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Name _____________________________ Date ____________________ Period __________
Human Reflexes Lab
Introduction:
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 serve as
immediate, protective responses to potentially harmful stimuli.
Reflexes can be classified as either autonomic or somatic reflexes. Autonomic reflexes
(visceral) 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.
All reflex arcs have a minimum of five functional elements:
- The receptor reacts to a stimulus.
- The sensory neuron conducts the afferent impulses to the CNS.
- The integration center consists of one or more synapses in the CNS.
- The motor neuron conducts the efferent impulses from the integration center to an
effector.
- The effector, muscle fibers or glands, respond to the efferent impulses by
contraction or secretion a product, respectively.
Somatic Reflexes
1. Spinal Reflexes
a. Stretch Reflexes - Stretch reflexes are important postural reflexes that
act to maintainposture, balance, and locomotion. Stretch reflexes are
produced by tapping a tendon, which stretches the attached muscle.
This stimulates muscle spindles (specialized sensory receptors in the
muscle) and causes reflex contraction of the stretched muscle, which
resists further stretching. Even as the primary stretch reflex is
occurring, impulses are relayed to higher brain centers to advise of
muscle length and speed of shortening- information needed to
maintain muscle tone and posture.
i. Patellar Reflex - The patellar, or knee-jerk, reflex assesses the
L
- L
region of thespinal cord. The reflex is processed at this
level and does not progress to the brain. This reflex is a
monosynaptic, two-neuron, reflex arc
ii. Achilles Reflex - The Achilles, or ankle-jerk, reflex assesses the
first two sacral segments of the spinal cord. The reflex is
processed at this level and does not progress to the brain. The
reflex is monosynaptic, two-neuron, reflex arc.
iii. Biceps Reflex - The biceps-jerk reflex is a more complex, three
neuron reflex (polysynaptic). This reflex assesses C5- C8 and
T1.
b. Superficial Cord Reflexes - The superficial cord reflexes (abdominal
and plantar reflexes) are initiated by stimulation of receptors in the
skin and mucosa. The superficial cord reflexes depend both on brain
participation and on the cord-level reflex arc.
i. Plantar Reflex - The plantar reflex is elicited by stimulating the
cutaneous receptorsin the sole of the foot. In adults, stimulation
of these receptors causes the toes to flex and move closer
together. Damage to the cortocospinal tract (the major
voluntary motor 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.
c. Cranial Nerve Reflex Tests - Two reflexes mediated by cranial nerves
are the corneal reflex and gag reflex.
i. Corneal Reflex - The corneal reflex is mediated through the
trigeminal nerve (cranial nerve V). The absence of this reflex is
an ominous sign because it often indicates damage to the brain
stem. This reflex is tested by touching the cornea with a wisp of
absorbent cotton. It causes pain and the eye to tear up. This
reflex will not be tested in class.
ii. Gag Reflex - The gag reflex tests the motor responses of
cranial nerves IX and X. When the oral mucosa on the side of
the uvula is stroked, each side of the mucosa should rise to
some extent. We will not perform this experiment, as vomiting
is a possibility for those with queasy stomachs.
Autonomic Reflexes - The autonomic reflexes include the pupillary reflexes as well as
many others.
- Pupillary Reflexes - There are several types of pupillary reflexes- the pupillary
light reflex and the consensual reflex. In both of these reflexes, the retina of the
eye is the receptor, the optic nerve (cranial nerve II) contains the afferent fibers,
some of which synapse at the optic chiasma in the midbrain with other optic
nerve fibers from the opposite eye. As a result, the neurons in the midbrain
process the signal on both sides of the brain. The oculomotor nerve (cranial
nerve III) is responsible for conducting efferent impulses back to the eye, and
the smooth muscle of the iris is the effector. Many CNS centers are involved in
the integration of these responses. Absence of the normal pupillary reflexes is
Data:
Reflex Response
Patellar- normal
Patellar- after
fatigue
Achilles
Biceps
Plantar
Pupillary-
Response to light
Before:
Right eye _____mm Left eye _____mm
After:
Right eye _____mm Left eye _____mm
Difference:
Right eye _____mm Left eye _____mm
Pupillary-
Consensual
response
Draw and label the reflex arc in your lab notebook.
