Somatosensory Pathways Touch & Pain PDF

Somatosensory Pathways Touch & Pain Page 1 of 16 Dr. Paul Walker Session Objectives By the end of this session, students should be able to accurately: 1. Summarize the functional anatomy of the somatosensory pathways for touch and pain. 2. De...

Somatosensory Pathways Touch & Pain Page 1 of 16 Dr. Paul Walker Session Objectives By the end of this session, students should be able to accurately: 1. Summarize the functional anatomy of the somatosensory pathways for touch and pain. 2. Describe clinical deficits and tests for somatosensory pathway lesions. Session Outline I. Functional Anatomy of the Somatosensory Pathways for Touch and Pain II. Clinical Deficits and Tests for Somatosensory Pathway Lesions Supplemental Reading Haines & Mihailoff, Fundamental Neuroscience for Basic & Clinical Applications, 5 th Ed (2018) Elsevier, Chapters 17-18. TW Vanderah & DJ Gould, Nolte’s The Human Brain, 8 th Ed (2021), Elsevier, Chapters 10-11  Somatosensory Pathways Touch & Pain Page 2 of 16 Dr. Paul Walker I. Functional Anatomy of the Somatosensory Pathways for Touch & Pain A. Definition and Significance of Somatosensation 1. Somatosensation is the detection of stimuli from the body surface (e.g., touch, pain, temperature) and the sensing of the position and movement of limbs (proprioception). 2. Somatosensation is important clinically because it is impaired by common neurological disorders. Patient signs and symptoms of somatosensory deficits can help determine what type of neurological disorder may be involved or help localize what part of the nervous system has been damaged. B. Modalities of Somatosensation Fig 1 Fine Discriminatory Touch 1. Certain touch sensations allow for discrimination of surface characteristics such as rough, smooth, sharp, dull, edges, bumps, soft, hard, etc. This type of touch sensation is called ‘fine discriminatory touch’ or abbreviated ‘fine touch’. 2. Fine discriminatory touch sensations also include the detection of pressure and vibration, as well as being able to sense different areas of the skin that were touched. This latter sense is called ‘2-pt discrimination’. 3. Stereognosis is the ability to identify (recognize) an object by the sense of touch and usually involves manipulation of the object via combined sensorimotor systems. Proprioception Limb position and movement is detected by the brain at 2 levels: conscious & unconscious. 1. Conscious proprioception- sensations allow the ‘feeling’ of limb position and movement. These sensations are relayed to the cerebral cortex via the thalamus. 2. Unconscious proprioception- information about muscle stretch and tension is part of spinal reflex circuitry. This information is relayed to the cerebellum and is not perceived consciously and will be discussed in a separate lecture on cerebellum. Pain, Temperature, Itch, Crude Touch 1. Sensation of damaging or potentially damaging stimuli such as mechanical (puncture, crush), temperature (scalding, freezing), and chemical (cell rupture, inflammatory). 2. Innocuous changes in skin temperature (warm, cool). 3. Sensation of itch (pruritis). 4. Sensation of having been touched (crude or light touch), but not able to determine exact location or type of touch as can be done with fine discriminatory touch sensation. Somatosensory Pathways Touch & Pain Page 3 of 16 Dr. Paul Walker C. Dorsal Column-Medial Lemniscus (DC-ML) pathway Transmits somatosensory information about: Fig 2 1. Fine Discriminatory Touch: surface characteristics, vibration, pressure, 2-point discrimination, object recognition by touch (stereognosis). 2. Conscious Proprioception: sensations of limb position and movement that are consciously perceived. Summary of DC-ML Pathway A. 1st Order Neuron: Cell body is a sensory dorsal root ganglion (DRG) neuron located at C2-S5 spinal levels. The central processes (axons) of DRG pseudounipolar neurons enter the spinal cord via the dorsal root and ascend superiorly in the dorsal funiculus as the dorsal column (DC) pathway. 