Nervous System Lab - Reflexes
Reflexes are involuntary, rapid responses to the internal or external environment that allow
the body to maintain homeostasis. A reflex action happens automatically and in the same
way every time a particular kind of stimulus is detected by a particular set of sensory
receptors. The nerve pathways involved in reflexes may be to the spinal cord and back if
spinal nerves carry the sensory and motor impulses, or to the brain stem centers and back if
cranial nerves do. Each spinal and cranial nerve consists of both sensory and motor neurons.
Most reflex arcs contain all of the components in the diagram below (sensory neuron,
interneuron, motor neuron. In more complex reflex arcs, two or more interneurons are
present and one neuron may diverge and synapse with 2 other neurons, presenting greater
possibilities for modifying the reflex response. In the diagram below, the quadriceps muscles
(agonists) are stimulated to contract while the hamstrings (antagonists) are inhibited from
flexing. Preventing a reflex response requires conscious inhibition of effector (muscle or
gland) activity. A good example is stopping yourself from blinking when you know someone
is going to move his or her hand toward your eyes.
1. Tendon Reflexes: Stimulating tendon stretch receptors elicits a reflex contraction in the
corresponding muscle. These reflex arcs often do not involve interneurons. Their purpose is
generally to monitor the stretch of muscles and help improve the efficiency and coordination
of muscle movement.
a. patellar tendon reflex: Seat your partner on the edge of a table so their legs dandle
freely. Instruct your partner to keep their eyes closed. Use the percussion hammer to gently
tap the subject's patellar tendon just below the center of the kneecap. Test this reflex in both
legs under the following conditions:
1. Control conditions - are the responses equal?
2. Have the subject contract the flexor muscles in the back of their thigh while
you are testing them. How do the reflex responses compare to the control?
3. Have the subject count backward from 100 by 7's so that they are focused on
something other than you doing the test. What effect did this have?
b. Achilles tendon reflex: Have the subject knell on a chair with their feet hanging
over the edge. Gently tap the achilles tendon with the percussion hammer, just above their
heel. Describe the response that you observe.
2. Postural reflexes: Awareness of posture and maintaining equilibrium depend on sensory
information from muscle stretch receptors, visual receptors and the vestibular receptors of the
inner ear.
a. Static equilibrium - ask the subject to stand still with feet close together and medial
malleoli (bone sticking out on inner ankle) touching. Compare the amount and kinds of body
movement needed to maintain this position for 1 minute, first with eyes open and then with
eyes closed.
b. Rotational equilibrium:
1. Have the subject sit on a rotating stool with head upright and eyes open.
Rotate the subject slowly through 720 degrees. Note the movement of their head and
eyes, relative to the direction of rotation.
2. Have the subject keep their eyes closed and rotate them 20 times at a rate of
one turn per second. After you are finished rotating them ask them to let you know when
they have stopped spinning. Time how long it takes from when their chair has actually
stopped rotating.
3. Eye reflexes.
a. Pupillary reflexes - in response to light falling on the retina, the sensory portion of
cranial nerve #3 carries information to the upper part of the brainstem, the midbrain. Motor
neurons from the midbrain travel back out through the motor portion of the cranial nerve to
the muscles of the iris. The iris muscles will contract or relax to change the diameter of the
pupil.
1. Measure the diameter of your partner's pupil to the nearest millimeter while
they are looking at a distant target.
control pupil diameter right eye = __________mm
left eye = __________mm
2. Have you partner cover and close their eyes for two minutes. Turn on the
lamp and shine it in their eyes as you instruct them to open their eyes. Observe the changes
in pupil diameter. Measure the diameter of each pupil.
right eye __________mm
left eye __________mm
3. Repeat this experiment but after 2 minutes, shine a penlight only into the
right eye (have the subject hold a thin book vertically between their eyes). Observe the pupil
size in both eyes.
b. Convergence reflex. - Ask your partner to look at the wall on the opposite side of
the room. Observe the position of your partner's eyeballs relative to an imaginary vertical
line passing through the center of the pupil. Now have them shift their gaze to their thumb,
held at arm's length. What happens to the position of the eyeballs?
Sight vs Sound Reflexes:
http://physci.kennesaw.edu/javamirror/explrsci/dswmedia/reflex.htm
Conclusion:
1. Which of the reflexes that you observed involved:
a. cranial nerve pathways only?
b. spinal nerve pathways only?
c. both cranial and spinal nerve pathways?
2. Diagram the nerve pathways involved in parts 1b (Achilles tendon reflex) and 3 a.3 (pupil
response to penlight shined in one eye). Be sure to label: sensory neuron, interneuron, motor
neuron, divergence, effector in both & dorsal root ganglion, posterior gray horn, ventral root
of spinal nerve in 1b. Highlight the peripheral nervous system in one color and the central
nervous system in another color.