Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 9Reflexes 165

the brain. All of these pathways converge on and determine
the activity in the final common paths.

CENTRAL EXCITATORY

& INHIBITORY STATES

The spread up and down the spinal cord of subliminal fringe
effects from excitatory stimulation has already been men-
tioned. Direct and presynaptic inhibitory effects can also be
widespread. These effects are generally transient. However, the
spinal cord also shows prolonged changes in excitability, pos-
sibly because of activity in reverberating circuits or prolonged
effects of synaptic mediators. The terms central excitatory
state and central inhibitory state have been used to describe
prolonged states in which excitatory influences overbalance in-
hibitory influences and vice versa. When the central excitatory
state is marked, excitatory impulses irradiate not only to many
somatic areas of the spinal cord but also to autonomic areas. In
chronically paraplegic humans, for example, a mild noxious
stimulus may cause, in addition to prolonged withdrawal-ex-
tension patterns in all four limbs, urination, defecation, sweat-
ing, and blood pressure fluctuations (mass reflex).

CHAPTER SUMMARY

■ A reflex arc consists of a sense organ, an afferent neuron, one or

more synapses within a central integrating station, an efferent

neuron, and an effector response.

■ A muscle spindle is a group of specialized intrafusal muscle fi-

bers with contractile polar ends and a noncontractile center that

is located in parallel to the extrafusal muscle fibers and is inner-

vated by types Ia and II afferent fibers and γ-motor neurons.

Muscle stretch activates the muscle spindle to initiate reflex con-

traction of the extrafusal muscle fibers in the same muscle

(stretch reflex).

■ A Golgi tendon organ is a netlike collection of knobby nerve

endings among the fascicles of a tendon that is located in series

with extrafusal muscle fibers and innervated by type Ib afferents.

They are stimulated by both passive stretch and active contrac-

tion of the muscle to relax the muscle (inverse stretch reflex) and

function as a transducer to regulate muscle force.

■ A collateral from an Ia afferent branches to terminate on an inhib-

itory interneuron that synapses on an antagonistic muscle (recip-

rocal innervation) to relax that muscle when the agonist contracts.

Clonus is the occurrence of regular, rhythmic contractions of a

muscle subjected to sudden, maintained stretch. A sequence of in-

creased resistance followed by reduced resistance when a limb is

moved passively is known as the lengthening reaction.

MULTIPLE-CHOICE QUESTIONS

For all questions, select the single best answer unless otherwise directed.

1. The inverse stretch reflex
   A) has a lower threshold than the stretch reflex.
   B) is a monosynaptic reflex.
   C) is a disynaptic reflex with a single interneuron inserted
   between the afferent and efferent limbs.
   D) is a polysynaptic reflex with many interneurons inserted
   between the afferent and efferent limbs.
   E) requires the discharge of central neurons that release acetyl-
   choline.


2. When γ-motor neuron discharge increases at the same time as
   α-motor neuron discharge to muscle,
   A) prompt inhibition of discharge in spindle Ia afferents takes
   place.
   B) the contraction of the muscle is prolonged.
   C) the muscle will not contract.
   D) the number of impulses in spindle Ia afferents is smaller
   than when α discharge alone is increased.
   E) the number of impulses in spindle Ia afferents is greater than
   when α discharge alone is increased.


3. Which of the following is not characteristic of a reflex?
   A) Modification by impulses from various parts of the CNS
   B) May involve simultaneous contraction of some muscles and
   relaxation of others
   C) Chronically suppressed after spinal cord transection
   D) Always involves transmission across at least one synapse
   E) Frequently occurs without conscious perception


4. Withdrawal reflexes are not
   A) initiated by nociceptive stimuli.
   B) prepotent.
   C) prolonged if the stimulus is strong.
   D) an example of a flexor reflex.
   E) accompanied by the same response on both sides of the
   body.

CHAPTER RESOURCES

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Rev Physiol Biochem Pharmacol 1984;101:1.

Hunt CC: Mammalian muscle spindle: Peripheral mechanisms.

Physiol Rev 1990;70: 643.

Jankowska E: Interneuronal relay in spinal pathways from

proprioceptors. Prog Neurobiol 1992;38:335.

Kandel ER, Schwartz JH, Jessell TM (editors): Principles of Neural

Science, 4th ed. McGraw-Hill, 2000.

Lundberg A: Multisensory control of spinal reflex pathways. Prog

Brain Res 1979;50:11.

Matthews PBC: Mammalian Muscle Receptors and Their Central

Actions, Williams & Wilkins, 1972.