Synaptic Transmission Flashcards
what is a neurotransmitter
chemical
primary means of communication b/w cells
allows neurons to communicate with one another
what is a synapse
point of contact
where one neuron comes in contact with another neuron
presynaptic side
axonal terminal
post synaptic side
may be another dendrite or soma of another neuron
synaptic cleft
space b/w the 2 sides
synaptic transmission
transfer of info across the synapse
how are NT synthesize
by the neuron
where are NT stored
the vesicles at the nerve terminal
what is the post synaptic side (specifically)
a protein
what happens when the NT crosses the synapse and binds w/ the post synaptic receptors
protein changes shape
alters the fxn of the receiving neuron
kinds of synapses
axondendritic
axosomatic
axoaxonic
types of synapses
electrical
chemical
electrical synapse
not very common in the NS
chemical synapse
very common in the NS
how do electrical synapses run
can act in either direction
passage –> electrical synapse
there is a passage of electrical current secondary to cell membranes of neurons communicating w/ one another
–>occurs b/c of low resistance gap jxns
chow to chemical synapses run
impulse only has ability to travel in one direction
release of NT from presynaptic side –> membrane of the postsynaptic side
steps 1-4 of impulse travel
- arrival of action potential, Ca+ channels opened
- Ca+ influx into presynaptic term
- Ca+ acts as intracellular messenger stimulating synaptic vesicles to release NT
- Ca++ removed from the synaptic knob by mitochondria or calcium-pumps
steps 5-7 of impulse travel
- NT diffuses across synaptic cleft and binds to receptor on postsynaptic membrane
- receptor changes shape of ion channel opening it and changing membrane potential
- NT is quickly destroyed by enzymes or taken back up by astrocytes or presynaptic membrane
types of chemical synapses
excitatory chemical synapses
inhibitory chemical synapses
excitatory chemical synapses cause
a depolarizing graded potential in the postsynaptic cell
EPSP
excitatory postsynaptic potential
moves the membrane potential towards threshold
what kind of channels do excitatory chemical synapses use
only chemically gated channels
do postsynaptic membranes generate APs?
no
what do EPSPs do to resting membrane potential
bring RMP closer to threshold
closer to an AP
what do inhibitory chemical synapses cause
either hyperpolarizing graded potential or a stabilization of the resting membrane potential in the postsynaptic cell
IPSP
inhibitory postsynaptic potentials
move the membrane potential away from threshold
what does stabilization of the membrane potential prevent
EPSPs from being created in the postsynaptic cell
summation
a single EPSP CANNOT initiate an AP
what must EPSPs do to bring membrane potential to threshold at the axon
must summate
2 types of summation
temporal
spatial
temporal summation
postsynaptic potentials are generated at a high frequency
sequential postsynaptic potentials “piggyback” one another
spatial summation
multiple postsynaptic potentials are generated at different locations at the same time and converge at the axon
neurons are made up of
cell body
dendrites
axons
dendrites
input structure
receive inputs from other neurons
relay them to the cell body
axons
output structure
a fiber that carries messages (spikes) from the cell to dendrites of other neurons
what do neurons do
conduct info
what do action potentials move
down the axon
what propagates the message down the neuron
the cell membrane
what happens as the AP moves down the neuron
does not diminish in intensity over a distance
signal is fixed
duration is fixed
info is coded
info coded in an AP
frequency of firing
how many neurons of the same nerve are firing together
what does the cell membrane do
surrounds the neuron
what are the main building blocks of the cell membrane
phospholipids
the cell membrane is
semipermeable
regulator
how is the cell membrane a regulator
determines the movement of ions into and out of the neuron
how are ions distributed across the membrane
unevenly
what does the sodium potassium pump do
develops an electrical gradient with a greater positive charge outside of the cell membrane
what is the key determinant of neuronal fxn
permeability of potassium channels
what is an ion channel
cell membrane proteins that pass ions in and out of the cell
2 types of ion channels
voltage-gated ion channels
chemical-gated ion channels
voltage-gated ion channels
gates are regulated by membrane voltage
chemical gated ion channels
also called receptors
gates are regulated by NTs
depolarization
increase in permeability of the Na+ channels
inward Na+ current
decrease in internal negativity
hyperpolarization
increase in permeability of the K+ channels
outward K+ current
increased internal negativity
the membrane is mostly
impermeable
forms a barrier to many proteins, molecules and other ions dissolved in the intracellular and extracellular fluids
what is the membrane selectively permeable to
sodium
potassium
chlorine
what is the most permeable
K+
what is the least permeable
Na+
RMP
-65 mv
critical to understanding cellular behavior
when does EPSP occur
depolarization
when does IPSP occur
hyperpolarization
what is depolarization and hyperpolarization
transient changes in the membrane potential
what is temporal summation dependent upon
amount of time in b/w receiving EPSPs and IPSPs
what is spatial summation dependent on
distance b/w the incoming IPSPs and EPSPs
when does an AP begin
if the graded potentials add up to threshold
threshold depolarization
when the summation of all the incoming info to the dendrites and the soma reaches a critical level
Na+ channels at the hillock open
Na+ channels opening –> action potential
rush of positive ions into the axon cause the membrane potential to become positive
the change and the subsequent reversal of the change (repolarization) is the actual AP
resting levels become more negative than normal or hyperpolarized
what happens to the axon hillock during the AP
less responsive to other stimuli
what happens to Na+ channels during an AP
inactivated for a period of time after the peak Na+ conductance
this is the absolute refractory period
what is the absolute refractory period
where no stimuli can impact on the axon hillock
an action potential is a _______ event
all or nothing
there is no thing as a strong or weak AP
are there summations of APs?
no
however there could be increases in firing frequency of the APs
what are APs throughout the course of transmission
consistent in amplitude and duration
throughout depolarization
Na+ continues to rush inside until the AP reaches its peak
–> Na+ gates then close
what is depolarization is not great enough to reach threshold
an AP is not produced
an impulse is not produced
where does an AP go after the axon hillock
axon terminal
how does the AP go from the hillock to the terminal
wave like fashion
change in RMP occurs down the axon
what is speed or conduction velocity of an AP dependent on
diameter of the axon
presence of myeline sheath
what does a myelin sheath do
increases conduction velocity
myeline
specialized glial cells that surround the axon
schwann cells = pns
oligodendrocytes = cns
what does myeline surround
axon in segments
nodes of ranvier
where there is no myeline
saltatory conduction
saltatory conduction
AP “jumps” from one node to the next
passive conduction
will ensure that adjacent membrane depolarized
so AP “travels” down the axon
but transmission by continuous APs is relatively slow and energy consuming
myelination provides
saltatory conduction
how do impulses travel down neurons
very rapidly
what does the presence of myeline do
greatly increases the velocity at which impulses are conducted along the axon of a neuron
unmyelinated fibers
entire axon membrane is exposed
impulse conduction is slower
when are NT released
into the synaptic cleft in response to the AP
what do the NT do
bind to the postsynaptic receptors
cause IPSPs or EPSPs on the opposite side
NT are either
recycled by the presynaptic terminal
degraded by enzymes in the synaptic cleft
passive diffusion
absorbed by glial cells
gap jxn
membranes of joining cell membranes line up
when activity occurs on one side, it occurs on the other side as well
parasynaptic neurotransmission
NTs released form the axon terminal diffuse into the extracellular space before encountering its postsynaptic target
