Exercise 17. Human Sensations, Reflexes, and Reactions
Student Learning Outcomes
After completing this exercise the student will be able to:
1. Define consciousness, sensory neurons, receptors, stimulus, motor neurons, effectors, somatic
motor, neurons, autonomic motor neurons, interneurons, chemical synapse, integration,
proprioception, sensations, modality, free neuron endings, encapsulated neuron endingsMeissner’s and Pacinian corpuscles, projection, phantom pain, adaptation, reflex, reflex arc,
stretch reflexes, patella reflex, muscle spindle, pupillary reflex, swallowing reflex, reaction;
2. Describe the flow of information through the nervous system;
3. State the nature and function of sensation;
4. Describe a stretch reflex;
5. Describe the pupillary reflex;
6. Distinguish between a reflex and a reaction;
7. Measure visual reaction time.
INTRODUCTION
How do you interact with the external environment? To answer this question, you first have to
be able to analyze your interactions. This means you have to be conscious. Consciousness is
the state of being aware of the things around you, your responses, and your own thoughts.
Being conscious allows you to learn, to remember, and to fell and show emotion. Second, you
have to understand the flow of information through the nervous system (Figure 37-1).
Sensory neurons carry messages from receptors to the spinal cord and brain, which
comprise the central nervous system (CNS). Receptors are located both within the body and
on its surface. Receptors within the body receive information from the internal environment,
while those on the surface of the body receive information from the external environment.
Each piece of information received by a receptor is called a stimulus (plural, stimuli).
Motor neurons carry messages from the CNS to effectors. Effectors are muscles or
glands that respond to stimuli. Somatic motor neurons control skeletal muscles, and
autonomic motor neurons control smooth muscles, cardiac muscle, and glands.
In the CNS, a sensory neuron can directly stimulate a motor neuron across a chemical
synapse; more frequently, though, one or more interneurons (association neurons) connect the
sensory and motor neurons.
A chemical synapse is a junction between two neurons, or between a neuron and an effector
that are separated by a small gap. A chemical transmitter substance released from the first
neuron diffuses across the gap and then binds to (and produces changes in) the receiving cell.
The function of the interneuron is integration. At this level, integration is the
processing of messages received from receptors via sensory neurons and the activation of the
appropriate motor neurons, if any, to initiate responses by effectors. Your conscious mind
is located in the cerebral cortex of the brain and is aware of (and indeed plays a part in) some
of this activity.
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Central Nervous System
(Interneurons)
INTEGRATION
Sensory Neurons
Somatic motor neurons
Autonomic motor neurons
Receptors
Skeletal muscles
STIMULI FROM INTERNAL AND
EXTERNAL ENVIRONMENTS
RESPONSES MOSTLY TO
EXTERNAL ENVIRONMENT
Smooth and cardiac
muscles and glands
Muscles and glands
RESPONSES MOSTLY TO
INTERNAL ENVIRONMENT
Figure 1 Show the flow of information through the nervous system.
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10.1. Sensations
A receptor is the smallest part of a sense organ (such as skin) that can respond to a stimulus.
The receptor is linked to the CNS by a single sensory neuron. Our bodies have receptors for
light, sound waves, chemicals, heat, cold, tissue damage, and mechanical displacement. Senses
for which we have sensations include sight, hearing, taste, smell, pain, touch, pressure,
temperature, vibration, equilibrium, and proprioception (knowledge of the position and
movement of the various body parts). Sensations are that portion of the sensory input to the
CNS that is perceived by the conscious mind. There are also a number of complex sensations
such as thirst, hunger, and nausea.
Most sensations inform the conscious mind about the state of the external
environment. Sensations from the internal environment inform the conscious mind about
problems such as dehydration. If you are thirsty, you will make a conscious decision to find
and drink water.
Receptors and the sensations they produce have three characteristics: modality,
projection, and adaptation. These characteristics can be easily demonstrated by investigating the
skin’s receptors.
