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Source: Adapted from Eccles, 1964.

Source: Adapted from Eccles, 1964.

FIGURE 1.3-4 Representative intracellular recordings of epsps from the soma of a motoneuron in the lumbar region of cat spinal cord. The medial gastrocnemius nerve was stimulated with progressively larger pulses, recruiting more and more fibers that excite the motoneuron being recorded from. (Based on data from Eccles, 1964.)

FIGURE 1.3-4 Representative intracellular recordings of epsps from the soma of a motoneuron in the lumbar region of cat spinal cord. The medial gastrocnemius nerve was stimulated with progressively larger pulses, recruiting more and more fibers that excite the motoneuron being recorded from. (Based on data from Eccles, 1964.)

epsps, wherever on the postsynaptic neuron they are generated, sum in space (over dendrites and soma) and in time (by superposition) to generate a net generator potential (Vg) at the SGL. If the net Vg(t) at the SGL exceeds the spike initiation threshold, the postsynaptic neuron generates an action potential.

The epsp "ballistic potential" is often modeled by a simple, two real-pole low-pass filter in which the input is a unit impulse coincident with the arrival of the presynaptic action potential, and the output is the epsp, vm(t), which is added to the dc resting potential of the postsynaptic neuron. In terms of Laplace transforms, this potential is vm(s) = K/[(s + a)(s + b)]. When a = b, the transfer function is called an a-function by some computational neurobiologists.

Inhibition of spinal motoneurons is generally accomplished by second-messenger-type synapses on the soma that admit chloride ions. As the result of the transient

FIGURE 1.3-5 Schematic of stimulation and epsp recording procedure from a spinal motoneuron. Multiple input fibers converge on the dendritic tree with excitatory synapses.

increase in influx of Cl-, an inhibitory postsynaptic potential (ipsp) is generated. In cat spinal motoneurons, the peak ipsp is about -2.3 to -3.0 mV below the normal resting potential of -65 mV. That chloride ions are involved in this ipsp can be shown by voltage-clamping the soma of the postsynaptic neuron to potentials below -70 mV, the Nernst potential for chloride ions for the motoneuron. When Vm is held at -100 mV (strong hyperpolarization), stimulation of the inhibitory synapse produces a large, positive-going ipsp, where the chloride ions flow out of the postsynaptic membrane. Eccles shows the normal motoneron ipsp to have a delay of about 1.2 ms, a peak at ~2.5 ms, and a decay time constant of about 1.8 ms.

It is true that the negative ipsp transients subtract from the summed epsps, but the inhibition is more complex than simple subtractive superposition. The open chloride channels during inhibitory synaptic activation raise membrane conductance in their neighborhood, shunting the excitatory currents, attenuating epsps, as well as subtracting from them. By locating inhibitory synapses on the soma near the SGL, inhibitory inputs gain more "leverage" than if they were out on the dendrites of the motoneuron. Although it is tempting to treat neurons as "all-or-none," discrete devices, it is clear from examining the synaptic control of motoneuron firing that the motoneuron acts as a complex, nonlinear analog threshold device. The decision to fire is simply determined by the generator potential, but the generator potential is the result of complex spatiotemporal summation of excitatory and inhibitory inputs over the dendrites and cell body.

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