Copyright ©Mark Nelson, 2002. All rights
reserved.
Chapter 8: Integration of Synaptic
Action
What you need to know
(exam questions will be a drawn from this subset
of material)
What three basic electrical properties influence the spread of synaptic
current along a neurite? (p. 188)
membrane resistance,
membrane capacitance, and axial resistance
What is the length (or space) constant of a
neurite? (p. 188-189)
it's a measure
of the distance that a postsynaptic potential can spread along the postsynaptic
membrane
mathematically, it is the distance at which the
change in potential ΔV decays to 1/e of its peak value.
ΔV(x) = ΔVmax e -x/λ, where
x is distance and λ (lambda) is the space constant.
How is the length constant related to the basic electrical properties
of a neurite ? (p. 188-189)
The length constant
increases with increasing membrane resistance
(i.e., the current spreads further
when the membrane is less leaky)
The length constant increases with decreasing
internal (axial) resistance
(i.e., the current spreads further
when the internal resistance is low, as occurs in large diameter axons)
λ = SQRT(rm
/ ri)
How is the length constant related to the diameter of a neurite?
(p. 188-189)
The length constant
increases with increasing neurite diameter..
(i.e., current spreads further in
fat neurites than in skinny neurites)
What would be a typical length constant for a neurite, i.e. how far does
a PSP typically spread? (p. 189)
typical values
range from several hundred microns to several millimeters
What is the time constant of a neurite? (p. 189-192)
it's a measure
of the time it takes for a postsynaptic potential to decay at one location
along a neurite
mathematically, it is the time at
which the change in potential ΔV decays to 1/e of its peak value.
ΔV(x) = ΔVmax e -t/τ, where
t is distance and τ (tau) is the time constant.
How is the time constant related to the basic electrical properties of
a neurite? (p. 191)
The time constant increases with increasing membrane
resistance
(i.e., the electrical charge stays
around longer when the membrane is less leaky)
The time constant increases with increasing
membrane capacitance
(i.e., the electrical charge stays
around longer if the membrane has a higher storage capacity)
τ = rm * cm
How is the time constant related to the diameter of a neurite? (not in text)
The time constant
is independent of neurite diameter.
What would be a typical time constant for a neurite? (p. 192)
about 10 msec
What is spatial summation? (p. 193-195)
the combined
postsynaptic effect of two or more inputs from different locations occurring
at about the same time;
What factors influences the effectiveness with which two synaptic inputs
can interact by spatial summation? (p. 194-195)
the distance
between the two inputs, and the length constant of the neurite
What is temporal summation? (p. 195-196)
the combined
postsynaptic effect of two or more inputs that occur at different times
What factors influences the effectiveness with which two synaptic inputs
can interact by temporal summation? (p. 195-196)
the time interval
between the inputs and the membrane time constant of the neurite
Summarize the factors that influence the spiking output of a neuron at
the single neuron level. (p. 196-197)
the output activity
of a neuron reflects the spatial and temporal interactions of incoming EPSPs
and IPSPs,
which can depend on
1) the sizes,
shapes, and electrical properties of neurite branches of the postsynaptic
neuron,
2) the spatial location
of synaptic inputs along these neurites
3) the temporal pattern
of synaptic inputs to different locations
4) the spatial distribution
and temporal characteristics of voltage-gated and ligand gated channels
5) the effects of neuromodulators
on these channel properties
6) synaptic plasticity
What is facilitation? (p. 197-199)
an increase
in the amplitude of successive postsynaptic responses as a result of repetitive
activation
What distinguishes facilitation from temporal summation?
(p. 197)
in facilitation,
the time between successive inputs is long compared to the membrane time
constant,
such that the membrane potential returns
to resting levels in between successive EPSPs
What is the difference between homosynaptic and heterosynaptic
facilitation? (p. 198)
in homosynaptic
facilitation, the synapse that is stimulated is the one that shows facilitation
in heterosynaptic facilitation, one synapse is stimulated
(the "priming" input) and a different synapse is facilitated (the "test"
input)
What is one possible mechanism for homosynaptic facilitation? (p. 198)
the accumulation
of residual Ca++ in the presynaptic terminal following repeated
stimulation
What is one possible mechanism for heterosynaptic facilitation? (p. 199; Figs. 8-8, 8-9)
heterosynaptic
facilitation can occur when the inputs are arranged to produce "presynaptic
facilitation" (Fig. 8-9)
in this arrangement, the "priming" input contact the
axon terminal of the "test" input
1) release of the neuromodulator serotonin from
the priming terminal
2) causing the closure of K+ channels in
the "test" terminal
3) causing a longer duration depolarization when the
"test" input arrives at the axon terminal (spike broadening)
4) causing greater influx of Ca++ into the
"test" terminal
5) causing greater neurotransmitter release, and a facilitated
response
What is potentiation? (p. 200)
potentiation
is a special form of facilitation in which the increase in amplitude of the
PSP continues well after the stimuli that caused it.
What is post-tetanic potentiation (PTP)? (p. 200;
Fig. 8-10)
In PTP, a high
frequency burst of presynaptic inputs lasting several seconds, called a
tetanic stimulus, causes facilitation during the
tetanic stimulus, as well as a persistent potentiation
lasting several minutes after the tenatic stimulus (see Fig. 8-10)
What is one possible mechanism for PTP? (not in
text)
similar to facilitation,
PTP is thought to arise from accumulation of presynaptic calcium
What is long-term potentiation (LTP)? (p. 200-201)
a form of potentiation
following high-frequency stimulation that can
last for hours to days
proposed as a possible mechanism for a variety
of learning and memory phenomena
What is one possible mechanism for LTP? (p. 201-202)
LTP is induced through NMDA receptors by
the entry of Ca++ ions, which initiate a biochemical cascade
that causes the neuron to be more responsive to neurotransmitter, and by
means of the diffusible retrograde messenger NO, can cause more transmitter
to be released from the presynaptic terminal.
What role does Mg++ play in heterosynaptic LTP? (p. 200-201)
NMDA channels
are normally blocked by external Mg++ ions.
depolariztion of the postsynaptic neuron by other inputs
can relieve the Mg++ block, initiating LTP.
thus heterosynaptic LTP requires electrical activity
in both the presynaptic and postsynaptic neuron