MATERIALS
Per lab Group:
1) felt-tip, nonpermanent pen
2)
3)
4)
5)
6)
bristle
dissecting needle
scientific calculator
2 blunt probes (glass stirring rods)
A source of ice water, hot - tap water
[Note: the hot water will have to be changed every five minutes.]
7) ice bag
8) camel-hair brush
9) reflex hammer
10) sources of containers containing:
a) ice water (4 0C)
b) water bath at 45 0C
a) room-temperature water
11) tissue paper (paper towels)
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A. MODALITY
Modality is the particular sensation that results from the stimulation of a particular receptor. For
example, the modalities of taste-bitter, salty, sour, and sweet-are associated with four different types of
taste buds. However, although every receptor has evolved to be most sensitive to one type of stimulus,
modality actually depends on where in the brain the sensory neurons from the receptor (or the
interneuron to which they connect) terminate. Modality cannot be encoded in the messages carried
by sensory neurons, because every impulse in that message is identical. The only information
neurons transmit is the absence or presence of a stimulus and (if one is present) its intensity-low
stimulus-intensities produce a low frequency of impulses and high stimulus-intensities produce a high
frequency of impulses.
PROCEDURE
1. Examine a prepared section of skin (Figure 37-2). Two categories of receptors are present: free
neuron endings and encapsulated neuron endings.
Free neuron endings are almost impossible to see in typically stained sections, but note their distribution
in Figure 37-2. Stimulating different free neuron endings produces sensation of pain, crude touch, and
perhaps cold and hot. Encapsulated neuron endings consist of neuron endings surrounded by a connective
tissue capsule.
Find Meissiner’s corpuscles in the dermal papillae (Figure 37-2). Meissner’s corpuscles are
receptors for fine touch and low-frequency vibration. Now look for Pacinian corpuscles between the
dermis and hypodermis. Pacinian corpuscles look a cut onion and are receptors for pressure and highfrequency vibration. Not all skin sections will contain a Pacinian corpuscle. If you cannot locate one,
ask the lab instructor.
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2. With a felt-tip, nonpermanent ink pen, draw a 25-cell, 0.5-cm grid on the inside of your
lab partner’s forearm; just above the wrist (see Table 1 0 . 1 . )
3. You are now the investigator, and your lab partner is the subject. At this point, ask your
lab partner is the subject. At this point, ask your lab partner to close his or her eyes.
Using a bristle touch the center of each box in the grid. If the bristle bends, you are pressing
too hard. Ask your lab partner to announce when the touch is felt. Do not count responses
given when you remove the bristle. Just count those that coincide with the initial touch.
Mark each positive response with a T in the upper left-hand corner of the corresponding
box in Table 10.1.
4. Repeat the above with a clean dissecting needle. This time, if your feel a prick,
mark P for “pain” in the upper right-hand corner of the corresponding box in Table 10.1.
Table 10.1 Grid for testing skin stimuli and recording modality data.
Caution: do not press; simply let the tip of the Dissecting needle rest on the surface of the skin.
5. Repeat the above with a chilled blunt probe. Before using the blunt probe, dry it with
tissue paper. The blunt probe will warm up over time, so switch it with the second
chilled blunt probe every five trials. This time, mark each positive response with a C for
“cold” in the lower left-hand corner of the corresponding box in Table 10.1.
6. Repeat the above with a heated blunt probe. Before using the blunt probe, dry it with tissue
paper. Use the two blunt probes alternately every five trials. This time, mark each
positive response with an H for “hot” in the lower right-hand corner of the
corresponding box in Table 10.1.
7. Record the total number of positive responses for each stimulus in Table 10-2.
Calculate the density of receptors for each modality (multiply the number of positive
response for each stimulus by 4) and record them in the third column of Table 10-2.
8. Repeat this procedure for your lab partner.
9. Does each cell in the grid contain a receptor for all four modalities studied?
(yes or no).
