Nerve cells are excitable. At rest, they are said to be polarized; the cell membrane separates the negatively and positively charged ions, so that the inside of the membrane is negatively charged with respect to the outside. In response to a stimulus, nerve cells produce action potentials, which are generated by the rapid movements of ions (negatively or positively charged atoms) across the nerve cell membrane. These ion fluxes lead to depolarization, or a transient reversal of the membrane voltage – the inside of the membrane becomes positively charged, and vice versa.
The action potential lasts only very briefly – several thousandths of a second later, the voltage across the membrane has returned to its resting state. Depolarization of one segment of cell membrane triggers ion movements in the adjacent segment, so that the action potential is propagated along the nerve fiber, or axon. When it reaches the nerve terminal, the action potential elicits neurotransmitter release into the synapic cleft. These chemicals diffuse across the synapse, and bind to receptors on the adjacent cell, so that it too generates a similar response.
This is the standard view of nerve cell function. An action potential is propagated to the nerve terminal, where it elicits the release of neurotransmitter into the synapse. Since Hodgkin and Huxley elucidated the mechanism of the action potential in 1952, it has been taken for granted that all neurons function in this way. But now, a group of French researchers have demonstrated that nerve fibres in the peripheral nervous system can control a reflex of the intestines without producing action potentials.
The new study was carried out on nerve fibers of the autonomic nervous system that innervate the digestive system and regulate a reflex called the gastroduodenal inhibitory reflex (GIR), in which contractions of the duodenum, or small intestine, are inhibited in response to a stretched stomach.
The GIR is mediated by circuits consisting of just two cells. One of these is a mechanosensory neuron, which projects from the stomach the coeliac ganglion in the spinal cord. In the ganglion, the mechanosensory neuron forms a synapse with the second cell in the circuit, which projects from the spinal cord to the small intestine.
Together, these neurons mediate the GIR. The mechanosensory neuron detects stomach distension, and becomes depolarized in response. The signal is transmitted to the second cell, which then stops the small intestine muscles from contracting so that the stomach can be emptied.
In previous studies, the authors established that the mechanosensory neurons release the gaseous neurotransmitter nitric oxide into the coeliac ganglion in response to stomach distension. They also showed that the coeliac ganglion neurons can organize the GIR without generating action potentials.
This was demonstrated by the addition of tetrodotoxin (a chemical which abolishes the action potential all together) to an in vitro preparation of coeliac ganglion, stomach and duodenum, and by the removal of calcium (which is also essential for the action potential) from the solution in which the tissue preparation was kept. However, the mechanism by which the cells generate the GIR without action potentials remained a mystery.
In the current study, which was led by Caroline Fasano of the Centre National de la Recherche Scientifique (CNRS), the group report that they have determined how this novel mode of nerve function operates. Their findings are published in the open access journal PLoS One.
The French team carried out the new research on the same tissue preparation used in their previous studies. They first performed an analysis of the lipid composition of the axons of coeliac ganglion neurons. This showed that concentrations of a molecule called ceramide increased 7-fold following distension of the stomach, and led the French team to suspect that ceramide could be involved in the novel signalling mechanism.
It was then found that ceramide can elicit in the peripheral nerve fibers the same response that is elicited by various kinds of neurotransmitter receptors. In response to an action potential generated by the mechanosensory neurons in the circuit, increased ceramide production at the membrane of the coeliac ganglion cell activated a second messenger system, which involves a complex cascade of biochemical reactions that convey signals by a neuron into its interior.
The authors propose a model whereby this biochemical signal is propagated along the nerve fiber: production of ceramide at one membrane site triggers the chemical to be produced at the site immediately next to it. Thus, the second messenger system is activated recurrently, with each successive activation taking place further and further from the initial response. And when the cascade reaches the nerve terminal, it induces the release of a neurotransmitter (nitric oxide) just as an action potenital would.
From the length of the nerve fibers in the tissue preparation, and the time taken for the reflex to begin following stomach distension, the velocity at which this signal is propagated along the nerve fiber was calculated to be approximately 1 cm per minute.
As well as the apparent propagation of their excitable state using second messenger systems, coeliac ganglion cells also generate action potentials. They therefore appear to have two different modes of activity. Action potentials are likely to be generated when fast regulation of intestinal function is required, while the second messenger system is utilized for slower responses that take place over periods of minutes or hours.
The proposed mechanism is fundamentally different from the Hodgkin-Huxley model of the action potential. Like various other recent findings, this study shows that our understanding of nervous system function is extremely rudimentary, and will force researchers to re-evaluate their thinking about nerve function yet again.