptic terminal
what is the postsynaptic density? how is it placed to increase efficiency?
an electron dense area with many transmitter receptors. it is alligned with presynaptic active zones to make it more efficient
what is the major source of energy for neurons and where is it mainly consumed?
glucose
mainly consumed in presynaptic terminals
what does fMRI measure and how is it interpreted?
measures changes in regional blood flow that is associated with changes in local cerebral glucose metabolism (that part of the brain is working)
how are neurotransmitters released? describe the process.
exocytosis
vessicle membrane V-SNAREs (including synaptobrevin and calcium sensor synaptotagmin) interact with membrane T-SNAREs (including SNAP-25 and syntaxin)
how are vesicle membranes recycled? why is it done?
endocytosis with the inclusion of V-SNAREs. This is done to prevent enlargement of the presynaptic membrane
what is the function of Botulinum toxin?
cleaves SNAP-25 or synaptobrevin proteins in alpha motor neurons causing muscle weakness
what is the range of time course action of chemical transmitters?
ranges from ms to days
list 6 small chemical transmitters and one large one.
small: glutamate, acetylcholine, norepinephrine, dopamine, GABA and glycine
large: vasoactive intestinal polypeptide
what types of neurotransmitters have a long time course and what are they used for?
peptides. they usually are modulators
what is an iolotropic receptor and how fast is the response?
a receptor that contains an ion channel as part of its structure
rapid response- either hyperpolarizing or depolarizing
what is a metabotropic receptor and how fast is the response?
a receptor that is commonly linked to G-proteins
slower biochemical signal- may sitll hyperpolarize or depolarize the cell
what is acetylcholine and what are its rapid and slower responding receptors?
a molecule made from choline and acetate
rapid: nicotinic ACh receptor (nAChR)
slow: muscarinic ACh receptor (mAChR)
what terminates the action of acetylcholine and how is it recycle?
acetyl cholinesterase (AChE) cleaves it and the presynaptic membrane contains a high affinity transport system that pumps choline back into the nerve terminal for reutilization
what is the result of activation of a nAChR in skeletal muscle and mAChR in cardiac muscle? is there a similar amount of ACh needed?
nAChR- muscle contraction
mAChR- slowing the heart rate by hyperpolarizing the membrane
yes.
how are postsynaptic potentials produced?
conductance changes (ion channels open or close) and the membrane potential changes.
what is an EPSP?
an EPSP is an excitatory postsynaptic potential that increases the probability that an action potential will be triggered. (cell gets depolarized)
caused by sodium influx
what is an IPSP?
an IPSP is an inhibitory postsynaptic potential that decreases the likelihood (cell gets hyperpolarized)
caused by potassium efflux, chloride influx or both
what is the major excitatory transmitter in the brain?
glutamate
describe AMPA receptor channels.
glutamate channels that are selectively activated by pharm agent AMPA, mediate fast EPSP in presence of glutamate because of Na and K flow
describe NMDA receptor channels.
glutamate channels selectively activated by pharm agend NMDA and mediate slower EPSP via flow of K, Na and Ca ions.
do not open unless membrane is depolarized because they are voltage dependent-need stronger stimulus
list the following receptors by speed of reaction and the reasoning: metabotrophic glutamate receptors, AMPA and NMDA
(fastest) AMPA, NMDA, and glutamate (slowest)
AMPA and NMDA are ionotropic
glutamate is metabotropic
what would be the additive result of AMPA at -80 mV? at -40 mV?
only AMPA would work at -80 mV
they would have a greater stimulatory effect at -40 mV because both would take part
what is the major inhibitory transmitter?
GABA (create IPSP hyperpolarizing responses)
what mediates the fast IPSP and how?
GABA-A receptor
lets Cl ions flow down their electrochemical gradient
what happens when pentobarbital acts in the presence of GABA?
there is a larger pSP because it increases the channel open time and current
why are GABA-B receptors slower?
because they are metabotropic receptors
how are small excitatory stimuli generated into an action potential?
if there are enough individual stimuli they may be able to depolarize the cell to the required threshold to create an action potential
how can synapses become more effective?
more receptors can be present in the postsynaptic density
what is the difference between vesicle fusion within other cells and within neurons?
neuronal exocytosis is calcium mediated. need the calcium signal to bind the synaptotagnin more tightly to fuse the membranes
what is a synaptic bouton?
branches of the terminal regions of axons where myelination is lost and vesicles fuse with presynaptic membrane
when a small depolarization occurs, what happens as it travels?
the signal will spread a short distance and fade as it moves away from the stimulus (temporal and spatial effects)
what does the cable theory of temporal and spatial effects take into account?
membrane capacitance, membrane resistance and longitudinal cytoplasmic resistance between different neuron parts
what is decremental conduction?
before an EPSP can reach the soma, a large amount of the potential is lost by leakage through the membrane
what is the process of temporal stimulation?
stimuli can build upon each other if they are close enough in time or if there is a longer time constant. if they are not close enough, the signal will decay before the next stimulation (short time constant)
what is the process of spatial stimulation?
an EPSP travels down a dendrite through passive conduction If there is another PSP down the dendrite, it will only summate with the first one if it has not dissipated by the time it reaches the second (it has a longer space constant)
what happens to isolated synaptic potentials in dendrites usually? how is this combated?
they decline in strength before reaching the cell soma
can be combated by multiple ESPSs combining to generate large potentials
what are two other names for the axon hillock? and what is in high density there?
initial segment or trigger zone
high density of voltage gated Na channels
