Have you ever been startled by a sudden noise, sight or touch? It can be quite a shock to the system. You tense up, your mind blanks out all previous thoughts, and you find yourself preparing for the worst. Then there are the lingering effects that can last for minutes after it is all over. While you may hate the feeling of being startled, neuroscience researchers have found the entire process is a natural part of life inherited in evolution from our ancient ancestors.
Imagine a loud sound hits your ears, like a crash, explosion, or if someone has turned your headphones up too loud, the auditory nerve sends a signal to the brainstem. Here, a group of neurons controlling your ability to move send out messages to your eyes, neck, shoulders, and legs. The simple message is to move, which ends up causing us to blink, change our posture, and possibly jump.
The whole point of this complicated – and for some embarrassing – reaction may not seem apparent at first. But it is a necessary part of the fight-or-flight response. The reflex is quite possibly the best way to prepare ourselves for an incoming attack and so we can respond appropriately.
In the modern world, many may wonder why we have this particular function. To answer this, we need to take a look at the development of the nervous system from an evolutionary perspective. Humans are not unprecedented in their neural biology; we are an evolved form of an anatomical blueprint that has been conserved over millions of years, maybe 500 million years. Many of the functions our brains, neurons, and peripheral nerves perform today are shared amongst many of Earth’s related creatures, including those with a fish-like or tadpole-like body form.
For some researchers, this begs the question: how far back in evolution can the startle response be found? For the laboratory of Dr. Ian Meinertzhagen at Dalhousie University, the quest for this answer led him and his research fellow, Dr. Kerrianne Ryan, down a fascinating path provided by our evolutionary tree. As they show in the journal Current Biology the startle response dates back much further than most of us may believe, at least 500 million years.
The team investigated a rather unassuming species officially known as Ciona intestinalis although you may know it better as a common sea squirt. But it has a tiny marine tadpole larva that actually has a tubular central nervous system, or CNS, that in many of its aspects compares to our own, and that can enable the larva to react to neural stimuli. Unlike humans, who have billions of neurons, however, the CNS of this creature is relatively minimalistic.
The team focused on the larvae of Ciona, which have only 177 neurons in their CNS. These contain a few cell types including five pairs of motor neurons, and what are known as descending decussating neurons, or ddNs. The name refers to the path of a specific pair of neurons that cross the brain’s midline, from the left side of the body to the right and its partner vice versa. The simplicity makes the animal a perfect model for study, with the added advantage that many features can be related to the brains of vertebrates such as fishes, and thereby our own brains.
The team set out to show the evolutionary link between sea squirts and fishes. They hoped to use the startle response as a model. But rather than try to scare the little creatures, the team had another approach. They used microscopy.
The process was relatively straightforward. Larval sea squirts were examined using electron microscopy to identify the different components, connections and networks occurring in their nervous system. The hope was to find network similarities among cells in more recently evolved creatures that are responsible for the startle response.
When the imaging was complete, the team had a fascinating comprehensive neural map of the larval nervous system, like a wiring diagram. This was only the second such complete map for any animal. More importantly, they found the cellular similarities in the startle response. While there were some obvious differences in the way the cells are organized in the body, the main pathways and networks necessary to support a startle response were identified.
From this analysis, the team could develop a means to, in effect, startle a sea squirt tadpole. First, the stimulus signal arrives at the ddNs. They in turn coordinate a response through the motor neurons innervating the tadpole’s tail, resulting in the most useful fight-or-flight response, a tail flick. Perhaps more fascinating is the nature of these flicks. Instead of being symmetrical, as with normal swimming, the reflexes coming from the ddNs are unilateral. As one might expect with being startled, the goal is to get out of the immediate area. The actions taken by the larvae may appear to differ from how we deal with startles yet neurologically speaking, they are very similar.
While the team have shown the evolutionary conservation of a primal behaviour, the results offer much more than just a perspective on our past. Mapping the Ciona nervous system provides a greater understanding of the animal. The study also provides a nice early-stage animal model for neuroscience investigation, especially for future functional studies.
We may now have a simple testing mechanism for drugs and other treatments particularly in the realm of startle-reflex disorders such as epilepsy, Tourette’s syndrome, and the condition known as hyperekplexia, which means exaggerated surprise. This process of validation using thousands of larvae may also help to reduce the cost of testing, which can be, in a word, startling. This in turn could reduce the financial fight-or-flight response in researchers, clinicians, and account executives.
Original research article:
Ryan K, Lu Z, Meinertzhagen IA. Circuit Homology between Decussating Pathways in the Ciona Larval CNS and the Vertebrate Startle-Response Pathway. Curr Biol. 2017 Mar 6;27(5):721-728. doi: 10.1016/j.cub.2017.01.026.
