Human Anatomy & Physiology
Reflex Physiology lab
To understand what reflexes are, the processes involved, and purpose of reflexes.
A reflex is an involuntary neural response to a specific sensory stimulus that threatens the
survival or homeostatic state of an organism. Reflexes exist in the most primitive of species,
usually with a protective function for animals when they encounter external and internal stimuli.
In humans and other vertebrates, protective reflexes have been maintained and expanded in
number. Examples are the gag reflex that occurs when objects touch the sides or the back of the
throat, and the carotid sinus reflex that restores blood pressure to normal when baroreceptors
detect an increase in blood pressure. A second type of reflex, the stretch reflex, has evolved to
help maintain muscle tone that is important for posture and movement.
Reflexes can be categorized into one of two large groups: autonomic reflexes and somatic
reflexes. Autonomic (or visceral) reflexes are mediated through the autonomic nervous system,
and we are not usually aware of them. These reflexes active smooth muscles, cardiac muscles,
and the glands of the body and they regulate body functions such as digestion, elimination, blood
pressure, salivation, and sweating. Somatic reflexes include all those reflexes that involve
stimulation of the skeletal muscles by the somatic division of the nervous system. An example
of such a reflex is the rapid withdrawal of a hand from a hot object.
An understanding the neural circuitry underlying each type of reflex also has clinical
significance. When physicians test for potential damage to various components of the nervous
system, they often begin by attempting to elicit reflex responses to the appropriate stimuli.
Examples of reflexes with protective and diagnostic importance are the flexor withdrawal reflex,
the corneal (blink) reflex, and the accommodation reflex. The loss of the papillary light reflex in
a comatose patient often indicates extreme damage deep in the brain. Loss of the knee-jerk
response may indicate problems with muscle tone, chronic diabetes, or damage at the L2, L3, or
L4 vertebral level.
All reflex arcs have five basic components: 1) a sensory receptor, 2) a sensory (or
afferent) neuron, 3) interneurons (the middle man), 4) a motor (or efferent) neuron, and 5) an
effector organ (muscle fibers, or glands).
Use a general two-column format for this lab (similar to what many of you did in chem. Labs)
You do NOT need to write all instructions for each activity, but should use the left column to
briefly describe the activity, and the right column to record observations and pertinent data.
Label each activity with a separate heading (e.g. Initiating the Stretch Reflex) and number
each group of questions.
Write down all of your answers in your lab book. You should not need to re-write the
questions; however, state your answer in such a way as to subtly restate the question so that your
answer makes sense upon further review.
Opening Questions: **Answers can be found in the introduction AND in your textbook. (pages
209 214) Answer these questions at the beginning of the lab after you've written the title and
purpose of the lab.
A. What is a reflex? What is a reflex arc?
B. Explain the differences between the two basic types of reflexes.
C. Why do physicians test your reflexes?
D. What are the components of the Withdrawal Reflex Arc & the Knee-Jerk Reflex?
Initiating the Stretch Reflex
Stretch reflexes are important postural reflexes, normally acting to maintain posture,
balance, and locomotion. Stretch reflexes are initiated by tapping a tendon, which stretches the
muscle the tendon is attached to (the Quadriceps femoris muscle). This stimulates the muscle
spindles and causes reflex contraction of the stretched muscle or muscles, which resists further
stretching. Even as the primary stretch reflex is occurring, impulses are being sent to other
destinations as well. For example, branches of the afferent fivers also synapse with interneurons
controlling antagonist muscles. The inhibition of antagonist muscles that follows, called
reciprocal inhibition, causes them to relax and prevents them from resisting (or reversing) the
contraction of the stretched muscle caused by the main reflex arc. Additionally, impulses are
relayed to higher brain centers to advise of muscle length, speed of shortening, and the like
information needed to maintain muscle tone and posture. Stretch reflexes tend to be absent or
hypoactive in cases of peripheral nerve damage or ventral horn disease, and hyperactive in
corticospinal tract lesions. They are absent in deep sedation and coma.
1) Have your partner relax & sit on the edge of a table with his/her legs dangling loosing
over the edge.
2) The experimenter should apply the stimulus by tapping, firmly, but carefully, the area just
below the kneecap with the reflex hammer. (Do not hurt your partner!)
3) Test both knees and record your observations.
4) Test the effect of mental distraction on the patellar reflex by having the subject add a
column of three-digit numbers or some other similar task (singing a song, reciting the
alphabet backwards, etc) while you test the reflex again. Is the response greater than or
less than the original response? Explain...
5) Test the effect of muscular activity occurring simultaneously in other areas of the body.
Have the subject clasp the edge of the table and vigorously attempt to pull it upward with
both hands. At the same time, test the patellar reflex again. Is the response more or less
vigorous than the first response? Explain...
6) Fatigue also influences the reflex response. The subject should jog in position until she
or he is very fatigued (no slacking!!). Test the patellar reflex again and record whether it
is more or less vigorous than the first response.
7) Switch roles & repeat. Record all observations
Initiating Achilles Reflex
1) With your shoe removed and your foot dorsiflexed slightly (pointed up) to increase the
tension in your gastrocnemius muscle (calf muscle) have your partner sharply tap your Achilles
tendon with the reflex hammer. Record your results. The experimenter should hold the subject's
ankle by supporting the foot lightly in the hand & then tap the tendon just above the ankle.
2) Switch roles & repeat.
