SECTION VII THE NERVOUS SYSTEM Sensory Loss After Knife Wound A 23-year-old woman was hrought hy an anbulance to the emergency room She had been in a fight and had been stabbed in the back at a midthoracic level with a butcher knife.A sensory exanination revealed a loss of vibratory sense in the right ankle and loss of position sense in the great toe of the right foot.Pain and temperature sensation on the left side was lost below the umbilicus.She was unable to move her right lower extremity.and stretch reflexes were absent in that limb.One moath later the sensory deficits were still present.At this time the reflexes in the right lower extrenity were hyperactive,and am extensor plantar response (sign of Babinski)was present on the right.Bowever.her right linb was still weak.Ber condition did not change substantially at follow-up exaninations over the next several years. I.What part of the nervous system was damaged by the knife wound? 2.What was the segmental level of the nervous system lesion?Of the back wound? 3.Interruption of what central nervous system patheay explains the loss of pain and temperature sensations on the left side? 4.Interruption of what central nervous system pathways explains the loss of vibratory and position semse in the right leg? 5.Why was touch not lost?Why was vibratory or position sense on the lower trunk not lost? 6.What sense organs signal vibratory and position semse? 7.Why was the right lower extrenity paralyzed? 8.Why were the stretch reflexes absent at first and them later hyperactive? 9.Why did a sign of Babinski appear? 10.Why was recovery of function so poor? ANSVER
SECTION VII THE NERVOUS SYSTEM Sensory Loss After Knife Wound A 23-year-old woman was brought by an ambulance to the emergency room. She had been in a fight and had been stabbed in the back at a midthoracic level with a butcher knife. A sensory examination revealed a loss of vibratory sense in the right ankle and loss of position sense in the great toe of the right foot. Pain and temperature sensation on the left side was lost below the umbilicus. She was unable to move her right lower extremity, and stretch reflexes were absent in that limb. One month later the sensory deficits were still present. At this time the reflexes in the right lower extremity were hyperactive, and an extensor plantar response (sign of Babinski) was present on the right. However, her right limb was still weak. Her condition did not change substantially at follow-up examinations over the next several years. 1. What part of the nervous system was damaged by the knife wound? 2. What was the segmental level of the nervous system lesion? Of the back wound? 3. Interruption of what central nervous system pathway explains the loss of pain and temperature sensations on the left side? 4. Interruption of what central nervous system pathways explains the loss of vibratory and position sense in the right leg? 5. Why was touch not lost? Why was vibratory or position sense on the lower trunk not lost? 6. What sense organs signal vibratory and position sense? 7. Why was the right lower extremity paralyzed? 8. Why were the stretch reflexes absent at first and then later hyperactive? 9. Why did a sign of Babinski appear? 10. Why was recovery of function so poor? ANSWER
1.The spinal cord and perhaps sone spinal roots were damaged.This is shown by prominent neurologic deficits that can only be explained by a central nervous system lesion.For example,a single spinal cord lesion can account for the loss of sore sensations on one side of the body and of other sensations on the opposite side of the body.No such simple explanation is available to account for the clinical picture if the lesion oaly affected peripheral nerves.Furthermore,increases in stretch reflexes and the appearance of a Babinski sign result fron interruption of central,but not peripheral,motor pathays. 2.The clue to the secmental level of the lesion is the distribution of the sensory loss.Pain and temperature were lost on the left side below the unbilicus. The dermatome in which the unbilicus lies is T10:that is,the sensory fibers that supply the skin in the region of the umbilicus enter the spinal cord over the T10 dorsal root.If a knife wound to the spine danages the T10 segrent of the spinal cord,the knife will have entered the vertebral canal at about the level of the T7 or T8 vertebra.The reason for the segmental discrepancy betseen the vertebral column and the spimal cord is that the spinal cord stops elongating during developeent before the vertebral column does. 3.Nociceptive and thermoreceptive prinary afferent fibers that supply the left lower trunk and lower extremity enter the spinal cord through dorsal roots below T10 (T11-S5)and synapse in the dorsal horn of the spinal cord.Activity in these sensory fibers activates spinothalanic tract cells at approxinately the sane levels. The axons of the spinothalamie tract cells cross to the other side and ascend to the brain in the anterolateral white matter of the right side of the spinal cord. If a knife wound on the right side at approximately the T10 level of the spinal cord hemisects the spinal cord,the wound would interrupt the spinothalanic tract on the right.Thus,it would prevent pain and temperature information that arises in the left lower trunk and lower linb from being signaled to the brain. 4.The nervous system pathway that is responsible for vibratory sense is the dorsal column-medial lemniscus system.At the spinal cord level,the part of this