1st order axons reach the caudal medulla and synapse in the Nucleus Gracilis (receive info from T7-S5 spinal levels) and Nucleus Cuneatus (receive info from C2-T6 spinal levels). B. 2nd Order Neuron: Nucleus Gracilis and Nucleus Cuneatus. These nuclei originate 2nd order axons that cross the midline to form the Medial Lemniscus (ML). The DC-ML pathway is now contralateral to the side of sensory stimulus origin. The ML pathway ascends through the brainstem to synapse in the thalamus. C. 3rd Order Neuron: Ventral Posterolateral (VPL) thalamic nucleus. This nucleus of the thalamus originates 3rd order axons that project directly to the ipsilateral somatosensory cortex. Somatosensory Pathways Touch & Pain Page 4 of 16 Dr. Paul Walker D. Spinothalamic Pathway (also known as the Anterolateral System, ALS) Transmits somatosensory information about: Fig 3 Innocuous Stimuli Temperature Itch Crude touch Damaging Stimuli Pain (Nociception) Mechanical Thermal Chemical Summary of the Spinothalamic Tract (ALS) 1st Order Neuron: Sensory dorsal root ganglion (DRG) neuron. DRG central processes enter the spinal cord via the lateral division of the dorsal root and synapse in the dorsal horn of the spinal cord. 2nd Order Neuron: Dorsal horn spinal neurons project axons across the midline via the anterior white commissure to form the contralateral ascending ALS pathway. located in a region that spans the anterior and lateral funiculus. These axons travel to the Ventral Posterolateral (VPL) thalamic nucleus (spinothalamic), while also contributing collaterals to the reticular formation of the brainstem (spinoreticular). 3rd Order Neuron: Spinothalamic information is relayed by VPL axons project to the ipsilateral somatosensory cortex. Spinoreticular information is conveyed to brain regions that control arousal and emotions, as well as the autonomic nervous system. Somatosensory Pathways Touch & Pain Page 5 of 16 Dr. Paul Walker E. Organization of Somatosensory Pathways in the Spinal Cord How do the 1st order central processes for each pathway enter the spinal cord? 1st order dorsal root ganglion (DRG) central processes enter the dorsal root at each spinal level (S5-C2) (Fig 4). DC-ML central processes are classified as A fibers. 1. A fibers: heavy-myelinated, medium-diameter sized (6-12 µm), and fast conducting (36-72 m/s). Convey information about fine discriminatory touch, vibration, pressure, proprioception. These fibers enter more medially in the dorsal root. Spinothalamic (ALS) central process are classified as either A fibers or C fibers. 1. A fibers: myelinated, small-diameter (1-6 µm) axons with conduction velocity of 4-36 m/s. Convey information about tissue damage (mechanical, thermal, chemical) that is perceived as sharp (fast) pain. This is the fast pain pathway important for limb-withdrawal spinal reflexes (step on a sharp object and withdraw foot). These fibers enter more medially in the dorsal root. 2. C fibers: unmyelinated, tiny-diameter (0.2-1.5 µm) axons with conduction velocity of 0.4-2.0 m/s. This is the slow pain pathway that conveys burning, aching sensations that are prolonged following tissue damage (Fig 2). C fibers also transmit itch and crude touch sensations. These fibers enter more laterally in the dorsal root (where the Weigert staining appears lighter). Fig 4 Somatosensory Pathways Touch & Pain Page 6 of 16 Dr. Paul Walker Where do the fibers go after they enter the spinal cord? DC-ML fibers 1st order DRG fibers carrying DC-ML information DO NOT synapse in the dorsal horn of the spinal cord. Instead, they gather in the heavily myelinated dorsal columns of the spinal cord and ascend toward their 2nd order target nuclei located in the caudal medulla. Within the dorsal columns, these fibers and are organized from medial to lateral in the following order: Sacral, Lumbar, Thoracic, and Cervical. This is called somatotopic organization because it refers to a specific body (soma) region. The below Fig 4 shows a cross section through the level of the lumbar enlargement of the spinal cord. Let’s imagine that section level is L3, so the dorsal column would only contain 1st order DC-ML fibers representing S5 to L3 dermatomes. The S5 fibers would be most medial in the dorsal column and the L3 fibers would be most lateral. Spinothalamic (ALS) fibers 1st order DRG fibers carrying spinothalamic (ALS) information DO synapse in the dorsal horn of the spinal cord. When these fibers enter the cord, the main branch goes into the dorsal horn gray matter (shown