Chapter 28.2 Reflexes and Homeostasis

This textbook is designed specifically for Kansas State's Biology 198 Class. The course is taught using the studio approach and based on active learning. The studio manual contains all of the learning objectives for each class period and is the record of all student activities. Hence, this textbook is more of a reference tool while the studio manual is the learning tool.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/[e-mail] is licensed under a Creative Commons Attribution License 4.0 license.

Episode Transcript

(00:00):
Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is Chapter twenty eight
point two Reflexes and Homeostasis. Allhyperlinks, images and sources can be found

(00:22):
at the link to the book.In the description, there is no other
species on Earth that does science.It is so far entirely a human invention,
involved by natural selection in a humancortex. For one simple reason,
it works. It is not perfect, it is misused. It is only

(00:43):
a tool, but it is byfar the best tool we have. Self
correcting, ongoing, applicable to everything. Carl Sagan, American astronomer in Cosmos,
nineteen eighty. As we come tothe last section of this book,
it seems appropriate to harken back towhere we started with a reminder that science
is a way of knowing. Theknowledge of animal biology discussed in this module

(01:06):
and the mechanisms of nervous system function, homeostasis, etc. Are all products
of painstaking experiments and observations, insome cases dating back hundreds of years.
This uniquely human endeavored gave us thatknowledge. More importantly, animal biology is
just a subset of the science ofbiology, and biology is just a subset
of science. There is a lotmore to explore in other areas of science,

(01:32):
and infinitely more for current and futurehumans to learn about all of them.
We hope that the knowledge and thinkingprocesses that you have used in this
course will serve you well in yourfuture education and will help you attain success
in whatever endeavor you choose for yourfuture work. Reflexes when the body reacts
involuntarily to an internal or external stimulus. The response is called a reflex,

(01:53):
and the neurons that make up thesimple circuit are called a reflex arc.
This is in voluntar, terry,spontaneous, and does not involve processing of
stimuli by higher centers each regions ofthe brain. Reflexes can be spinal or
cranial, depending on the nerves andcentral components that are involved. For example,

(02:14):
heat and pain sensations from a hotstove causing withdrawal of the arm through
a connection in the spinal cord thatleads to contraction of the muscles in the
arm, jerking the arm away fromthe hot stove. Other examples of reflexes
include withdrawal reflex, which occurs whenyou step on a painful stimulus like attack
or a sharp rock. The painreceptors no susceptors better activated by the painful

(02:38):
stimulus, activate the motor neurons responsiblefor contraction of the leg muscles to remove
your foot from the pain. Stretchreflex, which helps to maintain muscles and
an optimal length. Receptors called spindlereceptors within muscles are activated when the muscle
is stretched, causing direct contraction ofthe muscle. Cornial reflex or eyeblink reflex.

(03:01):
When the cornia is stimulated, whetherby touch or a bright light,
blinking of the eyelids is initiated.Obviously, this is to keep the cornea
safe from abrasion by dust or flyinginsects, etc. And to protect the
lens and retina from overbright light.There are more nerves per square inch in
your cornia than in any other partof your body. The autonomic nervous system

(03:23):
regulates organ systems through circuits that resemblethe reflexes of the somatic nervous system,
such as the knee jerk reflex.The main difference between the somatic and autonomic
systems is in what target tissues areeffectors. Somatic responses are solely based on
skeletal muscle contraction. The autonomic system, however, targets cardiac and smooth muscle

(03:46):
as well as glandular tissue. Whereasthe basic circuit is a reflex arc,
there are differences in the structure ofthose reflexes. For the somatic and autonomic
systems components of the reflex arc.There are typically three neurons in a reflex
arc. These are a sensory neuronwhich responds to a sensory stimulus touch,

(04:06):
pain, muscle, stretch, etcetera, an interer neuron, which receives
a signal if the sensory neuron isstimulated sufficiently, and a motor neuron,
which is stimulated by the interneuron andthen carries out the action required for the
stimulus which initiated the response. Thesensory neurons can be oriented externally, ie
to detect stimuli coming from outside thebody, and internally to detect stimuli originating

