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Biology: Human Nervous System: Function, Reflexes


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  • Type: Video Tutorial
  • Length: 10:28
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 112 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Animal Systems and Homeostasis (63 lessons, $84.15)
Biology: The Nervous System (2 lessons, $2.97)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Even though neurons share the same basic structure in terms of axon and dendrites and terminal branches, and the same basic function, which is to convey an impulse, you can well imagine that there has to be different types of neurons. And that their structures will differ from each other at least in the big picture. You can imagine that a sensory neuron, a neuron that is going to say, feel pressure, is going to be different than, say, a neuron in your spinal cord, whose job it is to convey an impulse from the peripheral nervous system to the central nervous system. Because remember something, structure and function always go together.
I want to tell you about three types of neurons, and just to give you an idea of some of the ways neuro-pathways have evolved, some of which, all of which are highly adaptive. Now, just to give you three thoughts.
Thought Number One. Sensory neurons. Sensory neurons are built for one function, to sense. They are, again, going to have the typical dendritic terminal branch structure. They are going to have an axon, but particular at their dendrites, they are going to have a lot of different variations, depending upon whether they are photoreceptors in the eye, or whether they are pressure sensors in the skin, or whether they are heat and temperature sensors. So the bottom line is, sensory neurons' generalized function is to pick up a change in the environment. Because that's in essence what a stimulus is. A change in the environment. The sensory neuron will convey its impulse to another neuron. You probably know that neurons don't hook up so it's not like if I touch this surface, it's not like one neuron goes all the way up to my brain and then back down. That's not the same neuron. In fact, what we find is that this sensory neuron will end in my spinal cord. And then my spinal cord will be another neuron, called an interneuron. We'll look at some of these structures in just a second.
The interneuron is in between neurons. And then sometimes, sensory neurons hook directly to motor neurons or interneurons hook to motor neurons, but motor neurons functionally are going to have a very different function than sensory neurons. Whereas sensory neurons are to pick up an impulse and convey it, a motor neuron is to go the other way. A motor neuron is to say, "Move." Just think about the complexity of writing. As I write the word, Neuron, what am I doing? Well, I'm sending messages, first of all, I have a photoreceptor from my eye to the paper. And it's giving me depth, and as I know the depth, and as I'm making pressure adjustments, because if I didn't push hard enough, I wouldn't get anything. And then to write the word neuron, forget about all the cerebral things going on, like thinking about, oh what's the letter that goes there? And talking to you at the same time I'm writing this letter. Don't even consider those things. Think about just the simple motor action of doing this. What muscles am I moving? I'm gripping with the pen, I'm gripping with the fingers, I'm going up like so, and now, I'm coming back down. I'm using arm muscles, I'm using finger muscles, all without thinking about it.
So, the point here is that we can do these things almost unconsciously. Which means that there is a whole different of neuronal involvement outside of the brain. Of course, the brain, with its cerebrum and its cerebellum, and its medulla oblongata, and all of the unbelievable - I mean if you and started talking about the brain, we'd be here until like you were seniors in graduate school, for goodness sakes. Because there is so much to learn about the brain. But, generally speaking, I'm not even talking about the voluntary movement being controlled by my cerebrum, as I write something. I'm talking merely about three neurons that are interacting in this region, sensory, inter and motor.
Now the sensory and the motor neuron are going to be similar in many, many aspects in terms of that interneuron that's kind of interesting. Because an interneuron has to be built to convey impulses from one neuron to another. So, it's neuronal, or cell body, is going to be very, very different than other cell bodies. And so, an interneuron may look something like this. Where it's built, are many more branches than that. Where it's built so that it can have impulse coming this way, and maybe fork that impulse to two different directions. For instance, this one might lead to the brain. This one might go to inhibit and this one might be sensory. So you get the idea that an interneuron is going to be more like a switchboard, then it's going to be directional or unidirectional. I think the best places I can exhibit the interactions of neurons without getting like so overblown with this stuff, is to talk about a reflex, and something called a reflex arc. Reflexes, we say, they are inborn automatic responses to change his inner environment. Inborn, automatic responses. From the day you're born, if I rub your cheek, you'd turn to that side of that rubbing without thought. It's called a rooting reflexes and it's a nursing thing. Rubbing a baby's cheek, automatically the baby will go and try to suckle. You know that you have a knee-jerk reflex. In fact, that's the one I want to talk about.
But did you ever wonder - this is kind of an interesting thing. Did you ever touch a hot stove? And you pick your hand up and then it hurts. You don't sit there and say, "My skin's melting. I'm burning flesh. Perhaps this is not a good homeostatic thing to do. I ought to pick my finger up." You don't do that. You have automatic inborn things that, as soon as that sensory process of heat is delivered, you pick it up and then the brain kicks in the pain. What's pain all about? Pain is there to protect you. It's saying "You idiot, you burned by flesh. I'm not going to let you use that limb for awhile." And that's exactly what it does. You broke the bone, so every time you try to walk on that until it heals, I'm going to send you this message to keep you off of it. It's very selective. The brain is much smarter than you are. So the point here is that these reflexes are inborn and automatic. And they're protective. And that is what a reflex is all about.
Let's take a look at the knee-jerk response. And see what happens here. Now, it's very cool. There are a couple of things that happen. I'm going to happen and then I'm going to show you it's a little more complex than that. You know what happens with a knee-jerk response. You know, somebody hits you right underneath your patella, your kneecap. Well, think about it. Why is that protective? Because there are all of these very sensitive tendons and ligaments under your kneecap. That's why you have a kneecap. And so you've got this inborn reflex to keep you from damaging those things and so you close your leg, and what that does is that slides your leg up and that makes your patella slide over those ligaments and protects them. And there it is. There's the doctor whacking on your knee. And you're foot goes shooting up. Why? Let's see why.
Immediately what happens is there is a change in tension here. And that change in tension is picked up by a sensory neuron. And watch what happens. This is so cool. The sensory neuron goes into the spinal cord and you'll notice something and we'll talk about this a little bit later, but you notice that this purple neuron and this blue neuron are not connected. Neurons don't connect. They have spaces in between them called synapses. And so, trust me on this one. This is not a misprint. And you'll notice that the sensory neuron, instead of going to the brain, goes directly to a neuron called a motor neuron, which is going to cause this muscle to contract. And that muscle contracts. Well, if this muscle contracts, this muscle here better relax. Now we come to the interneuron. Because notice what happens to the sensory neuron. It forks. And it goes to an interneuron. And now the interneuron is going to send another impulse. It's going to send an impulse along here to inhibit the movement of the lower muscle. So that when this muscle contracts, you're not doing this fighting thing. You are as quickly as possible, shooting that leg up. And what I'm not showing you here is, well then you obviously feel that, which means there must be a message sent up to your brain. The brain says, "Somebody's banging me on the knee with a hammer."' It sends you a message so you feel it. So, how simple is that? And the other things is, there's not just one neuron. These are nerves and nerves have many neurons within them. But, we're showing you with one just because it's so much simpler to see.
So think of that. Think of reflexes. Reflexes are inborn and automatic. They protect you. Whether it's choking out a chicken bone, turning to nurse, removing a hand from a burning stove, or just protecting yourself from some idiot with a hammer. The point is reflexes, nerves, nerve interactions, protection.
Animal Systems and Homeostasis
The Nervous System
Human Regulation: Nervous System: Nerve Function and Reflexes Page [1 of 2]

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