below in Fig 4). Branching collaterals also diverge from these fibers and travel up and down the spinal cord within Lissauer’s Tract to integrate pain & temp information across several spinal levels. This is thought to be important for reflexive withdrawal of an entire limb in response to a painful stimulation from a single dermatome. The main fiber synapses in several parts of the dorsal horn of the spinal cord. The substantia gelatinosa part of the dorsal horn contains inhibitory (opioid) neurons that modulate incoming pain signals from 1 st order DRG neurons. As such, signals that excite opioid interneurons are ‘pain dampening’ while signals that inhibit the opioid neurons are ‘pain enhancing. Fig 4 (repeated) Somatosensory Pathways Touch & Pain Page 7 of 16 Dr. Paul Walker Axons from 2nd order dorsal horn neurons project ventromedially across the midline in the anterior white commissure to form the spinothalamic tract or ALS pathway (Fig 4). Why 2 names for this pathway?  Spinothalamic tract: because the main pathway technically begins as a tract in the dorsal horn of the spinal cord and ends in the thalamus of the diencephalon. This is the traditional (old) name of the pathway transmitting pain and temperature information from the contralateral body.  ALS: same pathway but refers to its position in both anterior funiculus and lateral funiculus parts of the spinal cord. It is also more than the spinothalamic tract because the thalamus is not the only target. As mentioned above, collaterals of this pathway synapse in the reticular formation of the brainstem. As such a more modern name given to this pathway is the Anterolateral System (ALS). We will use both names in this course because NBME and USMLE questions may depend on your knowledge of either name. Fig 4 (repeated) The crossing of ALS axons to the opposite side of the spinal cord is oblique and takes at least 1-spinal level to achieve. So again, let’s imagine that the level of section in Fig 4 is L3 spinal cord. DRG axons carrying pain and temperature information from L3 dermatomes will not gather in the spinothalamic tract of the contralateral spinal cord until the L2 spinal level. This means the spinothalamic tract or ALS fibers observed in the image are carrying pain and temperature from S5 to L4 dermatome levels. From lateral to medial, the somatotopic organization of the spinothalamic tract (ALS) in Fig 4 is sacral representation lateral and lumbar representation medial. As incoming fibers enter the spinothalamic tract, axons from lower levels are pushed laterally. The next page follows the both DC-ML and ALS pathways as they ascend in the spinal cord. Somatosensory Pathways Touch & Pain Page 8 of 16 Dr. Paul Walker Fig 5 The adjacent Fig 5 section is through the thoracic spinal cord inferior to T6 spinal level. Look at the size of the dorsal columns. They are larger because they now carry DC-ML information from sacral, lumbar, and thoracic levels up to the level of the section. The spinothalamic tract is also more elongated. It includes from lateral to medial: sacral (S5-S1), lumbar (L5-L1), thoracic (T12-to 1- level below this thoracic spinal cord section). Fig 6 Now look at the Fig 6 section through the cervical spinal cord between C5- C8 spinal levels. This is at the level of the cervical enlargement of the ventral horn. The dorsal columns are divided into Fasciculus Gracilis and Fasciculus Cuneatus by the posterior intermediate sulcus and septum. The fasciculus gracilis (‘gracile’ means ‘slender’) runs the entire length of the spinal cord and carries DC-ML axons to the nucleus gracilis of the caudal medulla. The fasciculus gracilis contains 1st order DC-ML axons from S5 to T7 dermatomes. The fasciculus cuneatus (‘cuneatus’ means ‘wedge-shaped’) begins at T6 spinal levels and continues to the nucleus cuneatus in the caudal medulla. The fasciculus cuneatus contains 1st order DC-ML axons from T6-C2 dermatomes (Fig 6 would only have cervical fibers to the level of the section). The posterior intermediate septum and sulcus is an important landmark because it is only found at T6-T1 spinal levels and all cervical levels. It divides the dorsal columns into fasciculus gracilis and fasciculus cuneatus. When you see it, it helps you know where you are