Moving in a straight line - sounds simple, right?

One hundred years ago Asa Schaeffer blindfolded his friend and challenged him to walk in a straight line. He did three loops of a spiral, before tripping over a tree stump. This wasn't a cruel prank, it was an experiment, and all people are surprisingly bad at this simple challenge.

More modern experiments showed it is lack of external reference points that trips people up. Blindfolded in a desert? You walk in circles. Not blindfolded in a desert? Straight lines. Forest on a sunny day? Straight lines. Forest on an overcast day? Circles. Current Biology, DOI: 10.1016/j.cub.2009.07.053

People need some external reference point, a distant hill, the sun, or even the direction of shadows, to manage a straight line. Why they drift into circles without a reference isn't clear. Is it asymmetric leg strength? A handedness bias? Or some psychological miss-correction? What is clear is that it is a universal problem.

Navigation without reference points is always difficult. This is why spin stabilisation is widely used to stop flying objects, from bullets to rugby balls, curving off course in the air. Even advanced tools like inertial navigation systems, which use dead reckoning from measuring acceleration, always drift off course. 

So how about swimming cells? Many swimming cells and microorganisms have the ability to swim in a straight line. Most have no ability to look at a reference point, they can only perceive the liquid they are in immediate contact with. They are also typically top small to be affected by gravity, so have no way of using up or down for reference. They are essentially blind.

The trick for straight line swimming in cells seems to be some kind of spin stabilisation. Way back in 1901, H.S. Jennings noted that many microorganisms spin as they swim, and thought this could be a spin stabilisation somewhat like a spinning bullet. The problem is it can't be. Rugby balls, bullets and spacecraft use spin stabilisation which depends on rotational inertia to keep them spinning and stable, similar to a spinning top. If you made a top that was the size of a cell, and span it in water, it would stop spinning immediately; there is too much friction. The American Naturalist, 35(413):369-379

It turns out the mechanism is just geometry. A person walking is a back-forward/left-right, a 2D, situation. If you curve the walking path it makes it into a looping circle. For a cell swimming there is another direction to think about; in 3D there are two ways to curve the swimming path. The first which curls the path into a circle, and a second which twists the circle to elongate it into a helix-shaped path. Elongate the helix far enough and it turns into a straight line, with the cell rotating as it swims.

The interesting property of the helical swimming paths is their stability. If a cell deliberately twists its swimming path into a helix then small asymmetries (the cellular equivalent of having one leg stronger than the other) won't bend the swimming path into a circle. Instead it just slightly alters the shape of the helix. It can ensure the cell swims in a dead straight line.

My latest paper is all about how cells manage straight line swimming, looking at trypanosome and Leishmania human parasites. Each aspect of swimming has been looked at before, but I believe has never previously been put together as a full story: from mutants with altered cell shape (to add more or less twist), measuring the effect on swimming, and matching this to simulation of how cells achieve their straight line swimming. PLOS Computational Biology, DOI: 10.1371/journal.pcbi.1005353

There are still big unanswered questions though: Why do parasites need to swim in a straight line? And what are they swimming towards? This is an area of active research, with several major trypanosome research groups (especially Kent Hill and Markus Engstler) interested in addressing these questions.