Pain Pathways
Spinothalamic Tract
Dorsal Root & Spinal Cord
- The dorsal horn receives sensory innervation.
- The dorsal root receives sensory fibers.
- The dorsal root ganglion houses the primary sensory neuron.
- It is pseudounipolar, meaning it sends fibers both towards the spinal cord and also toward the periphery.
- Indicate that where the roots merge, they form the mixed spinal nerve.
- The sensory nerve activates a neuron in the dorsal horn (nociception most notably involves neurons of lamina I).
- Spinothalamic tract (STT) fibers cross within the spinal cord to the contralateral side.
Spinothalamic Projection
- Leg fibers synapse within the lumbar spinal cord and then decussate and ascend the anterolateral cord and brainstem to the thalamus.
- Arm fibers synapse within the cervical spinal cord, and, also decussate and ascend the anterolateral cord and brainstem to the thalamus.
- Trigeminothalamic fibers decussate within the pons and ascend the brainstem, medially, to the thalamus.
Thalamocortical Projection
- Leg fibers project to the midline region; they terminate in the posterior paracentral gyrus.
- Arm fibers project from the lateral thalamus to the upper convexity of the postcentral gyrus, lateral to the leg fibers.
- Facial fibers project from the medial thalamus to the lateral cerebral hemisphere.
Medial-lateral orientation
- Note that medial-lateral orientation of these fibers in the cerebral cortex is opposite of that in the thalamus.
Additional Pain Projection Fibers
- Pathways, such as the spinoreticular, spinomesencephalic, and spinohypothalamic tracts, and the cervicothalamic tracts all also carry noxious stimuli from their self-named origins and termination sites and synapse in many regions, including the nuclei we addressed with the descending pain modulatory system. Pain is more than simply the detection of a direct noxious stimulus, it is the emotional integration of many different influencing stimuli.
- For a detailed understanding of these pathways, as well as the trigeminothalamic tracts, and the correlating neurotransmitters that play a role in pain, review our tutorials on the: Midbrain, Pons, & Medulla.
descending pain modulatory system
Origins
- Originates from numerous regions, including the anterior cingulate gyrus, amygdala, insula, etc... And along its descent innervates the:
Periaqueductal gray area
- The periaqueductal gray area at the midbrain level.
Parabrachial complex
- The parabrachial complex in the dorsal pons at the superior cerebellar peduncle, which comprises the "brachium conjunctivum".
Rostral ventromedial medulla
- The rostral ventromedial medulla.
Dorsal horn
- The dorsal horn of the spinal cord where they inhibit pain pathways.
Clinical Terminology
- As a final part of this tutorial, let's learn some key clinical terminology, so we can get a sense of all of the ways that pain can manifest.
Pain types:
- Neuropathic pain is a broad term, which can refer to pain that stems from any somatosensory nervous system lesion or pathologic process.
- Whereas neuralgia is a more specific term, which refers to pain in a nerve distribution (or in the distribution of multiple nerves).
- And neuritis specifically refers to inflammatory nerve pain.
Abnormal pain responses:
- Hyper/hypo-esthesia refers to an increased (hyper) or decreased (hypo) sensitivity to a any sensory stimulus – it's a broad term.
- Whereas hyper/hypo-algesia specifically refers to an increased (hyper) or decreased (hypo) pain response to a noxious stimulus.
Unexpected pain:
- Allodynia refers to pain to a non-noxious stimulus, such as the bed-sheets brushing over your feet. This is especially common in complex regional pain syndrome (aka reflex sympathetic dystrophy).
Abnormal sensations:
- Dysesthesia, which refers to an unpleasant response and can encompass allodynia or hyperalgesia.
- Whereas, paresthesia is a similar term that describes an abnormal sensation but the sensation isn't unpleasant.
Absence of pain:
- Analgesia refers to the absence of pain to a painful stimulus.