in the spinal cord and you can use other landmarks (e.g. cervical enlargement) to further localize the approximate level. Note also that the spinothalamic tract is further elongated. It includes from lateral to medial: sacral (S5-S1), lumbar (L5-L1), thoracic (T12-T1), and cervical to 1-level below this cervical spinal cord section. Somatosensory Pathways Touch & Pain Page 9 of 16 Dr. Paul Walker Fig 7 We finally reach the top of the spinal cord at C1 level (Fig 7). The 2 somatosensory pathways that we have been following have accumulated all of their fibers. DC-ML fibers in the dorsal columns are 1st order fibers. Their cell bodies of origin are in the ipsilateral DRGs at each respective spinal level (S5-C2). Spinothalamic (ALS) tract fibers are 2nd order fibers. Their cell bodies of origin are in the contralateral dorsal horn of the spinal cord at each respective spinal level (S5-C2). Think about this. If a patient had a penetrating injury that destroyed ½ of the spinal cord at C1, they would lose fine discriminatory touch and conscious proprioception from the ipsilateral body and pain and temperature sensation from the contralateral body. Can you figure out why on your own? We will review this concept in the neuro labs. F. Organization of Somatosensory Pathways in the Brainstem Fig 8 Rostral to the spinomedullary junction, two nuclei appear in the dorsal part of the caudal medulla: Nucleus Gracilis and Nucleus Cuneatus (Fig 8). The two fasciculi of the dorsal columns terminate in their respective nuclei and maintain their somatotopic organization.  Fasciculus Gracilis (FG) fibers (S5-T7) synapse in Nucleus Gracilis (NG).  Fasciculus Cuneatus (FC) fibers (T6-C2) synapse in Nucleus Cuneatus (NC). 2nd order somatosensory axons from Nucleus Gracilis and Nucleus Cuneatus course ventrally and medially as "internal arcuate fibers" and CROSS THE MIDLINE to form the medial lemniscus (ML). As such, the ML is the location of 2nd order DC-ML axons. Their target is the thalamus located in the diencephalon. The spinothalamic tract stays in its position but is crowded and pushed laterally by a large nucleus that begins to appear dorsal to the pyramids. The somatotopic organization of the spinothalamic tract is maintained: sacral lateral and cervical medial. Somatosensory Pathways Touch & Pain Page 10 of 16 Dr. Paul Walker Fig 9 At mid-medulla, all 2nd order DC-ML fibers have gathered in the ML, which is located dorsal to the medullary pyramids (Fig 9). The somatotopic order is preserved in the ML: sacral representation is ventral and cervical representation is dorsal. The inferior olivary nucleus separates the ML medially from the spinothalamic (ALS) tract laterally. If a lesion affects ½ of the medulla at this level, the patient would lose fine discriminatory touch and conscious proprioception from the contralateral body and pain and temperature sensation from the contralateral body. This is because the DC-ML pathway crossed the midline in the caudal medulla and the spinothalamic (ALS) pathway crossed the midline in the spinal cord. We are above the crossing level of each so the fibers represent contralateral body sensation. Let’s keep following the DC-ML and spinothalamic (ALS) pathways as they ascend through the brainstem. Fig 10 An interesting rotation of the ML happens as this pathway travels through the pons. The left section of Fig 10 shows the orientation of the ML in the rostral medulla. It is the same as in Fig 9 at mid-medulla level. Beginning in caudal pons, the ventral-to-dorsal orientation of the ML changes to a medial-to-lateral orientation at mid-pons. The somatotopic organization is maintained such that cervical levels end up medial and sacral levels are lateral. This orientation now matches the somatotopic organization of the spinothalamic (ALS) tract and the two pathways are positioned side-by-side. Somatosensory Pathways Touch & Pain Page 11 of 16 Dr. Paul Walker Fig 11 Within the midbrain, the ML is displaced dorsolaterally by a large nucleus- called the Red Nucleus (Fig 9) The Red Nucleus has motor function that will be discussed later in the course. For now, use it as an anatomical landmark to recognize the midbrain and note that the ML and spinothalamic (ALS) tracts are positioned laterally. The medial