(04:30):
in the body. An example ofthe latter would be the stretch receptors that
tell your stomach to start contracting.Harder after a big meal, or the
pressure receptors barrow receptors in your carotidarteries, which detect blood pressure and tell
your heart to beat faster or sloweraccording to the pressure that they are sensing.
Similarly, the inder neurons fall intotwo general categories. In long reflexes,

(04:55):
the inder neuron is in a centralnervous system CNS structure, such as
the brain spinal cord. In shortreflexes, the inder neuron is located in
a peripheral ganglion bypassing the CNS,as shown in figure Finally, the motor
neurons, which generates the response,can also be classified into two general categories.

(05:15):
One type of motor neuron innervates amuscle and stimulates contraction of that muscle
and the subsequent rapid removal of yourhand from a hot stove or a more
rapid heartbeat, for example. Thesecond class innervates a gland and causes secretion
of hormones that mediate the appropriate response. An example of the latter would be
the nerves which cause the adrenal glandto release adrenaline as part of the fight

(05:39):
or flight response when you see alion or tiger or bear coming at you.
Short and long reflexes. The toppanel in this figure shows a long
reflex where the spinal cord is connectedto the sensory receptor cell and the peripheral
ganglion. The bottom panel shows ashort reflex where the sensory receptor cell is
directly connected to the periphero ganglion.Sensory input can stimulate either a short or

(06:03):
a long reflex. A sensory neuroncan project to the CNS or to an
autonomic ganglion. The short reflex involvesthe direct stimulation of a post ganglionic fiber
by the sensory neuron, whereas thelong reflex involves integration in the spinal cord
or brain homeostasis. As we cometo the end of this module, it

(06:26):
is appropriate to revisit the concept ofhomeostasis as a unifying theme in biology.
The reflexes described above are just oneexample homeostatic mechanisms that allow organisms to maintain
the parameters of their internal environment aboutan optimal setting. The organ systems described
in this section also all have manyexamples of homeostatic mechanisms that allow those organ

(06:47):
systems to maintain optimal levels of otherparameters such as oxygen levels in the blood
or the pH of the contents ofthe duodenum. All of these organ systems
work together for the benefit, survivaland reproduction of the organism. The organ
systems are highly interconnected as well.The circulatory and respiratory systems must coordinate their

(07:09):
functions since the function of one isto circulate the needed gases and waste products
that come in and go out viathe other system. The osmoregulatory system also
has to work hand in hand withthe circulatory and respiratory system to maintain the
content of the blood at optimal levels. The digestive system has to provide the
nutrients for all of these other organsto function. The nervous and endocrine systems

(07:30):
have to respond to a variety ofexternal and internal signals, etc. Finally,
the organisms that compose populations and communitiesand ecosystems also seem to interact with
the abiotic components e g. Nutrientsof the ecosystem in a homeostatic fashion.
For example, the CO two neededby plants is emitted by other organisms,
and the level of atmospheric CO twohas remained relatively constant over many millennia,

(07:57):
but it is becoming ever more clearthat the activities of one species on the
planet, Homo sapiens, is resettingthe set points for these homeostatic processes.
As we currently burn about a millionyears worth of past net primary productivity every
year, we are seeing the atmosphericCO two levels slowly rise. As you
learned in an earlier module, theeffects of changing the amount of that globally

(08:18):
important compound over a short time span, geologically speaking, are just starting to
be realized. But it is clearthat there will be many ramifications, even
if we don't yet understand all ofthem. It has hoped that your newfound
understanding of biological interactions and homeostatic mechanismswill enable your generation to devise appropriate strategies

(08:39):
that will again demonstrate that the sapienspart of our name is a good descriptor
for our species. It will takeboth the knowledge you have gained in this
class and the wisdom you will gainin all your classes and experiences to meet
the challenges ahead. This podcast willbe released episodically and follow the sections of

(09:00):
the textbook in the description. Fora deeper understanding, We encourage you review
the text version of this work voiceby voicemaker Dotaane. This was produced by
Brandon Castiro as a creative Common Senseproduction