Welcome back to what a plant knows. In today's lecture, we're going to talk about what do plants feel. Now, we feel plants all the time. Sometimes they're soft and comforting, like grass or lying on grass in a park, or the softness of rose petals. Other times, plants, we feel them as rough and thorny, prickly like the thorns on a blackberry bush that are getting in the way of us harvesting some blackberries. but in most cases, plants for us are passive objects, like props that we interact with, that we ignore. We pluck petals off of daisies. We saw the limbs off of trees. But what if the plants knew that we were touching them. You know, it's probably a bit surprising, and maybe even a bit disconcerting to discover that plants know when they're being touched. For example, when I take this Mimosa plant, and I touch it, it's leaves close. Somehow or another, this plant knew that I was touching it, or that something was touching it, and it responded. Other plants, like vines, when their tendrils, the long part that grab onto the fences, when they're touched, they start coiling. And we've all seen the Venus Fly Trap close in on an unsuspecting fly, as it's crawling across its open leaves. Trees also know when their branches are being, are swaying in the wind, when they're being pushed by the wind. And there are some plants, that if you touch them so much, can even die. So of course, when I'm using the word feel here, that plants feel when they're being touched, I'm not talking about emotion feeling. I'm talking about the ability to perceive tactile sensation. And in this case, some plants actually feel better, or are more sensitive, than we are. How sensitive are plants to touch? Well, most humans can feel the weight of about two micrograms of a thread going across your skin. Some plants, for example, the burr cucumber, where their, its tendrils, start coiling under the weight of only a quarter of a microgram. That's ten times more sensitive than humans. So, although plants are more sensitive to humans when it comes to touch, plants and animals, including humans, share some surprising similarities when it comes to feeling that touch. So, how do we feel? How do we sense that we are being touched? Humans, all animals, sense touch through what's called our somatic sensory system. This is the nervous system that responds to different types of physical stimulations and it's actually quite diverse. But in general, it consists of various types of receptors that respond to touch or to pain or to temperature. Neurons that mediate that connect between these receptors in the skin or in our muscles through our spinal cord and up to the brain. And our brain which processes the information and defines it as this particular type of sensation. Well here we're going to see now, a cross section through our skin, to get a better idea of what these, sensors, what are the receptors, what are the structures for different sensations. In our skin there are different types of nerves, different types of structures for different types of sensations. For example, close to the tips, close to the top of the skin there are nerves that are specific only for pain. And they contain specific receptors which are called nociceptors. These are the receptors that mediate our sensation of pain. There are about three different types of nerves that have receptors that feel different types of pressure. Whether it be light pressure, or a deep pressure, and they respond differently, again, to what type of pressure's going on the skin. These nerves contain what we call mechanoreceptors. And other parts of our skin contain receptors that respond to changes in temperature. These contains receptors that are called thermal receptors. And there's different thermal receptors for heat and different thermal receptors for cold. So what happens once one of these receptors is stimulated. So once the receptor is stimulated, the principle involved in neural communication is the same for all nerve cells. It's electricity. The initial stimulus, whether it be touch from a canoreceptor or cold from one of the thermal receptors, starts a rapid electro-chemical reaction which is called a depolarization. And we'll define that soon, what this depolarization is. And this depolarization is propagated along the nerve, much like a wave going through water. And this electrical signal can then proceed from nerve to nerve until it reaches our brain where it elicits the reaction. So the depolarization is a result of what we call an action potential. So while the mechanisms involved in these electrical chemical signaling are actually quite complex, and there could actually be an entire course, just on neurosignaling, the basic principles are relatively simple. Just as a battery, for example, maintains a charge, an electric current, by keeping different electrolites, different ions in different compartments of the battery. A cell also has a charge due to the different amounts of ions, the different amounts of salts that are inside and outside the cell. For example, there are more sodium ions on the outside of cells and more potassium ions on the inside. And this change in the ions is what gives cells an electrical charge. So when a mechanoreceptor is activated for example, or any type of receptor for that matter, this opens up specific holes, channels in the membranes of the cells which lets ions pass through the channel. This happens near the point of contact for mechanoreceptor, and this change in the ions leads to an electric charge, which leads to the opening of more channels further down the same nerve. Which leads to another change of charge. And this change of charge again progresses as a wave down the length of the neuron. Again like a wave going through the ocean. When this charge then reaches the end of the neuron at the junction where one nerve meets another nerve, this leads to the release of a second type of ion, another ion, which is called calcium. And this calcium change causes a chemical change at the junction between the two neurons which leads to the electrical chemical signal preceding again to the next neuron. What the calcium ions do as they're released, leads to the release of chemicals that are called neuro-recep-, are called neurotransmitters. And maybe you've heard of these type of chemicals, neurotransmitters, because they transmit a signal between neurons. When one nerve releases a neurotransmitter, and it's caught by another nerve, this again causes the action potential to propagate through the second nerve. And this continues until it reaches the brain. Okay, now I know that this was a very complex series of of explanations. So let's take a short break and review action potentials and neural communication again, before we move on.