to lateral CTLS somatotopic organization is maintained in both pathways as they continue toward their common thalamic target in the diencephalon (Fig 12) Fig 12 The midbrain section shown in above Fig 11 is the rostral midbrain. Can you see the superior colliculus and substantia nigra? These are indicators of the rostral midbrain. Note: Depending upon the angle of the image in the axial plane, it is possible to see substantia nigra ventrally and inferior colliculus dorsally. We will see this is in some of our axial images (not shown here). However, the red nucleus is positioned more rostrally and indicates rostral midbrain. Fig 13 The thalamic nucleus that both the DC-ML and spinothalamic (ALS) pathways synapse is called the ventral posterolateral (VPL) thalamic nucleus. It is encircled by the yellow oval in both Figs 13 and 14. The somatotopic organization of fiber synapse is maintained for both pathways: cervical is medial and sacral is lateral. Each pathway synapses on separate thalamic neurons in each somatotopic region so the sensory modalities are kept separate. Fig 14 Somatosensory Pathways Touch & Pain Page 12 of 16 Dr. Paul Walker G. Thalamocortical Axons Relay Somatosensory Impulses to the Cerebral Cortex Fig 13 repeated rd 3 Order VPL thalamus axons transmit DC-ML and spinothalamic (ALS) pathway information to specific areas of the primary somatosensory (SI) cortex. These axons ascend within the posterior limb of the internal capsule (PLIC) (Figs 13 & 15) to reach the gray matter of the cerebral cortex. The appearance of the white matter between the PLIC and the gray matter of the cerebral cortex resembles a radiating crown and is called corona radiata (CR). (Fig 13) The primary somatosensory (SI) cortex is located in the postcentral gyrus (Fig 16). SI cortex is also called Brodmann areas 3,1, 2. Fig 15 Fig 16 Fig 17 Individual neurons of SI cortex receive input arising from a particular area of skin. Skin areas containing a higher density of receptors (e.g. fingers, hand) are overrepresented in the SI cortex. The somatotopic representation in the cerebral cortex is known as the "sensory homunculus" (Fig 17). The primary somatosensory (SI) cortex sends information to secondary somatosensory cortex for higher integrative processing. One such region is in the superior parietal lobule and is known as Brodmann Area 5,7. Somatosensory Pathways Touch & Pain Page 13 of 16 Dr. Paul Walker II. Clinical Deficits and Tests for Somatosensory Pathway Lesions Fig 18  Ipsilateral Deficits: A unilateral lesion of the Dorsal Columns in the spinal cord or Dorsal Column Nuclei in the caudal medulla would produce ipsilateral symptoms. This is because the lesion occurs before the pathway crosses the midline.  Bilateral Deficits: A lesion of internal arcuate fibers as they cross the midline would produce bilateral symptoms. Also with bilateral dorsal column degeneration (e.g. tabes dorsalis).  Contralateral Deficits: A unilateral lesion of the medial lemniscus, VPL thalamus, or SI cortex would produce contralateral symptoms because the lesion occurs after the pathway crosses the midline. DC-ML lesions result in permanent loss of:  Fine touch/texture discrimination (tested using different tactile sensations)  Conscious proprioception (tested by detection of limbs in new position with eyes closed)  Two-point discrimination (tested with a compass)  Vibration sense (tested with a tuning fork)  Stereognosis (ask the patient to identify an object placed in the hand with eyes closed). Fig 19 Somatosensory Pathways Touch & Pain Page 14 of 16 Dr. Paul Walker Fig 20 Cerebral cortex control of voluntary movements depends on somatosensory feedback at both conscious and unconscious levels. The DC-ML pathway provides significant conscious somatosensory information such that DC-ML pathway lesions cause clumsy movements that are poorly coordinated. Romberg Test: Used to assess DC-ML function. The patient stands with feet close together, hands by their sides, and eyes closed (physician stands by ready to steady them). Without visual cues to maintain orientation, the loss of conscious proprioception due to DC-ML damage causes the patient to begin to fall. Loss of upright stance under these conditions denotes a positive Romberg's sign. Tabes