Relationship of spikes, synaptic activity, and local changes of cerebral blood flow

The coupling of electrical activity in the brain to changes in cerebral blo od flow (CBF) is of interest because hemodynamic changes are used to track brain function. Recent studies, especially those investigating the cerebell ar cortex, have shown that the spike rate in the principal target cell of a brain region (i.e. the efferent cell) does not affect vascular response am plitude. Subthreshold integrative synaptic processes trigger changes in the local microcirculation and local,glucose consumption. The spatial specific ity of the vascular response on the brain surface is limited because of the functional anatomy of the pial vessels. Within the cortex there is a chara cteristic laminar flow distribution, the largest changes of which are obser ved at the depth of maximal synaptic activity (i.e. layer IV) for an affere nt input system. Under most conditions, increases in CBF are explained by a ctivity in postsynaptic neurons, but presynaptic elements can contribute. N eurotransmitters do not mediate increases in CBF that are triggered by the concerted action of several second messenger molecules. It is important to distinguish between effective synaptic inhibition and deactivation that inc rease and decrease CBF and glucose consumption, respectively. In summary, h emodynamic changes evoked by neuronal activity depend on the afferent input function (i.e. all aspects of presynaptic and postsynaptic processing), bu t are totally independent of the efferent function (i.e., the spike rate of the same region). Thus, it is not possible to conclude whether the output level of activity of a region is increased based on brain maps that use. bl ood-flow changes as markers.