A few months ago I was hurriedly preparing for a dinner party that I had promised to bring a salad to. I was halfway through a late afternoon beer on that particular Sunday and was feeling pretty relaxed, having a great time chatting away with my guy while prepping vegetables. If you’re me, a little late afternoon tipsiness + the distraction of conversation + a chef’s knife is a very dangerous combination! Next thing I knew, I had sliced pretty deeply into my finger along with the ear of corn that I was working on.
On the left: my finger a few days after the cut. On the right, my finger 3 months later.
It was a pretty deep cut. (And no, I did not get stitches. Maybe I should have.) We never made it to the dinner party. I spent the rest of the evening with my hand elevated above my head, complaining while my boyfriend fed me dinner.
As the healing slowly progressed, I tested the tip of my finger for feeling and was intrigued to find that while immediately after the accident I could just barely feel sensation in the tip of my finger, over the next few weeks sensation slowly increased from the tip down to the cut. Over two months later, it’s still not completely back to normal, and sensation is sort of dull over the cut.
If you’re me, you observe this and think, WHAT’S GOING ON IN THERE?
So I’m going to take you on a microscopic tour of what (probably) went on inside my finger while it healed.
Sensory nerves are made up of bundles of specialized cells called neurons. Sensory nerves are what give us our sense of touch, and what yell at us when we are in pain. Neurons stretch long protrusions called axons and dendrites over long distances- in the case of the sciatic nerve, all the way down the length of the human leg (almost a meter!)- and transmit information about what’s going on in our fingers and toes back to our brains. Our brains then make decisions and send directions down motor neurons back to muscles to move accordingly.
Why are these cells so bizarrely huge, compared to most other cells in our bodies? Why not just make up a long chain of normal-sized cells to transmit information about heat or cold or sharp or soft or smooth or (….)? Because these long cells allow extremely fast transmission of information over long distances. This usually means that my brain would tell my hand to adjust before being sliced by an incoming chef’s knife!
I probably cut one of the terminal branches of the median nerve, which runs down the arm and to the tip of the index finger.
When I cut my finger, the skin and muscle cells around the cut quickly amped up their growth rates, dividing and migrating into the wound to heal it. Because neurons are such elongated, specialized cells, and they are part of bigger circuits of communication, it is not easy for them to simply replace themselves with a new copy. Rather, a sensory neuron must go through the labor-intensive process of regenerating the wounded protrusion. This growth happens at a rate of approximately 1 millimeter per day.
First, the distal portion of the severed nerve dies off in a process called Wallerian degradation.
Then, the cell body (also called the soma) bulks up in preparation for sending more energy and materials down the injured protrusion. Much of these construction materials are sent down the protrusion by traffic in lipid vesicles traveling along microtubules.
Meanwhile, the cell also directs its energies towards making a growth cone at the tip of the injured protrusion. The growth cone contains lots of cytoskeletal elements (check out my previous post mapping out the components of a cell) that push forward from the tip of the injury site.
Slowly, slowly, this growth cone moves down the track left behind by the dead stump of the nerve protrusion. Check out this video to see live microscopy of a growth cone moving.