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6 electrical events in Post synaptic neurons (ÌPSP (it begins when an…
6
electrical events in
Post synaptic neurons
EPSP
excitatory post synaptic potential
An afferent impulse enters the
spinal cord
it rises to its peak
& if its not sufficient to generate
an AP, it then slowly declines
during this potential,
the excitability of the PSN to other stimuli
are increased
therefore its called an
excitatory postsynaptic potential
the current generating the EPSP
passes through the
-synaptic cleft
-& inward across the activated
subsynaptic membrane,
-also outward across the remaining of
the postsynaptic membrane.
thus the area of inward current flow
is so small,that its not enough to
depolarize the whole membrane.
instead an EPSP is prodeced.
EPSP due to activity in 1 synaptic knob is small
but EPSPs produced by each active knob summates
(in 2 ways)
spatial & temporal
spatial summation
temporal summation
if 1 afferent nerve fiber is
stimulated for a 2nd time
before the decay of the 1st stimulation,
-neither of which can produce a spike
frm the motor neuron,
-but each produce some EPSPs
that may get summated to the
critical level necessary for the initiation
of a spike potential.
this is called temporal summation.
when 2 afferent nerves
which has synaptic connections with
the motor neuron
are stimulated simultaneously
their EPSPs get summated
-to produce a spike discharge
frm the motor neuron
-this is called spatial summation
the excitation of either fiber alone
does not produce a spike potential
& activity in 1 synaptic knob
is said to facilitate activity in another
-to approach the firing level
-this is due to increased release
of chemical transmitter
ionic basis of EPSPs
the released excitatory transmitters
bind to appropriate postsynaptic receptors
& cause opening of
chemically gated ( Na+ or Ca2+ )
ion channels in the subsynaptic membrane
of the postsynaptic cell
the production of "end plate potential''
by A-ch at the neuromuscular juction
provides a good example
of the operation of these mechanisms.
EPSPs can also be produced
by agents that close
K+ channels.
A Single stimulus applied to the
sensory nerves-does not lead to a
propegated AP in the PS neuron
rather it produces either a
-transient partial depolarization
or a
-transient hyperpolarization
ÌPSP
is produced by stimulation of
certain presynaptic fibers
-which regularly initiate a
-hyperpolarizing response
in spinal motor neurons
it begins when an afferent impulse enters
into the spinal cord
it rises to it's peak & declines exponentially
during this potential the excitability of the
neuron to other stimuli is decreased
spatial & temporal summation of
IPSPs also occures
-this type of inhibition is called
postsynaptic/direct inhibition
ionic basis of IPSPs
PSP is due to the liberated
inhibitory chemical transmitter
on the subsynaptic membrane
it becomes highly permeable to
K+, & Cl-
but becomes highly impermeable to
Na+
this results in hyperpolarization
of the whole postsynaptic membrane
but this permeability change is
short lived,
-& RMPs are rapidly restored
IPSPs can also be produced
by the opening of
K+ channels
-also by closure of
Na+ or Ca2+ channels
6a
genesis of AP in Postsynaptic neuron
whenEPSP depolarization reaches
the threshold level,
it produces an AP in the initial segment
(at the axon hillock)
called IS spike
at about 10-15mV
the IS spike requires a relatively
low degree of depolarization
for its own production
-but once initiated,
it produces a further depo
(of abt 30-40mV)
this is why the portion of the cell
with the lowest threshold id the initial segment