Initiating the Plantar Reflex
The plantar reflex, an important neurological test, is elicited by stimulating the cutaneous
receptors in the sole of the foot. In adults, stimulation of these receptors causes the foot to flex
and move closer together. Damage to the corticospinal tract, however, produces Babinski's sign,
an abnormal response in which the toes flare and the great toe moves in an upward direction. (In
newborn infants, Babinski's sign is observed due to incomplete myelination of the nervous
1) Have the subject remove a shoe and lie on the table with knees slightly bent and thighs
rotated so that the lateral side of the foot rests on the table. Alternatively, the subject may sit up
and rest the lateral surface of the foot on a chair. Draw the end of a pencil, a ruler, or your
finger, firmly down the lateral side of the exposed sole from the heel to the base of the great toe.
Record the response. Is this a normal plantar reflex or Babinski's sign?
2) Switch roles & repeat.
Initiating the Corneal Reflex
1) Stand to one side of the subject; the subject should look away from you toward the
opposite wall. Wait a few seconds and then quickly but gently, touch the subject's cornea (on
the side toward you) with the tip of a Q-Tip. What reflex occurs when something touches the
cornea? What is the function of this reflex?
2) Switch roles & repeat.
Stretch Reflex Questions:
A. What are the functions of the stretch reflex?
B. What is reciprocal inhibition and why does it exist?
The autonomic reflexes include the papillary, ciliospinal, and salivary reflex, as well as a
multitude of other reflexes.
Initiating the Pupillary Reflex
There are several types of papillary reflexes. The papillary light reflex and the
consensual reflex are two. In both of these reflexes, the retina of the eye is the receptor, the
optic nerve is contains the afferent neurons, the oculomotor nerve is responsible for conducting
motor impulses to the eye, and the smooth muscle of the iris is the effector. Many central
nervous system centers are involved in the integration of these responses. Absence of the normal
papillary reflexes is generally a late indication of severe trauma or deterioration of the vital brain
stem tissue due to metabolic imbalance.
1) The lights must be dimmed in the room to conduct this portion of the lab. Before
beginning, obtain a metric ruler to measure and record the size of the subject's pupil as
best you can. Record the size (in mm) for both the right and left pupils. Record.
2) Stand to the left of the subject to conduct the testing. The subject should shield his or her
right eye by holding a hand vertically between the eye and the right side of the nose.
Shine a flashlight into the subject's left eye. Record the response and measure the size
of the pupil to the best of your ability.
3) Observe the right pupil. Has the same type of change (called a consensual response)
occurred in the right eye? Measure the size of the right pupil to the best of your ability.
4) Switch roles and repeat.
The consensual response or any reflex observed on one side of the body when the other side has
been stimulated, is called a contralateral response. The papillary light response, or any reflex
occurring on the same side stimulated is referred to as an ipsilateral response.
A. Was the sympathetic or the parasympathetic division of the autonomic nervous
system active during the testing of these reflexes?
B. What is the function of these papillary reflexes?
Reaction Time of Basic and Learned or Acquired Reflexes
The time required for a reaction to a stimulus depends on many factors sensitivity of the
receptors, velocity of nerve conduction, the number of neurons and synapses involved, and the
speed of effector activation, to name just a few. Some reflexes are basic or inborn; others are
learned or acquired reflexes, resulting from practice or repetition. There is no clear-cut
distinction between basic and learned reflexes, as most reflex actions are subject to modification
by learning or conscious effort. In general, however, if the response involves a specific reflex
arc, the synapses are facilitated and the response time will be short. Learned reflexes involve a
far larger number of neural pathways and many types of higher intellectual activities, including
choice and decision making, which lengthens the response time.
1) Using the reflex hammer, elicit the patellar reflex in your partner. Note the relative
reaction time needed for this basic reflex to occur.
2) Now test the reaction time for learned reflexes. The subject should hold a hand out, with
the thumb and index finger extended. Hold a meter stick so that it's end is exactly 3 cm
above the subject's outstretched hand. The meter stick should be in the vertical position
with the numbers reading from the bottom up. When the meter stick is dropped, the
subject should be able to grasp it between thumb and index finger as it passes, without
having to change position. Have the subject catch the meter stick five times, varying the
time between trials. The relative speed of reaction time can be determined by reading the
number on the meter stick at the point of the subject's fingertips. **(Thus, if the number
at the fingertips is 15 cm, the subject was unable to catch the meter stick until 18 cm of
length had based through their fingers; 15 cm plus 3 cm. to account for the distance of the
meter stick above the hand.
3) Record the number of cm that pass through the subject's fingertips for each of the five
4) Perform this test again, but this time say a simple word each time you release the meter
stick. Designate a specific word as a signal for the subject to catch the meter stick.
(Record this word, for example your word might be "cat") On all other words, the subject
is to allow the ruler to pass through their fingers. Ignore trials in which the subject
erroneously catches the meter stick. Record the distance the ruler travels in five
successful trials. Did the addition of a specific word to the stimulus increase or
decrease the reaction time? Explain.
5) Perform the testing once again to investigate the subject's reaction time to word
association. As you drop the meter stick, say a word for example hot. The subject
responds with a word he or she associates with the stimulus word for example cold
catching the meter stick while responding. If unable to make a word association, the
subject must allow the meter stick to pass through their fingers. Record the distance the
meter stick travels in five successful trials, as well as the number of times the meter stick
is not caught by the subject. Do you notice a large variation in reaction times in this
series of trials? Explain.
A. What determines the speed of a reflex?
B. What are the differences between innate and learned reflexes?
C. Name at least three factors that may modify reaction time to a stimulus.
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