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How is the phenomenon of presynaptic inhibition (see p. 470) relevant to the spinal gating of pain?

Before You Go On

Answer the following questions to test your understanding of the preceding section:

5. What stimulus modalities are detected by free nerve endings?

6. Name any four encapsulated nerve endings and identify their stimulus modalities.

7. Where do most second-order somesthetic neurons synapse with third-order neurons?

8. Explain the phenomenon of referred pain in terms of the neural pathways involved.

9. Explain the roles of bradykinin, substance P, and endorphins in the perception of pain.

,0acronym, from endogenous morphinelike substance " dyn = pain

,2Named substance P because it was first discovered in a powdered extract of brain and intestine

Saladin: Anatomy & I 16. Sense Organs I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

592 Part Three Integration and Control

Spinothalamic and spinoreticular tracts

Reticulospinal tracts

Enkephalin

Substance P

Second-order neuron

First-order neuron

First-order neuron

Spinothalamic and spinoreticular tracts

Reticulospinal tracts

Enkephalin

Substance P

Second-order neuron

Substance Sensory Receptor

Nociceptors

Other sensory nerve fibers

Dorsal horn of spinal cord

Dorsal root ganglion

1. In the absence of a pain stimulus, an inhibitory 4. The second-order neuron sends a pain signal 7. Neurons of the reticular formation release

Nociceptors

Other sensory nerve fibers

Dorsal horn of spinal cord

Dorsal root ganglion

1. In the absence of a pain stimulus, an inhibitory 4. The second-order neuron sends a pain signal 7. Neurons of the reticular formation release interneuron of the spinal cord prevents transmission of pain signals.

2. When tissue damage stimulates a nociceptor, the nociceptor inhibits the inhibitory interneuron.

3. The nociceptor also releases substance

P, which stimulates the second-order neuron.

to the brain.

5. Some sensory neurons other than nociceptors also stimulate the second-order neuron.

6. These sensory neurons, however, have an even stronger effect on the inhibitory interneuron, thus blocking the transmission of pain signals.

enkephalin, which, by presynaptic inhibition, blocks the release of substance P. Thus the brain can reduce the transmission of the pain signal to itself.

Figure 16.4 Spinal Gating of Pain Signals.

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