BIO103 Nervous Part One, MiraCosta Biology


Uploaded by MeerdesIrrtums on 02.08.2012

Transcript:
I’ve created what I sort of call an “overriding summary of physiology” for beginning, non-major
students like you, hoping that--being an animal yourself—you should be able to relate and
get into the proper mind-set quickly. I like to identify five major classes of physiological
function. In no particular order, they are as follows. First, there’s what I call “meeting
the body’s need for X through exchange with the outside world,” where “X” is something
that needs to pass into or possibly out of the body. Nutrients would be a good example,
and animals have feeding adaptations and digestive physiologies that make it possible for them
to get adequate nutrients. Oxygen would be another example of something an animal’s
body absolutely must acquire from the outside world, and so as animals we all have physiological
systems of digestion and respiration and gas exchange and all of these can be classified
within this general category.
The second big class of physiological function is homeostasis—where the body maintains
a relatively constant level, despite what would otherwise be a tendency to fluctuate
with the changes in the environment. One of the more obvious examples here is the way
we keep a constant body temperature of around 37.1°C while the world around us fluctuates
from warm to cold to hot. Remember that with the exception of mammals, birds, and some
fish, animals are not warm-blooded—to use the proper term, they are mostly ectotherms—getting
their body warmth from the outside world. This is actually going to be the focus in
the next unit, so I won’t say anything more about it here. But even an ectotherm is doing
homeostasis on other fronts besides that of body temperature—there are many other things
that animals and their cells must regulate—some of the most important are nutrient levels,
oxygen, and wastes. Even an animal as simple as a sea jelly regulates its internal conditions
by using cellular and organ-level physiology.
The third big area of physiological function has to do with sensing the external world—gathering
information that can then become the basis for adaptive responses. A flatworm, for example,
doesn’t crawl through its world blindly—it possesses chemical sensors that detect molecules
associated with food, and it uses these sensors to find its food. If the worm’s food is
on its left the concentration of food-associated molecules will be greater on its left than
on its right. So how does the worm know which direction it should go? Yeah, it follows its
nose—but how could the worm do this if it didn’t have a nose? Well, okay, technically
it doesn’t have a “true nose,” but let’s just call those chemical sensors on the different
parts of the worm’s head its “nose” and it’s all good.
The fourth and fifth general areas of function are defending the body against microbes and
parasites and reproduction. We’ll talk about the immune system in the last unit, but I
want to just leave things here for our present discussion which focuses on the nervous system.
The connection between an animal’s nervous system and the third category of physiological
function—that is, sensing the external world—is the most obvious, of course. Using the example
of the flatworm again, there are two things going on. There’s the detection of high
concentrations of food molecules on the worm’s left side, and then there’s the response
of the worm to turn to the left and head in the direction of the chemicals.
Here is the first basic lesson in animal nervous system physiology and anatomy: you can divide
the nervous system into sensory and motor divisions. The sensory part of the nervous
system is anatomically distinct from the motor part, and it carries signals from an animal’s
sensory receptors to the central nervous system. In the case of the worm, these would be the
nerve cells that are triggered by sensory cells—and we call them chemoreceptors because
they’re detecting chemicals—in the worm’s integument. Those signals from chemoreceptors
on the left side of the worm’s body ride sensory neurons to the brain (the worm’s
brain is very simple , but a brain nonetheless), and then motor neurons carry signals out to
the worm’s muscles, causing it to turn left and squiggle towards the food source.
In this regard we are no different from the worms. In a general bio class, you’d be
getting the same lesson on nervous system anatomy with a human focus. You have millions,
maybe billions, of sensory neurons traveling to the central nervous system carrying information
from the various sensory receptors in all parts of the body, and you have a separate
population of millions of neurons called—reasonably enough—motor neurons carrying signals out
to make their targets, skeletal muscle and glands, do what they need to do.
Okay, so that’s easy enough—the nervous system is at the core of our ability to gather
important information from the world around us. The less obvious part of this lesson is
that the nervous system also plays a big role in the first two of the major physiological
functionalities: meeting the body’s need for X, and homeostasis. Using our worm again,
it seems that the little bugger really has no choice in the matter. The way its sensory
and motor neurons are connected, it will move toward its food. Period. That’s it. You
might think that’s silly—maybe the worm can decide to move away from its food—how
can I know what a worm wants? Maybe it’s on a diet or something. What’s really there
to keep a worm from turning the other direction and crawling away from the food source? Well,
nothing. I’m sure it happens all the time that a worm is born with wiring that causes
it to move away from its food, toward its predators, away from moisture and towards
certain death. Those worms won’t be able to pass those interesting (but not very helpful)
traits on to future generations –not nearly as well as those worms that move toward food
and moisture and away from predators. Natural selection has shaped the neural connections
in a worm’s nervous system to respond adaptively toward stimuli, and the end result is an animal
that is able to meet its need for (in this case) nutrition.
How about homeostasis? That’s the second of the large categories of physiological function.
Well, yeah. In vertebrates, the motor nervous system is broken into the skeletal division—the
motor nerves that go out and tell your skeletal muscles to contract or not to contract—you
have more or less voluntary control over these—and then there’s the autonomic division of the
motor nervous system. These are the nerves that make you breathe faster and make your
heart beat faster when your body has a greater need for oxygen. These are the nerves that
make your sweat glands seep out perspiration when your body temperature rises too high.
Both of these are obvious homeostatic functions related to blood oxygen levels and to the
regulation of body temperature.
I could go on and talk more about how the nervous system is involved in both immunity
and reproduction, but this video has already past the thousand-word mark, and as clever
as you are (with your highly sophisticated neural anatomy) you should be able to reason
these connections out on your own, if it becomes necessary. I’ll cut this particular discussion
off here and use the next video to talk more about the actual mechanism of nerve cell function.