1. The spinal cord and perhaps some spinal roots were damaged. This is shown by prominent neurologic deficits that can only be explained by a central nervous system lesion. For example, a single spinal cord lesion can account for the loss of some sensations on one side of the body and of other sensations on the opposite side of the body. No such simple explanation is available to account for the clinical picture if the lesion only affected peripheral nerves. Furthermore, increases in stretch reflexes and the appearance of a Babinski sign result from interruption of central, but not peripheral, motor pathways. 2. The clue to the segmental level of the lesion is the distribution of the sensory loss. Pain and temperature were lost on the left side below the umbilicus. The dermatome in which the umbilicus lies is T10; that is, the sensory fibers that supply the skin in the region of the umbilicus enter the spinal cord over the T10 dorsal root. If a knife wound to the spine damages the T10 segment of the spinal cord, the knife will have entered the vertebral canal at about the level of the T7 or T8 vertebra. The reason for the segmental discrepancy between the vertebral column and the spinal cord is that the spinal cord stops elongating during development before the vertebral column does. 3. Nociceptive and thermoreceptive primary afferent fibers that supply the left lower trunk and lower extremity enter the spinal cord through dorsal roots below T10 (T11-S5) and synapse in the dorsal horn of the spinal cord. Activity in these sensory fibers activates spinothalamic tract cells at approximately the same levels. The axons of the spinothalamic tract cells cross to the other side and ascend to the brain in the anterolateral white matter of the right side of the spinal cord. If a knife wound on the right side at approximately the T10 level of the spinal cord hemisects the spinal cord, the wound would interrupt the spinothalamic tract on the right. Thus, it would prevent pain and temperature information that arises in the left lower trunk and lower limb from being signaled to the brain. 4. The nervous system pathway that is responsible for vibratory sense is the dorsal column-medial lemniscus system. At the spinal cord level, the part of this
systen that carries sensory information fron the lower extremity is the gracile fasciculus.Many of the axons in the gracile fasciculus are branches of primary afferent fibers that enter dorsal roots on the sane side of the spinal cord and ascend to the nedulla to synapse in the nucleus gracilis.Others belong to the postsynaptic dorsal column pathway,which originates froa neurons in the dorsal horn.A knife wound on the right side that completely interrupts the gracile fasciculus will disrupt vibratory sensation that arises from levels helow the lesion.In the cervical spinal cord,the equivalent pathway is the cuneate fasciculus.This pathway contains primary afferent and postsynaptic dorsal column axons that supply the upper extrenity and upper trunk.Bowever.the cuneate fasciculus differs from the gracile fasciculus in that it mediates both vibratory and position senses.The pathaay from the lower extremity that is responsible for position sense travels in the gracile fasciculus oaly as far as the thoracic and upper lumhar spinal cord:here the axoas from primary afferent fibers that signal position sense synapse in Clarke's column. Neurons in Clarke's column then project axons in the dorsal spinocerebellar tract, fron which collaterals pass to a small relay nucleus in the pedulla called nucleus Z.However,a knife wound that interrupts the dorsal spinal cord at a lower thoracic level will block hoth vibratory and position sense,because the axons of the afferent fibers of both pathways ascend in the lower part of the gracile fasciculus. 5.The dorsal column-medial lemniscus pathsay is responsible for discriminative touch.Hlowever.the spinothalanic tract also has a tactile function.although the resolution is less fine.Because the knife wound left the spinothalamic tract on the left side of the spinal cord intact,and because the left spinothalamic tract carries tactile information from the right side.there was no overt loss of touch on the right despite interruption of the right gracile fasefculus.However,careful sensory testing would reveal a loss of fine discrimination (such as the recognition of nunbers traced on the digits).Vihratory sense is rarely tested on the trunk. and position sense cannot be tested on the trunk. 6.Higher frequency vibrations are signaled by Pacinian corpuscles and position sense by muscle spindles.For distal joints,such as those of the fingers.a