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Stimulus
Touch
Pain
Cold
Hot
Number of Responses
Density (Responses/cm)
Table 10-2 Positive Identifications to Stimuli Applied to 25 Cells in a 0.25-cm Patch of skin.
10. Can you see a pattern in the distribution of positive responses marked in table 10.1?
_________________________ if yes, describe the pattern(s) ________________________
_____________________________________________________________________________
11. Are the densities for the receptors for each modality the same? _____________ (yes or no.)
B. PROJECTION
All sensations are felt in the brain. However, before the conscious mind receives a
sensation, it is assigned back to its source, the receptor. This phenomenon is called
projection. This is a very important characteristic of sensations because it allows the
conscious mind to perceive the body as part of the world around it. You have probably
experienced projection. A common example is the “pins and needles” you feel in your hand
and forearm when you accidentally jar the nerve that passes over the inside of the elbow
(so-called funny bone). The sensory neurons in the nerve are stimulated, and your brain
projects the sensation back to the receptors. Another example is the phantom pain and other
sensations that recent amputees sometimes “feel” in missing limbs. This occurs because the
sensory neurons that once served the missing body part are activated by the trauma of the
amputation.
[Note: sketch and label your sensations on figure 37-4 below.]
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PROCEDURE
1. Obtain an ice bag from the freezer.
2. Your lab partner holds the ice bag against the inside your elbow for 2-5 minutes.
3. Describe any sensation felt in the hand or forearm to your lab partner who records them in
figure 37-4 (above.)
4. While the ice bag is applied to your elbow, your lab partner checks for any loss of sensation
by gently stroking your forearm and hand with a camel-hair brush. Sensations may also be
felt after the ice bag is removed.
5. If no results are obtained, try tapping the inside of the elbow with the reflex hammer.
6. Similarly test your lab partner.
7. What can you conclude about projection and receptors on the surface of the hand and
forearm?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
C. ADAPTATION
The intensity of the signal produced by a receptor depends in part on the strength of the stimulus
and in part on the degree to which the receptor was stimulated before the stimulus. Receptors
undergo adaptation to a constant stimulus over time. For example, when you first enter a dark
room after being in bright light, you cannot see. After a while, your photoreceptors adapt to the
new light conditions, and your vision improves.
1. Partially fill each of three 1000-mL beakers with ice water, water at room temperature, and
water at 45 0C.
2. Place one hand in the ice water and one in the warm water. After 1 minute, place both hands
simultaneously in the water at room temperature.
3. Describe the sensation of temperature in each hand to your lab partner, who records these
descriptions in table below.
Relative Temperature of Pre-adaptation
Result
Cold
Warm
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4. What can you conclude about the skin receptors for temperature and their capacity for
adaptation? ___________________________________________________________
________________________________________________________________________
________________________________________________________________________
5. What about the ability of other kinds of receptors to adapt? Note: use your own
experiences for smell, touch, and pain to answer this question.
[Hints for touch: Can you feel your clothes? How about when you first get
dressed after a shower?]
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
D. Reflexes
A reflex is an involuntary response to the reception of a stimulus. A reflex arc consists of the
nervous system components activated during the reflex. The simple reflex arc consists of a
receptor, sensory neuron, motor neuron, and effector. Involuntary means that your conscious
mind does not decide the response to the stimulus. However, the conscious mind may be
aware after the fact that the reflex has taken place. Reflexes of which we are not aware occur
most often in the internal environment (for example, reflexes involved in adjustments of blood
pressure).
MATERIALS
Per lab group:
1) Reflex Hammer
2) penlight
PR O C E D U R E
A. Stretch Reflexes
Stretch reflexes are the simplest type of reflex because the interneurons are not directly
involved (Figure 37-5). The sensory neuron connects directly with the motor neuron in the
spinal cord. Stretch reflexes are important in controlling balance and complex skeletal
muscular movements such as walking. Physicians often test these reflexes during physicals to
check for spinal nerve damage. You’ve probably experienced one of these tests, the patella
reflex. In this test, the receptor is the muscle spindle in the quadriceps femoris muscle of the
front of the front of the thigh, which is attached through its tendon and the patellar ligament
to the top of the front surface of the tibia. The tibia is the larger of the two lower leg
bones. The patella (kneecap) is embedded in the middle of the combined tendon/ligament.
