Spinothalamic Pathway Case File

Spinothalamic Pathway Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 19

A 27-year-old female was hit by a car while riding her bicycle to work. She was brought to the emergency room where she complained of severe neck pain. On physical examination she was noted to have spastic paralysis of the right upper and right lower extremities, loss of vibration and position sense on the right side, and loss of pain and temperature sensation on the left side. An MRI reveals an acutely herniated disk compressing the right side of the upper cervical spinal cord.

• What syndrome could explain all of these patient’s symptoms?
• Where is the injury located?
• What is the explanation for the loss of pain and temperature on the opposite side of the injury?

ANSWERS TO CASE 19: SPINOTHALAMIC PATHWAY

Summary: Twenty-seven-year-old female involved in a bicycle accident. An MRI reveals an acutely herniated upper cervical disk creating compression of her right spinal cord. She has symptoms of ipsilateral spastic paralysis, ipsilateral loss of vibration and position sense, and contralateral loss of pain and temperature sensation.

• Syndrome that explains this patient’s symptoms: Brown-Sequard syndrome is characterized by an ipsilateral paralysis and loss of proprioception and vibration and a contralateral loss of pain and temperature sensation.
• Location of injury: Brown-Sequard syndrome can be caused by hemicompression or hemisection of the spinal cord.
• Cause of the contralateral loss of pain and temperature sensation: Injury to the spinothalamic tract will result in contralateral pain and temperature loss from one or two levels below the injury, and ipsilateral pain and temperature loss at the level of the injury.

CLINICAL CORRELATION

Brown-Sequard syndrome is a disorder that results from a hemi-injury to the spinal cord. Three main neural systems are affected, producing the resulting symptoms: the disruption of the corticospinal tract carrying the upper motor neurons produces a spastic paralysis on the same side of the body; loss of one or both dorsal columns will result in an ipsilateral loss of vibration and proprioception, and loss of the spinothalamic tract leads to pain and temperature sensation being lost from the contralateral side of the body. The deficit begins one or two segments below the lesion and all sensory modalities are lost on the ipsilateral side at the level of the lesion because of the fibers still being uncrossed. While this syndrome is usually the result of penetrating traumatic injury to the spinal cord, rarely it can be caused by a herniated cervical disc. In this case, surgical intervention to decompress the disc would be indicated.

APPROACH TO SPINOTHALAMIC PATHWAY

Objectives

1. Know the origins of the neurons that make up the anterolateral system.
2. Be able to describe the pathway of the spinothalamic and spinoreticular tracts.
3. Understand the central projection patterns of these pathways.

Definitions

Anterolateral system: The ascending pathway that conveys pain and temperature from the periphery to the brain.

Dermatome: An area of the skin which innervates afferent nerve fibers coming to a single dorsal spinal root.

Nociceptors: A sensory receptor that sends signals that cause the perception of pain in response to potentially damaging stimuli.

Reticular formation: The network of neurons in the brainstem involved in consciousness, regulation of breathing, and transmission of sensory stimuli to higher brain centers.

Spinoreticular tracts: The fibers of the anterolateral system that do not reach the thalamus directly, instead synapsing first in the reticular formation before ascending to the thalamic nuclei, hypothalamus, and limbic system.

DISCUSSION

The anterolateral system is comprised of the spinothalamic tracts and the spinoreticular tracts, which do not reach the thalamus and cannot therefore be termed spinothalamic. These pathways mediate the sensations of pain, itching, temperature, and simple touch (see Figure 19-1). Neurons of the anterolateral system originate in the contralateral dorsal horn of the spinal cord. Most of their axons cross the midline through the ventral white commissure before ascending in the spinal cord to give rise to a diffuse bundle of fibers projecting through the anterior and lateral funiculi. The originating neurons in the dorsal horn are activated by small-diameter, lightly myelinated and unmyelinated dorsal root afferents, including Aδ(III) and C(IV) fibers, as well as larger myelinated cutaneous afferents. Aδ fibers, because of a faster conduction velocity, convey sharp pricking pain and the C fibers convey dull aching pain.

Figure 19-1. A cross-section of the spinal cord showing the spinothalamic and other ascending sensory pathways. (With permission from Morgan’s Clinical Anesthesiology. 4th ed. Figure 18-4, page 366.)

A dermatome is the area of skin supplied by one dorsal root. The dorsal roots convey information from essentially all dermatomes below the face. The cell bodies of the dorsal root fibers are located in the dorsal root ganglia. Each ganglion cell possesses a single nerve process that divides into a central branch running to the spinal cord and a peripheral branch coming from a receptor organ or organs. The dorsal root fibers enter the spinal cord through the dorsal root entry zone in the region of the dorsolateral sulcus. The Aα and Aβ fibers are the most heavily myelinated and occupy the most medial position in this entry zone, while the small myelinated Aδ and unmyelinated C fibers occupy the most lateral position.