system that carries sensory information from the lower extremity is the gracile fasciculus. Many of the axons in the gracile fasciculus are branches of primary afferent fibers that enter dorsal roots on the same side of the spinal cord and ascend to the medulla to synapse in the nucleus gracilis. Others belong to the postsynaptic dorsal column pathway, which originates from neurons in the dorsal horn. A knife wound on the right side that completely interrupts the gracile fasciculus will disrupt vibratory sensation that arises from levels below the lesion. In the cervical spinal cord, the equivalent pathway is the cuneate fasciculus. This pathway contains primary afferent and postsynaptic dorsal column axons that supply the upper extremity and upper trunk. However, the cuneate fasciculus differs from the gracile fasciculus in that it mediates both vibratory and position senses. The pathway from the lower extremity that is responsible for position sense travels in the gracile fasciculus only as far as the thoracic and upper lumbar spinal cord; here the axons from primary afferent fibers that signal position sense synapse in Clarke's column. Neurons in Clarke's column then project axons in the dorsal spinocerebellar tract, from which collaterals pass to a small relay nucleus in the medulla called nucleus Z. However, a knife wound that interrupts the dorsal spinal cord at a lower thoracic level will block both vibratory and position sense, because the axons of the afferent fibers of both pathways ascend in the lower part of the gracile fasciculus. 5. The dorsal column-medial lemniscus pathway is responsible for discriminative touch. However, the spinothalamic tract also has a tactile function, although the resolution is less fine. Because the knife wound left the spinothalamic tract on the left side of the spinal cord intact, and because the left spinothalamic tract carries tactile information from the right side, there was no overt loss of touch on the right despite interruption of the right gracile fasciculus. However, careful sensory testing would reveal a loss of fine discrimination (such as the recognition of numbers traced on the digits). Vibratory sense is rarely tested on the trunk, and position sense cannot be tested on the trunk. 6. Higher frequency vibrations are signaled by Pacinian corpuscles and position sense by muscle spindles. For distal joints, such as those of the fingers, a
contribution to position sense is also made by Rufrinf (SAID)receptors and joint receptors. 7.The right lower extremity was paralyzed because the knife wound interrupted the lateral corticospinal tract,which is the main motor control pathsay for voluntary movenents.The lateral corticospinal tract that descends on the right side of the spinal cord originates from the left motor cortex. 8.When a rajor part of the descending notor control systens is suddenly interrupted.as when the spinal cord is transected.spinal reflexes are generally reduced or lost for a period of 3 to 4 weeks.The causes of this "spinal shock"are unclear,but they presumably include a sudden loss of a descending tonic excitatory drive originating in the motor control centers of the brain.One proposed explanation for the later development of hyperactive stretch reflexes is sprouting of afferent nerve fibers to fill synapses vacated by degenerating descending motor fibers. 9.The sign of Babinski (extensor plantar response)may represent a prinitive flexion reflex to noxious stimlation of the sole of the foot.It is normally present in infants,but becomes suppressed as the lateral corticospinal tract is myelinated and becomes functional.Interruption of the lateral corticospinal tract allows the reexpression of this flexion reflex.Under these cooditions,the sign of Babinski is regarded as a pathologic reflex. 10.Although axons in the central nervous systen can sprout and reestablish synaptic connections under certain circumstances,the success of regeneration in the central nervous systen is much more restricted than that in the peripheral nervous system A number of explanations for this have been proposed.However,an important possibility is that trophic suhstances needed to support rezeneration may not be appropriately expressed (or inhibitory substances that interfere with regeneration may be expressed)
contribution to position sense is also made by Ruffini (SAII) receptors and joint receptors. 7. The right lower extremity was paralyzed because the knife wound interrupted the lateral corticospinal tract, which is the main motor control pathway for voluntary movements. The lateral corticospinal tract that descends on the right side of the spinal cord originates from the left motor cortex. 8. When a major part of the descending motor control systems is suddenly interrupted, as when the spinal cord is transected, spinal reflexes are generally reduced or lost for a period of 3 to 4 weeks. The causes of this "spinal shock" are unclear, but they presumably include a sudden loss of a descending tonic excitatory drive originating in the motor control centers of the brain. One proposed explanation for the later development of hyperactive stretch reflexes is sprouting of afferent nerve fibers to fill synapses vacated by degenerating descending motor fibers. 9. The sign of Babinski (extensor plantar response) may represent a primitive flexion reflex to noxious stimulation of the sole of the foot. It is normally present in infants, but becomes suppressed as the lateral corticospinal tract is myelinated and becomes functional. Interruption of the lateral corticospinal tract allows the reexpression of this flexion reflex. Under these conditions, the sign of Babinski is regarded as a pathologic reflex. 10. Although axons in the central nervous system can sprout and reestablish synaptic connections under certain circumstances, the success of regeneration in the central nervous system is much more restricted than that in the peripheral nervous system. A number of explanations for this have been proposed. However, an important possibility is that trophic substances needed to support regeneration may not be appropriately expressed (or inhibitory substances that interfere with regeneration may be expressed)