Dorsalis: Late-stage syphillis destroys large DRG cells projecting A axons into the spinal cord bilaterally, especially affecting the lower extremities. Loss of dorsal column information leads to wide-based, staggering gait often accompanied by slapping the feet against the ground when walking (to increase feedback). Patients use visual feedback (watch their feet during walking) as a substitute for tactile feedback. Primary somatosensory cortex lesions can also produce contralateral somatosensory deficits in fine touch, pressure, vibration, joint position, 2-pt discrimination, as well as difficulties with more complex tactile tests such as stereognosis and texture discrimination. Interestingly, secondary somatosensory cortex lesions affecting the superior parietal lobule (Brodmann Area 5,7) do not produce deficits in simple tactile sensations such as occurs in primary somatosensory cortex lesions. Instead, these lesions affect skilled movements of the contralateral limb that require sensory feedback. A phenomenon called “useless hand (limb) syndrome” or “tactile apraxia” is associated with these types of lesions A patient will be able to detect sensations in the affected limb but doesn’t seem to know how to use the limb in making skilled movements. For example, if the upper extremity is affected, it hangs “useless” against the patient’s side as if it is non-functional. Somatosensory Pathways Touch & Pain Page 15 of 16 Dr. Paul Walker Spinothalamic Lesions in the Spinal Cord Fig 21 A unilateral lesion of the lateral spinothalamic tract would produce a loss of pain and temp localization in the contralateral body, but only affecting fibers that have already gathered in the tract. For example, a spinothalamic tract lesion (see encircled area below) at the C6 spinal level would cause a loss of pain, temperature, and crude touch sensations in C7-S5 dermatomes of the contralateral body. C6 dermatome sensations would be spared because they are just entering at this level and it will take 1-level of 2nd order pathway ascension before the fibers have crossed the anterior white commissure and gathered in the contralateral spinothalamic (ALS) tract. Fig 22 Syringomyelia is a cavitation of the spinal cord central canal that damages the anterior white commissure producing a bilateral changes in pain and temperature perception. This lesion can be localized to a single spinal level but more commonly spans multiple spinal levels. Below 2 examples discuss each scenario. Example 1: a small cavitation that interrupts the anterior white commissure only at C6 levels would cause a bilateral loss of pain, temperature, and crude touch sensations from the C7 dermatome only. Example 2: a larger cavitation interrupts the anterior white commissure at T6-C5 spinal levels. Patient has bilateral loss of pain, temperature, and crude touch sensations from T7- C6 dermatomes. Bottom line: This lesion affects 2nd order pain and temperature fibers crossing the midline, but not the fibers that have already gathered in the spinothalamic tract. pay attention to the spinal level extent of damage to the anterior white commissure. That will inform you of where the patient deficits are located (1 level below lesion span). Somatosensory Pathways Touch & Pain Page 16 of 16 Dr. Paul Walker Dejerine-Roussy Syndrome (Thalamic Pain Syndrome) Can develop following ischemic damage of the posterior thalamus. Red lesion area in below Fig 23 is in the region of the VPL thalamus where both DC-ML and spinothalamic fibers terminate. Fig 23 Deficits/Symptoms Loss of fine discriminatory touch sensation and conscious proprioception in contralateral body. Loss of ability to localize pain and temperature sensations in contralateral body. Patients may still sense some crude touch due to relays of the spinothalamic pathway into the brainstem reticular formation, which in turn sends signals to other regions of the thalamus and cerebral cortex. However, the ability to discriminate touch sensations from the contralateral body is completely lost (as discussed above). Over time, numbness of affected side replaced by unpleasant spontaneous pain sensations (burning, tingling). This type of pain is excruciating for the patient and is not lessened by common pain meds including opioids.