The muscle spindle detects any stretching of the muscle. The effector is the muscle itself.
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1. Sit on a clean lab bench and shut your eyes.
2. Your lab partner taps the patella ligament with a reflex hammer (figure 37-6).
Describe the response.
________________________________________________________________________
________________________________________________________________________
3. Even with your eyes shut, are you aware of the stimulus and the response?
__________ (yes or no). This is because of pressure receptors that sense the tap or
because of proprioceptors that sense movement of the leg.
4. Stretch reflexes are somatic reflexes because they involve somatic motor neurons and
skeletal muscles. Can you willfully inhibit a stretch reflex? __________ (yes or no).
5. Similarly test your lab partner.
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Figure 37.6. Doctor taps the patella ligament with a reflex hammer on a patient.
B. Pupillary Reflex
1. Shine the penlight into your lab partner’s eyes. Does the size of the pupil (the diameter of the
pupil (the diameter of the opening into the eye that is surrounded by the pigmented iris) get
larger or smaller?
2. Now turn off the penlight. Does the size of the pupil get larger or smaller?
3. Repeat steps 1 and 2. Which is faster, constriction of the iris (which makes the pupil
smaller) or dilation of the iris (which makes the pupil larger)?
4. Are you aware of the pupil’s changing diameter? ______________ (yes or no).
5. The pupillary reflex is an autonomic reflex because it involves an autonomic motor neuron
and, in this case smooth muscle. Can you willfully inhibit the pupillary reflex?
___________
(yes or no).
6. Similarly test your lab partner.
C. Complex Reflexes
Complex reflexes involve many reflex arcs and interneurons. A good example is swallowing.
The stimulus in the swallowing reflex is the movement of saliva, food, or drink into the
posterior oral cavity. The response is swallowing.
1. Cup your hand around your neck and swallow. Fell the complex skeletal muscular
movements involved in swallowing. Do you consciously control all these muscles?
(yes or no).
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2. Is it possible to swallow several times in quick succession?_______________(yes or no).
3. Explain this result. It has something to do with the stimulus.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
4. What part of swallowing does your conscious mind control, and what part is a reflex?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
III. REACTIONS
A reaction is a voluntary response to the reception of a stimulus. Voluntary means that your
conscious mind initiates the reaction. An example is swatting a fly once it has landed in an
accessible spot. Because neurons must carry the sensory message to the cerebral cortex and the
message to react back to the motor neuron, a reaction takes more time than a reflex. Reaction
time is the sum of the time it takes for
1.
2.
3.
4.
5.
6.
the stimulus to reach the receptive unit,
the receptor to process the message,
a sensory neuron to carry the message to the integration center,
the integration center to process the information,
a motor neuron to carry the response to the effectors
the effector to respond.
Visual reaction time can easily be measured with a reaction- time ruler. This device makes
use of the principle of progressive Acceleration of a falling object.
MAT E R IAL S
Per lab group:
1) Reaction time Kit (Carolina Biological Supply Company)
2) Chair or stool
3) Scientific calculator
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The following instructions are modified from the Reaction time Kit Instructions booklet.
1. Sit on a chair or school (Figure 37-7).
2. Your lab partner stands facing you and holds the release end of the reaction-time ruler with the
thumb and forefinger of the dominant hand, at eye level or higher.
3. Position the thumb and forefinger of your dominant hand around the thumb line on the ruler.
The space between the thumb and forefinger should be about 1 inch.
4. Tell your lab partner when you are ready to be tested.
5. Any time during the next 10 seconds, your lab partner let go of the ruler.
6. Catch the ruler between the thumb and forefinger as soon as it starts to fall. The line under
your thumb represents visual reaction time in milliseconds.
