exercise
The Reflex Arc
Read the section on reflexes irr yl
Objective 1 : D e ~ n f l c x e M i r c f r e x o ~ r .
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N c u m a caffunudmilr: in many ways, but much of what
Reflexes can be classed a either autonomic or somatic
reflexes. Autowmlc (or visceral) reflexes are not subject to
conscious control. These reflexes activate smooth muscles, '
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Components of a Reflex Arc
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of each clement of a reflex arc.
N 1 nflcx mcs have a mi-'-urn of five .fu@m4 elements
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1. ?ht receptor nwts to a stimulus.
2. The SQWOSYn e m e eanduets the afferent impulses to the
GNS.
3. The intrgration canter cmistsr of onc at more synapses
in thc CNS.
4. The m t o r neuron canducts the affemt impulses fiwn
the integration w t t r to aa effector.
3. ~ h Hector,
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muscle fitxm or &mis, respond to ~IEdfertnt impulses by contracting or secreting a product, respectivcly.
Tga shpfc pgltellnr 81: &-jerk reflex shown in F?gun
16.2s is an example of a simple, ma-neumn, mnosymiptiic
(iitmdly, "onc~yn~pse")ffcx arc. It will be d e m w
Flgure 16.1 ample 4 e x arcs. Components of all human reflex arcs:
(one or mon synapses in the CNS), motor neuron, and effector.
receptor, setwry neuron, integratlcrn ctnW
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the hborirbq YQwever, most reflexes are @ore complex
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1. To test the patellar, or knee-jek reflex, seat "asubject
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the laboratory bench with legs hanging free (or with knees
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Tap the patella ligament sharply with the reflex
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hammer iust below the knee to elicit the res~onse.The knee9,
jerk ref~&assesses the b - L 4 level of the spinal cord (Figure
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16.3). Test both knees and record your observations.
0 b j e c t i v e 3 : Describe several types of reflex
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There are many types of somatic reflexes, including severa1 that you will be inducing during this laboratory
session-the stretch, superficial cord, corneal, and gag reflexes. Some require only spinal cord activity; others require
brain involvement 2 ~ e ! - ~.- .. . . . # . #
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Which muscles contracted?
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Stretch Reflexes Stretch reflexes are important postural reflexes that act to maintain posture, 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.
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Spinal Reflexes
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(a) Monosynaptic reflex
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2. Fatigue influences the stretch reflex response. To
demonstrate its effect, the subject should jog in,position until
his or her lower limbs are fatigued (really fatigued-no
slackers). Test the patellar reflex again and record whether it
is more or-less vigoraus @an @e first response.
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(b) Polysynaptic reflex
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Figure 16.2 Monosynaptic and polysynaptic reflex arcs. The integration center is in the spinal cord, and in
each example the receptor and effector are in the same limb. (a) The patellar reflex, a two-neuron mdnosynaptic reflex.
(b) A flexor reflex, an example of a polysynaptic reflex.
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