I try not to ask people for favours too often, but, if you could, I’d like you to do something really simple for me. Wiggle the big toe on your left foot.
Pretty simple, right? Well…..maybe not quite as simple as you think.
Behind that tiny little movement of your toe was a surprisingly complex chain of events. First of all, before you could even get to moving, you had to read, and understand, the words on this page. First the light reflected from this page travelled to your eyes, entering through the cornea at the front of your eye. The light passed through your lens, and was focused onto the retina at the back of your eye. Special photoreceptor cells in your retina then converted this light into an electrical signal, which then travelled down your optic nerve, from the back of your eye, and into your brain.
Your brain then had the job of interpreting and making sense of these signals. Different areas of your brain played a part in this, all working together. One area recognised the symbols that you know as letters. Other areas put those together into words, then put those words together to form a mental image. Yet another area planned how you would respond to my request.
Once your brain had figured out that I was asking you to “wiggle the big toe on your left foot”, and decided to do it, it needed to get that information to your foot. The signal to move your toe was generated in an area of the brain called the primary motor cortex – it’s the bit that’s responsible for voluntary movements. This signal, which took the form of an electrical pulse, had to move all the way from your head to your toe. It did this by moving along a specialised type of cell called a neuron – in this case a type of neuron called a motor neuron. The motor neuron carried the signal from your brain down into your spinal cord. In your spinal cord that motor neuron met, or synapsed with, a second motor neuron, which carried the electrical signal on to the muscle of your toe. Once it reached that muscle, it caused the muscle to contract. The result? Your toe moved.
When you wiggled your toe you would have felt it move against the floor, or against the inside of your sock or shoe. Sensory receptors in your toe felt this pressure, and, in much the same way your brain sent the “move” signal to your toe, your toe sent a signal back to your brain. This signal raced along neurons, back into your spinal column, and all the way up to your brain. And that’s how you knew you had successfully moved that big toe.
All of that is taking place, in just a few fractions of a second, just so you can wiggle your big toe. Now imagine if I’d asked you to make me a cup of tea….
First published online in the Armidale Express, August 12th, 2019