With 302 neurons, 200 of which are in the head, hermaphroditic female roundworms reveal surprising sex differences in the nervous system and how these differences influence behavior, reproduction and aging.
Researchers at Northeastern University compared brain activity in men and hermaphrodites Caenorhabditis elegansa microscopic worm commonly used in biological research to understand how the male and female nervous systems are controlled differently by gender.
Their study, published in Current Biologyshowed that almost every chemosensory neuron in a tiny roundworm responds differently to at least one chemical signal depending on the sex of the animal, “suggesting that sexually dimorphic circuits are ubiquitous and stimulus-dependent,” the researchers wrote in the study.
Because male and hermaphrodite nervous systems showed wide-ranging differences in the way they process information, the lead researcher told Northeastern that their central question is how brain structure relates to brain function.
Mapping sex differences throughout the brain
In 1 millimeter long, transparent hermaphrodites Caenorhabditis elegans According to Northeastern University, humans have 302 neurons throughout their nervous system, while men have about 387.
Vivek Venkatachalam, an associate professor of physics at Northeastern University, explained that the worms are primarily chemosensory animals. They rely heavily on smell and taste. The researchers monitored individual neurons as worms encountered chemical stimuli, including smells, salt and pheromones. Using calcium imaging, a technique that measures changes in calcium levels in neurons, they tracked changes in nerve activity across the animals’ heads.
The team found that sensory responses varied by gender and also by stimulus.
“We have known for some time that the nervous systems of male and hermaphrodite worms are wired somewhat differently, but a wiring diagram reveals only limited information about how a brain actually works,” Javier Apfeld, an associate biology professor at Northeastern, said in the Northeastern report.
The team found that sensory responses varied not only by gender but also by stimulus, and identified gender-specific responses in neurons associated with food intake and digestion.
“This dimorphic and monomorphic activity was present to a lesser extent in downstream interneurons and even less in motor neurons, suggesting that sensory neurons are the primary source and site of sexually dimorphic activity,” the researchers reported.
The researchers also examined whether differences in neuronal activity corresponded to differences in the physical connections between neurons.
Scientists compared the worms’ observed neuronal activity with previously published maps of synaptic connections, or connectomes. The researchers found that functional differences in neurons were distinct from differences in their physical connections.
In other words, the researchers examined whether neuronal activity corresponded to the physical connections between neurons. They found that structural differences did not predict functional changes.
“Comparing the functional activity we measured with the published synaptic connectomes of both sexes revealed that sexual dimorphism in functional connectivity is distinct from and complementary to sexual dimorphism in synaptic connectivity,” the study says.
The results could provide a basis for studying sex differences in the nervous systems of other species. However, Venkatachalam cautioned that the results from worms may not necessarily be directly applicable to humans.
“The goal is to learn principles of neural computation that are applicable worldwide,” he concluded to Northeastern News.
