Aquatic animals require precisely coordinated movements to move efficiently through open waters. Jellyfish, which swim forwards by contracting their umbrella and expelling water, must also react to sensory stimuli on the outer skin of their bell-shaped body in order to initiate chase or flight. How they use their simple nerve networks to activate their muscles is not yet fully understood.
Using a mathematical model, Fabian Pallasdies and colleagues from Prof Dr Susanne Schreiber's Theoretical Neurophysiology research group at the Department of Biology at Humboldt-Universit?t zu Berlin (HU) have now investigated the coupling of neuronal activity and motor response in these swimming movements. In their study, which has been published in the Journal of Neuroscience, they uncover the sophisticated temporal interplay between nerve and muscle cells that causes the muscles to contract rapidly, enabling jellyfish to swim stably and tumour-free. The study provides a rare example of a complete mechanistic explanation of animal behaviour - from the biophysical properties of individual cells to the movement of the entire body.
Combination of nerve, muscle and fluid-mechanical simulation
For their study, the researchers used a mathematical model that not only simulates the electrical activity of the nerve and muscle system of the red-eye jellyfish, but also the bell-shaped body of the animal and how it interacts with the water during swimming. This combination of nerve, muscle and fluid-mechanical simulation showed that it is the rapid, symmetrical muscle contraction in particular that stabilises jellyfish during swimming. The simulation also shows how the muscle contraction is achieved: If the jellyfish is stimulated at any point on its body, the ring-shaped muscle strands running around the body contract in order to push water away outwards from the area enclosed by the jellyfish's body and thus swim forwards. To do this, the muscle ring is electrically activated. This is done by the nerve ring, in which the electrical activity first spreads and then stimulates the connected muscle cells.
But how do nerve and muscle cells interact to achieve the necessary speed of muscle contraction? "In the simplest case imaginable, the electrical excitation would spread in one direction once in a circle over the entire ring," says Susanne Schreiber, head of the working group. "But then the contraction would take too long and the jellyfish would start to tumble." Even if the electrical activity from the stimulation point were to move in two directions simultaneously and thus halve the time it takes to activate the muscles, the simulations showed that this would not be sufficient to stabilise the swimming movements.
Elegant excitation mechanism enables rapid muscle contraction
In their study, the researchers reveal that jellyfish utilise an elegant principle to significantly reduce the time span of muscle contraction: The electrical excitation initially spreads from the stimulation point in the nerve ring in two directions. Initially, this activity of the nerve cells is still too weak to stimulate the muscles. The electrical signals in the nerve ring only synchronise as they spread and are then sufficient to "ignite" the muscle cells. As a result, four waves of activity now propagate in the muscle ring (from both ignition points in both directions). This reduces the total period of time until all the muscles in the ring are activated to around a quarter. In addition, the muscle activation is more symmetrical, which enables a more linear movement.
"The simulation of the swimming movement of the jellyfish proves that the swimming movement is only possible in a stable manner with nerve cell-muscle coupling that supports this fourfold propagation principle," says Schreiber. The study also shows how important it is to consider the direct connection between the properties of the individual nerve cells, the muscle cells and the behaviour of the animal in its natural environment. "In animals with less complex nervous systems, such as the jellyfish, this is now possible thanks to mathematical simulation and mechanisms can be discovered by means of which nerve cells and their properties have a direct effect on behaviour."
Contact
Prof Dr Susanne Schreiber
Department of Biology
Humboldt-Universit?t zu Berlin
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