Efficacy of thalamocortical and intracortical synaptic connections: Quanta, innervation, and reliability

Thalamocortical (TC) synapses carry information into the neocortex, but the y are far outnumbered by excitatory intracortical (IC) synapses. We measure d the synaptic properties that determine the efficacy of TC and IC axons co nverging onto spiny neurons of layer 4 in the mouse somatosensory cortex. Q uantal events from TC and IC synapses were indistinguishable. However, TC a xons had, on average, about 3 times more release sites than IC axons, and t he mean release probability at TC synapses was about 1.5 times higher than that at IC synapses. Differences of innervation ratio and release probabili ty make the average TC connection several times more effective than the ave rage IC connection, and may allow small numbers of TC axons to dominate the activity of cortical layer 4 cells during sensory inflow.