GLIA IN THE LEECH CENTRAL NERVOUS SYSTEM: PHYSIOLOGICAL PROPERTIES AND NEURON-GLIA RELATIONSHIP

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Abstract
An electrophysiological study was made of the large glial cells in the central nervous system of the leech, Hirudo medicinalis. There are 3 morphologically distinct cell types: (a) the packet glial cells, 6 per ganglion, each of which envelopes about 60 monopolar nerve cell bodies; (b) a pair of glial cells that permeate the neuropil; (c) the connective glial cells, one pair between adjacent ganglia, each enveloping several thousand axons. These cells take up approximately half of the volume of the central nervous system. Their physiological properties were similar. Glial cells have resting potentials of 60-75 mV. and a membrane resistance comparable to that of neurons and perhaps higher. In contrast to neurons, they do not give impulse-like responses even to large changes of their membrane potential. Thus, they do not participate directly in the electrical signaling activity of the nervous system. Moreover, glial cells can be removed without interfering with the impulse activity of the nerve cells which may continue for many hours. This demonstrates that glial function is not directly concerned with the action potential mechanism of nerve cells. The commonly held view that the glial cells play a trophic role is discussed. There are low-resistance pathways through which current can flow freely from one glial cell to another. It is concluded that special contacts link these cells. In contrast, no similar connections exist between nerve and glia. There was remarkably little electrical interaction even during widespread neural activity or when large steady currents were passed into nerve or glial cells. The 100 to 150 A clefts between nerve and glia, constituting extracellular space, were measured in electron micrographs of the connective. The extracellular space accounts for about 5% of the nervous tissue. Since no mechanism was discovered by which a large glial cell can be called into action as a unit, it is suggested that its trophic activity is stimulated by a local influence of the nerve on that portion of the glial cell which is immediately adjacent to it. Possible electrical and chemical mechanisms of such neuro-glial coordination are discussed. If connective glial cells are cut they rapidly regain their resting potentials and a high resistance is established across the cut surface. No unusual properties were observed in the neurons. Their resting potentials of about 50 mV. were lower than those of glial cells. Impulses, recorded in various uninjured cell bodies, varied over a wide range between 20 and 90 mV. It is concluded that certain of the cell bodies normally are not invaded by the impulse of the axon.

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