The Body Electric

We are juiced. From head to toe, miles of membrane shuttle electric charges through the body. Impulses pour in to my brain from eyes, ears, nose, mouth, and skin as raw versions of what I see on this screen, the feeling of the keys at my fingertips, the tapping sounds; then out from the brain through the wires to the muscles in my hands and fingers to type the s e  l e t t e r s.

Simple nerve systems appeared in early jellyfish and other sea creatures about 500 million years ago.  Loose nets of nerves responded to light and the touch of other creatures as these swimmers captured smaller fish and dodged bigger ones.

Much earlier, in the first fully developed cells, neurons began to evolve from membranes. A membrane, in Wikipedia’s words, is “a selective barrier; it allows some things to pass through but stops others.” A cell’s membrane helped the cell manage the salt levels inside the cell as it floated through the salty ocean. And since the salts of sodium, potassium and calcium consist of atoms with a positive or negative charge, the pores in membranes became gates that opened and closed to control the electrical potential across the membrane itself.

As animals evolved, such membranes lengthened into neurons with conductive axons, the “wire” of the nerve cell. In us, the longest axon runs down the length of each leg, branching as it goes. The shortest axons, fractions of a millimeter, fill our heads by the billions.

Neurons in the brain (Wikipedia)

Neurons in the brain
(Wikipedia)

The axons don’t carry an electric charge in the way that a wire carries electricity or a lightning bolt of electrons crashes to the ground. Instead, think of the wave at a sports stadium, where groups of fans stand up, throw their hands in the air, and sit down in a spontaneous sequence that moves through the rows. A nerve impulse moves down the axon in a similar way, charged atoms crossing through opened pores from one side of the membrane to the other and then quickly back again while the “wave” of the electric charge moves along.

The impulse never varies in strength. It is either on or off, either moving or only ready to move. There are no drops in the current, no power failures, no biological surge protectors needed. If a muscle must contract to move a load, the nerve signal, always at the same strength, simply repeats rapidly enough so that the muscle cells remain contracted.

At both ends of the axon, where the impulse begins and ends, devices of various kinds translate between the electrical charge and other structures. In the ear, sound waves cause small hairs to vibrate and set off the impulses that we hear as “hello.” In our eyes, light causes molecular changes that trigger the impulses to the brain to form the image we recognize as a chair. Where a neuron terminates at a muscle cell, the final “wave” triggers chemicals that start the muscle’s contraction.

We barely notice all this wizardry. Compared to the breath that we can feel and the blood we can see, our circuitry is undetectable. But if we’ve been shocked by a faulty toaster or we suffer from numbness or irregular heartbeats, we’ve glimpsed what can go wrong.

In another way, though, we are always aware of the electricity in us. Notice the faint tingle that is always present in our limbs and head. It’s a sense of animation, a potential, an ability to move a muscle, look around or think a thought at any time. That tingly readiness is, essentially, our neurons at the ready. It’s a reminder that we’re alive.

Peter Wohlleben’s “The Hidden Life of Trees”

Until recently I was quite sure that a broad difference between animals and plants was that animals, because they are mobile, readily interact with each other (flocking, pursuing, etc.) while plants, anchored to the ground, don’t do so because they can’t. Except to attract insect pollinators, plants, I thought, live a life of exquisite solo struggle, seeking only the sun and water.

I’ve been steadily learning how far off I was. German forester Peter Wohlleben’s popular book, The Hidden Life of Trees: What They Feel, How They Communicate, is the most compelling lesson yet.

Among his many descriptions of communication and mutual assistance is Wohlleben’s account of how trees defend not only themselves but also each other. Observers have noted, for example, that umbrella thorn acacias in the African savannah pumped toxins into their leaves when they felt giraffes nibbling on them. “The giraffes got the message and moved on to other trees in the vicinity. But did they move on to trees close by? No, for the time being, they walked right by a few trees and resumed their meal only when they had moved about 100 yards away.” They passed by the nearest trees because the trees being nibbled, in addition to pumping a repellent, “gave off a warning gas that signaled to neighboring trees that a crisis was at hand.” The giraffes knew these trees would not taste any better and kept walking.

hidden life of trees (pri.org)

pre.org

Many trees also have the ability to call in the air force. Reacting to bites from hostile insects, such trees emit scents that attract predators that devour the pests. “For example, elms and pines call on small parasite wasps that lay their eggs inside leaf-eating caterpillars.” The growing larvae devour the caterpillars from the inside.

The book brims with information and appreciations of this kind. Three more examples:

  • Trees that spend their lives in the forest fare much better than trees raised in one place and then transplanted to the forest. “Because their roots are irreparably damaged,…they seem almost incapable of networking with one another.” Like “street kids,” they “behave like loners and suffer from their isolation.”
  • Time for trees is slow and long. Internally, they, like animals, send alerts to parts of their body via chemicals and electrical impulses. But in a tree the electrical impulses move only about a third of an inch per second. (In our bodies, pain signals move  through our nerves about two feet per second, muscle impulses a hundred times faster.) No wonder it seems to us that plants are unresponsive.
  • Conifers (evergreens) “keep all their green finery on their branches” throughout the winter and have been doing so for 270 million years. Then deciduous (leaf-bearing) trees came along 100 million years ago, growing and discarding annually millions of delicate green solar panels. Was this an improvement? Why go to all that trouble? Wohlleben asks. Because “By discarding their leaves, they avoid a critical force—winter storms.” Between high winds, muddy soil, and a surface area equivalent to that of a large sailboat, tall evergreens take a battering in European winters. Growing and then dropping their huge surface area every year proved well worth while for the leafy new comers.

Wohlleben’s liberal use of human descriptors to explain the actions of trees delights many readers and annoys others. Andrea Wulf, in her review of the book, has both reactions.

I’m usually not keen on anthropomorphizing nature—and here trees are “nursing their babies” and having “a long, leisurely breakfast in the sun” while…fungus mushrooms are “rascals” who steal sugar and nutrients. These cutesy expressions make me cringe….But I have to admit that Wohlleben pulls it off—most of the time—because he sticks with scientific research and has a knack for making complex biology simple and thoroughly enjoyable.

I agree. While the vocabulary may bestow on trees a dignity and affection that we usually reserve for our own kind, it is scientists’ growing understanding of trees that creates the real story here. At a time of rapid environmental change, the book is as fascinating a revelation as one could ask for that life is even more intricate and purposeful than we knew.