Elderly individuals may experience a condition known as trigeminal neuralgia, characterized by recurring episodes of intense, often sharp pain in one or more branches of the trigeminal nerve. This pain is frequently triggered by slight mechanical stimulation in the same area. The primary cause of this painful state seems to be mechanical damage to the trigeminal ganglion caused by an artery that presses against the ganglion. Surgical repositioning of the artery often resolves the condition.

The pain pathways within the central nervous system consist of the spinothalamic, spinoreticular, and spinomesencephalic tracts. The spinothalamic tract is the main sensory pathway for somatic pain and temperature sensations from the body. It also contributes to tactile sensation. Originating from second-order neurons in the spinal cord, primarily in laminae I and IV to VI, the axons of these neurons cross to the opposite side of the spinal cord at or near their origin level. They then ascend through the brainstem to the thalamus, where they terminate on third-order neurons (thalamocortical neurons), as explained previously. Pain and temperature signals carried by spinothalamic cells target the VPI portion of the ventroposterior complex, the posterior nucleus, and the intralaminar nuclei tract cells in the thalamus. These nociceptive signals are then transmitted to various cortical areas, including the somatosensory cortex, as well as cortical areas involved in affective responses, such as the cingulate gyrus and insula, which have functions related to the limbic system.

Most spinothalamic tract cells receive excitatory input from nociceptors in the skin, although many can also be stimulated by noxious stimuli in muscles, joints, or viscera. Only a few receive input solely from viscera. Effective cutaneous stimuli encompass noxious mechanical, thermal (hot or cold), and chemical stimuli. Hence, different spinothalamic tract cells respond appropriately to signal noxious, thermal, or mechanical events.

Certain nociceptive spinothalamic tract cells receive convergent excitatory input from various classes of cutaneous sensory receptors. These cells, known as wide-dynamic range cells or multireceptive cells, can be activated by stimuli with a wide range of intensities. They predominantly signal noxious events, tending to ignore weak responses to tactile stimuli at higher centers. However, under certain pathological conditions, tactile stimuli may sufficiently activate these neurons to evoke a sensation of pain. This may occur due to activity in sensitized afferents that were previously silent, providing an explanation for pain states in which mechanical stimulation causes pain (mechanical allodynia). Other spinothalamic tract cells are exclusively activated by noxious stimuli and are commonly referred to as high-threshold or nociceptive-specific cells.

Because cells conveying visceral input often also transmit information from cutaneous receptors, the brain may misinterpret the source of pain. This phenomenon is known as referred pain. A classic example is when the heart muscle becomes ischemic, and the pain is perceived in the chest wall and left arm.
Elderlypeoplearesometimessusceptibletoaconditionofchronicpainknownastrigeminal neuralgia. Peoplewiththisconditionexperiencespontaneousepisodesofsevere,oftenlancinatingpaininthedistributionofoneormorebranchesofthetrigeminalnerve.Frequentlythepainistriggeredbyweakmechanicalstimulationinthesameregion.Amajorcontributingfactortothispainfulstateappearstobemechanicaldamagetothetrigeminalganglionbyanarterythatimpingesontheganglion.Surgicaldisplacementofthearterycanoftenresolvethecondition.
The central pain pathways include the spinothalamic, spinoreticular, and spinomesencephalic tracts. The spinothalamic tract is the most important sensory pathway for somatic pain and thermal sensations from the body (see
Fig. 7.1A ). It also contributes to tactile sensation. The spinothalamic tract originates from second-order neurons located in the spinal cord (primarily laminae I and IV to VI). The axons of these cells cross to the opposite side of the cord at or near to their level of origin. They then ascend to the brain in the ventral part of the lateral funiculus and subsequently through the brainstem to the thalamus, where they terminate on third-order neurons (thalamocortical neurons), as described earlier. Spinothalamic cells conveying pain and temperature target the VPI portion of the ventroposterior complex (although some also end in the
Multireceptive spinothalamic VPL), the posterior nucleus, and the intralaminar nuclei tract cells of the thalamus. Nociceptive signals are then forwarded to 120 several cortical areas, including not only the somatosensory cortex but also cortical areas that are involved in affective responses, such as the cingulate gyrus and the insula, which have limbic system functions (see
Fig. 7.5
Most spinothalamic tract cells receive excitatory input from nociceptors in the skin, but many can also be excited by noxious stimulation of muscle, joints, or viscera. Few receive input only from viscera. Effective cutaneous stimuli include noxious mechanical, thermal (hot or cold), and chemical stimuli. Thus different spinothalamic tract cells respond in a manner appropriate for signaling noxious, thermal, or mechanical events. A
Some nociceptive spinothalamic tract cells receive convergent excitatory input from several different classes of cutaneous sensory receptors. For example, a given spino
Fig. 7.7A ). Such neurons are called wide–dynamic range cells or multirecep-120 tive cells because they are activated by stimuli with a wide range of intensities. Wide–dynamic range neurons signal mainly noxious events; weak responses to tactile stimuli appear to be ignored by the higher centers. However, in certain pathological conditions, these neurons may be suf ficiently activated by tactile stimuli to evoke a sensation of pain, possibly as a result of activity in sensitized afferents that were previously silent. This would explain some pain states in which activation of mechanoreceptors causes pain (mechanical allodynia). Other spinothalamic tract cells are activated only by noxious stimuli. Such neurons are often called high-threshold or nociceptive-specific cells (see
Fig. 7.7B
Because cells signaling visceral input also typically convey information from cutaneous receptors, the brain may misidentify the source of the pain. This phenomenon is called referred pain. A typical example is when the heart muscle becomes ischemic and pain is felt in the chest wall and left arm.