7. Read the reaction time from the ruler out loud, and your lab partner records the data in
Table 37-3.
8. Repeat steps 1-7 ten times and calculate the average reaction time from the ten trials.
9. Similarly test your lab partner.
10. The reaction times of most of the ten trials should be similar, but perhaps the first few or
one at random may be relatively different from the others. If this is true for your own or your
lab partner’s data, suggest some reasons for this variability.
________________________________________________________________________
________________________________________________________________________
11. If opportunity and interest allow, the Reaction Time Kit Instructions booklet has a number of
suggestions for other experiments that you can easily do with the reaction- time ruler.
Figure 37-7 Measuring visual reaction time.
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TABLE 37-3. Reaction-Time Data
Trial
Subject 1
Subject 2
1
2
3
4
5
6
7
8
9
10
Total
Average (Total/10)
PRE-LAB QUESTIONS
1. Neurons that carry messages from
receptors to the CNS are
(a) sensory
(b) motor
(c) interneurons
(d) autonomic
6. Which characteristic of receptors does
phantom pain illustrate?
(a) modality
(b) projection
(c) adaptation
(d) proprioception
2. Neurons that carry messages from the
CNS to effectors are
(a) sensory
(b) motor
(c) interneurons
(d) both a and b
7. A simple reflex arc is made up of a
receptor and
(a) a sensory neuron
(b) a motor neuron
(c) an effector
(d) all of the above
3. Neurons that carry messages within the
CNS are
(a) sensory
(b) motor
(c) interneurons
(d) autonomic
8. A stretch reflex is
(a) somatic
(b) autonomic
(c) both a and b
(d) none of the above
4. Knowledge of the position and
movement of the various body parts is
(a) modality
(b) projection
(c) adaptation
(d) proprioception
5. Skin contains
(a) free neuron endings
(b) encapsulated neuron endings
(c) no nervous tissue
(d) both a and b
9. A pupillary reflex is
(a) somatic
(b) autonomic
(c) both a and b
(d) none of the above
10. A reaction is
(a) a reflex
(b) involuntary
(c) voluntary
(d) both a and b
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___________________________________________
Last Name, First Name [lab partner N0. 1]
____________________________________________
Last Name, First Name [lab partner N0. 2]
_______________________________
_______________________________
Last Name, First Name [lab
partner N0. 3]
___________________________
Last Name, First Name [lab
______________________
Section
group #
partner N0. 4]
______________________
Date
Exercise 17. Human Sensations, Reflexes, and Reactions
Post-Lab Questions
Introduction
1. Where in the brain does your consciousness reside? ____________________________ .
37.1. Sensations
2. In your own words, define the following terms:
A. Modality ____________________________________________________________
______________________________________________________________________
B. Projection ____________________________________________________________
_______________________________________________________________________
C. Adaptation ___________________________________________________________
_______________________________________________________________________
3. Indentify the structure indicated in these photos.
a. _____________________________
b. _____________________________
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37.2. Reflexes
4. List and diagram the basic steps of a simple reflex arc like the stretch reflex.
[Hint: see page 9.]
.
5. How does the patella reflex differ from the pupillary reflex?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
37.3. Reactions
6. Indicate whether the following actions are due to reactions or reflexes by putting a check in
the correct column in the table.
Action
Reaction
Reflex
A baby wetting a diaper
Braking a car to avoid an
accident
Withdrawing your hand from
a hot stove surface
Sneezing
Waving to a friend across the
street
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Food for Thought
7. What are the advantages and disadvantages to an organism of receptor adaptation?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
8. All animals do not perceive the external environment in exactly the same way. List some
examples from your own knowledge and readings in the textbook.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
9. To survive, an animal needs all its receptors working, and even then it cannot fully sense the
external environment. What extra receptors do you think would be advantageous to the
survival of humans in this modern world?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
10. Why is it advantageous for organisms not to be consciously aware of all the activity in their
nervous systems?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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