Download Reflex Activity/Lab

SOMATIC REFLEXES
Reflexes are rapid, involuntary motor responses to an environmental stimulus detected by sensory
receptors. A nerve impulse travels from the receptor through a neural reflex arc pathway to an effector. If
the motor response is contraction of skeletal muscle, the reflex is called a somatic reflex. If the motor
response involves cardiac muscle, smooth muscle, or glands, the reflex is called an autonomic (visceral)
reflex. Reflexes mediated by spinal nerves are called spinal reflexes, whereas reflexes mediated by cranial
nerves are called cranial reflexes. Most reflexes help our bodies maintain homeostasis and therefore have a
protective function.
A. Reflex Arcs
These are five basic components of a reflex arc:
1) Sensory receptor: If a stimulus to the sensory receptor is strong enough, an action potential is generated
in the sensory neuron.
2) Sensory neuron: The sensory neuron propagates the action potential and synapses with neurons in the
spinal cord or brain stem.
3) Integrating center: The integrating center is located within the gray matter of the central nervous system
(CNS) and transfers information from the sensory neuron to the motor neuron.
4) Motor neuron: The motor neuron carries the action potential initiated by the integrating center to the
effector.
5) Effector: An effector can be skeletal muscle (somatic reflex), cardiac muscle, smooth muscle, or glands
(visceral reflex).
Activity: Label the reflex arc on your student worksheet.
B. Reflex Testing
A series of reflex tests are used clinically to evaluate the nervous system and to diagnose an abnormality or
dysfunction. The following reflexes are examples of clinical reflex tests.
1. Patellar Reflex (Knee Jerk)
The patellar reflex is the extension of the knee that occurs when the patellar tendon is stretched. The
components of the patellar reflex arc are:
 Sensory receptors: located in the quadriceps femoris muscle group; tapping the patellar tendon
stretches this muscle group and stimulates the stretch receptors, initiating nerve impulses in axons
of sensory neurons
 Sensory neuron: sensory axons carry nerve impulses to the integrating center (gray matter) in the
spinal cord
 Integrating center: The sensory axon synapses with and initiates nerve impulses in the motor
neuron that innervates the quadriceps femoris muscle group
 Motor neuron: axons of the motor neuron travel in the femoral nerve to the quadriceps femoris
muscle group
 Effector: the quadriceps femoris muscle group (agonists) contracts and extends the leg when
stimulated
Activity: Testing the Patellar Reflex
1) Choose one member of your group to be the subject. Have the subject sit
on the edge of a table or tall lab chair with the knees off the table or chair and
the legs dangling and relaxed.
2) Palpate (feel under the surface of the skin) the patella and the tibial
tuberosity. The patellar tendon, the insertion of the quadriceps femoris, is
between those two structures. To be sure you have found the tendon, have the
subject flex the quadriceps muscles, but not move the leg while you palpate
the tendon.
3) Gently but firmly tap the patellar tendon with the reflex hammer.
4) Now ask the subject to clasp both hands in front of the chest and
isometrically pull in opposite directions. This action leads to an enhancement
of spinal reflexes causing an enhanced patellar reflex.
5) While the subject is pulling, tap the patellar tendon again and see if there is
a difference in the reflexual response.
2. Biceps reflex (Biceps Jerk)
The biceps reflex is the contraction of the biceps brachii muscle that occurs when the biceps tendon is
stretched. Although the biceps brachii flexes the forearm, in this test you may only see contraction of
tension in the biceps brachii because the forearm is resting on the table.
Activity: Testing the Biceps Reflex
1) Have the subject sit at a desk and lay one arm on the desktop. The
arm should be at a 90o angle and completely relaxed.
2) Ask the subject to isometrically contract the biceps brachii so you can
palpate the tendon in the antecubital fossa (front side of the elbow).
3) After locating the tendon, have the subject relax and place your thumb
over the tendon.
4) Tap your own thumb with the reflex hammer.
5) If the reflex is absent ask the subject to clench his or her teeth for a
reinforced reflex and try the test again.
3. Achilles Reflex (Ankle Jerk)
The response of the Achilles reflex is plantar flexion when the Achilles (calcaneal) tendon is tapped with the
reflex hammer
Activity: Testing the Achilles Reflex
1) Have the subject put one foot on the floor and place the knee of the
other leg on a chair with the foot dangling over the edge.
2) Ask the subject to slightly dorsiflex the foot to stretch the tendon a
little, but remain relaxed.
3) Tap the calcaneal tendon with the reflex hammer and observe the
response of the foot.
4. Plantar Flexion
This test stimulates the cutaneous receptors and involves the brain in addition to the spinal cord. In adults,
plantar flexion with flexed (curled) toes occurs when the plantar surface is stroked. A reaction of
dorsiflexion with extended or flared toes (Babinski sign) is seen in infants because all the nerve fibers are
not myelinated. The Babinski sign is abnormal in adults.
Activity: Testing Plantar Flexion
1) Have the subject seated with a foot propped up and relaxed.
2) Using the metal end of the reflex hammer, stroke the plantar surface of the foot
starting at the heel, moving up the lateral side of the foot, and then under the toes.
3) Observe the reflexual response.
5. Pupillary Reflex
This is a protective reflex. Shining a light into one eye activates sensory receptors in the retina of that eye,
eventually stimulating output cells of the retina called ganglion cells. The ganglion cells respond to
brightness in the eye, and send the signals via the optic nerve to the midbrain, located in the brainstem.
Some optic nerve fibers synapse in the midbrain on the same side that the eye was stimulated; others cross
and synapse in the midbrain on the opposite side. As a result, the neurons in the midbrain process the
signal on both sides of the brain. Outgoing messages exit from the brain along the oculomotor nerve on
both sides, causing smooth muscle cells of the pupillary sphincter muscle to contract. Pupils of both eyes
constrict.
Activity: Testing the Pupillary Reflex
1) In a dimly-lit room, have the subject look out toward a wall and hold a green card on the bridge of
his or her nose to separate each eye’s field of vision.
2) Bring a penlight from the side to within 5 to 10 cm of ONE of the subject’s eyes.
3) Stimulate the eye for 3 to 5 seconds. Observe the response of the pupil in both eyes.
4) After allowing the subject’s eyes to return to the pre-dilated state, repeat with the subject’s other
eye. Again, note the response in both eyes.
Name ________________________________
Per. ______
Date _____________
ACTIVITY: Reflex Arc
1a. Name the five components of a reflex arc.
1b. Label these parts (a-e) on the diagram below.
a. __________________________________
b. __________________________________
c. __________________________________
d. __________________________________
e. __________________________________
2. Define reflex.
3. Describe the difference between a somatic and visceral reflex.
4. Describe the difference between a cranial and spinal reflex.
5. Name the muscle that is contracted in the patellar reflex.
6. Describe the effect that clasping and pulling the hands had on the patellar reflex.
7. Describe the effect that clenching the teeth had on the biceps reflex.
8. Name the muscles that contract for the Achilles tendon reflex.
9a. Describe what happened to each eye when stimulated.
9b. What did you notice about both eyes when one eye was stimulated?