A central tenet in biology holds that structure yields function. Whatever the cell does is significantly influenced by its morphology, or shape. For example, the morphology of neurons in the nervous system guides communication. On a macroscopic level, neurons talk to each other in circuit-like networks. Neurons with long axons (one meter or even longer) will be able to communicate with other neurons located far away, whereas neurons with short axons (one micrometer, or 0.1 percent of a millimeter) talk to each other in small localized circuits.
On a microscopic level, however, a great deal happens inside a neuron as it processes messages. Typically, the information is received by dendrites, passed through the cell body and finally ends up at the axon, where the message is relayed to the next neuron. Termed dynamic polarization, this is the classical model that describes the information transfer within each neuron.
This popular model of intracellular communication, however, may be too simplistic to encompass all of the impressive neuronal diversity in the body. Defying the dynamic polarization model, which posits that axons arise from cell bodies, a group of neuroscientists from Germany have discovered that a significant number of axons can also arise from dendrites. Recently published in the scientific journal Neuron on Sept. 17, these findings have major implications for understanding how neurons communicate and the neuronal mechanisms underlying our abilities for learning and memory.
In order to talk to each other, neurons release chemicals called neurotransmitters. The binding of a neurotransmitter to dendrites of the receiving neuron will generate an electrical signal that ripples through the cell body until it reaches the initial segment of the axon. As the electrical current flows and builds up, the neuron performs computations to decide whether amount of current is sufficient to ultimately reache the axon or not. Thus, certain signals will dissipate, whereas other signals will travel to the axon to generate an action potential, allowing the neuron to pass on the message to the next neuron.
Interestingly, the authors of the study discovered that information transfer can skip the cell body. Focusing on the hippocampus, a brain region critical for learning and memory functions, the study authors found that axons can actually originate from dendrites, rather than from cell bodies. In particular, about half of the pyramidal neurons in the CA1 region of the hippocampus exhibited such unique properties, which the researchers termed axon-carrying dendrites.
More importantly, the researchers performed electrophysiological studies to demonstrate that by skipping the cell body, axon-carrying dendrites transfer electrical signals much more easily than other dendrites. As a result, axon-carrying dendrites may provide neurons with an alternative “neuronal track” to deliver important messages more quickly and efficiently. While other studies have also reported evidence of axon-carrying dendrites, such a phenomenon was shown here to occur much more frequently than previously expected, and functional analysis indicate how this unique morphology influences information transfer.
Although the study provided the field with a greater insight into the structural diversity of neurons, it also raises more interesting questions. Given that axon-carrying dendrites allow incoming electrical signals to be forwarded much more efficiently, how does such a structural property contribute to brain function? Since the study focused on a brain region that is important for learning and memory, what roles are axon-carrying dendrites playing in the formation of memory? Answering these questions will advance our understanding of fundamental neuroscience, potentially allowing us to translate this knowledge into treatments for nervous system disorders in which neuronal communication is disrupted.
