Functional characteristics of unmyelinated fibres in the hippocampal cortex

doi:10.1016/0006-8993(78)90431-6

Brain Research

Volume 144, Issue 1, 7 April 1978, Pages 11-18

https://doi.org/10.1016/0006-8993(78)90431-6 Get rights and content

Abstract

(1)
In transverse hippocampal slices (350 μm thick), taken from guinea pigs initially anaesthetized with ether, intracortical afferent fibres were activated by small current pulses delivered through tungsten microelectrodes. Extracellular potentials were recorded from the zone of activated fibres in dendritic layers while intracellular recordings were made from the soma of CA1 pyramidal cells.
(2)
When recording was made from the same level as the stimulating cathode, the extracellular potential consisted of a diphasic deflection followed by a larger negative wave with a superimposed population spike. The negative wave corresponded to an intracellularly recorded EPSP, and is called an extracellular EPSP, whereas the initial diphasic deflection had no intracellular counterpart.
(3)
The initial diphasic deflection was linearly related to the size of both the intracellular and extracellular EPSP. It was not changed by removal of calcium ions from the bathing fluid, whereas all postsynaptic activity disappeared. The diphasic deflection was propagated along fibres lying parallel to the pyramidal layer with a velocity of 0.3 m/sec. It could follow short bursts of stimulation at 300 Hz. The absolute refractory period was 2.0 msec.
(4)
The initial diphasic deflection is interpreted as the compound action potential of the largely unmyelinated afferent fibres to the CA1 neurones.

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PS was quantified by its amplitude measured as the length of the projection of the minimum peak on the line connecting the two maxima peaks of the PS waveform. Given that the size of Fv indicates the number of activated afferent fibers (Andersen et al., 1978), we used the EPSP/Fv ratio to quantify synaptic effectiveness. The PS/EPSP ratio was always used as an index of postsynaptic excitability.
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^*: Present Address: Department of Physiology, University of Umea˚, Sweden.

^**: Present Address: Department of Physiological Psychology, University of Bergen, Norway.

^***: Present Address: Department of Physiology, University of Gothenburg, Sweden.

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Functional characteristics of unmyelinated fibres in the hippocampal cortex

Abstract

Brain Research

Lamellar organization of hippocampal excitatory pathways

Exp. Brain Res.

Specific long-lasting potentiation of synaptic transmission in hippocampal slices

Nature (Lond.)

Distribution of hippocampal mossy fibers in the rat. An experimental study with silver impregnation methods

J. comp. Neurol.

Parallel fiber stimulation and the responses induced thereby in the Purkinje cells of the cerebellum

Exp. Brain Res.

The effect of calcium on the myelinated nerve fibre

J. Physiol. (Lond.)

An extreme supernormal period in cerebellar parallel fibres

J. Physiol. (Lond.)