Larry Keenan

The primitive brains of platyhelminthes

Abstract The study of polyclad flatworms promises new insight into the organization and role of the brain during the evolution of Metazoa. Behavioral, physiological and anatomical studies are revealing that much, if not most, of the basic neuronal machinery seen in complex vertebrate central nervous systems, as well as higher invertebrates, is already present in extant flatworms.

Ionic bases of action potentials in identified flatworm neurones

SummaryThe ionic bases for generation of action potentials in three types of identified multimodal neurones of the brain ofNotoplana acticola, a polyclad flatworm, were studied. The action potentials were generated spontaneously, in response to water-borne vibrations, or by intracellularly injected current pulses. At least three components comprise the depolarizing excitable phase of the action potentials: (a) a rapidly inactivating TTXsensitive Na+ component (Fig. 2); (b) a Ca++ component that is unmasked by intracellular TEA+ (Figs. 4, 6, 7); (c) a TTX-resistant Na+ component (Fig. 8). Two K+ currents appear to account for the repolarization phase of the action potentials: (a) a rapid K+ current that is blocked by intracellular TEA+ (Figs. 4, 7, 8) and (b) a Ca++ -activated K+ conductance that is blocked by Ca++ and Ba++ (Fig. 6). Ionic mechanisms in the generation of action potentials in the central multimodal neurones ofNotoplana pharmacologically resemble those in higher metazoans.

Responses of midbrain dorsal raphe neurons to ethanol studied in brainstem slices

The effects of ethanol on midbrain dorsal raphe (DR) neurons were studied using the brainstem slice preparations. Ethanol in low doses (100 mg% and 200 mg%) exerted more excitation than inhibition on DR cells. Higher doses of ethanol produced inhibition in the majority of DR neurons. The present data indicate that the effects of ethanol were biphasic with low concentrations tending to excite and high concentrations to inhibit DR neurons.

In vivo visualization of living flatworm neurons using Lucifer yellow intracellular injections.

Turbellarian flatworms lend themselves to neurobiological investigations using intracellular iontophoresis of Lucifer yellow provided that one is able to anesthetize the animal and expose the nervous system. This paper details the methods used with the polyclad Notoplana acticola and the rhabdocoel Mesostoma ehrenbergii. Marine turbellarians can be anesthetized with equal parts of sea water and isotonic MgCl2 and fresh-water animals with an 8% ethanol in spring water. Animals can be held steady with minuten pins and spines of the cactus Opuntia basilaris or O. littoralis. Sheaths surrounding the brain can be digested away with a protease. Conventional glass microelectrode techniques are used to fill the cells with fluorescent dye, Lucifer yellow. The preparation needs to be viewed using darkfield illumination. Cells can be photographed through the microscope or traced using a camera lucida attachment to a fluorescence microscope. Tracings tend to be more useful for preserving details of the three-dimensional nature of the neuronal cytoarchitecture.

Primitive nervous systems: electrical activity in the nerve cords of the parasitic flatworm, Gyrocotyle fimbriata.

A large compound potential (LCP) could be evoked in the longitudinal nerve cords of Gyrocotyle fimbriata. This potential was graded and showed decremental conduction. Decrement was more pronounced in the posterior direction and was exponential with distance, dropping to 50% of its initial value in less than 5 mm. Conduction velocities in the anterior to posterior direction were greater than the reverse direction (means = 37.3 cm sec-1 vs. means = 30.3 cm sec-1). Single suprathreshold stimuli evoked long-lasting trains of spikes. Frequency of spiking varied linearly with stimulus intensity and averaged 0.7 spikes per volt stimulus. Mechanical stimulation of the acetabulum produced a short-lasting (10 sec) facilitatory effect to the second of a pair of electrical stimuli, whereas mechanical stimulation of the holdfast resulted in facilitation lasting several minutes.