Download Cortical Microcircuit

Cortical Microcircuit
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Pyramidal neurones in neocortex
Membrane thickness ~ 10nm, Cm ~ 1µF/cm2
Rm 1000 -100,000 Ωcm2 ; Ri 100 Ωcm
If diam =2µm, Rm=104 Ωcm2
Axial resist.= 8x108 Ω/cm
Memb resist=1.6x107 Ωcm
Space const. ( characteristic length) = 0.14cm
Time const = 10ms
Spread of transients in dendrites
Compartmental models
Summation of synaptic potentials is non-linear
Neuronal membranes contain many kinds of
conductances with voltage and time dependent kinetics
Active conductance mix determines excitability and firing pattern
Temporal firing
patterns differ widely
for different neuron
types
The same neuron can
vary firing patterns by
activating different
conductances
Membrane conductances are non-uniformly distributed
Transmitter release is stochastic and quantised
Synapses with high release probability depress during
repetitive activation (p = 0.75). Release probability is
determined by both neurone types
synapses with low release probability strengthen
during repetitive activity (p = 0.13)
LTP, LTD
Threshold for action potential initiation
Isopotential neuron, when inward current = outward current, and nett current discharges the
membrane capacitance
Non-isopotential neuron, nett current resulting from point current application must
discharge local capacitance + provide sufficient additional current to discharge membrane
capacitance at points further away. Hence threshold current will be greater than for
isopotential stimulation
Threshold voltage occurs when sufficient membrane is depolarised
above threshold to provide the inward current needed to spread
along the cable and activate more inward current.
Does this occur when nett current integrated over the entire
membrane is zero?
Potential at point of
excitation vs time
Integral of membrane current over
entire cable surface