Spontaneous electrical activity and behavior in the leech Hirudo medicinalis

Elizabeth Garcia-Perez

doi:10.3389/neuro.07.008.2007

Abstract

In the absence of external stimuli, animals explore the environment by performing irregular movements, but the neuronal mechanisms underlying this arrhythmic motion are largely unknown. In this paper, we studied the relationship between the spontaneous neuronal activity in the leech (Hirudo medicinalis) and its behavior. We analyzed the electrical activity of isolated ganglia, chains of two connected ganglia, and semi-intact preparations. The spontaneous electrical activity in ganglia was characterized by the occurrence of irregular bursts of spikes with variable duration and size. Properties of these bursts were modified by synaptic inputs arriving from the neighboring ganglia and from the two primitive brains located in the head and tail. In fact, in semi-intact preparations, unusually large bursts of spikes occurring spontaneously were recorded and caused the leech to move even in the absence of any external sensory stimulation. These large bursts appear to act as internal triggers controlling the spontaneous leech behavior and determining the duration of stereotypical motor patterns.

Highlights

Basic mechanisms underlying muscle activation and motor control seem largely shared among different species (Grillner, 2003)
The spontaneous activity in leech ganglia The spontaneous activity observed in leech ganglia was influenced by electrical signals arriving from the rest of the nervous system of the animal
These effects were observed in semi-intact preparations (Figure 2A), where it was possible to record the activity of ganglia connected to the intact nervous system of the leech

Summary

Introduction

Basic mechanisms underlying muscle activation and motor control seem largely shared among different species (Grillner, 2003). Animals often perform irregular movements that are not directly evoked by external trigger events, but the neuronal mechanisms responsible for such proactive behaviors have been investigated only recently (Lee and Assad, 2003; Maimon and Assad, 2006) and are still largely unknown. We think that this kind of behavior could be driven by the spontaneous electrical activity that is always present in the nervous system (Harris, 2005; Raichle, 2006) and that is already known to affect the response of many neuronal networks to external stimuli (Arieli et al, 1996; Fiser et al, 2004; Hasenstaub et al, 2007; MacLean et al, 2005)

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Conclusion