The horizontal (tangential) fibres system of lamina I of the cerebral neocortex.

Lamina I has two principal components: a meshwork of fine unmyelinated fibres, the so-called horizontal fibres (h.f.); the profusely branching apical dendrites of all pyramidal cells. These dendrites are studded with spines on which the h.f.'s make excitatory synapses in a crossing-over manner. The h.f. system is produced by the fibres that ascend through the modules and bifurcate in lamina I, each branch extending for up to 5 mm from its origin. These ascending fibres are axons of Martinotti cells, and of corticocortical fibre inputs to the module, both ipsilateral and callosal. Electrophysiological investigations have shown that the h.f. system has a powerful excitatory action on the pyramidal cells. There will be developed the hypothesis that the h.f.-pyramidal dendrite system of lamina I is a structure specially designed for the storage of memories. Conjunction of cartridge activation of pyramidal cell dendrites with a selection of impulses in h.f.'s could produce a synaptic potentiation encoding the experience that is dependent on that selection of h.f. impulses. On a later occasion these potentiated synapses could be effective in replaying the coded neuronal pattern and so retrieving the memory.