Electrophysiology of the Neurosecretory Cell

Abstract

This chapter discusses the electrophysiology of the neurosecretory (NS) cell. The NS cell possesses the unique characteristic that at least one of its axon collaterals terminates in the neurohemal structure. The chapter discusses the electrical properties of the NS cell membrane, the role of action potentials in the endocrine activity of NS cells, and synaptic control of NS cells. In electrophysiological studies of NS neurons it is extremely important to identify definitively a cell under study as a NS cell either morphologically and/or electrophysiologically. All the identified NS neurons so far examined in invertebrate and vertebrate species have been proved to generate and conduct action potentials. Ionic mechanisms for resting and action membrane potentials have been studied mostly in invertebrate NS neurons. A number of studies have depicted the mechanism of excitation-secretion coupling as follows. Depolarization of NS axon terminals, which is produced physiologically by action potentials conducted along the NS axons, induces an increase in calcium permeability of the terminal membrane, and as a result causes a calcium influx along the electrochemical potential gradient. Intracellular calcium ions in turn evoke the exocytotic release of neurohormones and binding proteins. Studies employing both techniques of antidromic identification and iontophoretic drug application have provided suggestive evidence for a particular substance to be a neurotransmitter which directly controls the activity of NS neurons in the mammalian hypothalamus.