The peripheral pain receptors consist of the naked terminals of the small Aδ and C nerve fibers. Many of these are specialized chemoreceptors that are stimulated by tissue substances released in response to noxious and inflammatory stimuli such as histamine, bradykinin, serotonin, acetylcholine, substance P, potassium, and cyclooxygenases. The concentration of the hydrogen ion in these substances has been found to be critical in the activation of pain receptors. A stimulus that evokes pain is usually one that can cause damage or destruction to tissue.

As the axons of the Aδ and C fibers enter the dorsal root zone, they immediately divide into short ascending and descending branches that run longitudinally in the posterolateral fasciculus, or Lissauer tract. After traveling one or two segments, these fibers leave the tract to synapse with neurons in laminae IV in the dorsal horn. These neurons contain receptors for numerous neurotransmitters, such as excitatory amino acids and neuropeptides. The Aδ and C fibers release glutamate and substance P at their neurotransmitters. Interneurons in laminae II through IV project to the neurons in laminae V where they synapse with the cells of origin of the anterolateral system, including the spinothalamic tracts and the spinoreticular projections. Neurons in lamina I contribute fibers to the spinothalamic tracts.

The axons of the spinothalamic tract from laminae I and V of the dorsal horn decussate anterior to the central canal in the ventral white commissure and then ascend rostrally in the anterolateral funiculus. The spinothalamic and spinoreticular tracts ascend through the spinal cord and brain stem and supply information to other spinal cord segments, to the reticular formation, the superior colliculus, and to several thalamic nuclei, including the intralaminar nuclei and the ventral posterolateral nucleus (VPL). The VPL transmits information from the spinothalamic pathway to the lemniscal system through the ventrobasal complex. Projections of the anterolateral system to the VPL are carried somatotopically, and fibers carrying input from the upper body are located medially to those carrying input from the lower body. This somatotopic organization is maintained as fibers project from the VPL, conveying painful and thermal sensations to the primary somatosensory cortex of the postcentral gyrus. VPL axons end in the primary somatosensory cortex and provide information for accurate localization of sharp pricking pain. The fibers from the upper parts of the body project to cortical areas near the lateral fissure, while those from the lower parts terminate on the medial surface of the hemisphere in the paracentral lobule. The postcentral gyrus is interconnected with the posterior parietal lobe. These areas function together to localize pain stimuli and integrate the pain modality with other sensory stimuli.

The anterolateral system is a predominantly slow conducting, polysynaptic system. The fibers that do not reach the thalamus directly synapse in the reticular formation of the brain stem. These are the spinoreticular tracts. From the reticular formation, ascending fibers relay pain information to the medial and intralaminar nuclei of the thalamus as well as to the hypothalamus and limbic system. Additionally, the intralaminar nuclei project information to multiple cortical areas to ensure that pain is not lost when there is damage to the primary somatosensory cortex. While most fibers of the anterolateral system cross the midline before ascending, many of the fibers of the spinoreticular tracts conveying visceral sensory information ascend ipsilaterally.

Painful sensations from the face, cornea, sinuses, and mucosal linings travel through fibers of the trigeminal nerve to its sensory trigeminal ganglion. After these fibers enter the brain stem in the pontine region, they form an ipsilateral descending tract, the spinal tract of V, which courses to the upper cervical segments of the spinal cord. The neurons synapse in the spinal nucleus of V (spinal trigeminal nucleus), and from there they decussate and form the ventral trigeminal lemniscus which joins with the crossed main sensory trigeminal efferents, and ascends to the ventral posteromedial nucleus (VPM). In this way, information from the spinal nucleus of V projects to the contralateral side and ascends to the VPM of the thalamus. The ascending trigeminal pathway also projects to the reticular formation and the medial and intralaminar thalamic nuclei, which also receive projections from the anterolateral system of the spinal cord. The VPM projects to the somatosensory cortex closest to the lateral fissure. The areas of the body most sensitive to somatosensory stimuli, the lips and fingers, have disproportionately large areas of neuronal representation in the somatosensory cortex. Thalamocortical fibers from neurons in the intralaminar thalamic nuclei and parts of the VPL and VPM nuclei relay pain information to the secondary somatic sensory area of the cerebral cortex.

This tract has also been known to respond to temperature. Naked nerve endings in the skin carry sensations of cold and warmth. The peripheral nerve fibers mediating these sensations consist of thinly myelinated Aδ and some C fibers. Other types of C fibers mediated the painful components of the extremes of hot and cold sensations. The central nervous system pathway from thermal sensation follows the same course as the pain pathway. These two systems are so closely associated in the central nervous system that they cannot be distinguished anatomically, and injury to one usually affects the other.

The lateral spinothalamic tract can be purposefully transected in the spinal cord in an attempt to relieve intractable pain. This procedure, known as a tractotomy, is performed by making a cut in the anterior part of the lateral funiculus. There is usually some damage to the ventral spinocerebellar tract and possibly to some of the extrapyramidal motor fibers. No permanent symptoms, however, are typically seen aside from a loss of pain sensitivity on the contralateral side which begins one or two segments below the cut. The pain relief is sometimes only temporary, suggesting other routes in crossed and uncrossed tracts that mediate nociceptive sensations in the spinal cord.

