Name ____________________
Reflex Arc Lab
Purpose
To study the simplest functional unit of the nervous system: the reflex arc.
To understand the difference between a reflex and a reaction.
Background
Neurons communicate in many ways, but much of what the body must do every day is
programmed as reflexes. Reflexes are rapid, predictable, involuntary motor responses to stimuli
and they occur over neural pathways called reflex arcs.
Reflexes can be classed as either autonomic or somatic reflexes. Autonomic reflexes are not
subject to conscious control. These reflexes activate smooth muscles, cardiac muscle, and the
glands of the body and they regulate body functions such as digestion and blood pressure.
Somatic reflexes include all reflexes that stimulate skeletal muscles. An example of such a
reflex is the rapid withdrawal of your foot from a piece of glass you have just stepped on.
Most reflexes have a minimum of five functional elements
1. The receptor reacts to a stimulus.
2. The sensory neuron conducts the afferent impulses to the CNS.
3. The integration center consists of one or more synapses in the CNS (not present in a
monosynaptic reflex)
4. The motor neuron conducts the efferent impulses from the integration center to an
effector
5. The effector, muscle fibers or glands, respond to the efferent impulses by contracting or
secreting a product
Procedure: Somatic Reflexes
Stretch Reflexes – Knee-jerk reflex; Achilles reflex
1. Have the subject sit on the edge of a table with one leg dangling freely.
2. Tap the patellar ligament sharply with the reflex hammer to elicit response
3. The knee-jerk reflex assesses the L2-L4 level of the spinal cord.
4. Test both knees and record your observations.
_______________________________________________________________________
Which muscles contracted? _________________________________________________
5.
Sketch a diagram of the reflex arc and label the receptor, sensory neuron, dorsal root, ventral
root, motor neuron, and effector.
6.
Now test the effect of muscular activity occurring simultaneously in other areas of the body. Tap the
patellar ligament while the subject performs the Jendrassik's maneuver (the subject clasps his/her
hands in front of him/her and, with fingers locked, tries vigorously to pull his/her hands apart). Is the
response greater or less than the first time? _______________________________
If you observed that mental distraction or the Jendrassik's maneuver influenced the patellar reflex,
explain the reason in the space below in complete sentences.
________________________________________________________________________
________________________________________________________________________
7.
Fatigue influences the stretch reflex response. To demonstrate its effect, the subject should jog in
position or back in forth in the hall until his/her lower limbs are fatigued. (Really fatigued – no
slackers). Test the patellar reflex again and record whether it is more or less vigorous than the
first response.
____________________________________________________________________
Would you say that nervous system activity or muscle function is responsible for the changes
you have just observed?
________________________________________________________________________
8.
The Achilles, or ankle-jerk, reflex assesses the first two sacral
segments of the spinal cord. Remove your shoe and have your
partner use one hand to dorsiflex your foot to increase the tension
of the gastrocnemius (calf) muscle, and sharply tap your calcaneal
(Achilles) tendon with the reflex hammer.
What is the result?
___________________________________________
___________________________________________
Does the contraction of the gastrocnemius normally result in the activity you have observed?
Explain.
________________________________________________________________________
Superficial Cord Reflexes - abdominal and plantar reflexes
The plantar reflex is elicited by stimulating the cutaneous receptors
in the sole of the foot. In adults, stimulation of these receptors causes
the toes 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 upward. (In newborn
infants, Babinski’s sign is seen because the nervous system is still
incompletely myelinated.)
1. Have the subject remove a shoe and sit on a chair or desk and put
their foot on a chair.
2. Hold the ankle in one hand and using the reflex hammer metal end
draw the handle of the reflex hammer firmly down the lateral side
of the exposed sole from the heel to the base of the great toe.
What is the response? ____________________________________________________
________________________________________________________________________
Is this a normal plantar reflex or Babinski’s sign? _____________________________
Sight Reactions – Measuring Reaction Time
Many people consider only the simplest types of responses as "reflexes", those that are always
identical and do not allow conscious actions. We must not confuse these with "reactions", which are
different from reflexes in that they are voluntary responses to a stimulus from the environment. For
example, while the body has various subconscious physiological responses to mitigate cold, as
humans we can simply choose to put on more clothes. This is a conscious order made by the
cerebrum, not an involuntary response to a stimulus. This is a very complex response involving
millions of neurons and some time to process the voluntary response. In contrast, spinal reflexes
occur much faster, not only because they involve fewer neurons, but also because the electrical
signal does not have to travel to the brain and back. Spinal reflexes only travel to the spinal cord and
back which is a much shorter distance. Because of this and the complexity of conscious reactions,
they take more time to complete than a reflex. On average, humans have a reaction time of 0.25
seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus.
Reaction times vary from individual to individual. Because of the higher degree of neural
processing, reaction times can be influenced by a variety of factors. Reaction times can decrease
with practice; often times athletes have faster reaction times than non-athletes. Sleepiness, emotional
distress, or consumption of alcohol can also impact reaction time.
Testing Reaction Time
1. Get a meter stick and one partner hold the ruler near the end (highest number).and let it hang
down.
2. Have another person put his or her hand at the bottom of the ruler and have them ready to grab
the ruler but not touching the ruler.
3. The person holding the ruler drops it and the other partner grasps it as fast as possible.
4. Record the level (centimeters) at which they catch the ruler.
5. Test the same person 3 times (vary the time of dropping the ruler within the 5 second "dropzone" so the other person cannot guess when you will drop the ruler).
6. Use the table to convert the distance on the ruler to reaction time. For example, if you caught the
ruler at the 20 cm mark, then your reaction time is equal to 0.20 seconds
Distance of Catch
2 in (~5 cm)
4 in (~10 cm)
6 in (~15 cm)
8 in (~20 cm)
10 in (~25.5 cm)
12 in (~30.5 cm)
Reaction Time (seconds)
0.10 sec
0.14 sec
0.17 sec
0.20 sec
0.23 sec
0.25 sec
Partner 1: ______________________________
Average Distance (3 times)
1. _____________ 2. _______________ 3. _______________ Avg. ________________
Reaction time ______________________________________
Partner 2:______________________________
Average Distance (3 times)
1. _____________ 2. _______________ 3. _______________ Avg. ________________
Reaction time ______________________________________
Practice a few more times and determine if reaction time improved with practice?
______________________________________________________________________________
Reaction Time and IQ Tests (http://www.intelligencetest.com/reflex/index.htm )
Go to the website and perform the reaction tests: color, image, letter, dots and then take the average time
test.
Have one person perform the five trials and record them below and include the average time as well.
______________________________________________________________________________
Do you think reaction time is correlated to IQ? Why or why not?
______________________________________________________________________________
______________________________________________________________________________
The website describes these as reflexes. Are they reflexes or reaction time tests? Why?
______________________________________________________________________________
______________________________________________________________________________
Our body has many more reflexes than described above. Discuss with your partner another reflex, and
then using a drawing of the reflex arc describe it below. (Make sure it is a reflex!) Include in the
drawing of the reflex arc the labels, whether it is autonomic or somatic, and an explanation of the reason
for this adaptation in the human body.