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)

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