COMPREHENSION QUESTIONS

Refer to the following case scenario for questions 19.1-19.2:

A 12-year-old boy is brought into your office with a 5-cm laceration on his forehead, that he got falling off his bicycle. Although he is trying to be brave, a few tears are running down his cheek, and when asked, he says it “really, really hurts.”

[19.1] What type of somatosensory receptor is involved in the initial detection of tissue injury as pain?

A. Merkel cell

B. Pacinian corpuscle

C. Unmyelinated free nerve ending

D. Heavily myelinated free nerve ending

[19.2] Through what thalamic nucleus is the pain relayed to the primary sensory cortex?

A. VPL

B. VPM

C. Centromedian nucleus (CM)

D. Parafascicular nucleus (PF)

[19.3] Where are the cell bodies of the primary sensory neurons involved in pain transmission located?

A. Dorsal horn of the spinal cord

B. Ventral horn of the spinal cord

C. Dorsal root ganglia

D. Near the site of sensation, in the subcutaneous tissue

[19.4] A 72-year-old man comes into the emergency department complaining of vertigo, hoarseness, difficulty swallowing, and a droopy left eyelid. On examination you note a loss of painful sensation over the left half of his face and the entire right half of his body. Interestingly, however, he has intact discriminative touch in those areas. His history in remarkable for 60 pack years of smoking and poorly controlled high blood pressure. What is the most likely location of the lesion in this man?

A. Lower cervical spinal cord

B. Medulla

C. Thalamus

D. Cerebral cortex

Answers

[19.1] C. The “receptor cells” involved in the detection of pain are not discreet sensory organs, as they are for discriminative touch and proprioception, but rather the free nerve endings of the first-order neurons in the pain pathway. These neurons are unmyelinated, or very slightly myelinated, as opposed to the neurons innervating touch receptors, which are heavily myelinated. The free nerve endings of pain fibers respond to a variety of chemical stimuli, including potassium ions, serotonin, histamine, and bradykinin. There are also free nerve endings that respond to physical deformation (stretch-sensitive pain fibers).

[19.2] B. This boy has a lesion on his face, so the pain sensation is traveling through the spinal trigeminal tract, and the thalamic relay for this tract (and all localized sensation for the face) is VPM, which projects to the face area of the primary sensory cortex. Had this been a lesion on the trunk or extremities, then the pathway would have been the spinothalamic pathway, which (like all localized sensation on the trunk and extremities) is relayed through VPL. It is true that CM and PF, which are also known as the intralaminar nuclei, are involved in the sensation of pain, but they relay more diffuse, emotional, poorly localized pain to diffuse areas of the cortex.

[19.3] C. The cell bodies of pain-sensing fibers are located in the dorsal root ganglia, just like the cell bodies of touch and proprioception neurons. Unlike touch neurons, however, the axons of pain neurons do not ascend the spinal cord all the way to level of the brainstem. They synapse with second-order neurons of the pain pathway in the dorsal horn of the spinal cord at or one to two spinal levels above where the fibers enter the cord.

[19.4] B. This man is experiencing what is known as Wallenberg syndrome, and is caused by a lesion in the lateral medulla. Often, this lesion results from occlusion of the posterior inferior cerebellar artery (PICA), which supplies this region. The sensory symptoms are explained by the fact that the spinal trigeminal tract descends from the sensory trigeminal ganglion to the spinal trigeminal nucleus in the ipsilateral lower medulla and upper cervical spine. The descending spinal trigeminal neurons (carrying pain from the ipsilateral face) are near the ascending spinothalamic tract (carrying pain from the contralateral body), and both of these tracts are supplied by the PICA, so both are damaged. The medial lemniscus is not in the same region, so not damaged by an occlusion of PICA, accounting for the preservation of discriminative touch. A lesion in the lower cervical cord would not affect the face as this is below the level of the spinal trigeminal nucleus, and lesions of the thalamus and cortex would affect the body and face on the same side.

NEUROSCIENCE PEARLS ❖ The anterolateral area of the spinal cord (spinothalamic tracts and spinoreticular tracts) mediates pain, temperature, and simple touch. ❖ Most of the anterolateral system axons cross the midline through the ventral white commissure. ❖ Cell bodies of dorsal root fibers are located in dorsal root ganglia. ❖ Aδ fibers convey sharp pricking pain while C fibers convey dull aching pain. ❖ Painful sensation from the face travels from the (1) trigeminal nerve to the (2) trigeminal ganglion, to the (3) spinal tract of V in the pons region, and then synapse in the (4) spinal nucleus of V, after which they decussate to form the ventral trigeminal lemniscus, and ascend to the (5) ventral posteromedial nucleus (VPM) of the thalamus.

REFERENCES

Buck LB. Smell and taste: the bodily senses. In: Kandel ER, et al. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 2000. 

Martin JH. The somatic sensory system. Neuroanatomy, Text and Atlas. 2nd ed. Stamford, CT: Appleton & Lange; 1996. 

Ropper AH and Brown RH. Other somatic sensation. Adam’s and Victor’s Principles of Neurology. 8th ed. New York, NY: McGraw-Hill; 2005.

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