A motoneuron study: Membrane and firing properties

PEREZ ETCHEGOYEN, CARLOS BERNARDO; SZCZUPAK, LIDIA

Montevideo, Uruguay

Workshop; XIV Escuela Latinoamericana de Neurociencia; 2009

Instituto de Investigaciones Biológicas Clemente Estable

The leech has proved to be an interesting model to research on the neural connectivity for many reasons: the nervous system is relatively simple, their neurons are easily seen and readily identified by the location of their somata, given the large size of the somata it is easy to record intracellularly, and the nervous system is iterated. The leech neural system comprises 21 body ganglia, and two groups of fused ganglia forming the head brain (4 ganglia) and the tail brain (7 ganglia). All the body ganglia are nearly identical, each ganglion has the whole set of interneurons, the sensorial and motoneurons that innervates the segment. In the leech there are four types of muscles implicated in the movement: the longitudinal, the circular, the dorsoventral and the oblique muscles. Contractions of  each of these muscles produce characteristic types of movements: contractions of longitudinal muscles produces the body shortening, contractions of circular muscles produces the elongation of the leech, contracction of oblique muscles produces a stiffening in the body, the dorsoventral muscles flattens the leech body. All these muscles are innervated by a set of motoneurons (excitatory and inhibitory). These motoneurons innervate one type of muscles, and of region (dorsal, dorsoventral, and ventral) of one side (left-right). The longitudinal muscles are innervated by seven motoneurons. Among the excitatory ones the dorsal excitor 3 (MN DE3) is easily to recognize because of it larger spikes in the dorsal posterior nerve (DP). The L motoneuron (MN L) innervates all the longitudinal muscles of one side, and the spikes are easily recognized in the DP and in the first branch of DP. We have characterized membrane properties of these two motoneurons: MN L and MN DE3 in order to compare them, this showed us that the two motoneurons have different membrane properties when they face hyperpolarizations: the MN L reaches higher hyperpolarizations than MN DE3. We have also characterized the spike frequency and firing properties of these motoneurons: the DE-3 reached higher firing frequency but was unable to sustain it; MNL showed no signs of firing frequency adaptation but reached lower firing frequency than MN DE3.