Martina, Marco, MD, PhD
Physiology; Feinberg School of Medicine
Tarry 5-705 Chicago
|Areas of Research||
Cell Imaging & Electrophysiology, Learning & Memory, Signal Transduction
|NU Scholar Profile|
|Recent Publications on PubMed|
It is well known, since the seminal work of Camillo Golgi and Santiago Ramon Y Cajal, that neurons are composed of three main anatomical components; the soma (cell body), the axon and the dendrites.
For quite a long time these compartments were thought to have clear and distinct physiological functions. The dendrites were seen as “antennas” collecting the input signals coming from other neurons and transmitting them passively to the cell body. The soma, endowed with multiple active conductance, was considered the site of integration of all the signals collected by the cell. When the amount of excitation (depolarization) summed up to a value above a specified threshold, the axon hillock would have generated an action potential that would have propagated forward along the axon to reach the synapses onto the dendrites of another cell. In the last twenty years, however, and particularly since the development of patch clamp in brain slices (Edwards et al. 1989), this picture has been modified, and it has become clear that the dendrites, far from being passive cable conductors, are endowed with a rich repertoire of ligand- and voltage-gated ion channels that enables a dendritic signal processing that is essential for basic neuronal processes as learning and memory.
The main focus the lab is the study of the ion channels expressed in dendrites of cells of the central nervous system and how they influence the physiological properties (pace-making, synaptic integration and plasticity, network properties) of neurons. We concentrate our attention on two main cell types; GABAergic interneurons of the hippocampus and cerebellar Purkinje cells.
The properties of interneuron dendrites are still vastly unknown, mainly due to technical problems connected to their thin diameter that prevents routine electrophysiological approaches. In very recent years, however, they have become accessible to recordings as a consequence of technical improvements, particularly in the optic systems available to visualize the cells.
It has turned out that, at least in one class of hippocampal interneurons, not only do dendrites allow a full amplitude and fast propagation of axo-somatic generated action potentials, but, in response to strong local stimulation, they can be the site of initiation of action potentials that subsequently invade the soma.
On the other hand, the huge dendrites of cerebellar Purkinje neurons do not contain voltage-gated sodium channels. Nevertheless, they express voltage-gated calcium and potassium channels that are of basic importance for many fundamental physiological functions as shaping of excitatory postsynaptic potentials, regulation of dendritic backpropagation and the generation of the burst firing that is one of typical spiking modes of the Purkinje cells (see figure).
Figure: Spontaneous burst recorded simultaneously from the soma (black) and dendrite (red) of a rat cerebellar Purkinje cell. The green trace represents a point by point subtraction of the membrane potentials recorded at the two sites and shows the direction of current flow between the two intracellular compartments (positive deflections represent current flowing from soma to dendrite).